Nuclear
Physics
News
Volume 13/No. 2
Contents
Editorial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
Laboratory Portrait
Nuclear Theory at the University of Surrey
by J. S. Al-Khalili et al. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
Meeting Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Feature Articles
New Magnetic Dipole Phenomena in Atomic Nuclei
by Norbert Pietralla and Krzysztof Starosta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Gravitation at a Micron and Mixing of Quarks
by H. Abele . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Nuclear Exchange Currents
by Dan-Olof Riska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Facilities and Methods
CYCLONE44 and ARES: New Tools for Nuclear Astrophysics
by Pierre Leleux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
News from NuPECC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Calendar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . pg. 44 and Inside Back Cover
Cover illustration: The University of Surrey in Guildford hosts the Centre for Nuclear and Radiation
Physics, home of the largest nuclear physics research activity in the UK. Photo courtesy of Steve Heritage,
University of Surrey.
Vol. 13, No. 2, 2003, Nuclear Physics News
1
editorial
Title to be Supplied?
In recent years nuclear physics
has developed into a broad field of
science covering all aspects of subatomic physics and a range of applications in energy, biomedical, and
material sciences. Experimental and
theoretical findings and methods
developed in nuclear physics are
having important impacts on other
fundamental sciences as well. The
opposite is also true. We are clearly
arriving at an era where nuclear
physics has to be seen as a part of
global science, and therefore it is of
great importance to all of us to keep
it that way.
Activities on advancing basic nuclear sciences, their education, and
public awareness have also been of
concern to the community. The effort in these areas is organized in Europe under the umbrella of NuPECC, the expert committee of the
European Science Foundation, allowing for broad participation of the
whole community in these important
activities. Current work on the new
Long Range Plan of NuPECC is a
beautiful example of coherent efforts
to plan the future of the field in Europe and worldwide. This plan is
being prepared under the conductance of nearly 100 leading experts,
and it aims at producing a coherent
document for guiding our common
research efforts over the time frame
of 5 to 15 years to come. This exercise is carried out as an open
process, allowing the whole scientific community, from small and
large countries to participate. Such
an open process carried out as a bottom-up procedure is also important
for successful realization of future
large-scale projects identified and
discussed in the Long Range Plan.
The plan will offer an excellent documentary on modern nuclear sciences. It will offer a true challenge
for funding of an internationally coherent plan within the European
Union as well as in all countries concerned, and with a long-term perspective. The first true test will be
exercised in the context of the Sixth
Framework Programme, where two
Integrated Infrastructure Initiative
proposals EURONS for nuclear
structure physics and I3HP for
hadron physics, will be presented.
It is important that the unique
strength of nuclear physics deriving
from broad international collaborations also be brought to the attention of those responsible for national
research and education efforts. This
is particularly important for smaller
countries where the field often is in a
minority position. It goes without
doubt that the university environ-
QY:
Author:
Supply title?
ment offers the best forum for communication between nuclear physicists and other scientists and for
contributing towards creating future
generations of scientists and teachers
for the field. Open academic competition and communication is the only
way for nuclear physics to permanently leave behind its reputation as
a closed field with a glorious past
history only.
After serving European nuclear
physics as a NuPECC chair for three
years, I have great trust in this community and its ability to develop the
field towards a New Era that is characterized by important large international projects in coexistence with
national efforts on research and education.
JUHA ÄYSTÖ
Jyväskylän Yliopisto
The views expressed here do not represent the views and policies of NuPECC except where explicitly identified.
2
Nuclear Physics News, Vol. 13, No. 2, 2003
laboratory portrait
NOTE:
This is revised
e-mailed file.
Nuclear Theory at the University of Surrey
Introduction
The Centre for Nuclear and Radiation Physics (CNRP) of the Department of Physics at the University
of Surrey hosts the largest nuclear
theory activity in the UK. The University is situated in the town of
Guildford, 48km SW of London.
Theoretical research includes both
the structure and reactions of light
and exotic—particularly halo—nuclei, as well as the structure of heavy
nuclei. Together with the experimental activity, the theory group is part
of the newly formed CNRP, which
also includes applied nuclear physics
for medicine, imaging, and materials
characterisation.
The focus of theoretical research
at Surrey is on the structure and reactions of extreme states of nuclei.
Surrey theorists have pioneered techniques for nuclear reactions involving loosely bound projectiles. These
techniques are highly applicable at
the incident energies of great current
experimental interest.
Few-body structure phenomena
reveal great sensitivity to nucleonnucleon interaction effects beyond
the mean-field. Experimental work
has shown that the mean field and
effective interactions deduced from
studies of near-stable nuclei fail to
have predictive power near the neutron drip line. In particular, knowledge of the essential structure of
halo nuclei comes largely from detailed reaction studies, but their interpretation depends crucially on the
availability of good reaction theory.
Surrey has been in the forefront of
the development of realistic fewbody models of nuclear reactions.
Surrey has a tradition of supporting researchers on extended vis-
QY:
Author:
Please cite Figs. 1,
2, 3, and 4
The Surrey Nuclear Physics Group—both theorists and experimentalists—
at the January 2003 International Workshop on Nuclear Structure at the
Limits of Stability.
its from outside the UK. It also hosts
a Marie Curie Training Site, funded
by the EU Fifth Framework Human
Potential Programme for the training
of European doctoral students who
visit Surrey for specific nuclear theory training. This is the only such
site for nuclear physics in the UK.
We are currently organising an EU
Framework 6 theory network.
In January, 2003 Surrey hosted
an international workshop, Nuclear
Structure at the Limits of Stability, to
focus on recent advances in nuclear
theory. The 3rd International Workshop on Direct Reactions with Exotic Beams will be held at Surrey in
July 2003.
History
The origins of nuclear theory at
Surrey can be traced to 1958 when
Daphne Jackson was invited by L. R.
B. Elton to join him as a research
student. Together, Elton and Jackson
laid the foundations of an internationally respected group studying the
theory of nuclear reactions and nuclear structure. In 1966 Daphne
Jackson was appointed Head of the
Nuclear Physics Group. During this
period, the group also appointed
Ron Johnson (1964) and Roger Barrett (1967). In 1971 Jackson was appointed as Head of the Physics Department, becoming the first woman
professor of physics in the UK.
Johnson became Head of the
Nuclear Physics Group in 1972, a
position he held until 1992, when he
handed over the role to Phil Walker,
the first experimentalist to be appointed by the group (in 1987).
Under Johnson’s leadership the Nuclear Group grew to 5 theorists and
4 experimentalists and became the
Nuclear Physics Group with the
strongest Research Council Support
in the UK. In 2002 the group amalgamated with applied nuclear physi-
Vol. 13, No. 2, 2003, Nuclear Physics News
3
laboratory portrait
Figure 1. Two-neutron halo nuclei such as 6He have been dubbed “Borromean” due to the nature of their three-body structure. Since no two-body
subsystems are bound, such nuclei behave like the interlocked Borromean
rings, whereby if any one is removed, the other two separate. The Surrey
group has made significant contributions to the study of the structure and reactions of such nuclei.
cists in the department to form the
CNRP under the directorship of Ian
Thompson.
The Surrey Ethos
Early research at Surrey featured
studies of nuclear sizes. Important
work from that period included two
widely cited books by Elton [1] and
by Barrett and Jackson [2]; but since
the early days of the group there has
been an emphasis on nuclear reaction studies. This effort has had two
strands:
(i) The development of approximations to direct reaction theories
that have provided insights into
the physics of the processes involved. Examples include: the
adiabatic approximation for
deuteron stripping and elastic
scattering [3]; the projectile excitation model of polarization
effects in Li scattering [4]; the
concept of tidal symmetry in
heavy-ion scattering [4]; and
the core-recoil model for halonucleus scattering [5].
4
(ii) The development of state-ofthe-art computer codes that
were designed to be useful to
the experimental community as
well as breaking new ground
theoretically. Examples of these
are the deuteron stripping code
of Santos and Johnson [6],
which was the first DWBA code
to include the effects of the deuteron D-state and was widely
used in the 1980’s to analyse
experiments with polarized deuterons, and the code FRESCO
of I. J. Thompson, which solves
the differential equations for
coupled two-body channels of a
variety of direct reaction theories [7]. This code has been exported to many groups all over
the world.
Recent work in this area has focussed on few-body models of the
structure and reactions of halo-nuclei [8]. However, the group has recently expanded its interests to include many-body nuclear structure
research and hadron physics.
Nuclear Physics News, Vol. 13, No. 2, 2003
Few-Body Methods in
Scattering and Reactions
Nuclear reactions play a crucial
role in the study of nuclei, and halo
nuclei are no exception; well-developed perturbative theories of nuclear
reactions already exist. The interesting question is whether these theories will work for reactions involving
halo nuclei, and if not, how they
should be modified. The weak binding of halo nuclei makes a positive
answer to the first question unlikely.
The weak binding of the valence
neutron in 11Be, for example, means
that the halo degree of freedom is
easily excited by the nuclear and
Coulomb fields of a target nucleus.
The weak binding also means that
even a small transfer of momentum
from the relative motion of the two
nuclei will excite the halo nucleus
into the continuum of unbound
states from which fragments may
propagate to large distances. The
challenge for theory is to find methods that treat such configurations realistically, while at the same time
lending themselves to the practical
analysis of experimental data. Cases
of interest typically involve many
strongly coupled channels, in addition to those involving halo excitation and a large range of angular
momenta. This challenge is especially difficult when the fragments
are charged.
The Adiabatic Approximation
One way of meeting the challenge of nuclear reactions with
loosely bound projectiles is to use
the adiabatic approximation, as developed by Johnson and Soper [3].
This permits a significant simplification of the few-body scattering
problem while treating coupling to
bound and continuum channels on
the same footing. In an experiment
in which a beam of projectiles con-
laboratory portrait
sisting of halo nuclei is scattered
from a stationary target, the adiabatic approximation assumes that
the motion of the halo nucleons relative to the projectile’s centre-ofmass is much slower than the projectile’s motion relative to the target.
