Nuclear Physics News
International
Volume 24, Issue 1
January–March 2014
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Pairing Interaction • FRIB
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Vol. 24, No. 1, 2014, Nuclear Physics News1
Nuclear
Physics
News
Volume 24/No. 1
Contents
Editorial
NuPECC: A 25-Year-Old Expert Board of the 40-Year-Old European Science Foundation
by Angela Bracco and Jean-Claude Worms................................................................................................................... 3
Laboratory Portrait
The Nuclear Science Laboratory at the University of Notre Dame
by Ani Aprahamian, Philippe Collon, and Michael Wiescher....................................................................................... 5
Feature Articles
Beta-Delayed Fission: A Rare Decay Mode as Probe for Phenomena Near and Beyond the Fission Barrier
by Andrei N. Andreyev, Mark Huyse, and Piet Van Duppen.......................................................................................... 14
Pairing Interaction and Two-Nucleon Transfer Reactions
by Gregory Potel, Andrea Idini, Francisco Barranco, Enrico Vigezzi, and Ricardo A. Broglia................................... 19
Facilities and Methods
Investigating the Structure of Neutron-Rich Nuclei with Neutrons
by Gary Simpson............................................................................................................................................................ 26
NSCL and the Facility for Rare Isotope Beams (FRIB) Project
by Alexandra Gade, C. Konrad Gelbke, and Thomas Glasmacher............................................................................... 28
Meeting Reports
INPC 2013: Florence, 2–6 June 2013
by Angela Bracco, Pier Andrea Mandò, and Cosimo Signorini.................................................................................... 31
Baryons 2013: International Conference on the Structure of Baryons, Glasgow, 24–28 June 2013
by I. J. Douglas MacGregor........................................................................................................................................... 33
XXXIII Mazurian Lakes Conference on Physics: Frontiers in Nuclear Physics,
Piaski, Poland, 1–7 September 2013
by Chiara Mazzocchi, Krzysztof Rusek, and Krzysztof Rykaczewski............................................................................. 35
Latest News in Antiproton Physics Discussed at the LEAP 2013 Conference in Uppsala
by Tord Johansson.......................................................................................................................................................... 37
News and Views
The LUNA-MV Project at Gran Sasso Underground Laboratory
by Alessandra Guglielmetti............................................................................................................................................ 40
Calendar.................................................................................................................................................. Inside Back Cover
Cover Illustration: Airlift of the tank for the new 5U accelerator to its final location in the Notre Dame Nuclear Science Laboratory.
In the background the administration building of the university is the so called “Golden Dome” (see article on page 5).
2
Nuclear Physics News, Vol. 24, No. 1, 2014
editorial
NuPECC: A 25-Year-Old Expert Board of the
40-Year-Old European Science Foundation
It has been decided by the editorial
board to inform the readers in brief
on the European Science Foundation
(ESF). NuPECC is an expert board of
this organization, which is largely and
continuously benefitting from this position.
As it is known, ESF is going
through a drastic rearrangement of its
structure and activities and a decision
on whether to terminate this organization is expected to be taken at the end
of 2014. However, at the same time,
plans are being discussed to establish
another organization that would deal
with scientific services such as peerreview or evaluation, but could also
continue to host the Expert Boards
and Committees such as NuPECC.
The rather recent birth of the new organization, Science Europe, which is
based in Brussels and has set up six
scientific committees, has produced a
consequent winding down of scientific
activities of ESF.
As a consequence, since 2012 ESF
has been actively engaged in reducing
its traditional tasks (EUROCORES,
Forward Looks, Conferences, Research Networking Programmes, Exploratory Workshops) and in exploring a scenario for a potential successor
organization.
The year 2014, however, is an important year for ESF, which is celebrating its 40th anniversary. This year
marks indeed 40 years of achievement
of collaboration in science policy and
research management. This event will
be celebrated in May in Strasbourg.
On that occasion an archive of ESF
documentation will be also launched
and publicized. In the 40 years of
existence, ESF has accumulated a
very large amount of documentation
(books, publications, office archives)
that represent an interesting and very
valuable resource for various purposes
(e.g., research policy studies, future
policymaking). Thus the ESF Governing Council has decided to properly
preserve and archive this patrimony in
a scientifically valid manner after the
restructuring or termination of the organization. The archiving project aims
at referencing selected ESF documentation in a centralized database and
depositing it in an appropriate hosting
institution ensuring broad access to
the documentation to research management practitioners as well as to
researchers studying research policies
and practices. A very special collaboration with the University of Strasbourg was set up to include ESF’s
book collection in its own collections
and make it available to a large audience of students and academics. As regards publications and office archives
there is a proposal for the ESF documentation to be hosted by the Historical Archives of the European Union
(HAEU) at the European University
Institute in Florence. Other proposals
and projects related to it are possibly
forthcoming.
Turning now to NuPECC we would
like to stress some points that express
well the importance of being recognized as an expert board of ESF:
• The expert boards were mostly
created in ESF to respond to spe-
cific scientific needs. They provide scientific and policy advice
and initiate strategic developments and thus their scientific services in Europe, or even in a more
global framework, are indispensable for Europe’s scientific landscape. Being in ESF NuPECC
was facilitated in providing to EU
its responses to framework program consultations.
• Many NuPECC publications, including the forthcoming report
“Nuclear Physics for Medicine,”
are published with ESF and this
opens opportunities for the communication of our activities to
a broader audience. In addition,
the information on facilities recently collected and organized
by NuPECC are also used for the
MERIL database (the Mapping
of the European Research Infrastructure Landscape, created
with support of the ESF Member
Organizations). This database
represents a comprehensive inventory of research infrastructures of “more-than-national”
relevance across all scientific
domains and makes the information publicly available through
an interactive online portal with
analytical capabilities.
• ESF has always recognized
the independence of its expert
boards. Indeed the qualified independence of NuPECC has
been a key point to assure appropriate performance of its mission.
The views expressed here do not represent the views and policies of NuPECC except where explicitly identified.
Vol. 24, No. 1, 2014, Nuclear Physics News3
editorial
• For NuPECC to have rather
regular reviews made by special
panels and to have annual status
reports to be presented at the
ESF Governing Council meetings is stimulating and it helps
to define and realize its activity
plans.
These are some of the reasons why
NuPECC would like to be hosted in a
platform with other ESF expert boards
after 2015 and possibly within the potential successor organization. In general NuPECC feels that direct transfer
of expert advice and information to
top-level management and governance is important.
Contacts with Science Europe have
also been established to have constructive interactions on issues pertaining
to the building of the European Research Area, and for which NuPECC
should be consulted regarding nuclear
physics.
It is important to underline that ESF
is really dedicating particular effort to
help creating a common platform for
the expert boards.
However, the most important point
is that there seems to be a real opportunity to fund a downsized successor
of ESF. The possibility of a potential
successor relies on the fact that, if
the winding down of ESF left gaps in
services—such as peer review, evaluation, and so on—then those gaps
should be filled in order to help European Research Area quality and research.
We would like in conclusion to
recommend making a good use of the
ESF archive when it becomes available, to celebrate its 40th anniversary
and wait for more information about
this organization, hopefully positive,
in a few months!
Angela Bracco
NuPECC Chair
Jean-Claude Worms
European Science Foundation
Open Access
Authors now have the option to publish their Nuclear Physics News articles with Taylor & Francis Open Select. For more information, please visit: 4
http://journalauthors.tandf.co.uk/preparation/OpenAccess.asp Nuclear Physics News, Vol. 24, No. 1, 2014
laboratory portrait
The Nuclear Science Laboratory at
the University of Notre Dame
The Nuclear Science Laboratory
(NSL) at the University of Notre
Dame (ND) is a mid-sized accelerator laboratory, situated 100 miles east
of Chicago, serving the local nuclear
physics group and a large growing national and international user community [1]. The research focus of the ND
nuclear physics group is on nuclear
astrophysics, nuclear structure, and reaction physics. This is complemented
by new initiatives in the development
of accelerator applications and in
questions associated with nuclear reactions in plasma environments. The
NSL faculty has also continued to be
very successful in the development of
new instrumentation, and substantial
improvements to the laboratory infrastructure. Presently the laboratory
supports seven tenure track faculty
members, five research faculty, six
postdoctoral associates, seven technical and administrative personal, and
thirty-three graduate students.
The NSL accelerator facilities
(Figure 1) have substantially improved
over the last three years with the purchase and installation of a new 5MV
single ended (5U) Pelletron accelerator complementing the research activities of the 11-MV-tandem-(FN)-Pelletron accelerator. The installation and
testing of the 5U has been completed
and first measurements for its experimental have started. The 5U accelerator provides intense light and heavy
ion beams and is primarily dedicated
to the nuclear astrophysics program.
The FN accelerator is used for multiple scientific initiatives; it is a key
instrument for the NSL nuclear structure and astrophysics programs, for
the production of radioactive beams
Figure 1. Present layout of the NSL showing the location of the accelerators and
other major research.
at the TwinSol facility, which operates
as a radioactive beam facility for light
isotopes since 1995, and the Accelerator Mass Spectrometry (AMS) program for nuclear astrophysics. Over
the last few years the tandem is also
increasingly used for a broad range of
applications ranging from isotope production and radiation chemistry for
medical and biological application, to
nuclear reaction studies for homeland
security and nuclear forensics applications, AMS for climate and geological applications, and finally PIXE and
XRF for the analysis of historical, archaeological, and other materials [2].
Complementing these two machines,
the NSL operates a 1MV JN Van de
Graaff accelerator as well as the 200
kV Cockcroft Walton accelerator—in
the past used as an implanter—for expanding nuclear astrophysics experiments to lower energies.
Besides the installation of the new
accelerator there have been a number of new instrumental development
initiatives at both the NSL and other
facilities to support the scientific program of the laboratory’s faculty. The
key instrument is the St. GEORGE
separator (Strong Gradient Electromagnetic Online Recoil separator for
capture Gamma ray Experiments) for
low energy inverse kinematics experiments with intense heavy ion beams,
the GEORGINA gamma detector array (GE-detector Online aRray for
Gammas In Nuclear Astrophysics),
and the NERINA neutron counter
(NEutron detector aRay In Nuclear
Astrophysics). These detection systems provide a wide range of opportunities for low energy nuclear astrophysics experiments of interest for the
different phases of stellar burning that
drive stellar evolution. For the nuclear
Vol. 24, No. 1, 2014, Nuclear Physics News5
laboratory portrait
structure program the Internal Conversion Electron Ball Array (ICeBall)
was recently moved from Yale University’s Wright Nuclear Structure Laboratory and has been re-commissioned
at the NSL. ICeBall is a mini-orange
spectrometer arrangement with an absolute efficiency of 15% over 4π. The
use of ICEBall with the GEORGINA
Ge detector array is a very promising
new capability for the NSL. Further
developments at outside facilities include the design for an underground
accelerator complex, DIANA to be
located at the 4850 ft level of the Sanford Underground Research Facility
(SURF) and the recoil separator system, SECAR (SEparator for Capture
Reactions), for nuclear astrophysics
experiments with radioactive beams at
FRIB (Facility for Rare Ion Beams) at
Michigan State University.
The NSL has an ever-growing number of outside users. Since May 2011,
the lab had more than 100 visitors using the local facilities. They came from
15 foreign countries, 14 U.S. universities and colleges, four U.S. national
laboratories, and two industrial laboratories. Besides U.S. user groups this
also includes a substantial number of
users from European countries, such
as Austria, France, Germany, Hungary, Italy, and the United Kingdom,
followed by user groups from South
America, the Middle East, India, and
China. While the NSL is not a user
facility we try to accommodate these
requests to the best of our capabilities.
NSL Equipment
and Instrumentation
The FN tandem accelerator is the
workhorse of the laboratory. Installed
in 1967, the FN tandem was up-graded
in 1995 with two Pelletron chains, and
routinely reaches an operating voltage of 10.5 MV at the terminal with
a CO2/N2 tank gas mixture [1]. The
tandem serves for a number of nuclear
6
structure and astrophysics experiments, it is the backbone for the AMS
program it serves as the driver for the
TwinSol dual superconducting solenoid separator. In addition the tandem
serves three target stations for basic
nuclear physics experiments, including a large scale scattering chamber,
a neutron time of flight beam-line, a
station for the measurement of conversion electrons. Finally the tandem
serves two dedicated target stations
for applied physics experiments, one
for radiochemistry measurements operated independently by the ND Radiation Laboratory, and one PIXE material analysis station. The FN is served
by two ion sources, a duoplasmatron
source, which is primarily used for
the production of 3He and 4He beams,
and a 40 cathode Multi-SNICS sputter source. The laboratory is in the
process of up-grading the ion source
area and replacing the injection magnet for improving the mass resolution
for heavy ion beams for the AMS program. The FN provides opportunities
for a broad and diverse NSL user community including researchers from
small local research institutions like
Hope College and Indiana University
South Bend to users from Princeton
and Fermilab.
The TwinSol radioactive nuclear
beam facility, has been developed as
a collaboration between groups from
the University of Notre Dame and the
University of Michigan [3]. TwinSol
is a dual superconducting solenoid
separator system which selects radioactive reaction products up to mass
A = 20 from nuclear reactions triggered by a heavy ion primary beam on
different target materials, and focuses
the intense radioactive beam species
on a secondary target for low energy
radioactive beam reaction studies.
TwinSol has found a wide range of applications in the study of radioactive
beam processes for nuclear astrophys-
Nuclear Physics News, Vol. 24, No. 1, 2014
ics, the investigation of neutron halos,
and the impact of halos on low energy
fusion processes.
The Tandem is also central for the
Accelerator Mass Spectrometry (AMS)
program at the NSL [4]. The separation
is improved by an additional velocity
filter positioned after the second analyzing magnet in the AMS bean-line.
The gas-filled Browne-Buechner Spectrometer serves as a final for isobar
separation station. The AMS system
provides an additional technique to the
nuclear astrophysics research efforts at
the NSL, but is also increasingly used
for a wider range of applications in the
analysis of geological, paleoclimate,
and forensic samples. It uniquely combines the high sensitivity provided by
the gas-filled magnet AMS analysis
technique with the energies and beams
made available by the FN accelerator.
Presently the facility focuses on the
analysis of medium mass long-lived radioactive species from 36Cl, 44Ti, 56Ne,
60Fe, and 93Zr improvements in the ion
source injection system will expand the
mass range well above A = 100.
The 5U single-ended Pelletron
was installed in spring 2013 to replace the 3.5 MV KN Van de Graaff
that had served for operating the low
energy nuclear astrophysics program
for the last 20 years. The new machine reaches 5MV terminal voltage.
With four Pelletron chains it achieves
the high beam intensities necessary
for the nuclear astrophysics program
of the laboratory. It is a vertical machine with a compact Nanogan ECR
ion source mounted in the terminal
(Figure 2). The ECR source allows the
production of intense heavy ion beams
in 2+ or 3+ charge states, a feature that
is important for reaching higher energy beams for inverse kinematics
experiments. A number of beams have
been developed, proton and alpha
beams are produced with intensities
of up to 200 mA, but higher currents
laboratory portrait
Figure 2. Column and ECR of the 5U accelerator at the NSL.
are expected for the future due to improvements. Heavy ion beams like
14N, 16O, 38Ar have been developed
and tested for different charge states.
Beam intensities in excess of 100 mA
have been obtained. The 5U Pelletron serves three beam-lines, two are
equipped with high beam power target
stations, one a solid beam-stop target
for radiative capture experiments, and
one a recirculating gas target system,
which can be operated both in the
extended as well as jet target mode.
These two beam-lines will be primarily used for intense light ion proton
and alpha beams, while a third beamline is dedicated to heavy ion beam
experiments with the recoil separator
St. GEORGE.
St. GEORGE is a state of the art
recoil separator for separating heavy
recoil reaction products from low energy inverse kinematics experiments
(Figure 3). The device is in the tradition of the DRAGON separator at
TRIUMF (Canada) and the ERNA
separator at Ruhr-University Bochum
(Germany), now at CIRCE in Caserta
(Italy), and is designed and dedicated
to the study of critical nuclear reactions in stellar helium burning. The
reactions take place in a high density
helium jet gas target HIPPO, with the
heavy ion beam being delivered by
the 5U accelerator. St. GEORGE consists of 14 quadrupole and six dipole
magnets to ensure proper charge separation and mass separation between
primary beam particles and reaction
recoils. The main separation is based
on a velocity filter positioned after the
first separation units. St. GEORGE has
a calculated rejection power of 1015
and a predicted mass resolving power
of 100. Further background reduction
of multiple scattered beam particles
will be achieved in the detection system itself, which takes advantage of
energy and timing measurements to
differentiate recoiling reaction products from the remaining primary beam
particles. The timing detection system
requires beam and recoil products to
pass through two thin foils before energy measurements are made with a
silicon detector, and energy straggling
in these foils will degrade the mass
resolution of the detection system.
Figure 3. The St. GEORGE recoil separator with the HIPP gas jet target at the
bottom part of the picture.
Vol. 24, No. 1, 2014, Nuclear Physics News7
laboratory portrait
The NSL’s 1 MV JN Van de Graff
was decommissioned during the time
of the 5U installation, it covered
the very low energy range between
150 keV and 900 keV, and traditionally reached fairly high proton and alpha beam currents up to 150 mA. The
machine is presently being rebuilt and
up-graded to be moved to the 4,850-ft
level of SURF as a first step toward
the development of an underground
accelerator program in the United
States.
Research at the NSL
The nuclear astrophysics program
of the NSL is recognized worldwide
and has centered on the study of low
energy nuclear reactions in stellar hydrogen, helium, and carbon burning
[5]. This is of particular importance
for the understanding of nucleosynthesis in early stars, the origin of seed
materials in explosive stellar environments, and the source of neutrinos in
our sun and other main sequence stars.