Note that this does not assume that
the projectile remains in its ground
state during the collision, but rather
that the spatial coordinates describing the halo degrees of freedom
within the projectile are frozen. The
scattering amplitude is then calculated separately for all relevant values of the halo coordinates.
The adiabatic approximation
has been widely used for reactions
induced by deuterons [8–11] at intermediate and high-projectile energies, where simple analytical considerations suggest it should be valid.
However, the method is also well
adapted to the case of halo nuclei for
which few-body models are a good
starting point, and this has been the
focus of much work at Surrey
With the current interest in radioactive nuclear beams operating at
energies in the region of tens of
MeV, it is important to investigate
how far the adiabatic method can be
used at these energies. The adiabatic
method is usually thought of as a
high-energy approximation. For the
elastic scattering of strongly absorbed systems, we find [12], in fact,
that the adiabatic model is accurate
at much lower energies than one
would expect from qualitative estimates.
It was shown in [5] that when
the interaction between the halo particles and the target is absent, the
adiabatic scattering wave function
has a simple form. This result has led
to new insights into the role of the
halo in elastic scattering and has also
been used as the basis of a new nonperturbative quantum mechanical
theory of the Coulomb break-up of
neutron halo nuclei. The same idea
has recently been used in a new approach to the theory of transfer reactions involving a halo nucleus in
initial and final channels [13].
The Glauber Model
and Its Development
Glauber’s microscopic theory of
the scattering of composite systems
has the adiabatic approximation as
its starting point. In addition, it assumes that each constituent of the
projectile follows a straight line path
and the scattering wavefunction is
obtained by calculating the phase
change produced by the interaction
potential along this path (semiclassical eikonal approximation).
Over the past decade the Surrey
group has exploited Glauber’s theory to study the scattering of loosely
bound few-body systems, such as
halo nuclei, to include fully correlated few-body wave functions for
the projectiles and spin-dependent
interactions between the projectile
constituents and the target. When
treating the scattering of nonradioactive nuclei, it is common to make
the additional assumption (optical
limit approximation) that only the
ground state density of the projectile
is relevant, but this assumption was
shown by the group [15] to have serious shortcomings when loosely
bound neutron halo nuclei are involved. When applied to the interpretation of fragmentation cross sections, Surrey’s improved calculations
required halo radii that were significantly larger than those deduced
using the static density approximation. This effect has recently been
shown to have a very general consequence of the static density approximation [16].
There now exist procedures for
calculating the scattering wave func-
Figure 2. Model calculations show
the total reaction cross section of a
bound two-cluster projectile against
the assumed rms separation between
the clusters. The three configurations all have the same overall onebody density (hence the decrease of
intrinsic cluster sizes with increasing
intercluster separation). In the optical limit (OL) of the Glauber model,
they all predict the same cross section, but within a few-body (FB) picture, the cross section is reduced
with increasing cluster separation.
The OL and FB cross sections only
coincide when the two constituents
overlap (no clusterisation). The inset
shows the assumed projectile density.
tion corresponding to projectiles
with two [11] or three [16] constituents whose relative coordinates
are treated adiabatically but whose
remaining degrees of freedom are
treated quantum mechanically. Using
eikonal methods as an additional
approximation, the case of five constituents (8He) has also been evaluated using the best available projectile wave functions. These methods
have been successfully applied to
the analysis of the elastic scattering
and reaction cross sections of halo
nuclei.
For practical calculations of reaction observables such as differential and total cross sections at low
energies where the eikonal assumption is no longer valid, we have de-
Vol. 13, No. 2, 2003, Nuclear Physics News
5
laboratory portrait
particle spectroscopic tool for use
with rare exotic nuclei [17]. Recent
important results include the possibility of determining not only relative, but also absolute spectroscopic
factors from single-nucleon knockout and the identification of twoproton knockout on very neutronrich systems as proceeding via a
direct two-proton removal mechanism [17].
Figure 3. The cross section as a function of the momenta of 14C fragments
(in their ground state) emerging, at various laboratory angle intervals, for the
removal of a single neutron from 15C. The angular distributions reveal unprecedented details of the reaction mechanism. The coupled channels fewbody break-up theory, using the CDCC method, is in excellent agreement
with the data.
veloped a new method [8] based on
the use of Exact Continued clustertarget S-matrix elements. This approach retains many of the simplicities of implementation of the Glauber
method but without the restriction to
extreme forward scattering. This replaces eikonal phase shifts for the individual cluster-target scattering by
the physical ones. This approach
therefore gives a powerful and numerically simple means of extending
many-body multiple scattering theories with no more than an adiabatic
assumption. Such an approach,
which has been shown in specific
cases to be reliable for elastic scattering down to ~10 MeV/A of projectile energy, does not require solving a full coupled channels problem.
Spectroscopy Using
Nucleon Knock-Out Reactions
The ordering and distribution of
single-particle strength and the occu-
6
pation of nucleonic single-particle
states in nuclei are fundamental to
their structure and stability. Experimental verification of such structures is also vital to test shell model
and other many-body theoretical
structure predictions away from the
stable nuclei. At the high beam momenta delivered by fragmentation
facilities, it is possible to use the
semiclassical eikonal and adiabatic
methods discussed elsewhere in this
report and, for reactions on light targets, we have shown that these are
indeed very accurate. Most importantly, these methods allow both the
structure and reaction dynamics aspects of the problem to be treated
with comparable rigour.
In an ongoing Surrey/MSU collaboration, one- and two-nucleon
knockout reactions, which selectively excite a minimal number of
nucleonic degrees of freedom, are
being developed as a novel single-
Nuclear Physics News, Vol. 13, No. 2, 2003
Transfer Reactions
The group has a long-standing
interest in reactions in which one or
more nucleons are transferred between the projectile and target to
leave the final nuclei in low-lying
states. The reaction A(d,p)B is a typical example.
Normally these reactions are
treated as one-step processes in
which, in the (d,p) example, the neutron is transferred directly to the target nucleus in one step. This is called
the Distorted Wave Born Approximation (DWBA). The unique contribution of the group was to explain
how the contribution to the transfer
from paths in which the deuteron is
excited—i.e., broken up into a continuum state since the deuteron has
no bound excited states—by tidal
forces generated by the target could
be relatively easily incorporated into
the theory. In the early 1970s these
effects were treated in the adiabatic
approximation. The ideas generated
at that time still influence the analysis of modern experimental data.
A key modern development from
the adiabatic approximation was the
Continuum Discretised Coupled
Channels Method (CDCC), first pioneered in Japan and the U.S. Whereas
the adiabatic approximation replaces
the continuum of deuteron break-up
channels by a single channel degenerate in energy with the deuteron
channel, the CDCC approach treats
laboratory portrait
the nondegeneracy of the continuum
directly by replacing it in an unambiguous way by a set of discrete
channels. The theory can be further
generalised to include couplings between different rearrangement channels [7].
Recent applications at Surrey are
mainly to the one- or two-halo neutron transfer in reactions with radioactive beams. In particular, there
is a special interest in reactions from
which astrophysically important information can be extracted. Information deduced by comparing transfer cross sections with experiment
(“asymptotic normalisation constants”) can be used to predict cross
sections for A(p,γ)B reactions at the
very low energies needed for astrophysical studies of nuclear synthesis.
In parallel, theoretical studies of
ANCs and other quantities associated with transfer reactions, such as
overlap integrals and spectroscopic
factors, are also carried out with the
aim of relating them to the structure
of the nuclei involved.
Multiple Scattering
An important problem in nuclear
theory is to show how nucleon–
nucleus and nucleon–nucleon scattering cross sections can be related.
Attempts to do this by expanding
the nucleon–nucleus scattering amplitude in a series in which the incident nucleon scatters off more and
more target nucleons in successive
terms in the series is known as multiple scattering and has been a longstanding interest of the group. Detailed studies in momentum space of
elastic proton scattering including finite nucleus medium effects were
carried out in the 1990s. Recent applications have been to halo nuclei.
Recent work on p + 11Li inelastic
scattering at 68 MeV/u leading to
continuum states in 11Li [18] obtains
good agreement with the peaks seen
in the experimental cross section
below 3 MeV excitation, but no evidence for an interpretation in terms
of a dipole resonance is deduced [19].
Halo Break-up and Fusion Studies
In break-up studies of exotic nuclei, we can calculate the most general observables at low- and highincident energies, by the CDCC approach. In selected observables there
is interference between the break-up
amplitudes from different spin-parity excitations of the projectile. The
resulting fragment angle and energy
distributions reveal the importance
of higher order continuum state couplings.
The break-up of weakly bound
light projectiles on heavy targets at
energies near the Coulomb barrier
was also studied in the adiabatic
two-centre shell model. The effect of
continuum couplings in the fusion of
the halo nucleus 11Be on 208Pb
around the Coulomb barrier was
studied using a three-body model
within the CDCC formalism. We investigated in particular the role of
continuum–continuum couplings.
These are found to hinder total,
complete, and incomplete fusion
processes. Results show that continuum–continuum couplings enhance
the irreversibility of break-up and
reduce the flux that penetrates the
Coulomb barrier. Converged total
fusion cross sections agree with the
experimental ones for energies
around the Coulomb barrier, but underestimate those for energies well
above the Coulomb barrier.
Photonuclear Reactions
A new probe of the halo structure is through the use of meson
photo-production reactions, reach-
Figure 4. Two proton decay: width or half-life as a function of resonance energy for 19Mg and 48Ni. Solid curves represent three-body calculations. Stars
correspond to diproton estimates. Dashed and dash-dotted curves are for direct decay to continuum, with, respectively, l = 0, and the l-value from the
shell model.
Vol. 13, No. 2, 2003, Nuclear Physics News
7
laboratory portrait
ing the nuclei of interest through an
electromagnetic charge exchanging
process. Work by the group has already shown that reactions will provide an interesting probe of the halo,
particularly for those short-lived
states, such as the first excited state
of 17F, that cannot be studied in radioactive beam experiments. In addition, while fragmentation reactions
tend to probe the long-range tail of
the halo wave function, pion photoproduction reactions are also sensitive to the interior, and as such can
give new insights into their structure,
such as the role of antisymmetrisation. The group is currently studying
the importance of including cluster
model wave functions in the calculations, to investigate the sensitivity to
details of the nuclear structure. Reactions of interest include 6Li(γ,π+)6He.