A number of key reactions for neutrino and neutron production in quiescent and explosive stellar environments have been investigated. Also
new sources of neutron production
during the helium and carbon burning
phases in stellar evolution have been
addressed. Furthermore, the group has
worked on important aspects associated with the thermonuclear runaways
in cataclysmic binary star systems
as well as with the study of the reactions in shock-front driven explosive
nucleosynthesis environments associated with the origin of long-lived galactic radioactivity. New theoretical
tools have been developed for deriving reliable reaction rates and for investigating the impact of the nuclear
reactions on nucleosynthesis, energy
production, and time scale of dynamic
stellar environments. Studies of critical nuclear structure parameters (e.g.,
masses, deformation, and incompress-
8
ibility) have been performed to extract information on critical input for
understanding the nucleosynthesis aspects of core collapse supernovae associated with the p- and r-process and
the origin of long-lived galactic γ-ray
sources. Beyond interests in nuclear
astrophysics, the nuclear structure effort has focused on the investigation of
collective modes and on novel modes
of quantal rotation using techniques of
γ-ray spectroscopy. The experimental
program is guided and complemented
by theoretical efforts aimed at reliable
predictions of nuclear structure and
stellar reaction rates and on possible
observable signatures associated with
these rates. Similarly, r-process related
measurements are guided by simulation studies to identify the most sensitive isotopes for mass and decay measurements. Strong efforts have been
made to develop a program in nuclear
physics and accelerator applications,
often in collaboration with other university groups and institutions. This
includes the development of AMS
techniques using new long-lived isotopes as well as the application of
PIXE and X-Ray Fluorescence (XRF)
as analytical tools on archaeological,
forensic, and biological samples [2].
The following sections give a
brief overview of some of the scientific highlights of the NSL research
program in nuclear astrophysics, Accelerator Mass Spectrometry, and radioactive beam physics as well as an
overview of our nascent applied program.
Nucleosynthesis in Quiescent
and Explosive Stellar Burning
The measurements of critical low
energy reaction cross-sections for
stellar burning are one of the major
research initiatives in nuclear astrophysics. Several fundamental problems are associated with the lack of
reliable low energy data of nuclear
Nuclear Physics News, Vol. 24, No. 1, 2014
reactions during the various stellar
burning phases. The so-called neutrino problem has been solved but
improvements in the associated reaction rates for the production of solar
neutrino emitters remains important as
a new tool to probe directly the conditions in the solar interior. At the NSL
we have studied experimentally and
theoretically a number of reactions
that impact the solar neutrino flux. The
3He(α,γ)7Be reaction is important for
neutrino production by the pp-chains
in our sun, new measurements as well
as an extensive new R-matrix analysis reduces previous uncertainties in
the extrapolation of the low energy
cross-section to less than 4% [6]. Detailed measurements of proton capture
on 12C, 14N, 15N, and 17O isotopes
that influence the neutrino production by the CNO cycles, have been
performed over a wide energy range.
The results directly address the role of
the CNO cycles in solar neutrino production and serve as important input
for using the measurement of CNO
neutrinos with BOREXINO as independent tool for determining the solar
metallicity. This program on radiative
capture measurements will continue
at the 5U accelerator using the new
GEORGINA gamma detection system
to improve the efficiency of the measurements. It will be expanded toward
radiative capture reactions in higher
mass regions to address nucleosynthesis patterns in nova explosions.
Stellar helium burning is of particular relevance since the reaction
chain 4He(2α,γ)12C(α,γ)16O(α,γ)20Ne
determines the 12C/16O in our universe. Particular attention was given to
the study of the 12C(α,γ)16O analysis,
probing the reaction components by
indirect techniques like elastic scattering on 12C and beta-delayed alpha decay analysis of 16N. This information
puts considerable constraints on the
R-matrix analysis of the 12C(α,γ)16O
laboratory portrait
radiative capture cross-section and reduces considerably the uncertainty in
the S-factor extrapolation of the low
energy data as shown in Figure 4. The
16O(α,γ)20Ne reaction has been measured successfully and low energy
extrapolation has been pursued by
R-matrix techniques. The installation
of the St. GEORGE separator opens
new possibility by studying the alpha
radiative capture processes in inverse
kinematics using intense heavy ion
beams on a high density helium gas
jet target. This new experimental program has just started with calibrating
the separator with well-known alpha
capture reactions such as 3He(α,γ)7Be
and 14N(α,γ)18F in inverse kinematics. Besides the 12C(α,γ)16O and the
16O(α,γ)20Ne reactions, other alpha
capture studies such as 15N(α,γ)19F
of relevance for helium shell burning
patterns and the origin of 19F through
the AGB phase of low mass stars will
be investigated.
The question of the stellar neutron
sources for the s-process in stellar helium and carbon burning has been one
of the important missions for the NSL
research program. We investigated
the impact of (α,n) reaction on 17O
and 18O that play a crucial role for the
overall neutron budget in stellar burning environments. In particular one of
the dominant neutron source of the sprocess is the 22Ne(α,n)25Mg reaction.
Direct measurements of it are handicapped by the large cosmic ray induced neutron background and therefore indirect techniques have been
developed to probe the alpha cluster
structure of the compound nucleus
26Mg. Alpha scattering and alpha
transfer reactions have been utilized
for this purpose to achieve a consistent
picture of the alpha cluster structure
in 26Mg near the alpha and neutron
threshold. For carbon burning, new
initiatives were developed to study
the low energy cross sections of the
Figure 4. Experimental data and R-matrix analysis of all reaction channels
12C(α,α), 16N(b-α), 12C(α,γ) feeding the 16O compound nucleus. This is an example of a comprehensive approach toward extrapolating the reaction crosssection of 12C(α,γ) to the stellar energy range around 300 keV.
various reaction channels of 12C + 12C
fusion and explore the possibility of
the 12C(12C,n)23Mg reaction as an sprocess neutron source in shell carbon
burning during late stellar evolution.
This effort resulted in a full simulation
of nucleosynthesis patterns during
carbon burning.
The NSL has developed an extensive program for investigating the origin of p-nuclei, the rarest stable neutron deficient isotopes above A = 100.
The p-nuclei are produced by photodissociation of heavy mass materials
in supernova environments. The cross
sections of critical photo-excitation reactions along the predicted p-process
path are derived by studying the inverse radiative capture reactions using
the activation method by monitoring
the characteristic emission of the reaction product. Using this technique,
a number of (p,g) and (α,g) reactions
have been studied in the Z = 50 closed
shell range. These measurements have
been complemented by systematic
analysis of the alpha scattering potential to simulate the data with improved
statistical model calculations.
Reactions that are important in explosive nucleosynthesis environments
often involve radioactive nuclei since
the reaction path is far from the line of
stability. For that reason the NSL has
been a driving force for large-scale
facilities like FRIB that promise the
delivery of high intensity radioactive
beams and participates on new developments such as the recoil separator
SECAR (SEparator for Capture Reactions) for nuclear astrophysics experiments at FRIB. However, there is also
considerable effort in using transfer
and capture reactions to probe indirectly the various reaction components
that determine the stellar reaction and
decay rates for critical processes along
the rp-process path and the r-process
path. These studies are being performed at the FN tandem accelerator
Vol. 24, No. 1, 2014, Nuclear Physics News9
laboratory portrait
but more recently take advantage of
other laboratories such as the RCNP in
Osaka, Japan, or GSI near Darmstadt,
Germany.
The experimental program in nuclear astrophysics is complemented by
theoretical studies that aim at identifying the most important experiments,
and most challenging questions to
pursue. Research is focused on the
implementation of new reaction rates,
masses, and lifetimes into nucleosynthesis simulations for evolutionary and explosive stellar conditions.
This includes the development of the
powerful multi-channel, multi-level
R-matrix code AZURE [6] that has
set new standards in the field of low
energy nuclear reactions and has been
applied for the analysis of numerous
reaction studies.
A new formalism was developed
for calculating fusion rates at extreme
densities as anticipated for the core of
white dwarf or the crust of neutron star
crusts. The formalism is suitable for
applications ranging across extremes
of temperature and density, from thermonuclear to pycnonuclear regimes.
While initial studies concentrated on
interactions between even–even nuclei, new initiatives have also included
odd–even and odd–odd nuclei. Recent
experimental work at ANL on fusion
reactions between stable carbon and
radioactive carbon isotopes showed
remarkable good agreement with the
predictions (Ernst Rehm, private communication). This work primarily addressed nucleosynthesis patterns in
high density neutron crusts by following the fate of x-ray burst ashes in accreting neutron star environments.
Extensive simulations have been
performed for the prediction of r-process abundance patterns. We have conducted a sensitivity study of the light rprocess path to identify the nuclei that
have the greatest impact on the final
abundance distributions (Figure 5).
10
Figure 5. The most impactful nuclei as determined from binding energies for
a classical hot r-process trajectory using three different mass models (FRDM,
Duflo-Zuker, HFB-21). The black line shows the potential experimental reach of
CARIBU at ANL while the gray line is the potential reach of FRIB in construction at the NSCL in Michigan State University.
Using the FRDM mass model as a
standard, the study was performed by
varying the binding energy of each
individual nucleus by ±25% and then
evaluating the impact of that change
on the entire range of the r-process
production [7]. The work is currently
being expanded to analyze the sensitivity of the r-process abundance
distribution to half-lives and neutron
capture reaction rates. The goal of
these r-process sensitivity studies is
to pinpoint the key nuclei that have
the most crucial impact on the abundance distribution. These studies have
become the basis on which several
proposals to Californium Rare Isotope
Breeder Upgrade (CARIBU), TRIUMF, NSCL, and RIKEN have been
submitted and will become the basis
Nuclear Physics News, Vol. 24, No. 1, 2014
for some of the future experimental
program of the NSL group.
AMS Development and Applications
The AMS program has focused on
measurements related to galactic radioactivity and main stellar burning as
well as the production of Short-Lived
Radionuclides (SLRs) in the Early
Solar System (ESS). The Notre Dame
AMS system, unique in North America in this configuration, provides the
nuclear astrophysics research efforts
of the laboratory with an additional,
highly sensitive detection technique.
It combines the high sensitivity provided by the gas-filled magnet separation technique with the energies and
beams made available by the FN accelerator, as well as the accessibility
laboratory portrait
to beam time [4]. Recently the AMS
astrophysics program has focused on
44Ti, 60Fe, 36Cl, and 93Zr detection and
is currently developing 55Mn as well
as a 14C capability for the growing
NSL applied program. Below some
details on some of these programs.
One isotope, 44Ti, offers a unique
synergy between observational, theoretical, and experimental nuclear astrophysics associated with explosive
silicon burning. It is typically assumed
to be mainly produced by the alpha
capture reaction 40Ca(α,γ)44Ti in the
alpha-rich freeze-out phase of an expanding supernova shock front. The
concurrent studies of this reaction at
the NSL via AMS and via direct radiative capture in-beam γ measurements
have placed very precise restrictions
on the reaction rate in the burning
window of astrophysical interest.
60Fe is predominantly produced in
the galaxy by type II supernovae and
plays an important role in astrophysical
investigation of a possible supernova
near the solar system about three million years ago. Recent measurements
of the NSL group of the 60Fe(n,γ)61Fe
at the FZ Karlsruhe, Germany and the
59Fe(γ,n)60Fe depend critically on the
60Fe half-life, which is currently open
for debate as a recent t1/2 = 2.6 × 106y
has been reported by the Munich AMS
group. The NSL group now pursues
AMS detection of 60Fe based on the
gas-filled magnet technique to separate the 60Fe-60Ni isobars. Our present 60Fe/Fe sensitivity of ~1 × 10–13
has allowed us to make important
60Fe concentration measurements on a
number of samples at ND in an effort
to re-determine its astrophysically important half-life. Recently a new 60Fe
sample was obtained from the VERA
group in Vienna for a direct decay activity measurement of 60Fe that will
complement the measurements of the
60Fe material produced by our group
at MSU during the previous funding
cycle.
Radionuclides with lifetimes τ < 100
My (e.g., 146Sm, 129I, 60Fe, 53Mn,
41Ca, and 36Cl) are often referred to as
SLRs known to have been extant when
the Solar System formed 4.568 Gy
ago. Identifying the origins of SLRs
can provide insight into the origins
of our solar system and the processes
that shaped it. Now extinct 36Cl in the
ESS is thought to have been produced
by local particle irradiation. However,
the models that attempt to recreate the
production of 36Cl in the ESS lack experimental data for the nuclear reactions considered. In particular, data for
the 33S(α,p)36Cl reaction, which is an
important reaction in the production of
36Cl, could help reduce the uncertainties in the models and better constrain
the environment where the SLRs were
produced. A very successful AMS
project to measure this reaction crosssection was recently performed at the
NSL and the results and comparisons
to model predictions have been published [8].
AMS detection of the rare isotope
93Zr (t
1/2 = 1.6 Ma) has application
potential in two fields of research:
the improved astrophysical modeling
of nucleosynthesis processes through
an independent determination of the
92Zr(n,γ)93Zr cross-section at stellar
energies, currently only measurable
using the TOF technique, and using
this radionuclide as a tracer in hydrological and radioactive waste studies.
A combination of chemical reduction
of the Nb interfering isobar in combination with the gas-filled magnet technique and a newly developed Bragg
peak detector show promise to allow
the detection of 93Zr in the ~10–8 isotopic ratio range.
Radioactive Beam Studies
at the NSL
The Radioactive Nuclear Beam
(RNB) group at ND has maintained
an extensive research program at the
TwinSol facility (Figure 6) using the
FN tandem accelerator as a driver. It
involves the study of nuclear reactions
with beams of short-lived radioactive
nuclei together with the development
of instrumentation and techniques to
facilitate these investigations. TwinSol
provides intense radioactive beams up
to A = 20 with the mass range limited
by energy range of the tandem driver.
Figure 6. The TwinSol radioactive beam facility Notre Dame.
Vol. 24, No. 1, 2014, Nuclear Physics News11
laboratory portrait
Secondary beams of 6He, 7Be, 8Li, 8B,
10Be, 12B, 12N, and 18Ne have been
used in experiments with maximum
intensities from 104 to 107 s–1, depending on the primary beam.
More recently, produced beams of
6He, 10Be, and 12B were used simultaneously to study the cluster structure of the composite systems 10Be
and 14C, respectively, using an active
target-time projection chamber (ATTPC). The NSL presently plans to expand the range of TwinSol application
by using it as a HELIOS (HELIcal
Orbit Spectrometer) type of separator
system, with the first solenoid acting
as a focusing element to enhance the
acceptance and the second solenoid
acting as helical orbit spectrometer.
Beams such as 11,14C, 15O, and
17,18F have been developed with a
similar range of intensities but have
not been used in experiments to date.
Detector Development
and Testing
Considerable effort was dedicated
for the support of the SCENE (Scintillation Efficiency of Nuclear recoils
in Noble Elements) experiment for the
development of a liquid argon detector
for direct dark matter search experiments. The SCENE experiment was
devised as a collaboration between
Princeton University, Fermi National
Laboratory, and several other institutions to measure the energy- and electric-field dependencies of the ionization and light yields from low-energy
nuclear recoils in liquid argon. These
data are of interest for understanding
the mechanism of liquid argon scintillation at moderate to high linear
energy transfer. For SCENE, a small,
dedicated LAr-TPC is used in coincidence with liquid scintillator neutron
detectors to collect elastic neutron
scattering events. With a pulsed neutron beam, clean data has been ob-
12
tained for recoil energies as low as
3.0 keV. Future runs are also planned
to improve and expand the dataset,
in particular for measurements of the
ionization yield.
The prototype for the AT-TPC
of MSU for ReA3 experiments was
tested with different gases and beams.
Beams of 6He, 10Be, and 12B produced by the TwinSol facility were
used simultaneously to study the
cluster structure of the composite
system, 10Be and 14C, respectively.
The products of complete and incomplete fusion from the beam with 4He
or 36Ar were detected for the study of
the fusion process with halo nuclei.
The three body break up of 14C is being analyzed. The detector tests have
been very successful and presently we
are discussing a number of scientific
studies associated with cluster-breakup of near threshold states in eveneven nuclei using the ND accelerator
facilities.
Applications for Cultural Heritage
and Societal Impact at the NSL
PIXE at the NSL is a very useful
tool for determining the elemental
composition of a wide range of specimens. We have developed a small test
set-up to initiate a broader applied program in cultural heritage at the NSL.
Pottery samples from the U.S.
Southwest Anasazi sites have been
analyzed using PIXE techniques. Of
particular interest was the pigment
composition of the painted pottery
surface of Mesa-Verde Black on white
pottery. The goal of the archaeologists involved was to differentiate the
source materials used to make the pigments on various samples taken from
different locations that may provide
insight into the production methods
and possible geographical origins of
the ceramics.
Roman Silver Denarii from the
period of 240 BC to 250 AD were
Nuclear Physics News, Vol. 24, No. 1, 2014
analyzed using high energy PIXE
techniques at the FN tandem accelerator. After mapping the composition of the surface of the coin with a
3 MeV beam, the composition of the
inner coin layers were probed with 7
to 9 MeV proton energy. The silver/
copper ratio of the coins was determined and showed a gradual decline
in silver between 240 BC and 200
AD, followed by a rapid devaluation
in the subsequent decades of the Severi emperors.
To complement the PIXE approach
methods like XRF and Raman spectrometry are being developed for
probing medieval manuscripts from
the ND rare book collection. We are in
the progress of developing a program
for scanning and analyzing selected
samples from the Ancient and Medieval Manuscript collection of the ND
libraries. The main purpose is to study
the pigment structure and composition
on medieval document ornaments and
paintings on selected library samples
using XRF and Raman spectrometry
techniques. This initiative is being
broadened by the development of an
art analysis program. The analysis of
forfeited painting materials will lead
to a broader effort in art forensics for
the identification of forgeries.
Radiation chemistry studies at the
NSL examine the fundamental processes that occur due to the passage
of ionizing radiation in condensed
matter. Energy deposition, medium
decomposition, and subsequent chemistry are being investigated in a variety of materials of importance to the
nuclear power industry. Processes induced in matter by ionizing radiation
are strongly dependent on the type
and energy of the incident radiation
because of the geometry of the local
energy deposition. The FN tandem
accelerator in the NSL is ideal for
performing these types of studies because of the variety of light ions and
laboratory portrait
energies available. Systematic studies have been performed on water and
a variety of aqueous solutions that
mimic the coolants in nuclear power
reactors.
Heterogeneous radiation chemistry does not only address corrosion
issues of reactor components but is
also a crucial aspect for the long-term
storage of waste materials. Water radiolysis has been shown to be substantially altered at or near to interfaces as
compared to bulk water. The transport
of energy, charge, and matter through
interfaces is now receiving renewed
interest because of the fundamental
studies performed at the NSL.
Future energy needs involving nuclear power will necessarily involve
the treatment and storage of waste materials. Many nuclear waste materials
are alpha particle emitters and studies
at the NSL have often included 5 MeV
helium ions to mimic this radiation.
Comparison of the results with conventional gamma radiolysis indicates
that several polymers and resins that
are thought to be radiation resistant
can readily decompose with alpha
particle radiolysis. Fundamental studies with various components of resins
are still in progress, but the results indicate that reactions of highly excited
states within the incident radiation
track are responsible. Ionic liquids are
the new designer solvents and work
currently underway is helping design
more radiation robust liquids to be
used in electrochemical waste separation systems.
Advanced fast neutron detection systems are being developed for
Homeland Security and related applications based on organic scintillation
detectors. Such detectors have proven
capabilities of neutron and γ-ray detection; however, more research is
needed to optimize their γ-ray discrimination capability before they can
be widely deployed. The proposed
systems are designed to provide detection capability equivalent to or better
than existing 3He-based systems, and,
in many cases, for a fraction of the
cost, given the current shortage of 3He
material. In addition, the advanced
digital data-acquisition capabilities
developed for the proposed systems
provide real-time particle classification with a simplified data stream for
quick and reliable identification of
special nuclear material.
A second project is to develop neutron detection capability for precision
Non-Destructive Assays (NDAs) of
actinide-fluoride samples. Fluorine is
commonly used in actinide compounds (mostly UF6) in the nuclear
fuel cycle. The method will significantly advance safeguards verification
at existing declared facilities, nuclear
materials accounting, process control,
nuclear criticality safety monitoring,
and a variety of other nonproliferation applications. INL in partnership
with Oak Ridge National Laboratory, Rutgers University, and ND are
undertaking a precision (better than
10%) determination of the absolute
cross section of the 19F(α,n)22Na reaction as a function of energy. The specific goal is to identify the neutron and
γ-ray energy spectrum emitted from
19F(α,n)22Na at alpha energies pertinent to NDA. The project uses the
Versatile Array of Neutron Detectors
at Low Energy (VANDLE) detector
array in a two-part experiment, with
both fluorine and α beams, to completely characterize the 19F(α,n)22Na
reaction and overcome difficulties
with previous measurements that seriously limited the precision of neutronbased assays of UF6 samples.
References
1. A. Aprahamian and M. Wiescher, Nuclear Physics News 12 (4) (2002) 5.
2. P. Collon and M. Wiescher, Physics Today 65 (2012) 58.
3. F. D. Becchetti, M. Y. Lee, T. W.
O’Donnell, D. A. Roberts, J. J. Kolata,
L. O. Lamm, G. Rogachev, V. Guimarães, P. A. DeYoung, and S. Vincent,
Nucl. Instr. Meth. A505 (2003) 377.
4. D. Robertson, C. Schmitt, P. Collon, D.
Henderson, B. Shumard, L. Lamm, E.
Stech, T. Butterfield, P. Engel, G. Hsu,
G. Konecki, S. Kurtz, R. Meharchand,
A. Signoracci, and J. Wittenbach, Nucl.
Instr. and Meth. B 259 (2007) 669.
5. M. Wiescher, F. Käppeler, K. Langanke, Ann. Rev. Astr. Astrophys. 50
(2012) 165.
6. R. E. Azuma, E. Uberseder, E. C. Simpson, C. R. Brune, H. Costantini, R. J. de
Boer, J. Görres, M. Heil, P. J. LeBlanc,
C. Ugalde, and M. Wiescher. Phys. Rev.
C 81 (2010), 045805.
7. S. Brett, I. Bentley, N. Paul, R. Surman,
and A. Aprahamian, Eur. Phys. J. 48
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8. M. Bowers, Y. Kashiv, W. Bauder, M.
Beard, P. Collon, W. Lu, K. Ostdiek,
and D. Robertson, Phys. Rev. 88 (2013)
065802.
Ani Aprahamian,
Philippe Collon,
and Michael Wiescher
Nuclear Science Laboratory,
University of Notre Dame,
Notre Dame, Indiana, USA
Vol. 24, No. 1, 2014, Nuclear Physics News13
feature article
Beta-Delayed Fission: A Rare Decay Mode as Probe
for Phenomena Near and Beyond the Fission Barrier
Andrei N. Andreyev1, Mark Huyse2, and Piet Van Duppen2
of York, UK, and Advanced Science Research Center, Japan Atomic Energy Agency,
Tokai-mura, Japan
2KU Leuven, Belgium
1University
Recently, with the advent of advanced radioactive beam
facilities and novel experimental techniques, unexplored
regions of exotic nuclei and new phenomena became accessible for detailed spectroscopic studies [1]. Examples
are the discovery of neutron halos, the appearance of new
magic numbers, exotic types of particle decays (e.g., two
proton radioactivity), shape coexistence, and exotic shapes
of nuclei [2–4] to name a few. Along the same lines, it is
relevant to ask what happens to the interesting, but at the
same time, complex phenomenon of nuclear fission, when
one moves further away from the classical and relatively
well-studied region of fission in the trans-uranium nuclei?
Fission of atomic nuclei, discovered in 1938, represents
one of the most dramatic examples of a nuclear metamorphosis, whereby the nucleus splits preferentially into two
smaller fragments releasing a large amount of energy. Historically, several distinctive types of fission were identified,
such as particle-induced fission (e.g., neutron-induced fission), spontaneous fission (SF), and spontaneously fissioning isomers, beta-delayed fission (bDF, being the subject of
this article) [5], electromagnetically induced (Coulomb excitation, or Coulex) fission of radioactive nuclei at relativistic energies [6], photofission, and surrogate-type of fission.
The fission process is often broadly classified as highenergy fission, in which the excitation energy E* of the fissioning nucleus strongly exceeds the fission barrier height
Bf , or, as low-energy fission (E* ≤ Bf). Figure 1 schematically shows the present status of experimental low-energy
fission studies. Low-energy fission is a unique tool to probe
the nuclear potential energy landscape and its dynamical
evolution, as a complex function of elongation, mass-asymmetry, spin and excitation energy, from the single “compound nucleus” system over the top of the fission barrier
and further to the scission point, culminating in the formation of fission fragments. This evolution involves a subtle
interplay of collective (macroscopic or mean field) and
single-particle (microscopic) effects, such as shell-effects
and pairing, all of which are considered both for the initial
14
Figure 1. The nuclei for which fission fragments mass or
nuclear-charge distributions have been measured by lowenergy fission. The distributions are shown for selected
systems. Blue open circles: distributions, measured in conventional particle-induced fission experiments and spontaneous fission. Green crosses: nuclear-charge distributions,
measured by Coulomb-excitation [6]. Open and closed diamonds show 26 known bDF cases in the three regions of
bDF (see text), the fissioning daughter is indicated. Filled
diamonds mark 11 daughter nuclides for which mass distribution was measured, two of them—180Hg and 242Cf are
shown in the plot. The references to all data in the plot can
be found in Ref. [5].
nucleus and for the final fission fragments and at large deformations. Fission enables the study of nuclear-structure
effects in the heaviest nuclei and has direct consequences
on their creation in nuclear explosions and in the astrophysical r-process [6], which is terminated by fission, and on
the abundance of medium-mass elements in the universe
through so-called “fission recycling.” Three fission processes are thought to be important for r-process termination
by fission: spontaneous fission, neutron-induced fission,
and beta-delayed fission.
In contrast to high-energy fission, in which the microscopic effects are washed out, the interplay between macro-
Nuclear Physics News, Vol. 24, No. 1, 2014
feature article
Figure 2. Simplified diagram for the b+/EC delayed fission
in the neutron-deficient nuclei. Shown are the ground states
of the parent (A,Z) and daughter (A,Z-1) nuclei, and as a
function of elongation, the potential energy and associated
shapes of the daughter nucleus. QEC value of the parent
and fission barrier Bf of the daughter nuclei are indicated
by vertical arrows. The bDF of excited states with E* ~
Bf in the daughter nucleus is shown by horizontal arrows.
The color code on the right-hand side represents the fission
probabilities; the darker colors correspond to higher probabilities.
scopic and microscopic effects in fission can be sensitively
explored at low excitation energy. In particular, in SF from
the ground state, the excitation energy is E* = 0 MeV, while
in SF from isomeric states or in thermal neutron-induced
fission it does not exceed a few MeV. However, SF studies are limited to heavy actinides and trans-actinides. By
using Coulomb-excited fission of relativistic radioactive
beams [6], fission studies became available in new regions
of the Nuclidic Chart with exotic N/Z ratios, see the nuclei
marked by crosses in Figure 1. In this case, the excitation
energy is centered around E* ~ 11 MeV.
In terms of the excitation energy, bDF is intermediate
between SF and Coulomb-induced fission. Beta-delayed
fission, discovered in 1965–1966 in Dubna [5], is a twostep nuclear decay process that couples beta decay and
fission, see Figure 2. Similar to other beta-delayed decay
processes, in bDF, a parent nucleus (a precursor) first undergoes b decay, populating excited state(s) in the daughter nuclide. In the case of neutron-deficient nuclei, electron
capture (EC) or b+ decay is considered, while b– decay happens on the neutron-rich side of the Nuclidic Chart. We will
use throughout this text the term bDF for both the neutronrich and neutron-deficient nuclei. In bDF, the maximum
excitation energy of the daughter nucleus is limited by the
QEC (Qb– in case of neutron-rich nuclei) of the parent. The
typical QEC values are in the range of 3–6 MeV and 9–12
MeV for the known bDF nuclei in the trans-uranium and
lead regions, respectively. If the excitation energy of these
states, E*, is comparable to or greater than the fission barrier height, Bf, of the daughter nucleus (E* ~ Bf) then fission
may happen in competition with other decay modes (e.g., g
decay and/or particle emission). Therefore, the special feature of bDF is that fission proceeds from excited state(s) of
the daughter nuclide. As these states are populated in the
beta decay, the time scale of the bDF events is determined
by the half-life of the parent nucleus. As in most cases the
b-decay half-lives are longer than tens of ms, it makes bDF
more easily accessible for experimental studies.
In summary, the importance of bDF is highlighted by its
ability to provide low-energy fission data for very exotic
nuclei that do not decay by SF and that are difficult to access by other techniques.
Presently, 26 bDF cases are known experimentally, see
Figure 1 and review [5], where all relevant references are
given. This can be compared to the number of ~220 cases of
beta-delayed charged particle emission on the neutron-deficient side and ~200 cases of beta-delayed neutron emission
on the neutron-rich side [2]. Globally speaking, the bDF
nuclei are situated in three distinct but extended regions of
the Nuclidic Chart: in the neutron-rich Ac and Pa isotopes,
and the neutron-deficient isotopes in the trans-uranium and
lead regions.
Historically, the first cases of bDF were discovered in
Dubna in the precursors 232,234Am (thus, 232,234Pu are fissioning daughter nuclides). This region is relatively easily
accessible by complete fusion reactions with heavy ions
but the initial experiments used quite unselective production and detection techniques (e.g., the fission track mica
foils). In many cases, the identification of the bDF precursors and the assignment of fission to a specific nuclide was
done based on half-life and extensive cross-irradiation with
projectile energy and different projectile-target combinations. The Berkeley group significantly contributed to bDF
studies of 228Np, 232,234Am, 238Bk, and 242,244,246,248Es isotopes by using radiochemical separation techniques, which
allowed the Z value of the bDF precursor to be established.
The use of the silicon detectors to measure the fission fragment energies allowed to deduce the total kinetic energy
release and the fragment’s mass distribution. The latter was
shown to be asymmetric, as for most of neighbouring transuranium nuclei, studied by spontaneous fission. In Figure 1,
an example of the mass distribution for bDF of 242Es (242Cf
is the fissioning nuclide) is shown. Later experiments included the use of a recoil separator like (e.g., the velocity
filter SHIP at GSI) where the bDF of 246Md was measured,
but still no direct Z or A could be performed.
Vol. 24, No. 1, 2014, Nuclear Physics News15
feature article
The second region of bDF nuclei includes six neutronrich nuclei of Ac-Pa (e.g., 228,230Ac) mostly produced by
transfer reactions on the heaviest stable targets. However,
although these measurements are more relevant for the rprocess, due to the extreme difficulties to produce neutronrich nuclei, often uncertain data exist in the literature.
The first cases of bDF in the neutron-deficient nuclei in
the lead region (being the third region of bDF occurrence
on the Chart of Nuclides) were discovered in the very neutron-deficient nuclei 180Tl (N/Z = 1.22), 188Bi and 196At in
1987 in Dubna [8]. They were produced in complete fusion
reactions with heavy ions, only half-life information could
be deduced for fission fragments. Overall, in the pre-2008
bDF studies a broad variety of different techniques was
used with different degrees of selectivity in production and
detection. In most cases however, no direct A and Z of the
bDF precursor was deduced.
Recently, extensive bDF studies in the very neutrondeficient nuclei between Tl and Fr have been performed by
our collaboration, which constitute the core of this communication. As shown in Figure 1, the respective bDF
nuclei lie very close to the border of known nuclides and
possess very unusual neutron-to-proton ratios, for example,
N/Z = 1.23–1.25 for 178,180Hg in contrast to a typical ratio
of N/Z = 1.55–1.59 in the uranium region, where numerous
SF and bDF cases are known. This allows to investigate
Figure 3. Schematic view of the ISOLDE and RILIS operation as applied in the bDF studies of 180Tl. The 1.4-GeV
2 mA proton beam impinges on the thick 50 g/cm2 238U target, producing a variety of reaction products via the spallation, fragmentation, and fission reactions. The neutral
reaction products diffuse towards the hot cavity where the
thallium atoms are selectively ionized to 1+ charge state by
two overlapping synchronized laser beams precisely tuned
to provide thallium ionization in a two-color excitation and
ionization scheme. The ionized thallium ions are extracted by the high-voltage potential of 30 kV, followed by the
A = 180 mass separation with the ISOLDE dipole magnet.
The mass-separated 180Tl ions are finally implanted in the
carbon foils of the Windmill system, for subsequent measurements of their decays.
16
potential differences in the bDF process and its observables
in the two regions, which differ in many nuclear-structure
properties. It is important to stress that while the bDF of
the most neutron-rich nuclides, relevant for the r-process,
cannot be presently studied experimentally, the bDF properties deduced for the neutron-deficient isotopes, will help
to determine the N/Z ratio dependence.
In a series experiments, performed by our collaboration
at the velocity filter SHIP (GSI), the occurrence of bDF in
186,188Bi and in 192,194At was firmly established [9, 10].
Since 2008, our collaboration initiated dedicated bDF
studies at the ISOLDE mass separator at CERN (Geneva)
[11]. The coupling of the Resonance Ionization Laser Ion
Source (RILIS) [12] to ISOLDE opened up new possibilities for bDF studies. The RILIS allows unique selective
ionization of the element of interest, thus, Z-identification.
Figure 3 shows the simplified operational principles of this
technique, as was first used at ISOLDE for detailed bDF
studies of 178,180Tl [5, 13]. The use of the laser ionization
technique also allows unique isomer separation, which is
especially important for the odd-odd bDF precursors, many
of which have more than one nuclear state capable of bDF.
As an example, the case of bDF of 180Tl is discussed here.
After selective ionization, acceleration up to 30 keV, and
mass separation, a pure 180Tl beam of ~150 atoms/s was
analysed by the Windmill detection system, which included
several silicon detectors. The use of two silicon detectors
in a compact geometry allowed both singles a/fission decays and double-fold fission fragment coincidences to be
efficiently measured. In a ~50-hour long experiment, 1111
singles and 356 coincidence fission events were observed
and attributed to the bDF of 180Tl, see Figure 4.
The uniqueness of this technique is the unambiguous A
and Z identification of the precursor, via the combination
of the mass-selection by ISOLDE and Z-selection by the
RILIS. Other advantages include a point-like source, the
implantation in a very thin foil whereby both fission fragments can be efficiently measured with little deterioration
of their kinetic energies, and the proximity of germanium
detectors for g-ray spectroscopy. Simultaneous measurement of fission and a decays in the same detectors reduces
the systematic errors for branching ratio determination substantially.
The mass distribution for fission fragments of 180Hg is
clearly asymmetric; the most abundantly produced fragments are 100Ru and 80Kr and their neighbors. No commonly expected symmetric split in two semi-magic 90Zr
nuclei was observed, and the authors claimed observation
of “new type of asymmetric fission in proton-rich nuclei,”
which differs from asymmetric fission in the trans-uranium
region [13].
Nuclear Physics News, Vol. 24, No. 1, 2014
feature article
Figure 4. Top panel: a coincidence energy spectrum for bDF
of 180Tl measured by two silicon detectors. The two-peaked
structure originates because the two fission fragments have
different energies, a direct result of the asymmetric mass
distribution. Bottom panel: the derived fission-fragment
distribution of the daughter isotope 180Hg as a function of
the fragment mass and the total kinetic energy.
Several different theoretical approaches were applied to
understand the asymmetric mass split, such as macroscopicmicroscopic model by Möller et al. [14, 15], the self-consistent nuclear density functional theory employing Skyrme
SKM* and Gogny D1S energy density functionals by Warda
et al. [16] and different versions of scission-point models
by Andreev et al. [17] and Panebianco et al. [18]. Figure 5
shows the contrasting behavior of the potential energy surface in the fission of a traditional fissioning nucleus 236U and
of 180Hg. A robust description of the asymmetric mass split
of 180Hg was obtained in all models despite the fact that quite
different underlying mechanisms are proposed by the different studies.
In the recent experiments at ISOLDE, the bDF of
178,180Tl, 194,196At and of 200,202Fr was firmly established [5]
and in most cases the mass distribution of the fission fragments was established. The clearly mass-asymmetric mass
distribution of fission fragments of 178,180Hg (being the fis-
sioning daughters of 178,180Tl) established a new region of
asymmetric fission in addition to the previously known one
in the trans-actinides. An extended region of predominantly
symmetric fission is situated between the two regions of
asymmetric fission around 208Pb (Figure 1).
To date, 26 bDF isotopes are known in three regions of the
Chart of Nuclides. However, in many cases only scarce information is presently available. Substantial progress can be
expected in all three regions, due to developments of novel
and/or improved production and detection methods. Below
we highlight some of the interesting bDF studies feasible in
the near future.