Few-Body Structure Theory
The subtle interplay between
quantum mechanical 3-body effects
and nucleon–nucleon correlations
make the study of two-neutron halo
nuclei of particular interest to theory. Bound states of such systems
have been studied at Surrey by many
methods. Some of the most interesting challenges arise when the system
is in the continuum of unbound
states, and this is the focus of much
current research. For example, the
hyperspherical harmonics method is
being used to investigate low-lying
resonances and the soft dipole mode
in the two-neutron halo nucleus 6He.
A unique structure for true threebody resonances has been revealed.
The method of many-body hyperspherical functions has also been
developed and used to calculate the
binding energies of light neutronrich isotopes of hydrogen and helium, and to search for multi-neutrons.
8
Recently the group applied its
expertise in three-body structure
models to the study of two-proton
decay. Emission of two protons from
nuclear states has been studied since
1960 when Goldansky predicted
two-proton radioactivity for nuclei
beyond the proton dripline. Twoproton decay may occur through
three possible mechanisms: (i) sequential emission of protons via an
intermediate state, (ii) simultaneous
emission of protons (direct decay to
continuum), and (iii) di-proton emission, i.e., emission of a strongly correlated 2He cluster. Experimentally,
only the first two mechanisms have
been identified. The third case is traditionally associated with two-proton radioactivity but has not been
observed. Surrey calculations [20]
show that the pp pairing interaction
strongly influences penetration
through the complicated multidimensional Coulomb and centrifugal barriers. Semiclassical models for
the penetration probability give only
upper and lower limits, which differ
by orders of magnitude and can give
results which differ substantially
from the more exact 3-body methods used at Surrey.
Hadron Physics
Recently, the group has pursued
an interest in nuclear medium effects
on nucleon resonances via meson
photoproduction reactions and developed a quark model approach for
meson photo and electroproduction
[21]. The advantage of this QCD-inspired phenomenology is that with a
limited number of parameters, all
the resonances can be self-consistently included and analysed. We
have also extended our quark model
approach to the study of meson
photo-production on nuclei, where
the interest is to learn how a nucleon
Nuclear Physics News, Vol. 13, No. 2, 2003
and its resonances behave in the nuclear medium.
Nuclear Structure Theory
The shape of the nucleus seems
at first glance to be one of the simplest of its macroscopic properties. It
turns out, however, that it can assume many possible shapes. It can be
a spheroid or a prolate, oblate, octopole, etc. ellipsoid. Moreover, we
find many examples of nuclei in
which states or groups of states with
different shapes exist. This, together
with the fact that single particle excitations, vibrations, and rotations
in nuclei have comparable excitation
energies, makes nuclear structure
and its associated nuclear spectroscopy complex and difficult to reproduce theoretically.
Among the many intriguing aspects of nuclear structure, we find
that nuclei with oblate shapes are
rare compared to those with prolate
deformations. In simple terms this is
to be expected. If we consider the
collective rotation of an axially symmetric nucleus, around an axis perpendicular to the symmetry axis, the
moment-of-inertia is larger for the
prolate than for the oblate shape.
Since the excitation energies of a
rotor are inversely proportional to
the moment-of-inertia, they are
lower for the prolate shapes for a
given angular momentum.
Experimentalists at Surrey are
keenly interested in the properties of
neutron-rich nuclei, which are becoming more accessible to study as
beams of radioactive nuclei are developed, as well as the spectrometers
that allow us to identify individual
nuclear species produced in the deep
inelastic collisions of heavy ions.
Theoretically, two distinct regions of
the nuclear chart have been found
where well-deformed oblate rotors
laboratory portrait
are expected to be favoured in energy over prolate rotors.
This occurs when there are several proton- and neutron-holes relative to the doubly-magic 208Pb and
132Sn nuclei. The nuclear Fermi surface then lies among single (Nilsson)
particle orbitals with high j values
aligned close to the axis of rotation.
This results in a strong Coriolis force
that stabilises the oblate shapes.
Both of these regions where oblate
deformation is favoured lie sufficiently far from stability on the neutron-rich side that it has not yet been
possible to verify these predictions.
Competition between oblate and
prolate shapes is also sensitive to the
strength of the spin-orbit interaction, and it is generally considered
that this is a key factor in deciding
how nuclear structure will evolve as
we move away from stability. Our
predictions are part of meeting the
challenge of finding mean field
models to describe this evolution.
Shapes and Isomers
At Surrey, shape calculations
based on the TRS macroscopic/
microscopic shell correction approach, with the Woods-Saxon potential and Lipkin-Nogami pairing,
have been carried out. One of the
favoured oblate regions is found in
neutron-rich hafnium isotopes, as
first reported by Hilton and Mang
for 180Hf. They predicted giant backbending at a spin of 26 h, with a dramatic switch from prolate to oblate
shape. We have found that the effect
becomes stronger, and the backbending spin lower, when isotopes with a
few more neutrons are considered.
One intriguing aspect of our results is that these nuclei, at least the
neutron-rich hafnium isotopes, carry
angular momentum in such a way
that the structure that competes with
oblate collective rotation at high
spin is not prolate collective rotation, but prolate multi-quasi-particle
states. Such states are well known to
form long-lived isomers in ordinary
hafnium isotopes, and they are expected to do likewise in the neutronrich isotopes, with perhaps even more
retarded decays [22]. The level ordering in calculations with a WoodsSaxon potential provides an intuitive
explanation of this oblate collective/
prolate-non-collective competition.
For the oblate shapes to come lowest
in energy, a key feature is that the
Fermi levels for both protons and
neutrons lie close to the tops of the
shells they are in.
All of these calculations await
the test of experiment.
Tilted Rotations with
Triaxial Symmetry
In nuclear physics, an emergence
of the triaxial symmetry allows socalled wobbling motion at high spin
and chiral rotation at medium spin.
We have been studying in detail
these exotic dynamical modes by
means of microscopic (tilted-axis
cranked HFB method) and quantal
approaches (generator coordinate
method and quantum number projection techniques). In particular, numerical investigations were carried
out for wobbling motion in 182Os as
a coupling mode between low and
high-K states [23,25]; and for a possibility of nuclear chirality in 134Ce
[26]. Figure 5 shows the energy surfaces of 134Ce (J = 26 and 28) with
QY:
Author:
lower case
“h” ok here
134
Figure 5. Energy surfaces of high spin states (I = 26 h and 30 h) for Ce. The polar angles θ and ϕ are tilt angles (3 times)?
of the total angular momentum vector.
Vol. 13, No. 2, 2003, Nuclear Physics News
9
laboratory portrait
several minima, corresponding to
1d, 2d, and 3d-rotations. Theta and
phi denote tilt angles of the total angular momentum vector.
Summary
Surrey theorists are actively developing and contributing nuclear
theory expertise to reactions and
structure studies explored with radioactive nuclear beams, topics that
are currently providing the main
drive towards new physics and new
nuclear physics facilities around the
world. The description of nuclei far
from stability in terms of the underlying forces is one of the most important current areas of scientific research. Studies of nuclei far from the
valley of stability probe nuclear
structure at lower than normal densities, in contrast with ultra-relativistic heavy-ion collisions, which seek
to create nuclear matter at high temperature and density. This area will
continue to be a main part of Surrey’s effort. We will also continue
our tradition of fostering close links
with experimental groups all over
the world, through both the promulgation of new ideas and the provision of computer codes.
The recent appointment of two
new members of the group reflects
Surrey’s confidence in the future of
fundamental nuclear physics. These
young theorists will bring an extra
dimension to the Group’s research.
Oi has developed a theory of nuclear
rotation of direct relevance to experimental work in the group [23].
Stevenson has developed a new effective nucleon–nucleon interaction
for use in mean-field theories and
beyond [24]. These novel contributions they have already made to the
field auger well for the future.
10
Finally, the group is proud of its
many activities in promoting its
work to a wider audience of physicists, and nuclear physics in general
to the wider public, through lectures, magazines articles and even
the first ever coffee-table glossy
book on nuclear physics [27].
References
1. L. R. B. Elton, Nuclear Sizes,
Clarendon Press, Oxford, 1961.
2. R. C. Barrett and D. F. Jackson, Nuclear Sizes and Structure, Clarendon
Press, Oxford, 1977.
3. R. C. Johnson and P. J. R. Soper,
Phys. Rev. C1 (1970), 976–990; J.
D. Harvey and R. C. Johnson, Phys.
Rev. C3 (1971), 636–645.
4. J. Gomez-Camacho and R. C. Johnson, Polarisation in Nuclear Reactions, in Scattering, Edited by P.
Sabatier and E. R. Pike, Academic
Press, London and San Diego, 2002,
Chapter 3.1.5, pp. 1414–1432.
5. R. C. Johnson, J. S. Al-Khalili, and
J. A. Tostevin, Phys. Rev. Lett. 79
(1997), 2771–2774.
6. R. C. Johnson and F. D. Santos,
Phys. Rev. Lett. 19 (1967), 364–
366.
7. I. J. Thompson, Methods of Direct
Reaction Theories, in Scattering,
ibid., Chapter 3.1.2, pp. 1360–
1372.
8. J. S. Al-Khalili and J. A. Tostevin,
Few-Body Models of Nuclear Reactions, in Scattering, ibid., Chapter
3.1.3, pp. 1373–1392.
9. J. S. Al-Khalili and R. C. Johnson,
Nucl. Phys. A546 (1992), 622.
10. R. C. Johnson, E. J. Stephenson, and
J. A. Tostevin, Nucl. Phys. A505
(1989), 26–66.
11. I. J. Thompson, Computer Programme ADIA, Daresbury Laboratory Report, 1984.
12. N. C. Summers, J. S. Al-Khalili, and
R. C. Johnson, Phys. Rev. C66
(2002), 014614.
Nuclear Physics News, Vol. 13, No. 2, 2003
13. N. K. Timofeyuk and R. C. Johnson,
Phys. Rev. C59 (1999), 1337.
14. J. S. Al-Khalili and J. A. Tostevin,
Phys. Rev. Lett. 76 (1996), 3903.
15. R. C. Johnson. and C. J. Goebel,
Phys. Rev. C62 (2000), 027603.
16. J. A. Christley, J. S. Al-Khalili, J. A.
Tostevin, and R. C. Johnson, Nucl.
Phys. A624 (1997), 275.
17. P. G. Hansen and J. A. Tostevin,
Ann. Rev. Nucl. Part. Sci. 53 (2003),
in press.
18. R. Crespo and R. C. Johnson, Phys.
Rev. C60 (1999), 034007.
19. R. Crespo, I. J. Thompson, and A.
A. Korsheninnikov, Phys. Rev. C66
(2002), 0210023.
20. L. V. Grigorenko, R. C. Johnson, I.
G. Mukha, I. J. Thompson, and M.
V. Zhukov, Phys. Rev. Lett. 85
(2000), 22.
21. Q. Zhao, J. S. Al-Khalili, Z. P. Li,
and R. L. Workman, Phys. Rev. C65
(2002), 065204.
22. P. M. Walker and G. D. Dracoulis,
Nature 399 (1999), 35.
23. M. Oi, P. M. Walker, and A. Ansari,
Phys. Lett. B525 (2002), 255.
24. P. Stevenson, M. R. Strayer, and J.
Rikovska Stone, Phys. Rev. C63
(2001), 054309.
25. M. Oi, A. Ansari, T. Horibata, and
N. Onishi, Phys. Lett. B480 (2000),
53.
26. M. Oi and P. M. Walker, submitted
to Phys. Lett. B.
27. R. Mackintosh, J. S. Al-Khalili, B.
Jonson, and T. Pena, NUCLEUS: A
Trip into the Heart of Matter, Canopus publishing, 2002.
Contributors to this
article include
J. S. AL-KHALILI, W. GELLETLY,
R. C. JOHNSON, M. OI,
P. D. STEVENSON, I. J. THOMPSON,
N. TIMOFEYUK, J. A. TOSTEVIN,
P. M. WALKER, AND Q. ZHAO
QY:
Author:
Ref. No’s.
24, 25,
26, 27, ok?
There were
two “27’s”
in file.
meeting reports
XVIIth Nuclear Physics Divisional Conference on
“Nuclear Physics in Astrophysics”
The 17th International Nuclear
Physics Divisional Conference of the
European Physical Society, “Nuclear
Physics in Astrophysics,” was held
in Debrecen, Hungary from September 30 to October 4, 2002. This conference was originally planned to
take place in Eilat, Israel in 2001,
but was moved to Debrecen because
of security considerations. Many of
the topics and much of the program
were similar to those planned for the
Eilat meeting.
The main objective of the conference was to deal with all those
subjects of nuclear physics that impact astrophysics and are an essential input in the understanding of astrophysical processes. The emphasis
was on the recent theoretical and experimental developments in nuclear
physics that are of relevance to astrophysics. The topics discussed at
the conference included: cross-section measurements and nuclear data
for astrophysics, stellar and big bang
nucleosynthesis, the application of
nuclear structure far from the stability line to astrophysics, neutrino
physics, nuclear reactions pertaining
to astrophysics, rare ion beam facilities and experiments, and a few
other diverse topics.
The program of the conference
included 26 invited lectures and 30
contributed ones delivered by speakers representing 22 countries. In addition, a poster session was organized. There were a number of talks
related to neutrino physics including
some of the most recent experimental
results (for example, those of SNO,
LSND, neutrino mass measurements), results in the field of nuclear
reactions of astrophysics relevance,
and measurements of radiation from
extinct, rare, and very short-lived
nuclei that may be a window into a
better understanding of nucleosynthesis. The importance of studying
exotic nuclei in order to understand
astrophysical processes was widely
stressed. The activities of various
large laboratories in the world (and
in particular in Europe) in the present and future were presented at the
meeting. There were also a number
of talks of more general interest
about dark matter, relativistic heavy
ion collisions, and cosmology as well
as a talk about public awareness of
nuclear science. The talks have led to
very lively and lengthy discussions
and interaction among the participants. The proceedings of the conference will be published in Nuclear
Physics A.
The breadth of the European activity in these fields of nuclear astrophysics, as revealed at the conference, is truly impressive. This seems
to be one of the most important future directions of research in Europe
and countries in America and Asia
represented at the meeting. Particularly pleasing was the the large participation of young researchers.
Among the 100 conference participants, 35% were scientists under the
age of 35 years. It is obvious that
this subfield of nuclear physics has a
bright future. (The Nuclear Physics
Board of the EPS is considering the
possibility of having this type of con-
ference, “Nuclear Physics in Astrophysics,” on a regular basis, every
few years.)
During the meeting Prof. Claus
Rolfs (Ruhr University, Bochum,
Germany) was awarded an Honorary Membership in the Roland
Eötvös Physical Society of Hungary
for his contributions to the field of
nuclear astrophysics and for his help
in the development of this field in
Hungary. The short ceremony was
followed by a lecture presented by
the recipient of the award.
The conference was organized
by the Nuclear Physics Board of the
EPS and by the Institute of Nuclear
Research (ATOMKI) in Debrecen.
The conference was sponsored by
the European Commission, Human
Potential Program, the EPS Young
Physicist Fund, Hungarian Academy
of Sciences, Hungarian Ministry of
Education, and the Hungarian National Fund for Scientific Research.
In addition, the initial stages of organization were sponsored by the
Ben-Gurion, Hebrew, and Tel-Aviv
Universities and the Weizmann Institute in Israel.
N. AUERBACH
School of Physics and Astronomy
Tel-Aviv University, Israel
ZS. FÜLÖP
ATOMKI
Debrecen, Hungary
Vol. 13, No. 2, 2003, Nuclear Physics News
11
meeting reports
XXXIII European Cyclotron Progress Meeting
The XXXIII European Cyclotron Progress Meeting, held September 17–21, 2002, was organized
jointly by the Heavy Ion Laboratory
of the Warsaw University and
Niewodniczański Institute of Nuclear Physics, respectively. The event
was held at both locations, a new
feature in the history of ECPMs.
Also, for the first time, the proceedings will be published in International Journal of Nuclear Research
Nukleonika.
The conference gathered almost
100 participants from Europe, USA,
Canada, and Japan. The reports
were devoted not only to the cyclotron techniques, but also to applications—medical and industrial—as
well as to the hottest subjects in nuclear science, namely the use of cyclotrons as radioactive beam sources.
Much of the time was spent on discussions about novel designs of the
highly proficient ion sources and associated transport systems. The subject of the ion sources was covered
by Santo Gammino (LNS Catania),
Claude Bieth (PANTECHIK, Caen),
Hannu Koivisto (Jyväskylä), and
Vladimir Loginov (Dubna). JeanLoup Belmont (Grenoble) and Wolfgang Pelzer (HMI) addressed the
problems associated with the transport of low-energy beams from the
source to the accelerating structure
12
of the cyclotron. New ideas concerning the acceleration of the hadron
beams were presented by Yves
Jongen (IBA), Michael Schillo
(ACCEL), Anne Paans (Groningen),
and Leonid Onischenko (Dubna).
Willem Kleeven (IBA), Helge Jungwirth (IKF Jülich), and Jarosl/aw
Choi ński (HIL Warsaw) discussed
the different modes of beam extraction. Finally, Marc Loiselet (Louvain), Marcel Lieuvin (GANIL),
Grigorij Gulbekian (Dubna), Gerardo Dutto (TRIUMF), and Sytze
Brandenburg (KVI) talked about the
developments in radioactive beam
facilities. Of course this classification, following the concluding remarks of Heinrich Homeyer (HMI),
is not showing the real scope of the
talks of the keynote speakers. All the
problems discussed interweave, so
the meeting was an excellent opportunity to bring together scientists
specializing in sometimes narrow
areas.
Besides the participants representing academic and research laboratories, commercial companies were
also present, stressing the increasing
role of applications of accelerator
techniques. ACCEL Instruments
GmbH, General Electric Medical
Systems and Ion Beam Applications
(IBA) presented their offers during
permanent exhibitions.
Nuclear Physics News, Vol. 13, No. 2, 2003
The working visits to the Warsaw heavy-ion cyclotron, the only
operating installation of its kind in
Central Europe, and to the Cracow
AIC-144 stimulated an agitated exchange of ideas on the basic technical level. Social events were also
helpful to give an opportunity for
eye-to-eye discussions not only
about the achievements, but also
problems. Although welcome reception at the Warsaw cyclotron, followed by the the concert performed
by Chamber Choir of the Warsaw
Chamber Opera as well as the conference dinner at the Royal Wawel
Castle in Cracow did not seem to be
an opportunity to argue about spacecharge effects, field imperfections
and so on, but scientists are always
scientists . . .
The organizers of the XXXIII
ECPM are thankful for the engagement of the Polish State Committee
for Scientific Research, Ministry of
Education and Sport and National
Atomic Energy Agency, as well as to
the industrial companies involved,
for the support which, to a great extent, made the event successful and
fruitful.
TOMASZ CZOSNYKA
Heavy Ion Laboratory
Warsaw University
meeting reports
Report on the 3rd International Balkan School
on Nuclear Physics (3rd IBSNucPhys)
The 3rd IBSNucPhys was held in
the auditorium of the Central Library of the Aristotle University of
Thessaloniki, Greece on September
18–24, 2002, continuing the tradition started in Istanbul, Turkey
(1998) and Bodrum, Turkey (2000).
The school was under the auspices of the Faculty of Physics of the
Aristotle University and the Balkan
Physics Union. It was sponsored by
the Ministries of Education, Culture
and Economics, as well as by the
School of Phyiscs and the Research
committee of the Aristotle University,
the Hellenic Physical Society (Central-Western Macedonia branch), the
Hellenic Nuclear Physics Society, the
Municipality of Thessaloniki, and
the National Bank of Greece.