The main efforts in all three bDF regions should concentrate on detailed experiments to reliably measure bDF probabilities, partial half-lives, and energy/mass distributions of
fission fragments, similar to those, performed for (e.g., 180Hg
in Ref. [13]). A direct measurement of the Z values is also
needed to firmly establish the A and Z distributions of the
fission fragments. The experiments with the laser-ionized
isomerically pure beams of 192,194At, 186,188Bi, and 202Fr
should determine whether both isomers of each isotope undergo bDF and whether any difference exists in the bDF process of different isomers. The importance of these isotopes is
further highlighted by the fact that their fissioning daughters
186,188Pb, 192,194Po, and 202Rn lie in the transitional region
between 178,180Hg, exhibiting asymmetric low-energy fission, and 204Rn, which fissions symmetrically at similar excitation energies, see Figure 1.
The search for new bDF cases is another important task.
For example, with the presently available beam intensities
dedicated searches for bDF of the neutron-rich 228,230,232Fr
and of 228,230,232Ac are possible at ISOL facilities (such as
ISOLDE or ISAC [TRIUMF]). In contrast to the earlier radiochemical studies of 228,230Ac at Lanzhou [19], a unique Z
and A identification of the parent isotope could be obtained,
along with the measurements of fission fragments energy and
mass distributions.
Furthermore, in the past, by using the multinucleontransfer reactions of 11.4 MeV/u 238U ions with W/Ta targets at the GSI ISOL mass-separator, new isotopes 232Ra
and 232,234Ac were produced [20]. The use of this method to
search for bDF of 232,234Ac could be an interesting extension
of the bDF studies of 230Ac, also produced in the transfer
reaction. In the lead region, a search for bDF of the odd-A
precursors should be performed.
The aforementioned goals require improved production
and detection techniques. The new in-flight recoil separators,
such as S3 (SPIRAL2 at GANIL) [21] will provide unprecedented opportunities to reach the neutron-deficient nuclei
in the trans-uranium region, which is not accessible using
the high-energy proton-induced reactions and based on ISOL
Vol. 24, No. 1, 2014, Nuclear Physics News17
feature article
Figure 5. Calculated PES surfaces for 180Hg and 236U taken from Ref. [15], as a function of dimensionless quadrupole
moment and the mass asymmetry. The shapes of the nuclei at several key locations as they proceed to fission are drawn,
connected to the points on the surface by arrows.
techniques. Due to a substantial beam intensity increase, a
gain by at least an order of magnitude in production rates
can be expected. Combined with better separation capabilities and improved detection systems, these facilities will certainly open a new era in bDF studies in the trans-uranium
region. The same technique can also be used to study the
shortest-lived bDF nuclides in the lead region, such as 192At,
which is not yet accessible at ISOL facilities due to its short
half-life compared to the relatively long release time from
the target-ion source.
Laser-based techniques, such as RILIS@ISOLDE [11,
12], CRIS@ISOLDE [22], and the recently developed IGLIS method [23], coupled to the S3 separator, will further
increase the sensitivity of the experiments and allow us to
address the problem of existence of two isomers in a bDF
precursor.
More generally, as far as low-energy fission studies are
concerned, several promising projects are presently being
developed. As a continuation of the Coulomb-induced fission experiments with relativistic secondary beams the next
generation of such studies has recently been initiated by the
SOFIA collaboration at GSI [24]. These experiments will
benefit from the improved beam intensity of the initial 238U
beam and from detector developments that should enable the
unique mass and charge identification of fission fragments
with a precision of one unit.
In another recent approach, the VAMOS spectrometer
(GANIL) was used to study fission initiated by multi-nucleon transfer reactions in inverse kinematics between a 238U
beam and a 12C target. The first experiments produced different minor actinides, within a range of excitation energies
below 30 MeV [25].
A new ambitious method to study low-energy fission
exploits the inelastic electron scattering off exotic radioactive beams in a colliding beam kinematics. Two such proj-
18
ects are currently underway: ELISe (FAIR) [26] and SCRIT
(RIKEN) [27]. All these new developments and efforts show
that substantial progress in bDF and low-energy fission studies and in the understanding of the fission process is expected
in the near future.
References
1. C. Fahlander and B. Jonson, Nobel Symposium 152: Physics
with Radioactive Beams, Physica Scripta I152 (2013) 010301.
2. M. Pfützner et al., Rev. Mod. Phys. 84 (2012) 567.
3. K. Heyde and J. Wood, Rev. Mod. Phys. 83 (2011) 1467.
4. L. P. Gaffney et al., Nature, 497 (2013) 199.
5. A. N. Andreyev, M. Huyse, and P. Van Duppen, Rev. Mod.
Phys., 85 (2013) 1541.
6. K.-H. Schmidt et al., Nucl. Phys. A665 (2000) 221.
7. I. Petermann et al., Eur. Phys. J. A48 (2012) 1.
8. Y. A. Lazarev et al., Europhys. Letts. 4 (1987) 893.
9. A. N. Andreyev et al., Phys. Rev. C 87 (2013) 014317.
10. J. F. Lane et al., Phys. Rev. C 87 (2013) 014318.
11. E. Kugler, Hyp. Int. 129 (2000) 23.
12. V. N. Fedosseev et al., Rev. Sci. Instr. 83 (2012) 02A903.
13. A. N. Andreyev et al., Phys. Rev. Lett. 105 (2011) 252502.
14. P. Möller et al., Nature 409 (6822) 785.
15. T. Ichikawa et al., Phys. Rev. C 86 (2012) 024610.
16. M. Warda, A. Staszczak, and W. Nazarewicz, Phys. Rev. C 86
(2012) 024601.
17. A. V. Andreev, G. G. Adamian, and N. V. Antonenko, Phys.
Rev. C 86 (2012) 044315.
18. S. Panebianco et al., Phys. Rev. C 86 (2012) 064601.
19. X. Yanbing et al., Phys. Rev. C 74 (2006) 047303.
20. K. L. Gippert et al., Nucl. Phys. A 453 (1986) 1.
21. A. Drouart et al., EPJ Web of Conferences 17 (2011) 14004.
22. K. M. Lynch et al., J. Phys.: Conf. Ser. 381 (2012) 012128.
23. R. Ferrer et al., Nucl. Instrum. Methods B291 (2012) 29.
24. J. Taieb, Private Communication.
25. X. Derkx et al., EPJ Web of Conferences 2 (2010) 07001.
26. H. Simon, Nucl. Phys. A 787 (2007) 102.
27. T. Suda et al., Phys. Rev. Lett. 102, (2009) 102501.
Nuclear Physics News, Vol. 24, No. 1, 2014
feature article
Pairing Interaction and Two-Nucleon
Transfer Reactions
Gregory Potel1, Andrea Idini2, Francisco Barranco3,
Enrico Vigezzi4, and Ricardo A. Broglia4,5,6,7
1CEA-Saclay, IRFU/Service de Physique Nucléaire, Gif-sur-Yvette, France
2Institüt für Kernphysik, Technische Universität Darmstadt, Darmstadt, Germany
3Applied Physics Department III, University of Seville, Seville, Spain
4INFN Milan, Milan, Italy
5Department of Physics, University of Milan and INFN Milan, Milan, Italy
6The Niels Bohr Institute, Copenhagen, Denmark
7FoldLESs S.r.l., Monza, Italy
Introduction
Soon after the formulation of BCS theory [1], it was recognized by Bohr, Mottelson, and Pines that the existence
of an energy gap in the intrinsic excitation spectrum of
deformed nuclei displayed a suggestive analogy with that
observed in the electronic spectra of metallic superconductors and could, like this one, be described at profit in terms
of correlated pairs [2]. Their paper represented the starting
point of more than fifty years of experimental and theoretical BCS flavored studies of pairing in nuclei [3].
One of the important results that has emerged from this
quest is that pairing has not one (bare nucleon-nucleon (NN)
interaction plus eventual 3N corrections, e.g., Refs. [4–6]
and refs. therein) but two origins, the second one resulting
from the exchange of collective nuclear vibrations between
pairs of nucleons moving in time reversal states lying close
to the Fermi energy (Refs. [7–9] and refs. therein; see also
Ref. [10] and A. Idini, Renormalization Effects in Nuclei,
http://air.unimi.it/handle/24341216315). This is why in discussing the pairing phenomenon one is simply forced to
“complicate” the force through many-body correlations, a
reflection of the retardation effects displayed by the nuclear
pairing dielectric function.
In keeping with the fact that the building blocks of pairing correlations are Cooper pairs, two-nucleon transfer is
specific to probe them, the associated absolute differential
cross-sections being the main, model independent observables relating theory with experiment.
In the first part of the present contribution we report on
recent progress made within this context [11–13], progress
that has allowed light to be shed into the interplay of bare
and induced pairing interactions and to obtain, inter alia,
quantitative evidence of phonon mediated pairing in halo
exotic nuclei ([14–17] and refs. therein). This is the subject
of the second part of the article.
Pair Transfer and Pairing Correlations in Nuclei
At the basis of BCS theory of superconductivity one finds
the condensation of strongly overlapping Cooper pairs, a
model that has been applied with success to the description
of pairing correlations in atomic nuclei. There is however
a main difference, as compared with the case of the condensed matter scenario in which BCS theory originated. In
the nuclear case, fluctuations of the pairing field as well as
of the normal density are very important and renormalize
in a conspicuous way the different quantities entering the
theory. In particular, around closed shell nuclei, systematic
evidence exists of the correlation and stability of the pair
addition (Figure 1) and pair subtraction modes, which are
strongly excited in two-particle transfer reactions. Pairing
vibrations (Refs. [18–20] and refs. therein), the nuclear embodiment of single Cooper pairs, smooth out through zeropoint fluctuations (ZPF) the sharp change of the occupancy
of levels around the Fermi energy (Figure 1a, bottom), taking place in mean field, thus paving the way for an eventual
phase transition from normal to superfluid phases.
A number of pairing vibrational bands have been observed throughout the mass table, containing up to three
phonon states [21]. Because of the strong correlations displayed by these vibrational modes, their microscopic properties can be accurately described in terms of RPA and of a
constant pairing strength, leading to reliable values of the
Vol. 24, No. 1, 2014, Nuclear Physics News19
feature article
Figure 1. RPA, Nuclear Field Theory (NFT) diagramatic representation of the structure [33, 34] and reactions [26] of,
and with pair addition modes. This pairing vibration is mainly a correlated superposition of two-particle states with (c)
forwardsgoing amplitudes Xk on the different orbitals above the Fermi energy (εk > εF ). The possibility of creating this
state by populating hole states below the Fermi energy (b), with backwardsgoing amplitudes Yi(εi  εF ) arises from the
presence of two-particle, two-hole configurations in the ground state of the closed shell system (a, top), ZPF which smooth
out the discontinuity in level occupancy at εF (a, bottom). The solid dot represents the strength and form factor with which
particles, moving in time reversed states, couple to the collective, quasi-boson pairing degree of freedom. It results from
the combined effect of a four-point vertex (bare interaction), see graph (d), and of vertex correction (induced interaction)
processes, an example of which is given in diagram (f ). Diagram (e) is representative of processes which dress the singleparticle states. By intervening processes (e) and (f ) with an external field which picks up two nucleons from the system, one
can force the virtual phonon to become a real final state. Assuming that the pair addition mode is the two-neutron halo of
11Li, the wavy line representing the quadrupole vibration of the 8He core, coupled to a p
3/2 (π) proton state, the process g)
describes the population of the first excited state of 9Li in the reaction 1H( 11Li, 9Li (1/2-;2.69 MeV)) 3H [14, 15].
X- and Y-amplitudes (Figures 1b and c), and thus of twonucleon spectroscopic amplitudes (Ref. [21], Tables XVI–
XVIII). The study of pairing vibrations provides, among
other things, insight into the mechanism by which a nuclear
superfluid phase eventually emerges from the condensation
of pairing vibrational modes, as the system under study
moves progressively away from closed shell nuclei. The
condensation of these extended and thus strongly overlapping, bosonic objects gives rise to a highly correlated superfluid state, displaying overall phase coherence. Superfluidity is tantamount to the existence of a finite ground-state
average value of the pair addition and removal operators,
P+, P in the ground state, that is, to a finite value of the
order parameter
a10 = |〈BCS|P+|BCS〉| = |〈BCS|P|BCS〉|
a quantity that provides an estimate of the number of correlated pairs in the BCS ground state (≈4–8).
It also gives a measure of deformation in gauge space,
the counterpart of deformation in ordinary space (e.g.,
Refs. [10, 19] and refs. therein). Just as adjacent (I,
I ± 2) states lying along a, for example, quadrupole rotational band are connected by strongly enhanced values
of the quadrupole operator, the adjacent 0+ ground states
(N, N ± 2) of, for example, a chain of superfluid isotopes
20
are connected by strongly enhanced values of the pair transfer operator, measured in terms of single- (particle-hole)
and of two-particle units, respectively [19, 22]. This result
testifies to the fact that these 0+ states are members of a
pairing rotational band. Within this scenario, pairing vibrations and rotations together with single-particle motion and
vibrations and rotations in “normal” (three-dimensional)
space constitute elementary modes of excitation.
The suggestive analogy concerning the nuclear phenomena associated with spontaneous symmetry breaking in 3-D
and in gauge space (Table XI, Ref. [21]) although extending also to the reaction (decay) processes in which these
rotational modes are specifically probed, is not operative
as far as the calculational details are concerned. In fact,
Coulomb excitation (electromagnetic decay) and Cooper
pair transfer display very different levels of calculational
challenges (complexity) concerning their implementation. This is keeping with the fact that in Coulomb excitation, let alone electromagnetic decay, one has to deal with
a single mass partition, a fact which makes it possible to
treat structure and reaction, to a large extent, separately.
This is not the case for two-nucleon transfer reaction, in
which case mass partition is different between entrance and
exit channels, a fact that leads to recoil effects and thus to
an important coupling between relative motion (reaction)
Nuclear Physics News, Vol. 24, No. 1, 2014
feature article
and intrinsic motion (structure). In fact, the situation is
even richer, in keeping with the fact that nucleons may be
transferred not only simultaneously but also successively.
Thus, one is confronted, in the calculation of the absolute value of two-nucleon transfer cross-sections, with the
opening of a new channel and thus of a new mass partition
(e.g., (N + 2) + p → (N + 1) + d → (N) + t). It is then
not surprising that the theory of Coulomb excitation and
electromagnetic decay was quantitatively operative already
few years after the first observation of rotational bands [19,
23], while it took decades (Figure 10 of Ref. [11] for an
overview of the groups and the practitioners involved in the
quest) after the first observation of a pairing rotational band
[20, 25] before one was able to calculate absolute Cooper
pair transfer cross-sections that account for the observations within experimental errors (Figure 2).
The fact that, as a rule, successive transfer dominates
over simultaneous transfer, and that, in both processes,
the transferred nucleons display equivalent pairing correlations, is a consequence of the fact that Cooper pairs
are weakly bound (<<εF), highly extended (>>R) objects.
Consequently, the minimum theory of two-nucleon transfer
corresponds to second order DWBA, where the two above
mentioned processes are taken into account properly corrected by non-orthogonality effects (Refs. [26] and [11]
and refs. therein. It is only recently that these well known
elements were implemented into a versatile software (G.
Potel, Cooper, private communication) with which, making use of well tested, state of the art spectroscopic amplitudes, and global optical potentials, one can calculate
absolute two-particle transfer differential cross-sections
that account for the experimental findings within experimental errors throughout the mass table [19–22, 24, 25].
Examples of these quantitative results are displayed in
Figure 2 (Figures 5 and 6, Table 3 of Ref. [11]).
It is worth pointing out the difference existing in the
physics which is at the basis of the agreement between
theory and experiment displayed in the upper and middle
panels (“upper”) of Figure 2, as compared to the two lowest
panels, in particular the lowest right panel (“lowest”). In
fact, the results displayed in “upper” depend little on the details of the pairing interaction employed, or the exact value
of the energies and Z-values (see below) of the single-particle levels used in the calculations, a fact intimately connected with the constancy of the lowest quadrupole mode
through the Sn-isotopes (evidence of the validity of generalized seniority), and of the large two-neutron separation
energy associated with 208Pb. This is the reason why simple
models like BCS or RPA which embody the physics of coherent pairing modes, i.e., pairing rotations and vibrations,
provide essentially “exact” two-nucleon spectroscopic amplitudes (i.e., UνVν and (Xj ,Yj) factors). On the other hand,
the results displayed in “lower” are very sensitive to the
details of the single-particle energies and associated Zvalues, as well as to components in the 11Li ground-state
Figure 2. Absolute cross-sections associated with two–
neutron transfer reactions involving superfluid (Sn-isotopes) and pair vibrational (208Pb and 9Li) nuclei. Making
use of the “exact” two-nucleon spectroscopic amplitudes
(see text), of global optical potentials, and of two–nucleon
transfer software developed within the framework of second–order DWBA (Potel, private communication), the corresponding absolute differential cross-sections associated
with these reactions were calculated and are displayed
(continuous curves) in comparison with the experimental
data [11, 12, 14, 15, 20, 27]. It is of notice that the absolute differential cross-section associated with the 9Li(1/2–,
2.69 MeV) state provides a realization of the NFT process
depicted in Figure 1g in direct comparison with the data
(right bottom panel).
Vol. 24, No. 1, 2014, Nuclear Physics News21
feature article
wavefunction displaying a 1% probability, a result of rather
refined NFT calculations.
In keeping with the above parlance, the type of results
displayed in “upper” and “lower” provide confidence in the
fact that one now knows how to accurately calculate absolute two-nucleon differential cross-sections in (nuclear
structure) simple cases as well as to quantitatively predict
new mechanisms to dynamically violate gauge invariance.
Let us now elaborate on the fact that the coherent character of pairing correlations manifests itself equally well in
simultaneous than in successive transfer processes. In fact,
for superfluid nuclei the quantity (a0) is given, in the case
of simultaneous transfer, by the relation
a0 = ∑v>0〈BCS|a+v a+v– |BCS〉
and by the expression
a0 = ∑i,v>0〈BCS|a+v |i〉〈i|a+v– |BCS〉
≈ ∑n,v>0〈BCS|a+v a+v |BCS〉〈BCS|ava+v– |BCS〉
in the case of successive transfer. Making use of the quasiparticle transformations both relations lead to ∑v>0UvVv.
This result is intimately connected with the large distance
(correlation length ξ ≈ ħ vF /D ≈ 36 fm) over which Cooper
pair partners correlate.
The Pairing Interaction and
Medium Polarization Effects
The nature of the attractive pairing force acting between
electrons represented a central question in the development
of the theory of superconductivity in metals (screened Coulomb field plus electron-phonon mediated interaction, Ref.
[38] and refs therein). In the nuclear case the bare NN-interaction is strongly attractive in the 1S0 channel for a wide
range of relative momenta, and mean field calculations lead
to neutron or proton pairing gaps of the order of those derived from experimental data. It is of notice, however, the
latest developments concerning 3N (mainly repulsive) corrections to the bare NN-interaction [4].
Before dealing with the question of how two-nucleon
transfer reactions can shed light on the question of the interplay (relative importance) of bare and induced pairing
interaction, let us remind the basics of medium polarization
effects. The relevance of these effects in connection with
one-nucleon transfer reactions has been recognized since
a long time (Ref. [28] and refs. therein). In such reactions,
as well as in (e,ep) processes, one often observes that the
single-particle strength associated with levels lying close
to the Fermi energy is fragmented over a number of peaks,
and the single-particle content of the main peak varies typi-
22
cally from 60% to 80% of the value expected in the independent particle limit.
Part of the reduction of single-particle strength can
be ascribed to the short-range part of the NN−interaction
(short wavelength mechanism) which shifts single-particle
strength away from the Fermi energy (high momentum
processes). Another part of the reduction is associated with
a long wavelength mechanism resulting from the interweaving of single-particle and low-lying collective vibrations (low-k processes). Examples of such processes are
displayed in Figure 3. A nucleon can bounce inelastically
off the nuclear surface, setting it into vibration, changing
its state of motion and, at a later time, by reabsorbing the
vibration return to its original state as shown in (I, upper
diagram). Important effects are also connected with the process depicted in (I, lower diagram), obtained from a time
ordering from process (I, upper diagram). It leads to a partial blocking of the ground-state correlations (oyster-like
diagram), process giving rise to an effect known in atomic
physics as the Lamb shift.
Through the processes displayed in Figure 3, a nucleon
moving in a single-particle configuration is forced into more
complicated configurations. In other words, the single-particle strength becomes fragmented, and the discontinuity of
the occupation numbers at the Fermi energy, Z = 1 in the
case of the non-interacting system, is reduced (Z < 1) .
The probability with which the associated components of
the ground-state wavefunction containing phonon degrees
of freedom are present in the dressed single-particle states
can, in principle, be experimentally determined in one-particle transfer processes populating the excited states of the
A-1 system. As an example, we refer to the p(11Be,10Be(2+))
d reaction [29]. The presence of such components has been
shown to be relevant also in break-up reactions [30]. We
note that the importance of contributions of multi-step processes, which can populate the final states in question, even
in absence of correlations in the initial state, must be quantitatively assessed. In fact, the possibility of observing the excitation of states associated with the “complex” components
of the single-particle wavefunction of the initial ground state
is connected with situations in which multistep processes are
hindered by structure and/or Q−value effects.
Renormalization effects of the nuclear pairing gaps have
been discussed for quite some time in connection with infinite matter (Ref. [31] and refs. therein). Work started at
the end of the 1990s provided evidence through the result
of detailed calculations that, in finite nuclei, the exchange
of virtual phonons—in particular quadrupole and octupole surface vibrations—between two neutrons coupled to
J p = 0+ gives rise to an energy dependent attractive force
Nuclear Physics News, Vol. 24, No. 1, 2014
feature article
Figure 3. The NFT scheme synthetized in Figures 1a–f becomes operative concerning the structure of 10Li and 11Li:
(I) self-energy processes, giving rise to parity inversion in 10Li; (II) bare (boxed inset) and induced pairing interaction binding the halo neutron pair to the 9Li core, through a bootstrap mechanism, in which the neutrons exchange
the pigmy dipole resonance of 11Li, as well as the quadrupole vibration of the core, as testified by the wavefunction b. In other words, the color snapshots displayed in (a) and (b) attempt at describing the becoming of the neutron halo Cooper pair of 11Li, from an uncorrelated s21/2(0) configuration to a strongly correlated, (weakly) bound
two-neutron state. It is of notice that the bare interaction (boxed inset in (II)), corresponding to the process depicted in Figure 1d (NFT four point vertex, rule (II) of NFT, Ref. [34], p. 314) lowers the s21/2(0) (as well as the p21/2(0))
pure configurations by only 100 keV, and is not able, by itself, to bind the pair. The color plots display the modulus
square of the two-neutron wavefunction as a function of the coordinates of the two nucleons (left) and the probability distribution of one neutron with respect to the second one held fixed on the x-axis (at a radius of 5 fm, solid dot).
The red circle schematically represents the core. After Ref. [37].
leading to state dependent pairing gaps that, in average,
account for a conspicuous fraction of the empirical values
[7–10] (within this context, see Ref. [19], p. 432). The processes at the basis of the induced pairing interaction are depicted in Figure 3 (II, upper diagram): a vibration excited
by a nucleon is reabsorbed by a second nucleon. Such a
process leads to an induced interaction among nucleons, associated with the polarization of the nuclear medium.
The superposition of the bare and the phonon induced
interactions (V eff = V bare + V ind ) increases in nuclei, the
value of the Cooper pair binding energy as compared to the
V bare result, while the coupling of surface phonons to single-particle states leads to a depopulation of the pure singleparticle states through self-energy processes (Figure 3(I)).
As a consequence, the BCS gap equation is modified by the
presence of Z-factors [9, 38], leading to:
X V eff .1; 2/Z2 Q2
Q 1 D Z1
:
2EQ 2
2
One can then identify two contributions to the gap,
~
~
~
D = Dbare + Dind. The effects of the basic renormalization
diagrams can be taken into account
order, by
upO to infinite
Q QO ;
HO ECQF D ! .1 /nO d 4N
Vol. 24, No. 1, 2014, Nuclear Physics News
4Csym
˛D
T
D
4Csym
"
;
Z
A
2
1
Z
A
2 #
2
23
feature article
solving the Nambu-Gor’kov equations, leading to a consistent theoretical picture, that accounts for these effects both
on the single-particle motion and on the pairing interac~
tion. The resulting total gap D is considerably larger than
the value DBCS obtained solving the usual BCS equations
with the bare (Argonne) pairing force without taking into
account renormalization of single-particle motion. The contributions coming from the bare and from the induced inter~
actions to D are comparable. At the Fermi energy the value
~
of D is larger than experiment by ≈20%. It is of notice that
coupling to spin modes will somewhat reduce the value of
~
D. However, at present no complete microscopic calculation
of the pairing gap including both the bare interaction and
medium polarization effects exists. Main open problems remain the determination of the initial mean field, the role of
three-body forces and the coupling to spin modes.
Theory indicates that the induced interaction is concentrated around the Fermi energy and is strongly surface
peaked. It is, however, not straightforward to have direct
information of these properties: its effects can, in many
cases, be simulated by adjusting the strength of the bare
interaction. In fact, the spatial dependence of the Cooper
pair, at least in well bound nuclei, depends only weakly on
the details of the pairing interaction. Within this context
one can posit that the pairing gap, although intimately connected with pairing in nuclei, is not the specific quantity to
probe the corresponding correlations, at least as far as the
nature of the interaction that generates them is concerned.
This is also in keeping with the fact that the pairing gap is
a derived quantity (e.g., 3-point empirical value, requiring
the knowledge of three different nuclear masses). On the
other hand, with the help of two-nucleon transfer reactions,
one can force the virtual processes displayed in Figures 1e
and f to become final, observable states. In fact, being able
to accurately calculate absolute differential cross-sections,
information about the phonon admixture in the Cooper pair
wavefunction can be obtained by studying pair transfer to
excited collective vibrational states of the core (Figure 1g).
Let us conclude this section with a technical note. At variance with infinite systems in general, and condensed matter in particular, in which case particle number fluctuations
are negligible, in the nuclear case they play an important
role. This is the reason why much work has been dedicated
to this question (projection methods, RPA techniques, etc.;
Refs. [3, 10, 35, 36] and refs. therein). Within this context,
~
the pairing gap becomes D = (D2 + G2 S0 (RPA)/2)1/2, where
S0(RPA) contains the (particle-conserving) matrix elements
of P+ and P (Ref. [10], p. 151). While projection effects are
dominant at the phase transition, they lead to corrections of
the order of 10–20% for the ground state pairing gap.
24
The Case of Halo Nuclei
Renormalization effects can have particularly striking
consequences in halo nuclei like 11Li, systems which are
weakly bound and easily polarizable. In particular, it was
proposed [32] that the coupling of single-particle levels to
quadrupole vibrations of these systems plays an important
role to explain the positive parity of the ground state of
N = 7 isotones, a dynamical effect going beyond mean field
theory. The particle-vibration matrix elements associated
with quadrupole vibrations are, in these nuclei, very large
(Figure 3(I)). In fact, the neutron 2s1/2 orbital is shifted
downwards by several MeV by virtue of its coupling to
configurations of the type [d5/2 ⊗ 2+]1/2+ (polarization diagram (I, upper) in Figure 3). Furthermore, the neutron 1p1/2
orbital is shifted upwards as a result of the suppression of
ground state correlations (Pauli principle processes) mostly
–1] + ⊗ 2+] +
associated with the configuration [p1/2 ⊗ p3/2
2
0
(correlation diagram (I, lower diagram) in Figure 3).
A dynamical Nuclear Field Theory (NFT, Refs. [33, 34]
and refs. therein) description of the two-neutron halo nuclei
12Be and 11Li, based on the coupling to the vibrations of
these systems and of their cores, provides an overall account of their nuclear structure properties [17, 37]. Dealing with a single, dressed Cooper pair, the corresponding
wavefunction can be obtained by summing the processes
shown in Figures 1e and f to infinite order with the help of
Dyson’s equation. Such a treatment of the variety of couplings is tantamount to a full diagonalization, including the
(discretized) continuum. In fact, and as is well known, the
continuum plays an essential role in the case of 11Li, for
which all the relevant single-particle orbitals are resonant
or virtual states, in keeping with the fact that 10Li is unbound. Furthermore in 11Li, an important role is played
by the low-lying dipole state (pigmy resonance ≈1 MeV),
responsible of much of the glue binding the neutron halo
Cooper pair to the 9Li core. The resulting wavefunction of
the dressed neutron halo can be written as shown in Figures
3b and (II).
It turns out that the (short range) bare 1S0 neutron-neutron pairing interaction leads, in the present case, to a small
contribution. This is in keeping with the very low angular
momenta available to the neutrons (essentially s,p states
being involved in the very extended and diffuse 11Li halo).
The wavefunction of the 3/2– ground state of 11Li is then
obtained by coupling the p3/2(p) proton, treated as a spectator, to the neutron halo.
A detailed analysis of the reaction 1H(11Li,9Li)3H reaction performed at TRIUMF [14] with a 11Li beam (inverse
kinematics) has been carried out [15]. Two states were observed: the 9Li ground state and the first 9Li(1/2–) excited
Nuclear Physics News, Vol. 24, No. 1, 2014
feature article
state, which is interpreted as the lowest member of the
p3/2(p) ⊗ 2+ multiplet. It is of notice that the angular distribution associated with the ground state is very sensitive
to the relative weight of the s2 and p2 configurations in the
wavefunction displayed in Figures 3b and (II), wavefunction which reproduces quite accurately the experimental
findings (Figure 2).
The renormalization processes in which the neutrons of
the halo Cooper pair of 11Li either emit and reabsorb a collective (p-h)-like quadrupole vibration (effective mass processes, Figure 1e) or exchange a phonon (vertex correction,
Figure 1f) can, in a two-particle pick-up reaction (Figure
1g), populate the first excited state 1/2– of 9Li. The absolute
value of the corresponding two-nucleon transfer crosssection provides an accurate measure of the probability with which the |(s1/2,d5/2)2+ ⊗ 2+;0〉 component appears in the 11Li ground state (Figure 3b) and thus of
the role the quadrupole vibration plays in binding the
neutron halo Cooper pair. This is also in keeping with
the fact that alternative channels, like final-state inelastic excitation and neutron break-up, lead to negligible
contributions [15].
The fact that theory reproduces the observed absolute
differential cross-sections, testifies to the fact that NFT of
structure and reactions [26, 33, 34] is able to accurately predict [37] and describe [15] the consequences of the induced
nuclear pairing interaction.
While this result can, arguably, be considered a milestone
in the understanding of the origin of pairing in nuclei, we feel
equally important and timely the developments taking place
at a breathtaking pace, concerning the connection of the NNbare interaction and the quark degrees of freedom, and of its
regularization in terms of renormalization group methods or
similar techniques (Vlow–k), to work out a pairing interaction
(taking also 3N terms into account), which can be used in
nuclear structure calculations. It is likely that these developments will contribute together with the ones presented above,
in an important and hopefully conclusive way to the quest of
assessing the relative role of bare and medium polarization
effects in the nuclear pairing interaction.
Acknowledgment
We thank Luisa Zetta and Paolo Guazzoni as well as
Ritu Kanungo and Isao Tanihata for discussions and clarifications concerning their state of the art A+2Sn(p,t)ASn and
1H(11Li, 9Li)3H data, respectively. Collaboration with Ben
Bayman is gratefully acknowledged. RAB acknowledges
his debt towards Daniel R. Bès for many discussions and
clarifications concerning the physics that is at the basis of
the subjects treated in the present contribution.
References
1. J. Bardeen, L. N. Cooper, and J. R. Schrieffer, Phys. Rev. 106
(1957) 162; ibid 108(1957).
2. A. Bohr, B. R. Mottelson, and D. Pines, Phys. Rev. 110 (1958)
936.
3. R. A. Broglia and V. Zelevinsky, eds., Fifty Years of Nuclear
BCS (World Scientific, Singapore 2013).
4. T. Lesinski, K. Hebeler, T. Duguet, and A. Schwenk, J. Phys.
G 39 (2012) 015108.
5. S. S. Pankratov, M. V. Zverev, M. Baldo, U. Lombardo, and
E. E. Saperstein, Phys. Rev. C 84 (2011) 014321.
6. H. Hergert and R. Roth, Phys. Rev. C80 (2009) 024312.
7. V. Avdeenkov and S. Kamerdzhiev, JETP Lett. 69 (1999) 715.
8. F. Barranco et al., Phys. Rev. Lett. 83 (1999) 2147.
9. F. Barranco et al., Phys. Rev. C72 (2005) 054314.
10. D. M. Brink and R. A. Broglia, Nuclear Superfluidity (Cambridge University Press, Cambridge, 2005).
11. G. Potel, A. Idini, F. Barranco, E. Vigezzi, and R. A. Broglia,
Rep. Prog. Phys. 76 (2013), 106301; arXiv:1304.2569.
12. G. Potel et al., Phys. Rev. Lett. 107 (2011) 092501; Phys. Rev.
Lett. 108, 069904 (2012) [Errata].
13. G. Potel et al., Phys. Rev. C87 (2013) 054321.
14. I. Tanihata et al., Phys. Rev. Lett. 100 (2008) 192502.
15. G. Potel et al., Phys. Rev. Lett. 105 (2010) 172502.
16. G. Potel et al., Yad. Fiz (2014), in press; arXiv:1210.5085.
17. G. Gori et al., Phys. Rev. C69 (2004) 041302.
18. D. R. Bes and R. A. Broglia, Nucl. Phys. 80 (1966) 289.
19. A. Bohr and B. R. Mottelson, Nuclear Structure, Vol. II (Benjamin, Reading, Mass., 1975).
20. J. H. Bjerregaard et al., Nucl. Phys. 89 (1966) 337.
21. R. A. Broglia, O. Hansen, and C. Riedel, Adv. Nucl. Phys.
6 (1973) 287; http://www.mi.infn.it/~vigezzi/BHR/Broglia
HansenRiedel.pdf
22. R. A. Broglia, C. Riedel, and T. Udagawa, Nucl. Phys. 184A
(1972) 23.
23. K. Alder et al., Rev. Mod. Phys. 28 (1956) 432
24. H. Flynn et al., Nucl. Phys. 154A (1970) 225.
25. G. Bassani et al., Phys. Rev. 139 (1965) B830.
26. R. A. Broglia and A. Winther, Heavy Ion Reactions (AddisonWesley, New York, 1991).
27. P. Guazzoni et al., Phys. Rev. C60 (1999) 054603; ibid
C69(2004) 024619; C78(2008) 064608; C83(2011) 044614;
C85(2012) 054609.
28. C. Mahaux et al., Phys. Rep. 120 (1985) 1.
29. J. S. Winfield et al., Nucl. Phys. A 683 (2001) 48.
30. A. M. Moro and R. Crespo, Phys. Rev. C85 (2012) 054613.
31. H.-J. Schulze et al., Phys. Lett. B 375 (1996) 1.
32. H. Sagawa, B. A. Brown, and H. Esbensen, Phys. Lett. B309
(1993) 1.
33. D. R. Bes and R. A. Broglia in Proceedings of the International School of Physics “Enrico Fermi,” Course LXIX,
eds. A. Bohr and R. A. Broglia, North Holland, Amsterdam
(1977), 55.
34. P. F. Bortignon et al., Phys. Rep. 30 (1977) 305.
35. M. Anguiano et al., Nucl. Phys. A 696 (2001) 467.
36. Y. R. Shimizu et al., Phys. Rev. Lett. 85 (2000) 2260.
37. F. Barranco et al., Eur. Phys. J. 11 (2001) 385.
38. J. R. Schrieffer, Theory of Superconductivity (Benjamin, New
York, 1964).
Vol. 24, No. 1, 2014, Nuclear Physics News
25
facilities and methods
Investigating the Structure of Neutron-Rich
Nuclei with Neutrons
Studies of neutron-rich nuclei close
to
or 132Sn give a rare opportunity
to test the interactions used in shellmodel calculations in a neutron-rich
heavy-mass region. Furthermore, the
proximity of the r-process path to these
nuclei makes an understanding of their
structure important. In the neutron-rich
A = 100 region the ground states of
some of the nuclei here undergo a rapid
change in shape from a spherical one at
N = 58 to a deformed one at N = 60 with
the addition of just two neutrons. This
shape-change process is still not fully
understood and allows insights into the
appearance of collective behavior in
medium-heavy nuclei to be gained.