The school was attended by
more than 90 participants from 22
countries all over the world. The
large participation of young colleagues at the M.Sc. and Ph.D. levels
should be particularly noticed. Their
interest and enthusiasm was reflected in the 31 contributions they
made during the special evening
seminars. The titles and short abstracts of the contributions, along
with the viewgraphs of the lecturers,
can be found on the school’s website:
http://nuclear.physics.auth.gr/bschool/
index.html.
The program included six onehour lectures per day. There were
also 30-minute seminars and student
contributions. The school was focused on hot subjects connected with
nuclear structure and reactions of
exotic nuclei and their influence on
nuclear astrophysics. The representatives of several major experimental
laboratories—G. Muenzenberg (GSI
Darmstadt), M. Lewitowicz (GANIL),
G. De Angelis (Legnaro), P. Butler
(Liverpool), M. Thoennessen (MSU),
R. Casten (Yale), M. L. Aliotta (Edinburgh), S. Harissopoulos (N.C.S.R.
Demokritos), A. Youngclaus (Madrid)—presented their latest achievements as well as their future plans.
From the theory point of view, many
of the state-of-the-art theoretical approaches were presented and discussed in detail by P. Ring (Munich),
T. Otsuka (Tokyo), J. Dobaczewski
(Warsaw), J. Tostevin (Surrey), D.
Vretenar (Zagreb), S. Goriely (Brussels), J. L. Egido (Madrid), L. Ferreira (Lisbon). The volume with the
proceedings of the school is in
preparation and will be published
soon.
During the school the Hellenic
Nuclear Physics Society honored
Professor G. Muenzenberg for his
leading role in the discovery of new
superheavy elements at GSI Darmstadt. Finally, the participants had
the chance to visit the recently discovered tombs of Macedonian kings
(including the one of Philip II, the
father of Alexander the Great) in
Vergina, the ruins and the museum
of the ancient city of Dion, the crusaders’ castle of Platamona, as well
as Mount Olympos, the traditional
abode of the ancient Greek gods.
GEORGIOS A. LALAZISSIS
Department of Theoretical Physics
Aristotle University of
Thessaloniki, Greece
Vol. 13, No. 2, 2003, Nuclear Physics News
13
feature article
New Magnetic Dipole Phenomena in
Atomic Nuclei
NORBERT PIETRALLA
Institut für Kernphysik, Universität zu Köln, Köln, Germany
KRZYSZTOF STAROSTA
Department of Physics and Astronomy, State University of New York at Stony Brook,
Stony Brook, New York, USA
Introduction
In recent years a number of nuclear phenomena was
discovered that are accompanied by strong magnetic dipole (M1) transitions. Measurements of M1 matrix elements enabled nuclear physicists to deduce new structure
information in a model-independent way, since the electromagnetic interaction is well understood. Due to the
dominantly isovector character of the electromagnetic
M1 transition operator (see Appendix A), information
on M1 matrix elements helps to clarify various aspects of
the nuclear isospin degree of freedom (see Appendix B).
This is of particular importance as a prerequisite for
making full use of newly established or planned radioactive ion beam (RIB) facilities probing nuclei at extreme
values of isospin.
M1 transitions are fundamental in nuclear physics.
The nucleon with spin and parity J π = 1/2+ is not elementary itself and can be excited to the J π = 3/2+ ∆-resonance by an M1 transition. The corresponding matrix element [1] amounts to 3 nuclear magnetons (µN =
- /2m c in Gaussian units) and sets the scale for M1 pheeh
p
nomena in nuclear physics. The anomalous magnetic
moments of the free nucleons, µp = +2.8 µN and µn = –1.9
µN, are of a similar absolute size. Their deviation from
the values expected for elementary J = 1/2 Dirac particles
[µD(p) = +1.0 µN and µD(n) = 0 µN] underpins the importance of the subnucleonic degrees of freedom. In studies
of nuclear structure, Schmidt’s successful prediction of
magnetic dipole moments of many (near closed-shell)
nuclei using the single particle approximation strongly
supported the microscopic shell model ansatz and represented a conceptual breakthrough.
Out of the variety of M1 phenomena, the current
paper presents a brief overview of those that have recently attracted attention because of new experimental
evidence or approaches. Certainly, all the details cannot
14
be discussed here and we apologize for the personal bias
regarding the covered topics and necessarily incomplete
reference list; for discussions of specific M1 phenomena
we recommend the recent review articles by Clark and
Macchiavelli [2], Frauendorf [3], and Heyde and Richter
[4].
This overview is organized according to increasing
complexity and collectivity (not necessarily increasing
M1 strength) of the covered M1 phenomena. We first address the observation of remarkably pure quasideuteron
configurations in the level schemes of heavy self-conjugate (N = Z) odd-odd nuclei formed by the coupling of
the unpaired proton and neutron in the same j-orbital.
Subsequent alignment of more complex several-particle
configurations leads in heavier nuclei to the phenomenon of magnetic rotation through the development of
regular shears bands. This alignment causes in triaxial
nuclei the recently discovered chiral bands and the doubling of states in nearly degenerate level sequences. Finally, recently discovered proton-neutron mixed-symmetry multi-quadrupole phonon vibrational structures will
be discussed. All these M1 phenomena are related to the
coupling of proton and neutron subsystems with finite
angular momenta.
Quasideuteron Configurations
The most elementary source of isovector M1 transitions are two-particle so-called quasideuteron configurations (QDCs) in odd-odd N = Z nuclei. These nuclei are
special because of the coexistence of states with isospin
quantum numbers T< = 0 and T> = 1 at low energies,
which leads in some cases to anomalous ground states
with isospin T>. The deuteron is the simplest odd-odd N
= Z nucleus. Its J π = 1+, T = 0 ground state and its J π =
0+, T = 1 excited state at ~2.24 MeV are dominantly
formed by the coupling of the 1s1/2 proton and neutron.
Nuclear Physics News, Vol. 13, No. 2, 2003
feature article
Analogous configurations can be formed in the heavier
odd-odd N = Z nuclei by an unpaired proton and neutron in the same j-orbital coupled to the 0+, T = 0,
ground state of the neighboring even-even N = Z nucleus. Such simple two-particle configurations:
J+
 J+, T〉 = [jp ⊗ jn]T ⊗ Ψ 0+
core,T=0
are uniquely specified by the total angular momentum
quantum number J = 0, 1, . . . , 2j with isospin T = [1 +
(–1) J ]/2 and are called QDCs due to their formal analogy
to the states of the deuteron. The QDCs with isospin T
= 1 possess corresponding configurations in the neighboring isobaric nuclei while those with T = 0 do not.
For the QDC wave functions it is straightforward to
analytically derive the ∆J =1 M1 transition rates in a
fashion similar to the derivation of the Schmidt-values
for magnetic moments of odd-mass nuclei. Such transitions are purely isovector and are expected to be enhanced due to the isovector character of the M1 transition operator (see Appendix A). This task has been
carried out by Lisetskiy et al. [5] for cases with spherical
and axially deformed cores. For spherical QDCs, the M1
transition strengths are given by
where G(l,j,J) = (2j + 2 + J)(2j – J)(J + 1)/(l + 1/2)2 is a
geometrical factor and gl(s)V are defined in Appendix A.
A destructive interference occurs between orbital and spin
parts for j = l –1/2 cases while for j = l + 1/2 constructive
interference can lead to particularly large, up to 20 µN2,
values of B(M1), as observed for the B(M1; 0+, T = 1 →
1+, T = 0) value for the 1041-keV 0+ → 1+ transition in
18F, where a d
5/2 proton and neutron couple to the spherical 16O-core. Deformation of the even-even core introduces further dependence of the M1 strength on the K
quantum number and on the size of the deformation.
Since the QDC scheme represents one of the most efficient ways to produce strong M1 transitions, large
B(M1) values can serve as good signatures for QDCs.
Sizeable B(M1) values (>1 µN2) were known in oddodd N = Z nuclei up to 42Sc. At larger masses the valley
of stability departs from the N = Z line and the accurate
measurement of transition rates becomes difficult. It is,
however, an intriguing problem to study whether the
QDC coupling scheme holds true for heavier nuclei or
those with large deformation. An ideal testing ground is
provided by the f7/2 shell isolated from other shells by the
gaps at Z, N ∈ {20,28}; the f7/2 shell includes deformed
(48Cr) and spherical (40Ca, 56Ni) even-even N = Z core
nuclei. The f7/2 orbital has j = l + 1/2 and, hence, large
B(M1) values are expected.
An experimental program for the investigation of the
low-spin structures of odd-odd N = Z nuclei in the f7/2
shell has been initiated at the γ-ray detector arrays in
Cologne and at Yale by Brentano. The previously poorly
known low-spin structures of 46V, 50Mn, and 54Co have
been considerably extended [6–8] in Cologne with
(p,nγγ) reactions. The high-spin parts of the level
schemes were intensively studied, predominantly at Legnaro and at the Gammasphere array; see e.g., recent references [9–11]. Precise measurements of E2/M1 multipole mixing ratios were possible in Cologne from
γγ-angular correlation data.
Information on E2/M1 branching ratios in comparison to known E2 transition rates between corresponding
T = 1 states in the even-even partners provided evidence
for large B(M1) values and, thus, for the existence of
considerably pure QDCs in 46V, 50Mn, and 54Co. Direct
measurements of crucial M1 transition rates were performed [12] at Yale University in “cold” heavy-ion induced reactions on 40Ca at the Coulomb barrier and
Doppler-shift analyses of the observed γγ-coincidence
data. Marked Doppler shifts were found in the sequence
of QDC candidates owing to the high transition rates.
Figure 1 compares the QDC structure found in 50Mn
with the full level scheme below 3.3 MeV of the T = 1
isobaric partner 50Cr.
The relative excitation energies R(J) = Ex(J)/Ex(21+)
of the T = 1 partner states with Jπ = 4+ and 6+ agree in
50Mn and 50Cr within 1%. The experimental B(M1;
31+ → 21+) = 2.9+1.0–0.7 µN2 value coincides within the
errors with the analytical result, B(M1; 31+ → 21+) = 3.1
µN2, obtained [5] considering an appropriately deformed
48Cr-core. This agreement proves the existence of quite
pure QDCs in f7/2-shell nuclei. The structure in 50Mn,
missing only the T = 0, 7+ state, represents the most extensive sequence of QDCs observed so far.