It is difficult to investigate excited
states of neutron-rich nuclei with
masses A = 80–160 using standard
nuclear-physics accelerators, such as
tandems. One particularly successful
technique for studying the intermediate-to-high spin excited states in these
nuclei is to place spontaneous-fission
sources at the centre of large arrays
of germanium (Ge) g-ray detectors
[1]. The division in two of a 252Cf, or
248Cm, nucleus generally results in two
fission fragments, one with mass A ~
100 and the other A ~ 145. As the parent nucleus has about 60% more neutrons than protons, and this process is
cold (only ~4–5 neutrons evaporated
on average), then the two resulting
fragments are nearly always neutron
rich [1]. Decays of the excited states in
these isotopes can then be detected in
a Ge detector array and triple-g coincidences used to select the cascade, or fission split, of interest [1]. The limitation
of this technique is however that only
two spontaneous-fission sources are
realistically available. An almost-identical reaction to spontaneous fission is
to induce fission in trans-actinde targets
132Sn, north-east of 78Ni, in the A = 100
78Ni
26
Figure 1. The EXOGAM array in operation at the PF1 beam line.
using neutrons with thermal or cold
(<0.025 eV), energies. Several targets
are available for such studies with fission cross-sections of a few hundred
barns. Changing the target specie can
significantly vary the fission-product
mass distribution and the population
of the fragments of interest can be
optimized by careful target selection.
Hence A ~ 80–95 or A ~ 125–135 neutron-rich nuclei, where little is known,
can be populated at intermediate spins
in such reactions. With this in mind,
the EXILL (EXOGAM at ILL) collaboration was formed to install the efficient EXOGAM array (Figure 1) [2]
of Ge g-ray detectors, from GANIL,
at the PF1B cold neutron guide of the
Institut Laue-Langevin (ILL) reactor
in Grenoble. This in effect temporarily “EXILed” EXOGAM to the ILL.
The array was operational for a total of
two 50-day reactor cycles. This was the
first time that a modern, high-efficiency
array of Ge detectors had been used
with an intense, cold-neutron beam.
The main aims of this novel project
are to study prompt g rays emitted by
neutron-rich fission fragments close to
Nuclear Physics News, Vol. 24, No. 1, 2014
region and to detect prompt g decays
following (n,g) reactions on other targets.
In low-energy fission, the excited
states in more than 100 nuclei can
be studied concurrently via prompt
g-g-g spectroscopy with an efficient
high-granularity Ge array. After neutron evaporation, the secondary fission fragments have average spins of
~6–8 ħ, allowing the study of intermediate-spin states. Nuclei with spins as
high as 20 ħ have been studied in such
reactions too. Compared to the fission
yields from a 252Cf source, inducing
fission with thermal neutrons in targets
such as 235U or 241Pu populates some
of the nuclei in in the A = 80–95 and
A = 130 regions with an order-of-magnitude higher production, for the same
fission rate. Both these targets have
been used in the EXILL campaign and
allow excited states with intermediate
spins in nuclei with just a few particles
or holes outside the doubly magic 78Ni
and 132Sn to be studied. The simple
structure of these neutron-rich A ~ 80
or A ~ 130 nuclei allows sensitive tests
of the predictions of shell-model calculations to be performed. Such information is useful not only for testing theoretical model predictions but has an
astrophysical interest too, as the rapid
neutron-capture (r-process) path passes
close to, or even through, the mass
distributions produced. Nuclear structure information is an important input
to such calculations. The neutron-rich
nuclei of the A ~ 100 region are also
well produced with these two targets
and these isotopes have the interesting
property that a rapid change in shape
of their ground state, from a spherical one to a strongly prolate-deformed
one, has been observed when increas-
facilities and methods
ing the number of neutrons from 58 to
60. Their study allows insights into the
onset of collective behavior in nuclei.
As more than 100 neutron-rich
nuclei are available for study in the
EXILL campaign then this contributes
to the worldwide effort of studying
nuclei with high neutron-to-proton
ratios. The ongoing construction, or
recent commissioning, of second-generation, intense radioactive ion-beam
facilities at several locations worldwide aims to study the properties of
the most neutron-rich nuclei. Although
it will not be possible to study nuclei
as far from stability as those produced
at these new radioactive ion-beam
facilities, the EXILL campaign contributes to these efforts by allowing
detailed spectroscopic information to
be obtained across a large area on the
neutron-rich side of the nuclear chart.
This allows the detailed evolution of
trends in nuclear-structure properties
to be followed, such as level energies,
nuclear shapes, and fission yields.
The (n,g) reaction has been used for
studying excited states in near-stability
nuclei for many years. All states within
a certain energy and spin range can be
populated in the resulting nucleus allowing “complete” spectroscopy to be
performed. The EXILL campaign was
unique in that it was the first use of a
large, high-efficiency, high-granularity
Ge array for (n,g) experiments allowing
greatly increased sensitivity for coincidence measurements. Thus even targets
could be studied that have extremely
low neutron capture cross-sections of
tens of millibarns or where only small
milligram quantities of enriched material are available.
The EXILL campaign has been
possible due to the low-background,
high cold-neutron flux available from
the ILL reactor. At the end of the PF1B
cold neutron guide a thermal-equivalent neutron flux of 2 × 1010 n/cm2/s
is available [3]. In order to perform
prompt g-ray spectroscopy, a carefully
constructed collimation system was
used which produced a pencil-like
beam with dimensions of ~1 cm2 at the
target position. This reduced the flux
by two orders of magnitude but resulted in a beam with low divergence,
almost free of g-ray background. A variety of stable targets were placed in
the beam for (n, g) studies and two fissile targets, 235U and 239Pu.
The EXOGAM array was used in
two configurations during the EXILL
campaign. The first consisted of 10
Clover Ge detectors, supplemented
with 6 GASP single-crystal coaxial Ge
detectors from INFN, Legnaro. This
configuration was optimized for g-ray
spectroscopy experiments and the array had a total photopeak efficiency of
about 6% at 1.3 MeV. Both the 235U
and 241Pu fission targets were used
with this setup, along with a selection
of stable targets for (n,g) studies. In the
second configuration 8 EXOGAM Clover detectors were used in combination
with 16 LaBr3 g-ray scintillation detectors from the FATIMA collaboration.
These scintillation detectors had time
resolutions each of 140–180 ps and allowed the lifetimes of excited nuclear
states to be measured via a direct timing
coincidence measurement. Knowledge
of the lifetimes of excited nuclear states
can give information on the collective
or single-particle nature of an excited
nuclear state. A mixed Ge-LaBr3 array
was necessary as the moderate energy
resolution of the LaBr3 detectors (~3%)
does not allow a clean selection of an
individual decay cascade. The Ge detectors provide this selection, but have
inferior timing performance (resolution
~10 ns). The same targets were used
with this configuration as in the first
reactor cycle and this is the first time
that such direct timing measurements
have been attempted using a mixed GeLaBr3 array with a fission target.
The signals from the preamplifiers
of all detectors were fed into a data
acquisition system consisting of 100
MHz digitizer modules which allowed
both energy and time information to
be recorded [4]. The time signals from
the LaBr3 scintillator detectors were
processed using analog electronics.
All data were recorded in a triggerless
mode and some 60 TB of data were
collected over the two reactor cycles.
This amount of data is the equivalent
to the total amount collected by the 40
instruments of the ILL in its previous
40 years of operation! Some 41 experimental proposals were submitted for
the first reactor cycle with EXILL in
the spectroscopy configuration and 32
in the combined EXILL-FATIMA one.
Around 120 scientists and students
participated in the data taking and several years work of data analysis now
lie ahead for this collaboration.
Acknowledgments
The EXILL campaign would not
have been possible without the support of several services at the ILL and
the LPSC. We are grateful to the EXOGAM collaboration for the loan of
the detectors, to GANIL for assistance
during installation and dismantling,
and to the INFN Legnaro laboratory
for the loan of the GASP detectors.
References
1. I. Ahmad and W. R. Phillips, Rep. Prog.
Phys. 58 (1995) 1415.
2. J. Simpson and the EXOGAM collaboration, Acta Physica Hungarica, New
Series, Heavy Ion Physics 11 (2000)
159.
3. H. Abele et al., Nucl. Instr. and Meth. A
562 (2006) 407.
4. P. Mutti et al., Proc. of the ANNIMA
Conference (2013).
Gary Simpson
on behalf of the EXILL
core team and collaboration
LPSC, Grenoble
and University of the
West of Scotland
Vol. 24, No. 1, 2014, Nuclear Physics News27
facilities and methods
NSCL and the Facility for
Rare Isotope Beams (FRIB) Project
The National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU), shown in
Figure 1, is the largest campus-based
nuclear science facility in the United
States. NSCL is funded by the National
Science Foundation (NSF) for operating its Coupled Cyclotron Facility
(CCF) as a national user facility and for
conducting research in nuclear physics,
nuclear astrophysics, and accelerator
physics.
The Facility for Rare Isotope Beams
(FRIB) will be a new U.S. Department
of Energy Office of Science (DOE-SC)
national user facility supporting the
mission of the Office of Nuclear Physics. The FRIB Project designs and establishes FRIB. FRIB will make effective use of NSCL’s infrastructure when
it becomes operational. The FRIB Project is funded by the DOE-SC, MSU,
and the State of Michigan. When FRIB
construction is complete, NSCL will
cease operations and merge into FRIB,
and FRIB operations will be funded by
the DOE-SC. The NSCL user group
has already merged into the FRIB User
Organization (FRIBUO) that has over
1,250 members and approximately 20
working groups.
NSCL and the FRIB project have
over 500 employees, including more
than 35 faculty members with joint
appointments in MSU’s Departments
of Physics and Astronomy, Chemistry, and Electrical and Computer Engineering. Presently, more than 140
students—approximately half of them
doctoral students—are employed and
educated at the laboratory.
NSCL maintains and operates two
coupled superconducting cyclotrons,
a high-acceptance superconducting
fragment separator, a superconducting
linear reaccelerator, and a diverse set
of experimental apparatus. The CCF
is capable of delivering a broad range
of primary beams from hydrogen to
uranium that are used for the in-flight
production of secondary, rare isotope
beams with energies up to nearly 170
MeV/nucleon. The in-flight technique
allows for sub-microsecond isotope
separation in a chemistry-independent
way with short beam development
times of a few hours to one day. The
high beam energies provide efficient
access to nuclei very close to the drip-
Figure 1. NSCL building complex on the campus of Michigan State University.
28
Nuclear Physics News, Vol. 24, No. 1, 2014
lines, both because thick targets can be
used and because ions in mixed beams
(“cocktail beams”) can be identified on
an event-by-event basis. Rare isotopes
produced with the in-flight technique
can be stopped in and extracted from a
He gas cell and subsequently used for
precision ion trap or laser spectroscopy
experiments at very low energy or for
charge breeding and reacceleration
with a state of the art superconducting
linac dubbed ReAx where x denotes
the maximum energy per nucleon of
uranium ions that can be delivered by
a particular linac section. A first experiment with reaccelerated beams from
ReA1.5 has been conducted in August
2013. Beams from ReA3 will be available for research in late 2014 and an
upgrade to ReA6 is in the advanced
design stage and may come on-line in
2015–2016 depending on funding.
The current layout of NSCL’s experimental areas is shown in Figure 2.
Major experimental apparatus includes
the large-acceptance high-resolution
S800 Spectrograph and the high-field
Sweeper Magnet, the high-resolution
array HiRA for charged-particle detection, high-resolution and highefficiency γ-ray detection systems (the
Segmented Germanium Array SeGA
and the Caesium Iodide Array CAESAR, respectively), neutron detection arrays suited for various energies
(Modular Neutron Array—MoNA and
its extension LISA, Neutron Emission
Ration Observer—NERO, Low-Energy
Neutron Detector Array—LENDA and
the Neutron Walls), the Beta Counting
System (BCS) as well as beta NMR/
NQR setups, diamond timing detectors, the low-energy beam and ion
(Penning) trap facility LEBIT, and the
facilities and methods
Figure 2. Schematic layout of the NSCL facility. The experimental area utilizing
reaccelerated beams from ReA6 is schematic with the final layout driven by user
demand (see text for the status of equipment).
beam-cooler and laser spectroscopy
facility BECOLA. A superconducting
linear re-accelerator (ReA3) and its
dedicated experimental area, beamlines, and apparatus are nearing completion. For research with reaccelerated
beams, an active-target time projection
chamber (AT-TPC), a multi-purpose
beam-line, a gas-jet target and a SEparator for CApture Reactions (SECAR)
are currently under construction or in
the advanced planning stages. In the
past year, NSCL hosted the advanced
γ-ray tracking array GRETINA. After
successful completion of an extended
scientific campaign, GRETINA was
moved to the ATLAS facility at ANL
in the summer of 2013. The first experiment with reaccelerated rare isotopes in the new ReA3 experimental
hall at NSCL was performed in August
2013 with the Array for Nuclear Astrophysics Studies with Exotic Nuclei
(ANASEN) built by Florida State University and Louisiana State University.
Research at NSCL addresses important questions in basic nuclear physics,
nuclear astrophysics, accelerator physics, and associated instrumentation research and development. About 5−10%
of the beam time is allocated to crossdisciplinary and applied research.
Beam time is approved by the NSCL
director who is advised by a Program
Advisory Committee (PAC) consisting
of several internationally accomplished
experts from other institutions.
DOE-SC and MSU signed the Cooperative Agreement to design and
establish FRIB on 8 June 2009 (Fig-
ure 3). In September 2010, the project
received Critical Decision 1 approval
from the DOE-SC acquisition executive. In August 2013, DOE-SC approved a performance baseline of $730
M with an associated completion date
in 2022 (Critical Decision 2). The Project is managed to an early completion
in December 2020. Also approved was
Critical Decision 3a, which allows the
project to proceed with long-lead procurements. Commencement of the start
of civil construction is subject to a Fiscal Year 2014 appropriation.
FRIB will provide researchers opportunities to study the properties of
rare isotopes and to put this knowledge
to use in various applications, including
in materials science, nuclear medicine,
and nuclear weapons stockpile stewardship. The research areas include:
• Nuclear Structure—What is the
nature of the nuclear force that
binds protons and neutrons into
stable nuclei and rare isotopes?
What are the limits of nuclear existence?
• Nuclear Astrophysics—What is
the nature of neutron stars and
dense nuclear matter? What is the
Figure 3. Architect’s rendering of the baselined Facility for Rare Isotope Beams
(FRIB).
Vol. 24, No. 1, 2014, Nuclear Physics News29
facilities and methods
origin of elements heavier than
iron in the Cosmos? What are the
nuclear reactions that drive stars
and stellar explosions?
• Tests of Fundamental Symmetries—Why is there now more
matter than antimatter in the Universe?
• Application of Isotopes to Society—What are the potential uses
in medicine, energy, material sciences, and national security?
The FRIB design provides for fast,
stopped, and reaccelerated beams of
rare isotopes. Features of the FRIB design include:
• A state-of-the-art superconducting-RF driver linear accelerator
provides 400 kW for all beams
with uranium accelerated to 200
MeV/nucleon and lighter ions to
higher energies (protons up to
600 MeV).
• Space in the linac tunnel and
shielding in the production area
allows upgrading the driver linac
energy to 400 MeV/nucleon for
uranium and 1 GeV for protons
without significant interruption of
the future science program.
• A high-power in-flight production target and a three-stage
high-acceptance, high-resolution
fragment separator produce and
deliver rare isotopes with high
rates and high purity.
• Provisions have been made in
the fragment separator to allow
future implementation of isotope
harvesting and the addition of
(limited) multi-user capability.
• Space is available and provisions
have been made in the facility design to allow the addition of a second target facility, for example for
ISOL beam production with protons or light ions up to 400 kW.
• Three beam stopping stations—
two gas stopping stations and
30
one solid stopper—will provide
“stopped” beams with highest
efficiency for precision experiments and for reacceleration.
• A superconducting-RF reaccelerator will be able to provide
beams up to 12 MeV/nucleon
(uranium) and higher energies
for lighter beams (e.g., 21 MeV/
nucleon for 48Cr).
• Large experimental areas (47,000
sq. ft.) can accommodate new experimental apparatus for science
with stopped beams, reaccelerated beams, and fast beams. The
site as space available to double
the size of experimental areas or
for housing additional rare-isotope research facilities.
• A full set of well-tested experimental equipment is already in
place for research in all FRIB science areas.
• Opportunity for a pre-FRIB science program using the existing
in-flight separated beams from
the Coupled Cyclotron Facility
and the ReA3 reaccelerator. Users will be able to mount and test
equipment and techniques and do
science with beams at all energies
in-situ so that they are immediately ready for experiments when
FRIB is complete; this will allow
for a continually evolving science
program during the time FRIB is
under construction, which will
seamlessly merge into the research program at FRIB.
• A User Relations Office supports ongoing research with the
CCF and the development of new
user programs and experimental
equipment.
The final design of the FRIB conventional facilities—the tunnel and
support buildings—is complete. Preconstruction site preparation is complete and pilings for the earth-retention
Nuclear Physics News, Vol. 24, No. 1, 2014
system have been placed. Research and
development activities have been successfully completed, with much of the
R&D work accomplished in collaboration with national laboratories. Final
design of the technical systems—accelerator and experimental equipment—is
underway and anticipated to be substantially complete in 2014.
For more information on the FRIB
Project, see http://www.frib.msu.edu.
The independent FRIB Users Organization website is http://www.fribusers.
org.
Alexandra Gade
NSCL/FRIB Laboratory, Michigan
State University, Michigan, USA
C. Konrad Gelbke
NSCL/FRIB Laboratory, Michigan
State University, Michigan, USA
Thomas Glasmacher
NSCL/FRIB Laboratory, Michigan
State University, Michigan, USA
meeting reports
INPC 2013: Florence, 2–6 June 2013
The 26th International Nuclear
Physics Conference (INPC) was held
in Florence on 2–6 June 2013 (Figure
1), three years after the previous edition of 2010 in Vancouver. The INPC
is the main conference in the field of
Nuclear Physics taking place every
3 years and is supported by IUPAP
(International Union for Pure and
Applied Physics), which also selects
the venue among the proposed ones.
For the organization of INPC2013 a
substantial contribution was given by
INFN (the Italian Institute of Nuclear
Physics) and support was also provided by three Universities—Università degli Studi di Firenze, Milano,
and Padova—and several sponsors.
In the evening preceding the starting of the conference, a public lecture,
on the activities of the LABEC laboratory in Florence concerning applications of nuclear physics techniques for
societal purposes, was given by Pier
Andrea Mandò.
As in most past editions this conference covered a wide spectrum of topics: (i) Nuclear structure and Nuclear
reactions; (ii) Hot and dense nuclear
matter; (iii) Fundamental symmetries
and interactions in nuclei; (iv) Hadron structure and Hadron in Nuclei;
(v) Nuclear astrophysics; (vi) Neu-
trinos and nuclei; (vii) Nuclear physics–based applications; and (viii) New
facilities and instrumentation.
The conference was well attended
with 730 participants (out of which
200 were women) from 54 different
countries. Thanks to the work of the
international advisory and program
committees it was decided to invite
several young colleagues. The main
task of the plenary invited speakers
was to illustrate the best theoretical
and experimental achievements in the
different nuclear physics topics that
are impacting worldwide the present
research programs in the field. This
goal was well accomplished by the 33
speakers (out of which 8 were women).
Their presentations were well organized in terms of content and clarity
and thus transmitted the main physics
messages to a very mixed audience.
This has triggered many unexpected
links, discussions, or collaborations
among participants and contributed
to make the attendance of the parallel sessions very lively. Indeed, the
afternoon parallel sessions, with 88
invited and 218 contributed talks, had
an impressive participation. The talks
of students and young researchers
were to a large extent outstanding and
in general manifested the interest and
the determination of the new generation to contribute to the progress with
experiments, theory, and associated
technology and applications.
It is difficult to transmit in a few
lines the richness of the program of
this conference, but we would like to
stress that the participation and the
enthusiasm in discussing the physics
results were remarkable. Therefore,
the short description below is far from
being exhaustive.
It was really impressive to see the
rather large number of scientific highlights, presented in the context of topical overviews in plenary sessions and
more in detail in parallel sessions as
invited and selected contributions. In
the sector of nuclear structure there
are several highlights concerning
the quest of shell evolution and new
magic numbers (in particular the recent evidence in Ca isotope), of superheavy elements, of collective modes
and new shapes (including pear type),
and of loosely bound light nuclei.
Many new interesting experimental
results benefited from the progress
on traps and laser techniques, and on
reaction and spectroscopy techniques.
For the latter, gamma-ray detection
resulted a key tool (e.g., the AGATA
array). From the interpretation of the
Figure 1. The conference group photo.
Vol. 24, No. 1, 2014, Nuclear Physics News31
meeting reports
results, it is clear that theory is making particular efforts also to provide
new predictions for few and many
bodies in terms of ab initio models,
three body forces, and energy density
Functional methods and in particular
cases also from lattice simulations.
Some talks emphasized that nuclear
reactions are essential tools to address
nuclear structure questions but also to
learn on more global properties (e.g.,
barriers and potentials) and on the
nuclear equation of state, which are
relevant for astrophysics problems.
Nuclear astrophysics is presently
blooming because of the availability
now and more in the future of new
radioactive beams, key tools to address extensively several basic questions concerning nucleosynthesis. In
addition, measurements at dedicated
low energy accelerators on the stellar
energy production will continue (e.g.,
LUNA). The discussed plans for major
facilities for radioactive ions (GANIL,
FAIR, RIKEN, TRUMF and FRIBS)
and for smaller facilities (e.g., ALTO,
ISOLDE, JYFL, SPES-LNL, and others) promise a very exciting future.
The progress on the structure of
nucleons, its tomography, and on meson productions is remarkable. It was
pointed out that the present measurements provide the needed stringent
tests to QCD in the non perturbative
regime and that it will be important
to continue the effort in this direction
to learn on the strong force with the
necessary detail. JLAB with 12 GeV
beams, FAIR under construction, and
the Electron-Ion collider project represent this endeavor. The presented
results on the properties of hadrons
in nuclei mainly concerned strangeness as produced with different probes
including heavy ion collisions. Soon
JPARC will make a major step in this
direction. The highlights in the field
of Hot and Dense QCD are related to
the recent results of ALICE at LHC on
32
Figure 2. The three winners of the IUPAP prize and the IUPAP chair and scientific secretary.
the production and decay of different
particles characterizing the very hot
hadronic matter, the quark degrees of
freedom and deconfinement.
The comparison with RHIC results
was discussed together with the major
theoretical developments.
For the study of weak interactions
the conference concentrated mainly
on selected topics concerning neutrino physics, particularly at nuclear
reactors, neutrino interaction, and
beta decay related to the testing of the
CKM Unitarity matrix. Among the
works on fundamental interactions it
is worth mentioning the measurement,
using nuclear physics methods, of the
neutron electric dipole moment providing a stringent test of the Standard
Model.
The talks on applications and new
instrumentation were in general very
attractive. There was a particular curiosity about the work made in Japan to
monitor radioactivity after Fukushima
because it was presented as a scientific
report of nuclear physicists. A new facility that, in the future, in addition to
basic research with intense high resolution gamma beams, will play an important role in applications is ELI_NP
(ESFRI facility in Bucarest).
Nuclear Physics News, Vol. 24, No. 1, 2014
Excellent talks were given, in a
dedicated plenary session, by the three
winners of the IUPAP prizes, Rabia
Burcu Cakirli (from MPI-Heidelberg)
on mass measurements of exotic nuclei, Stefano Gandolfi (from LANL)
on ab initio calculations, and Bjorn
Peter Schenke (BNL) on relativistic
heavy ions (Figure 2).
We were very proud that EPS decided to deliver the IBA prize 2013
during INPC2013. It was a great pleasure for all of us to applaud warmly
the winner, Prof. Marco Durante from
GSI.
INPC 2013 made a special effort
to attract many graduate students and
thus we had 106 presenting posters,
plus 33 were selected for oral contributions. As in the previous edition,
INPC 2013 teamed up with Nuclear
Physics A to provide awards to the two
best student oral presentations and five
top poster presentations at the conference. An international panel of judges
together with members from the editorial board of Nuclear Physics A finally
decided on the following award winners: (i) Ulrika Forsberg (University
of Lund, Sweden) for the oral presentation “Spectroscopy of Element 115
decay chains”; (ii) Tadashi Hashimoto
meeting reports
(University of Tokyo, Japan) for the
oral presentation “A search for the
K-pp bound state in the 3He(inflightK-,n) reaction at J-PARC”; (iii) Esther
Sabine Bönig (TU Darmstadt, Germany) for a poster “Quadrupole collectivity in neutron-rich Cd isotopes”;
(iv) Michele Gelain (University of
Padova, Italy) for a poster “Characterization of a highly-segmented Silicon
detector for the TRACE prototype”;
(v) Andrej Herzan (University of
Jyvaskyla, Finland) for “Spectroscopy
of 193Bi”; (vi) Timothy John Hobbs
(Indiana University, USA) for the
poster “The nonperturbative charm
content of the nucleon”; (vii) Andrea
Tsinganis (CERN and NTUA, Athens,
Greece) for the poster “Measurement
of the 242Pu(n,f) cross section at the
CERN n_TOF facility.”
As a conclusive remark we would
like to stress that by attending this
conference one had a very positive impression on the field being very vital,
healthy, and dynamic. Many young
people are eagerly and enthusiastically
working and are important actors in
new experiments, theory, and facilities.
At the end of the conference IUPAP
announced the selection of the host of
the next INPC conference: it will be
held in 2016 in Adelaide, Australia.
We look forward to discussing exciting progress there as well!
Angela Bracco
University and INFN Milano
Pier Andrea Mandò
University and INFN Firenze
Cosimo Signorini
University and INFN Padova
Baryons 2013: International Conference on the
Structure of Baryons, Glasgow, 24–28 June 2013
Baryons 2013, the thirteenth International Conference on the Structure
of Baryons, was hosted by the School
of Physics and Astronomy at the University of Glasgow, Scotland. The
conference continued a long series of
triennial meetings, which started at
Duke University in 1970, to discuss experimental and theoretical advances in
our understanding of the properties of
baryons, the essential building blocks
of the atomic nucleus. The 2013 conference attracted 149 participants from
73 different institutions in 22 countries,
spanning 6 continents (Figure 1). There
were 26 plenary talks and 8 parallel
session keynote talks given by experts
in the field as well as 81 contributed
talks and 4 poster presentations.
The conference took place immediately following the 3rd CLAS12 European Workshop, which was also held
in Glasgow. Many delegates from the
CLAS12 meeting stayed on to attend
Baryons 2013 and this increased participation and interaction.
The conference opened with a
plenary talk given by Volker Crede,
Florida State University, on progress
toward understanding baryon resonances. This introduced the theme
of baryon spectroscopy and included
reports on experimental photo- and
electro-pion production at CLAS (Jefferson Lab), CBELSA/TAPS (Bonn).
The conference also heard of hadroninduced experiments with proton and
deuteron beams at COSY (Jülich).
Complementary studies using pion
beams at J-PARC (Tokai) are expected
to provide new information which will
dramatically extend the precision and
quantity of (p,2p) reaction data and
lead to new constraints on coupledchannel effects.
Major progress in our theoretical
understanding of nucleon resonances
was reported through improvements
in partial wave analysis treatments.
Accounts of recent work using the
MAID, SAID, Bonn-Gatchina, and
Jülich codes were presented and compared. Also discussed were the effects
of dynamic coupling of resonances to
unbound continuum states.
Another major topic discussed was
the spin and flavor structure of the nucleon, with accounts of current experimental and theoretical investigations
Vol. 24, No. 1, 2014, Nuclear Physics News33
meeting reports
Figure 1. Baryons 2013 delegates in front of the Glasgow conference venue.
to shed light on the so-called spin
puzzle and plans to extend DIS and
SIDIS measurements to higher values
of Bjorken x at the upgraded 12-GeV
electron beam at Jefferson Lab.
The advantages of using the AdS/
QCD approach in a light-front wavefunction formalism to describe baryon
spectroscopy and hadronic form factors was described by several speakers. Other theoretical talks explored
the physics of dynamical chiral symmetry breaking, sea quarks and how
these relate to non-perturbative QCD.
A great deal of work on lattice QCD
has been carried out recently to determine the ground and excited state
properties of baryons. New techniques
and the availability of computing
power are revolutionizing calculations
and many baryon masses can now be
described with reasonable precision.
Other topics discussed included
discrepancies in recent proton radius
studies discovered between recent
34
high-precision electron scattering
measurements and data from muonic
hydrogen Lamb-shift measurements,
measurements to determine the Proton’s weak charge and the search for
dark photons at Jefferson Lab. Looking to the future the conference heard
of plans for the EIC, a future electronion collider that will shed light on the
role of sea-quarks and gluons in the
structure and properties of baryons.
A public lecture on the “Isotopic
Legacy of Frederick Soddy,” given
by David Sanderson of the Scottish
Universities Environmental Research
Centre, provided a historical account
of the first nuclear physics work at
Glasgow University. Soddy worked
in Glasgow on the chemical properties of radioactive materials from
1904 to 1914 and introduced the term
“isotope” in a Nature paper in December 1913 [1]. It is said that the term
“isotope” actually originated in the
course of a dinner party discussion at
Nuclear Physics News, Vol. 24, No. 1, 2014
Glasgow University. Soddy was subsequently awarded the 1921 Nobel Prize
in Chemistry for his “contributions
to our knowledge of the chemistry
of radioactive substances, and his investigations into the origin and nature
of isotopes” (see http://www.nobel
prize.org/nobel_prizes/chemistry/
laureates/). A modern analysis of several surviving radioactive samples
stored in the “Soddy box” provided
new insight into Soddy’s work at
Glasgow. Investigations are currently
being carried out to find safe ways of
curating this historical material for future educational purposes.
The social program included a
Whisky Tasting for those who arrived
early and a Civic Reception hosted
by Glasgow City Council at the City
Chambers. The conference dinner, at
the nearby Oran Mor restaurant, had
a traditional Scottish theme, including
Scotch broth and haggis. After-dinner
entertainment was provided by the
Hillhead High School Ceilidh Band.
Two conference prizes were
awarded to Igor Senderovich, Arizona
State University, and Karin Schoenning, Uppsala University, for the outstanding quality of their contributed
talks.
Copies of presentations made at the
conference are available at the conference website: http://nuclear.gla.ac.uk/
Baryons2013/.
Acknowledgments
The organizers thank the Scottish
Universities Physics Alliance (SUPA),
Hamamatsu, Glasgow City Council, and Grant’s Whisky for generous
sponsorship. The next Baryons conference will be hosted by Florida State
University.
References
1. F. Soddy, Nature 92 (1913).
I. J. Douglas MacGregor
University of Glasgow, UK
meeting reports
XXXIII Mazurian Lakes Conference on Physics:
Frontiers in Nuclear Physics, Piaski,
Poland, 1–7 September 2013
Figure 1. Conference participants at the shore of Lake Bełdany (photo: M. Zielinˊska).
The 33rd Mazurian Lakes Conference on Physics was held at Piaski,
Poland, on 1–7 September 2013. The
history of Mazurian meetings dates
back to 1968. This traditional conference is now organized every two years
by the University of Warsaw, the National Centre for Nuclear Research,
and the Pro-Physica Foundation. Its
goal is to bring together scientists to
discuss the hottest topics in nuclear
physics in an environment facilitating
contacts between the participants staying at a remote location (Figure 1).
Over 140 physicists from 17 countries all over the world enjoyed lively
discussions on the latest developments in the fields of low-energy nuclear physics, both experimental and
theoretical. The scientific program
included about 80 oral presentations.
Each conference day started with a
keynote lecture, followed by the invited and contributed talks. The number of excellent contributed abstracts
was so large that a crowded poster
session included the presentation and
discussions of almost 50 posters.
The conference began with a talk
given by M. Targowski (Mikołaj Kopernik University of Toruń, Poland)
presenting mostly unknown aspects
of Mikołaj Kopernik’s life. It included
well-documented stories about Kopernik’s investigations and conclusions,
the search for Kopernik’s birthplace,
the topic of his national origin and
even some letters referring to his private life, somewhat more rich than
might be expected for a 15th-century
canon (church official). Evidently,
rare cloudy nights in the Mazurian
Region prevented the continuous observation of shining stars by this great
astronomer.
Scientific topics discussed at the
conference ranged from nuclear
structure to nuclear reactions, from
nuclear astrophysics to the synthesis
of new elements, from results from
just commissioned powerful detector arrays to new facilities under
construction and the applications of
nuclear physics research. An introduction to the frontiers in low-energy
nuclear physics made by W. Nazarewicz was followed by a presentation
of nuclear spectroscopy results discussed with respect to the Standard
Model (P. Butler and B. Blank). An
impressive harvest of new isotopes
and isomers discovered at RIKEN
(Wako, Japan) was shared with the
audience by T. Kubo, G. Lorusso, and
P. Boutachkov, while GSI share of
new isotopes identification was mentioned by H. Geissel. First results of
two gamma tracking “demonstrator”
arrays, GRETINA and AGATA, were
presented by C. Campbell and S. Leoni. Several talks addressed the physics of loosely bound and unbound
states (M. Płoszajczak, I. Mukha,
K. Kemper, M. Pfützner, and S. Orrigo). Modern theoretical approaches
Vol. 24, No. 1, 2014, Nuclear Physics News35
meeting reports
to nuclear structure and reactions included the application of the energy
density functional (J. Dobaczewski),
three-body forces (S. Kistryn and
G. Hagen) and isospin-mixing phenomena (W. Satuła). State-of-the-art
nuclear astrophysics studies were
presented by M. Wiescher, P. Woods,
C. Deibel, K. Czerski, A. Caciolli,
M. Mumpower, A. Tumino, and P.
Descouvemont. A number of decay
spectroscopy results relevant to our
understanding of the structure of exotic nuclei contributed to the analysis of astrophysical processes in hot
stars (C. Mazzocchi, R. Grzywacz, A.
Jokinen, G. Lorusso, and S. Bottoni).
The importance of today’s nuclear reactions studies was explained by A.
Bonnacorso, I. Martel, S. Kistryn, M.
Mazzocco, K. Wimmer, A. Diaz-Torres, V. Goldberg, V. Pesudo-Fortes, J.
Johansen, M. Bondi, and T. Cap. V.
Zagrebayev’s talk offered a link between reaction studies and the production of super-heavy elements. His
talk was followed by the presentations of K. Siwek-Wilczyńska and Z.
Majka analyzing particular reactions
leading to new super heavy nuclei.
Theoretical aspects of the structure
of the heaviest nuclei were discussed
by M. Bender, A. Baran, and by J.
Skalski. All facilities contributing
to the discovery and spectroscopic
studies of new chemical elements
and super heavy isotopes beyond the
recently named Z = 112 Copernicium were presented at the Mazurian
Conference. Yu. Oganessian (Dubna)
summarized recent campaigns using radioactive actinide target materials like the Oak Ridge made Z
= 97 249Bk and intense 48Ca beams
to observe the properties of the yetunnamed elements 118, 117, 115, and
113, and their longer-lived daughter
activities at the “Hot Fusion Island.”
Plans for discoveries of even heavier
atomic nuclei using a Z = 98 mixed-
36
Cf target from Oak Ridge, and the
construction status of the new “SHE
Factory” were shared with conference participants. C. Düllmann, D.
Ackermann, and M. Block (Darmstadt) presented, among other results,
the confirmation of the discovery of
element 117, the search for new elements 119 and 120, longer-term perspectives and developments needed
for the discoveries of new elements
beyond 118, and the relevance of
direct mass measurements and laser
spectroscopy of the heaviest nuclei.
K. Morimoto summarized nearly 600
days of an experimental campaign
leading to the firm observation of the
278(113) isotope linked by several
alpha decays to well known heavy
nuclei on the nuclear mainland. K.
Gregorich and D. Rudolph presented
new detection setups for decay spectroscopy of super heavy nuclei, now
complemented by efficient gamma
detector arrays. The spectroscopy of
Z = 115 isotopes confirmed earlier
reported discoveries and led to the
observation of first g-ray and KX-ray
signals in prompt coincidence with
alpha decays—a first hint for Z-fingerprinting of nuclei at the “Hot Fusion Island” (D. Rudolph). In-beam
gamma and electron spectroscopy of
Z > 100 nuclei yielding the excited
levels and allowing the deformation
of these heavy isotopes to be deduced
were summarized by R. D. Herzberg
and T.-L. Khoo (the latter talk given
partially in Polish). Coulomb excitation studies of not-as-heavy nuclei,
using stable and radioactive beams,
were presented by K. HadyńskaKlęk, A. Trzcińska, M. Zielińska,
and K. Wrzosek-Lipska as well as
S. Leoni and P. Butler. A new technique to investigate nano- and microsecond gamma-decaying isomers in
neutron-rich nuclei was explained by
W. Królas. Single-hole states in the
doubly-magic 132Sn, namely the neu-
Nuclear Physics News, Vol. 24, No. 1, 2014
tron levels in 131Sn populated using a
radioactive 130Sn beam at Oak Ridge
were presented by A. Bey.