An interesting question is to what extent the isospin
symmetry is broken in N = Z nuclei; significant experimental effort is currently devoted to this problem.
Vol. 13, No. 2, 2003, Nuclear Physics News
15
feature article
Shears Bands and Magnetic Rotation
Discovery of the shears mechanism is one of the
most exciting recent achievements in high spin γ-ray
spectroscopy. Unexpectedly, it was observed that configurations based on high-j intruder orbitals in weakly deformed nuclei result in very regular γ-ray cascades [14]
as shown in Figure 2a for 199Pb. The surprise was related
to the fact that these cascades involve a sequence of ∆I =
1 transitions in contrast to already well-established rotational bands in deformed and superdeformed nuclei
comprised of stretched E2 transitions. It was shortly realized [15] that angular momentum in these novel dipole
bands is generated by a mechanism significantly different
from collective rotation.
In weakly deformed nuclei near closed shells, orbitals of either particle or hole character can be occupied
by nucleons of the opposite isospin projection (for the
bands first discovered in the lead region, protons occupy
h9/2 and i13/2 particle states, while neutrons occupy i13/2
hole states). For the lowest energy state the angular momenta of the resulting particle and hole configurations
are oriented in perpendicular directions since this orientation minimizes the interaction energy; this has been
verified with the g-factor measurement of the isomeric
band head in 193Pb (see [2] for the reference and details).
Figure 1. Comparison between the QDC structure found in
50Mn with the full level scheme below 3.3 MeV of the T = 1 isobaric partner 50Cr. The numbers below the level bars denote relative excitation energies R(J), those next to 50Mn-levels show
measured lifetimes in picoseconds. Deduced B(M1)↓ values are
given in units of µN2 at the transition arrows. The corresponding B(M1) predictions of the quasideuteron scheme coupled to
a deformed rotor are displayed in the column above “QDC.”
Data are from [12].
Coulomb energy displacements in isobaric mirror nuclei
revealed valuable information on the alignment mechanism; e.g., see [11]. Recently this information was used
to extract isospin mixing from a comparison to large
scale shell model calculations. The properties of robust
strong M1 transitions from QDCs have also been shown
to be useful for a direct determination of isospin mixing,
e.g., in 54Co [13], where mixing matrix elements of ~10
keV were found.
16
Figure 2. (a) Experimental spectrum showing M1 transitions in
one of the ∆I = 1 bands in 199Pb. (b) Geometry of angular momentum coupling for particle and hole states in shears configurations near the band head (top) and at higher angular momenta (bottom). (c) Reduced M1 transition probability B(M1)
as a function of angular momentum for the band in 199Pb
shown in panel (a). Data are from [2].
Nuclear Physics News, Vol. 13, No. 2, 2003
feature article
These long angular momentum vectors are said to form
“shears blades” with the shears open at the band head.
Due to small deformations near closed shells, it is more
favorable to generate higher angular momentum states
by closing the shears blades rather than by collective rotation, as shown in Figure 2b.
The increasing energy of the states formed as the
shears blades are closing reflects the energy of the repulsive interaction between the particles and holes forming
the blades; multipole expansion of the effective force
yields rotational-like behavior with energies proportional to the square of total angular momentum [2]. The
above scenario, known as the “shears mechanism” for
generating total angular momentum, inspired a number
of important theoretical developments, including the
semi-classical approach and the microscopic Tilted Axis
Cranking model reviewed in [2] and [3], respectively.
Multiplets of levels generated by the closing blades
form ∆I = 1 bands with states connected predominantly
by M1 transitions. The magnetic dipole character of
these transitions was established from angular correlation, linear polarization, and electron conversion measurements (see references in [2]). The E2/M1 admixtures
for ∆I = 1 transitions deduced from angular correlations
imply, in the lead region, deformations of oblate shape.
The relatively low magnitude of this deformation is indicated by the small intensities of ∆I = 2 E2 cross-over
transitions observed in a number of cases; the B(M1)/
B(E2) ratio of transition strengths extracted from the
branching ratios exceeds 20 µN2/e2b 2 in the lead region.
The M1 strength is related to the component of the
magnetic moment perpendicular to the total angular momentum. The fact that this component is reduced as the
shears blades close (see Figure 2b), results in an important prediction, namely that the B(M1) strength in shears
bands decreases as a function of increasing angular momentum. The B(M1) values have been addressed by a
number of lifetime measurements [2] which result for the
lead region in B(M1) ~ 2 µN2 and B(E2) ~ 0.1e2b2 in
good agreement with the expectations. Moreover, the decreasing trend for the B(M1) as a function of spin has
clearly been identified, as shown in Figure 2c.
Another prediction of the shears mechanism is the
band termination expected when the shears are fully
closed. This has been seldom observed, since specific
configurations are hard to trace up to the non-yrast terminating state; crossings with bands involving a larger
number of aligned particles and holes determine the
yrast line accessible to high-spin spectroscopy. Such
crossings, however, correspond in the above terminology
to a “reopening of the shears,” and thus should yield a
correlated increase in the B(M1) strength. The effect has
been indeed confirmed experimentally in the recent,
highly sensitive, lifetime measurement [16].
An appealing interpretation of the shears mechanism
named “magnetic rotation” is discussed in detail in [3].
It is noted there that the current loops associated with
high-j particles and holes embedded in the near spherical
mass distribution of the nucleus, as well as associated
transverse magnetic moment, allow one to specify the
angle of rotation with respect to the axis defined by the
total angular momentum. This anisotropy in space results in a rotational-like behavior, in analogy to wellknown rotational bands in deformed nuclei resulting
from the anisotropy of the mass distribution.
The shears mechanism is currently well established
in a number of mass regions near closed shells; more specific experimental information can be found in the data
tables of [17], while [2], [3], and [17] should be consulted for references to original work.
Nuclear Chirality and M1 Properties
Triaxial deformation defines in the intrinsic, bodyfixed frame three mutually perpendicular directions
along the principal axes of the mass distribution and
three principal planes spanned by these axes. Valence
particles and holes in a triaxially deformed potential
minimize their energies by aligning their angular momenta with the short or long axis, respectively, while the
collective core rotation aligns with the intermediate axis
which, for irrotational flow-like moments of inertia, is a
preferred axis of rotation [18]. These three mutually
perpendicular angular momenta couple to form a total
angular momentum vector which is tilted away from any
principal plane, and thus they can be arranged [19] into
a right-handed or a left-handed system, as shown in Figure 3a.
As a consequence of the two possible couplings, doublet states of the same spin/parity and nearly identical
excitation energy are formed for a given single particle
configuration. Indeed, intriguing ∆I = 1 doublet band
structures have been observed systematically for the
πh11/2νh11/2 configuration in the triaxial A ~ 130 region
[20–24] (see Figure 3b) and for the πg9/2νh11/2 [25] configuration in 104Rh. The best examples of level degeneracy are provided by 134Pr shown in Figure 3c) with levels at spin 15+ and 16+ separated by less than 60 keV and
by 104Rh with levels between spin 15- and 17- separated
by less than 90 keV but with remarkably small ~2 keV
separation at spin 17-. The separation between doublet
Vol. 13, No. 2, 2003, Nuclear Physics News
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feature article
Figure 3. (a) Two possible couplings of angular momenta in a
triaxial odd-odd nucleus to produce the total angular momentum. (b) πh11/2νh11/2 doublet bands in 128-132Cs isotopes in Energy-vs. Angular momentum plot [25]. (c) πh11/2νh11/2 doublet
bands [28] in 134Pr. (d) Staggering in relative reduced transition
rates for πh11/2νh11/2 doublet band in 128Cs, B(M1)/B(E2) in the
yrast band (top) and B(M1)in /B(M1)out in the side band (bottom). Data are from [25].
bands in other nuclei varies between ~150 and ~350
keV. An explanation [20] for this energy displacement is
related to the fact that the triaxial deformation in these
cases may not be stable, but perhaps more soft, resulting
in an average moment of inertia along the intermediate
axis which is reduced closer to the values along the other
two axes. In this situation, an angular momentum vector
of collective rotation is not constrained to positive/
negative orientation along the intermediate axis: instead
it can oscillate from one chiral system to the other over
(or tunnel through) the saddle-point energy barrier across
the short-long plane. In these “chiral vibrations,” however, the remnants of the chiral doublets are retained.
The properties of the electromagnetic M1 and E2
operators play a crucial role in proving the doubling of
states within the same configuration. The single particle
structure of the ∆I = 1 main bands (these bands are most
often yrast in the nuclei of interest) is established as comprising the unique parity high-j intruder orbitals from
systematics [26] and g-factor measurements for isomeric
band heads. The linking transitions connecting the side
bands to the main bands are of mixed M1/E2 multipolarities based on uniquely conclusive angular correlation
studies [27], as well as polarization measurements [22].
The selection rules for the one-body M1 and E2 opera-
18
tors allow for the transitions between configurations
with single particles of the same parity only; this combined with the unique parity for orbitals forming the
main band implies the same unique parity configurations
for the main and the side bands.
Chirality in nuclear rotation is currently an active
area of research with the main experimental effort concentrated on identifying nuclei with nearly degenerate
doublet bands. The M1 properties, however, provide an
important tool for studies of nuclear chirality due to the
fact that each of the three angular momentum vectors
forming the total angular momentum has an associated
magnetic moment. The geometry of angular momentum
coupling is reflected in the properties of M1 matrix elements. Consequently, g-factors of high spin states can
distinguish between chiral or planar coupling [28].
Moreover, the structure of the wave function imposed by chiral geometry has important consequences
for M1 and E2 transition rates. The right- and lefthanded systems of angular momenta are related by the
TRy(π) operator which combines time reversal and rotation by 180° around one of the principal axes (conventionally, the y axis is chosen). The wave functions for the
doublet states in the laboratory frame are those combinations of the right- and left-handed systems which are
invariant under the TRy(π) operator:
2 ( R〉 +  L〉),
 +〉 = 1/
2 ( R〉 –  L〉).