The long-term future of the Mazurian Conferences is justified by the
number of talks presenting the status
of new laboratories and powerful detector arrays. FAIR (H. Simon and H.
Geissel), SPIRAL-2 (M. Lewitowicz),
FRIB (G. Bollen), ARIEL (G. Hackman), CARIBU (G. Savard), SPES (F.
Gramena), SHE Factory (Yu. Oganessian), National Cyclotron Laboratory
(B. Fornal), and ELI-NP (D. Balabanski) equipped with efficient spectrometers like S3 (A. Drouart) and detector
arrays like the full versions of GRETA
(C. Campbell) and AGATA (S. Leoni),
VANDLE (R. Grzywacz), MTAS (A.
Fijałkowska), RIB isomer-scope (W.
Królas), traps and lasers (A. Jokinen,
M. Block, and J. Papuga) will allow
us to study even more exotic isotopes
and processes involving these nuclei.
These new and expensive constructions will continue to benefit the physics community and our society in general. The latter may be deduced from
several talks at the Mazurian Conference presenting the applications
of nuclear physics. Hadron therapy
(P. Olko and A. Biegun), radioisotopes for diagnostics and therapy (R.
Mikołajczak), medical imaging (L.
Królicki) and biological response
to radiation (U. Kaźmierczak) are
evident outcomes of earlier nuclear
physics developments and are helping thousands of patients every day.
Nuclear power has a substantial and
CO2-free contribution to the world energy budget. The decay heat, not well
studied as yet, plays an important,
sometimes dramatic, role during the
nuclear fuel cycle (A. Fijałkowska).
New detector techniques may keep us
safer from terrorist attacks and prevent
smuggling of nuclear materials (K.
Peräjärvi). Cultural Heritage research
profits from non-destructive inspec-
meeting reports
Figure 2. “Team Finland,” led by
Juha Äystö—regatta winners (photo:
M. Zielinˊska).
tion of art objects offered by nuclear
techniques (H.-E. Mahnke).
We should not forget the social aspect of the conference. With the aid of
excellent weather, in between the lectures, the participants could continue
scientific discussions while enjoying
outdoor activities offered by the Piaski
venue: kayaking, canoeing, cycling,
and of course sailing with the traditional Regatta, this year won by Juha
Äystö and his “Team Finland” (Figure
2). Everybody enjoyed the vocal skills
of several national teams at the campfire, in particular an unforgettable solo
performances by Kosuke Morita and
Magda Zielińska.
The conference was closed by the
announcement of the best poster (A.
Korgul) and by a comprehensive summary made by A. Maj, which provided
also a natural bridge to the next Polish traditional conference to be held
in Zakopane in 2014. The TASCA’13
workshop chaired by Ch. Düllmann
and A. Yakushev immediately followed the Mazurian Conference on 7
September at Piaski.
The 34th Mazurian Lakes Conference on Physics will be held in September 2015, not surprisingly in the
Mazurian Lakes Region.
Chiara Mazzocchi
Krzysztof Rusek
University of Warsaw
and
Krzysztof Rykaczewski
Oak Ridge National Laboratory
Latest News in Antiproton Physics Discussed
at the LEAP 2013 Conference in Uppsala
Figure 1. Participants of the LEAP 2013 conference in Uppsala.
Low-energy antiproton physics
is an interdisciplinary field centered
around the antimatter partner of the
proton that ranges from particle, nuclear, atomic and astrophysics to applied physics. It confronts directly the
symmetry between matter and anti-
matter and addresses many key questions of contemporary research: What
are the fundamental symmetries of
nature and in which way are they violated? Why is there basically no antimatter in the universe? How does the
strong interaction and its symmetries
shape the structure of hadrons? Does
matter and antimatter respond in the
same way to gravity?
The highly acclaimed synthesis
and trapping of antihydrogen atoms
at CERN’s Antiproton Decelerator
(AD) provides unique opportunities to
Vol. 24, No. 1, 2014, Nuclear Physics News37
meeting reports
probe the fundamental laws and symmetries. Satellite and balloon experiments are searching for cosmic antimatter, the results of which could have
profound implications on cosmology.
Antiprotons will be used to study the
properties and structures of atoms, nuclei and hadrons at the upcoming Facility for Antiproton and Ion Research
(FAIR) in Darmstadt. These items
were discussed at LEAP 2013, the
11th International Conference on Low
Energy Antiproton Physics, that took
place at the Angstrom Laboratory, Uppsala University, Sweden, during 11–
15 June. The conference was jointly
hosted by the Department of Physics
and Astronomy and the Department
of Chemistry of Uppsala University
with Tord Johansson as chair and Piotr Froelich as co-chair. The conference attracted nearly 100 participants
(Figure 1) and featured more than 20
invited plenary speakers, more than 20
contributed talks and a dozen posters
with an emphasis on promoting young
researchers. Some of the highlights of
the packed program are presented in
the following.
Symmetries and Antihydrogen
The conference began with a session on symmetries covering theoretical and experimental overviews
of testing fundamental symmetries.
At the center of this is the CPT symmetry, the combined action of changing particles and antiparticles (charge
conjugation, C), parity transformation
(P), and time reversal (T). A violation
of this symmetry would have severe
consequences for our understanding
of the laws of nature since invariance under CPT emerges directly
from quantum field theory and Lorentz invariance. Several approaches
to test this symmetry at the AD were
presented during the conference. Especially encouraging were the reports
on the rapid progress in trapping and
cooling antihydrogen atoms that allowed first experiments on the road
38
toward tests of the CPT-symmetry in
the domain of (anti)atomic physics.
The ALPHA collaboration reported
on the first measurement of hyperfine
transition in ground state antihydrogen and reviewed their future plans.
The ATRAP collaboration reported on
a large improvement of the measured
value of the antiproton magnetic moment, as well as their latest progress
in antihydrogen formation using a
double charge exchange process. The
ASACUSA collaboration reported on
the two-photon laser spectroscopy
measurements on antiprotonic helium
that has yielded the to date the most
precise measurement of the antiproton-electron mass ratio to date. They
also reviewed their approach toward
the hyperfine spectroscopy of antihydrogen. The Baryon Antibaryon Symmetry Experiment (BASE) presented
their intent to make a test of CPT invariance by a precise comparison between the proton and antiproton magnetic moment.
Antimatter in the Universe
The constraints on the presence
of antimatter in the Universe were
reviewed. An excess of antiparticles,
compared to expectations in the cosmic radiation could be a signal from
dark matter particles. The PAMELA
satellite-borne experiment presented
their results on the positron and antiproton flux in the cosmic radiation.
The Alpha Magnetic Spectrometer Experiment (AMS) at the International
Space Station presented high statistics
result on positron and electron fluxes
up to 350 GeV. The positron fraction
in the data shows no fine structure or
anisotropy but it seems as a large portion of the high-energy electrons and
positrons originate from an unknown
common source.
Gravity and Antimatter
The gravitational interaction of
antimatter with matter was discussed
Nuclear Physics News, Vol. 24, No. 1, 2014
both from the theoretical and experimental sides. An antiapple is expected
to fall down on Earth exactly as an
ordinary apple according to Einstein´s
Weak Equivalence Principle (WEP).
This principle has, however, never
been tested experimentally for antimatter and, as pointed out at the conference, there is room for a difference
between matter-matter and antimatter-matter gravitational interaction
within certain theoretical frameworks.
The result from the TRAP collaboration on a gravitational redshift limit
from simultaneously trapped protons
and antiprotons was presented as a
stringent test of the difference in the
gravitational constant for matter and
antimatter. The AEgIS and GBAR collaborations presented their approaches
toward a measurement of the gravitational acceleration of antihydrogen
at the AD. The ALPHA collaboration
presented the first measurement on
the effect of gravity on an ensemble
of antihydrogen atoms in a trap. The
error bars, in this pioneering test, are
too large to draw any definite conclusions, but the first experimental step
has been taken.
Hadron Physics
Antiprotons are also excellent tools
for hadron physics, as already proven
at the LEAR facility at CERN. The
gluon rich antiproton-proton annihilation makes these reactions ideal to
search for exotic hadrons with gluon
content, i.e., glueballs (hadrons consisting only of gluons) and hybrids
(hadrons having a gluon component).
The FAIR facility will open up a new
era in this field. The hadron physics
programme of the PANDA collaboration at FAIR, that will address this
topic, was presented. Charmed meson spectroscopy is also on its program. This is a hot topic, which was
reviewed during the conference, due
to recently discovered narrow X,Y,Z
charmonium-like mesons, some of
which cannot be explained as being
meeting reports
ordinary quark-antiquark configurations. Other topics that were covered
both from theoretical and experimental points of view were double lambda
hypernuclei and nucleon structure.
Facilities and Instrumentation
The near-future upgrades for antiatomic physics were discussed, such
as the construction of the Extra Low
Energy Antiproton ring (ELENA) at
the AD that will lower the energy of
the antiprotons and thereby increase
the number of useful antiprotons for
stop experiments by up to two orders
of magnitude.
The transfer of the storage ring
CRYRING from Stockholm will allow for an early start of physics with
low energy antiptrotons at FAIR.
Many developments toward a more
efficient production of antihydrogen
as well as the instrumentation of the
PANDA detector were discussed.
Applications
Talks on applications and new
techniques with antiprotons included
the ACE experiment at the AD, which
is studying the possible use of antiprotons for cancer therapy, and developments toward spin-polarized antiprotons and antihydrogen.
The conference included a boat trip
to the 17th century renaissance castle
Skokloster. At the conference dinner
Prof. em. Gösta Ekspong gave a celebrated exposé of his firsthand memories of the discovery of the antiproton
at the Bevatron in Berkley. LEAP
2013 ended with two well-attended
public lectures that presented the role
of symmetries in physics, both from
the experimental and theoretical perspective. Gerard Gabrielse, Harvard
University, talked about “Cold matter and antimatter—how similar are
they?” and Ulf Danielsson, Uppsala
University, “Mirror, mirror on the
wall--the beauty of the universe and
its symmetries.” The next LEAP meeting is planned for Kanazawa, Japan, in
2016, and will be chaired by Yasunori
Yamazaki.
For full details of the speakers and
the presentations, see http://www.
physics.uu.se/leap2013. The proceedings will be published in Hyperfine
Interactions.
Tord Johansson
Uppsala University
Correction
In the Laboratory Portrait “The Radioactive Ion Beams in Brazil (RIBRAS) Facility” by A. Lépine-Szily, R. Lichtenhäler,
and V. Guimarães in Nuclear Physics News 23(3), pages 5–11, Figure 1 was printed incorrectly. The figure is reprinted below.
Figure 1. The experimental set-up, from the left, with the production target, the W beam stopper, the first solenoid followed
by the intermediate scattering chambers, with the secondary target and detectors installed in it, followed by the second
solenoid and the large scattering chamber.
Vol. 24, No. 1, 2014, Nuclear Physics News39
news and views
The LUNA-MV Project at Gran Sasso
Underground Laboratory
Since 1991, in the Gran Sasso
underground Laboratory [1], in between the three main experimental
halls where large collaborations run
experiments on neutrinos and dark
matter, a relatively small collaboration has been performing the LUNA
experiment [2]. LUNA is an acronym
for “Laboratory for Underground
Nuclear Astrophysics”: in this laboratory, the LUNA collaboration was
able to reproduce a few thermonuclear reactions usually taking place
inside stars [3, 4]. But why go underground to perform these measurements? For a nuclear reaction between charged particles to occur in a
star, a large number of interacting nuclei has to be available and the Coulomb repulsion has to be overcome.
The convolution of these two effects
make most of the reactions occuring
in the so-called Gamow peak, an energetic region located well below the
Coulomb barrier. The probability of
the reaction, its cross-section, exponentially decreases with the interaction energy due to the tunneling of
the Coulomb barrier and therefore,
in stellar environments, turns out to
be extremely low. Of course, when
trying to reproduce the reaction in a
laboratory, with the typical beam currents and target densities achievable,
the low cross-section translates into
a low reaction rate that could be of
the order of a few events per month
or even lower. Therefore, it is mandatory to reduce as much as possible
any background component in order
to be able to detect such feeble signal. The rock overburden above the
Gran Sasso Laboratory decreases by
more than six orders of magnitude
40
the natural muon flux and by three
orders of magnitude the neutron one,
making the Lab an excellent location to measure such nuclear reaction cross-sections at typical stellar
energies, maximizing the signal to
background ratio. So far, the LUNA
collaboration has been engaged in
measuring reactions belonging to
the Hydrogen burning, which has the
very important task of transforming
four protons into Helium with a net
energy release, and of the Big Bang
Nucleosynthesis, responsible for the
formation of the lightest elements in
the early Universe. Two different accelerators have been used: a 50 kV
“homemade” machine and a 400 kV
commercial one, both able to deliver
intense beams, with long-term stability and precise energy determination.
The cross-sections already measured
have important consequences for
neutrino physics, element nucleosynthesis and cosmology, and constitute
important ingredients in stellar models. In order to make a step forward
and be able to measure reactions belonging to the Helium burning that
are important at higher temperatures
in stars that ultimately means larger
interaction energies, a higher voltage
accelerator is necessary. For this reason, the LUNA collaboration is now
involved also in a new adventure, the
LUNA-MV project, which foresees
the installation of a 3.5 MV machine
in the Gran Sasso Underground Laboratory. In particular, the experimental
program foresees the measurement
of the 12C(a,g)16O, the 13C(a,n)16O,
and the 22Ne(a,n)25Mg reactions.
The first determines the time scale of
Helium burning and the abundances
Nuclear Physics News, Vol. 24, No. 1, 2014
of Carbon and Oxygen at its end,
with important consequences on stellar evolution and further nucleosynthesis processes. The (a,n) reactions
on 13C and 22Ne instead provide the
neutron flux necessary for the slow
neutron capture process (s-process)
responsible for the formation of the
heavy elements. All three of these
reactions have already been deeply
investigated in laboratories at the
Earth’s surface, but the experimental
status of the art is still not satisfactory and a significant step forward
could only come by a measurement
performed in a deep underground laboratory. The LUNA-MV project has
been funded with about 5.5 million
Euros by the Italian Research Ministry in the framework of the so-called
“Progetti Premiali” funds. With such
money, we will be able to buy a commercial 3.5 MV machine equipped
with two beam-lines and to develop
proper neutron and gamma detection systems. The enterprise has now
started and a new, larger collaboration is growing up to perform this exciting and long-lasting experimental
program. Stay tuned!
References
1. http://www.lngs.infn.it
2. http://luna.lngs.infn.it
3. H. Costantini et al., Rep. on Prog. in
Phys. 72 (2009) 086301.
4. C. Broggini et al., Ann. Rev. of Nucl.
and Part. Sci. 60 (2009) 53.
Alessandra Guglielmetti
Università degli Studi di Milano
and INFN Milano
calendar
2014
May 12–16
Ischia, Italy. 11th International
Spring Seminar on Nuclear Physics
http://ischia2014.na.infn.it/index.
php
May 25–30
San Antonio, TX, USA. CAARI
2014
http://www.caari.com/
May 27–30
Nis, Serbia. Second International
Conference on Radiation and Dosimetry in Various Fields of Research (RAD 2014)
http://www.rad2014.elfak.rs/
welcome.php
May 29–June 3
Cracow, Poland. MESON2014
http://meson.if.uj.edu.pl/
June 1–6
Tokyo, Japan. ARIS2014
http://ribf.riken.jp/ARIS2014/
June 9–13
Kyiv, Ukraine. 5th International
Conference on Current Problems in
Nuclear Physics and Atomic Energy
(NPAE-Kyiv2014)
http://www.kinr.kiev.ua/
NPAE-Kyiv2014/
June 30–July 4
Darmstadt, Germany. Direct
Reactions with Exotic Beams
DREB2014
https://indico.gsi.de/
conferenceDisplay.
py?confId=2347
July 7–11
Debrecen, Hungary. Nuclei in the
Cosmos NIC14
http://www.nic2014.org/
July 20–25
Vancouver, Canada.
Structure 2014
http://ns2014.triumf.ca/
Nuclear
August 31–September 7
Zakopane, Poland. Zakopane
Conference on Nuclear Physics “Extremes of the Nuclear Landscape”
http://zakopane2014.ifj.edu.pl/
September 8–13
Kaliningrad, Russia. VII International Symposium on Exotic Nuclei (EXON-2014)
http://exon2014.jinr.ru/
September 15–19
Wien, Austria. EXA2014 International Conference on Exotic Atoms and Related Topics
http://www.oeaw.ac.at/smi/
research/talks-events/exoticatoms/exa-14/
September 15–October 10
Stockholm, Sweden. Computational Challenges in Nuclear and
Many-Body Physics
http://agenda.albanova.
se/conferenceDisplay.
py?confId=3987
September 16–24
Erice, Italy. Nuclei in the Laboratory and in the Cosmos
http://crunch.ikp.physik.tudarmstadt.de/erice/2014/index.
php
September 29–October 3
St. Goar, Germany. 9th International Conference on Nuclear Physics at Storage Rings STORI’14
http://web-docs.gsi.de/~stori14/
October 13–17
Worms, Germany. EPS Nuclear
Physics Divisional Conference “Science and Technology for FAIR 2014”
https://indico.gsi.de/
conferenceDisplay.
py?confId=2443
November 3–8
Ho Chi Minh City, Vietnam. International Symposium on Physics
of Unstable Nuclei 2014 (ISPUN14)
http://www.inst.gov.vn/ispun14/
2015
June 21–26
Catania, Italy. 12th International
Conference on Nucleus-Nucleus
Collisions (NN2015)
http://www.lns.infn.it/link/nn2015
September 14–19
Kraków, Poland. 5th International Conference on “Collective
Motion in Nuclei under Extreme
Conditions” (COMEX5)
http://comex5.ifj.edu.pl/
September 21–26
Canberra, Australia. 5th Joint
International Conference on Hyperfine Interactions and Symposium on
Nuclear Quadrupole Interactions
(HFI/NQI 2014)
http://www.hfinqi.consec.com.au/
More information available in the Calendar of Events on the NuPECC website: http://www.nupecc.org/