 –〉 = i/
In the high spin limit electromagnetic transitions between the right- and left-handed systems are not allowed. This results in specific selection rules for transitions between physical  +〉 and  –〉 states in the
laboratory frame [25]:
• stretched  +〉 →  +〉 and  –〉 →  –〉 M1 transitions
are enhanced;
• stretched  +〉 →  –〉 and  –〉 →  +〉 M1 transitions
are hindered;
• stretched  +〉 →  –〉 and  –〉 →  +〉 E2 transitions
are enhanced;
• stretched  +〉 →  +〉 and  –〉 →  –〉 E2 transitions
are hindered.
These selection rules manifest chirality in the staggering
of relative reduced transition rates as a function of spin
for B(M1)/B(E2) ratios in the yrast band and B(M1)in/
B(M1)out ratios in the side band. This has been observed
Nuclear Physics News, Vol. 13, No. 2, 2003
feature article
experimentally for chiral band candidates in 128,130Cs
[25] and shown in Figure 3d. Similar behavior has been
observed in 104Rh. Absolute M1 and E2 strengths are of
significant interest and several experiments are underway or planned.
Mixed-Symmetry Multi-Phonon Structures
Another M1 phenomenon of recent interest is the
formation of multi-quadrupole phonon structures with
mixed proton-neutron symmetry in heavy quadrupolecollective nuclei. Isovector quadrupole-surface vibrations have been anticipated in terms of collective models.
Static quadrupole deformation can give rise to orbital
out-of-phase oscillations of the deformed proton- and
neutron-bodies forming a so-called scissors mode [29]
with large M1 excitation strength from the ground state.
The scissors mode has subsequently been discovered in
deformed even-even [30] and odd-mass nuclei [31] and
has been extensively studied, predominantly in electronscattering and photon-scattering experiments at Darmstadt and Stuttgart, see, e.g., [4, 32]. In deformed nuclei
the scissors mode appears at about 3 MeV excitation energy as a somewhat fragmented concentration of M1
strength with a total value of about B(M1)↑ ~ 3 µN2.
Within the framework of the interacting boson
model (IBM-2) the J π = 1+ scissors mode is only one particular member of a more general class of quadrupolecollective states with non symmetric coupling with respect to the proton-neutron degree of freedom [33]. The
IBM-2 considers pairs of valence protons and neutrons
outside of an appropriately chosen closed-shell core and
approximates them by proton and neutron bosons that
interact via two-body forces. The formalism of isospin
can be applied on the boson level, where the “elementary” bosons form an “isospin doublet” with projections
+1/2 (proton boson) and -1/2 (neutron boson). This
“boson isospin” is called F-spin and allows one to quantify the proton boson–neutron boson symmetry character of the IBM-2 wave functions formed by Nπ proton
bosons and Nν neutron bosons. Symmetric wave functions have the maximum F-spin quantum number Fmax =
(Nπ + Nν)/2 and correspond to IBM-1 states where no
distinction between proton bosons and neutron bosons
is made.
The new feature emerging in going from the IBM-1
to the IBM-2 is the appearance of whole new classes of
collective mixed-symmetry states (MSSs) with quantum
numbers F < Fmax of which the scissors mode is one example. The boson-boson residual interactions are such
that states with symmetric wave functions lie lowest in
Figure 4. Top: Spectrum of a simple IBM-2 Hamiltonian for
the harmonic vibrator. Mixed-symmetry states with quantum
numbers Fmax – 1 form multiphonon structures. Bottom: Partial level scheme of the nucleus 94Mo. Mixed-symmetry states
have been identified on the solid basis of sizeable M1 matrix
elements of about 1 µN . The structure of 94Mo deviates somewhat from the U(5) symmetry.
energy followed by states with quantum numbers Fmax –
1, Fmax – 2, . . . . The top of Figure 4 shows the low-lying
parts of the spectrum of a schematic IBM-2 Hamiltonian. The operator nd = ndπ + ndν counts the number of
d-bosons and M = [Fmax(Fmax + 1) – F(F + 1)]/2 shifts
mixed-symmetry states to higher energies. This Hamiltonian has U(5) symmetry and supports a vibrational
spectrum with symmetric and mixed-symmetry onequadrupole phonon states and two-phonon multiplets.
The one-phonon state’s wave functions can be generated
from the ground state by applying the symmetric (Qs =
Qπ + Qν) and mixed-symmetric (Qm = Qπ /Nπ – Qν /Nν)
Vol. 13, No. 2, 2003, Nuclear Physics News
19
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quadrupole phonon operators, respectively. Multiphonon states are obtained by a successive action of the
one-phonon operators. The most distinct feature of
MSSs is the existence of allowed F-vector M1 transitions
to symmetric states with large matrix elements of ~1 µN .
M1 transitions between symmetric states are forbidden
within the sd-IBM-2 which considers only monopole (s)
and quadrupole (d) bosons. Further selection rules for
M1 transitions exist, e.g., with respect to the number of
d-bosons. In particular, the 1+ scissors mode cannot be
directly excited from the ground state by an M1 transition in pure vibrators.
One-phonon and two-phonon MSSs have recently
been studied in a couple of weakly deformed nuclei; see,
e.g., [34–36], and there have even been claims for MSSs
with quantum numbers down to F ≤ Fmax – 2 [37]. At
present, the best studied example for a one-phonon and
two-phonon mixed-symmetry structure has been found
[38, 39] in the level scheme of 94Mo with 52 neutrons in
the vicinity of the N = 50 neutron shell closure. The lowspin level scheme of 94Mo has been studied towards the
far-off-yrast region at Stuttgart, Lexington, and Cologne
with a variety of γ-ray spectroscopic tools including inelastic photon-scattering, inelastic neutron-scattering,
and γγ-coincidence spectroscopy in (α,n) fusion reactions
and β+-decay from 94Tcm. Comprehensive data on the
level scheme, spin quantum numbers, γ-ray multipolemixing ratios, and level lifetimes made the identification
of MSSs possible and allowed for the measurement of
their fragmentation. The bottom of Figure 4 shows the
MSSs of 94Mo uniquely identified by their large M1 transition strengths with matrix elements of about 1 µN. The
MS two-phonon multiplet occurs close to the sum-energy of the corresponding one-phonon energies indicating rather harmonic phonon coupling. Its energy splitting as a function of spin is less than 10%. The observed
MSSs are almost unfragmented in 94Mo. This can be
concluded from the comparably small M1 strengths
measured for surrounding levels with the same spin and
parity quantum numbers. Besides the identifying M1
transitions, also E2 transitions to symmetric states and
even to the MS one-phonon state were observed. These
data [38] give the first direct evidence that the MSSs indeed form a class of quadrupole-collective states with
similar structure. The discovery of the two-phonon 2+2,ms
state by Fransen et al. [39] is significant because it is the
first demonstration that even off-yrast states in the
mixed-symmetry sector can survive relatively purely in a
nuclear level scheme.
20
Similar structures have subsequently been observed
in various parts of the nuclear chart. Of particular experimental interest is the successful identification of the
one-phonon 2+1,ms state in 96Ru done in inverse-kinematics Coulomb excitation of a stable 96Ru-beam at Yale [40].
The technique of Coulomb excitation in inverse kinematics develops into a major spectroscopic method for the
investigation of neutron-rich nuclei at high-intensity RIB
facilities. Corresponding experiments on neutron-rich
RIBs of N = 52 isotones to search for mixed-symmetry
states are already scheduled within the RISING campaign at GSI. Information on isovector valence shell excitations, such as MSSs, will help to conclude on the
isospin dependence of the proton-neutron restoring force
and, finally, on the formation of neutron skins or even
decoupled neutron matter in exotic neutron-rich nuclei.
Summary and Outlook
Examples presented in the current overview show
that, because of a number of recent theoretical and experimental results, M1 transitions have taken on a new
role for the investigation of nuclear structure. No longer
are E2 transitions alone the signature of collective modes
in nuclei. In view of the proposed and planned advances
in nuclear structure experiments, further progress is expected in the near future; new radioactive beam facilities
will provide an opportunity to access nuclei far from the
stability line, while segmented Ge detector technology
holds the potential of revolutionizing the field. New and
exciting physics information, as well as an extension of
the studies presented above, is anticipated from investigations of magnetic dipole excitations due to their fundamental character and sensitivity to the details of nuclear many-body wave functions.
Acknowledgments
We thank all those who have collaborated with us on
the discussed topics. Help with the preparation of this
article by R. F. Casten, R. M. Clark, D. B. Fossan, and
A. F. Lisetskiy is gratefully acknowledged. This work
was supported by the Emmy Noether-Program of the
Deutsche Forschungsgemeinschaft under support No. Pi
393/1-2 and the U.S. National Science Foundation
award number 0098793.
Appendix A: M1 operator
In terms of the nuclear shell model the M1 operator
can phenomenologically be written as
Nuclear Physics News, Vol. 13, No. 2, 2003
feature article
where gl(s)p(n) is the effective orbital (spin) g-factor for
protons (neutrons) and l(s) is the corresponding orbital
(spin) angular momentum operator in units of -h. The appropriate values for the effective g-factors in nuclei can
deviate from those of free nucleons because of inmedium modifications due to sub-nucleonic degrees of
freedom or model space truncation. For the discussion of
M1 transitions it is convenient to decompose the M1 operator into isoscalar and isovector components and a
third part proportional to the total angular momentum
operator
where J = Lp + Ln + Sp + Sn is the total vector sum of all
proton and neutron orbital and spin angular momenta,
L(S)p(n) = Σp(n)l(s)p(n) is the total orbital (spin) angular
momentum of the protons (neutrons), and TIS(IV) = Lp ±
Ln + cs(Sp ± Sn) denotes the remaining isoscalar (isovector) part of the M1 transition operator. The parameters
gJ, gIS, gIV, and cs are rational functions of the factors
gl(s)p(n). Since nuclear states have good total angular momentum, the term proportional to J, which is diagonal in
the set of eigenstates, does not generate M1 transitions.
The ratio gIS /gIV turns out to be small, (<0.1) if one uses
g-factors for free nucleons. The M1 operator, consequently, has predominantly isovector character and,
hence, M1 transitions can serve us as a “magnifying
glass” for isovector nuclear properties. This is in important contrast to E2 transitions that usually enhance
isoscalar features. Using bare values for the nucleon gfactors, the relevant isovector orbital- and spin- g-factors
become of gl(s)V = (gl(s)n – gl(s)p)/2 =–1/2 and –4.7, respectively.
Appendix B: Isospin
The discussion of enhanced M1 transitions in nuclei
is intimately connected to the isospin degree of freedom
due to the predominantly isovector character of the M1
transition operator (see Appendix A). The concept of
isospin represents a convenient classification scheme for
nuclear properties that enables us to comprehend nuclear states, i.e., product states of protons and neutrons,
as quantum states of many indistinguishable fermionic
nucleons times a spin-like factor to the wave function
which keeps track of the proton-neutron degree of freedom (isospin d.o.f.). The isospin-concept was introduced
by Heisenberg in 1932 and has been an important classification scheme for nuclear and hadronic states ever
since.
In the nuclear case the isospin concept makes use of
the charge independence of nuclear forces, i.e., the experimental fact that protons and neutrons have the same
mass and react identically to nuclear forces in good approximation. Protons and neutrons are, therefore, considered to be identical particles with only a different
value of an internal spin-1/2-like degree of freedom, the
isospin projection Tz. Conventionally, neutrons are assigned isospin projection Tz(n) = +1/2 and protons Tz(p)
= –1/2. Nuclear states formed by N neutrons and Z protons have isospin projection Tz = (N – Z)/2 and can take
values for the isospin quantum number |N – Z|/2 ≤ T ≤
|N + Z|/2. In most nuclei the lowest-lying states have
isospin quantum numbers T = T< = Tz, while states with
T ≥ T> = T< + 1 are usually unbound or at least highly
excited. Nuclear states in neighboring isobars with
|Tz| > |Tz| form isospin multiplets with T > T< and share
identical nucleonic structure (although different isospin
projection). Considerable similarities between isospin
multiplet partner states in different nuclei offer a valuable source of information on nuclear properties. Most
important is the existence of selection rules for various
nuclear reactions or decay processes with respect to the
isospin quantum numbers. One example is the suppression of (T = 0) → (T = 0) M1 transitions due to the predominantly isovector character of the M1 transition
operator. Isospin symmetry is, however, weakly broken
by the small proton/neutron mass difference, by the
Coulomb force, and by small isospin-breaking parts of
the nuclear forces.
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Nuclear Physics News, Vol. 13, No. 2, 2003
Norbert Pietralla
Krzysztof Starosta
feature article
Gravitation at a Micron and
Mixing of Quarks
H. ABELE
Physikalisches Institut der Universität Heidelberg, Heidelberg, Germany
Introduction
Galilei Galileo would be somewhat surprised. When
neutrons become ultra-cold, his famous fall experiment
shows quantum aspects of the subtle gravity force in the
sense that neutrons do not fall as larger objects do. In a
free-fall experiment they don’t fall continuously. We find
them on particular levels, when they come close to a reflecting mirror for neutrons. Of course, such bound
states with discrete energy levels are expected when the
gravitational potential is larger than the energy of the
particle. Here, the quantum states have pico-eV energy,
a value that is smaller by many orders of magnitude
compared with an electromagnetically bound electron in
a hydrogen atom, opening the way to a new technique
for gravity experiments and measurements of fundamental properties. New motivations for gravity experiments
come from frameworks where the fundamental Planck
scale (the scale about 10-44 s after the big bang where
gravity becomes comparable in strength to the other interactions) is taken to the weak scale, the energy scale of
the Standard Model at 10-10 s after the big bang. Considering this very early stage of our universe, we have the
strong feeling, that a Standard Model description is incomplete, and many new observables pointing to physics
beyond the Standard Model emerge from superstring
theory, supersymmetry or other Grand Unified Theories
(GUT). For example, in theories with submillimeter dimensions, gauge fields can mediate repulsive gravity-like
forces ~1010 times stronger than gravity in submillimeter
distances. Furthermore, the quark-mixing CabibboKobayashi-Maskawa (CKM) matrix remains unexplained in the Standard Model as well as CP-violation,
which might explain the baryon-antibaryon asymmetry
of the universe. Some observables of these theories require neutron physics, for others the neutron provides
one of several possible ingredients.
Experiments in the field of particle physics usually
make use of highest beam energies. In the sub-field of
particle physics with neutrons however, physicists use
“low energy” neutrons, neutrons that are much colder
than the molecules around us. Some experiments profit
from high intense cold neutron beams, other experiments need even colder neutrons, so called ultra-cold
neutrons. These neutrons are reflected from surfaces and
can be stored in “neutron bottles.”
This articles discusses two aspects of neutron physics
with cold and ultra-cold neutrons. First, a recent gravity
experiment at the Institut Laue-Langevin demonstrates
quantum states in the gravitational potential of the earth
with ultra-cold neutrons [1] and places limits on gravitylike forces in the range between 1 µm and 10 µm [2].
Second, measurements by various international groups
of researchers determine the strength of the weak interaction of the neutron, which gives us unique information
on the question of the quark mixing. Neutron β-decay
experiments now challenge the Standard Model of elementary particle physics with a deviation, 3 times the
stated error [3].
Cold and Ultra-Cold Neutrons
Neutrons are produced in a spallation source or a research reactor. At production, these neutrons are very
hot; the energy is about 2 MeV corresponding to 1010
degrees centigrade. On the other side of the scale, the
gravity experiment uses neutrons having 1018 times less
energy in the pico-eV range (see Table 1). In a first step,
spallation or fission neutrons thermalize in a heavy
water tank at a temperature of 300 K. The thermal
fluxes are distributed in energy according to Maxwellian
law. At the Institut Laue-Langevin (ILL), cold neutrons
are obtained in a second moderator stage from a 25 K
liquid deuterium cold moderator near the core of the 57
MW uranium reactor. These cold neutrons have a velocity spectrum in the milli-eV energy range. For particle
physics, a new beam line with a flux of more than 1010
cm-1s-1 over a cross section of 6 cm x 20 cm is available
[4]. An overview of many observables in the rich field of
neutron particle physics and of related physical questions are taken from [5] and can be found in Table 2.
Ultra-cold neutrons are taken from the low energy
tail of the cold Maxwellian spectrum. They are guided
vertically upwards by a neutron guide. The curved guide,
Vol. 13, No. 2, 2003, Nuclear Physics News
23
feature article
Table 1. From hot to ultracold: neutrons at the ILL.
Energy
Temperature
Velocity
Fission
neutrons
Thermal
neutrons
Cold
neutrons
Ultracold
neutrons
Gravity
experiment
2 MeV
1010 K
25 meV
300 K
2200 m/s
3 meV
40 K
800 m/s
100 neV
1 mK
5 m/s
1.4 peV
—
υ⊥ ~ 2 cm/s
Table 2. Observables in neutron-particle physics and related physical questions [5].
Neutron particle properties
Mass: Bound states in gravitational field
mn/np, h/mn
Charge
Magnetic dipole moment
Electric dipole moment
Electric polarizability
Limits on gravity-like forces expected from large extra-dimensions
Value of electromagnetic interaction strength α
Charge quantization, GUT’s
Quark models
Time reversal violation, GUT’s
Quark confinement potential
Neutron β-decay
Lifetime
Correlation coefficients
β-asymmetry
Neutrino asymmetry
β neutrino correlation
β-helicity
triple neutron spin-correlations
triple electron spin-correlations
Weak lepton-quark interaction, as input for cosmology and
astrophysics, quark models, Standard Model tests
Unitarity of quark mixing
Right-handed currents
Conservation of weak vector current
Flavour symmetry
Limits on scalar and tensor admixtures
Gut’s, time reversal violation
Baryon asymmetry of the universe
Energy spectra:
of electrons, protons
of various correlation coefficients
of inner bremsstrahlung
Neutron decay into hydrogen
Weak magnetism in electroweak interaction
Second class currents
Radiative corrections
Yes/no experiment on right-handed currents
Neutron interaction
Scattering length:
Neutron-electron
Neutron-proton
Neutron-Neutron
Parity violating effects:
Spin rotation in nonmagnetic medium
Neutron polarizating action of
nonmagnetic medium
n-p γ-asymmetry
n-p circular polarization
24
Neutron intrinsic charge distribution
Quark models
Isospin invariance
Quark-quark electroweak interaction
Nuclear Physics News, Vol. 13, No. 2, 2003
feature article
which absorbs neutrons above a threshold energy, acts as
a low-velocity filter for neutrons. Neutrons with a velocity of up to 50 m/s arrive at a rotating nickel turbine.
Colliding with the moving blades of the turbine, ultracold neutrons exit the turbine with a velocity of several
meters per second. They are then guided to several experimental areas.
A mirror for neutrons uses the strong interaction between nuclei and a neutron, resulting in an effective repulsive force: neutrons propagate in condensed matter in
a manner similar to the propagation of light but with a
neutron refractive index less than unity. Thus, one considers the surface of matter as constituting a potential
step of height V. Neutrons with transversal energy E⊥ <
V will be totally reflected. Ultra-cold neutrons (UCN)
are neutrons that, in contrast to faster neutrons, are
retro-reflected from surfaces at all angles of incidence.
When the surface roughness of the mirror is small
enough, the UCN reflection is specular. Neutron mirrors
are interesting because they can be used to store neutrons, to focus neutrons, or to build a Fabry Perot interferometer for neutron de Broglie waves. UCN storage
bottle experiments have improved our knowledge about
the neutron lifetime significantly and, together with the
Ramsey method of separated oscillating fields, they have
been used for a search for an electric dipole moment of
the neutron.
Quantum States in the Gravitational Field
of the Earth
The idea of observing quantum effects in the gravitational potential occurring when ultracold neutron are
stored on a plane was discussed long ago by V. I.
Lushikov and A. I. Frank [6]. An experiment similar in
some aspects was discussed by H. Wallis et al. [7] in the
context of trapping atoms in a gravitational cavity.
Quantum theory and gravitation affect each other, and,
when neutrons become ultra-cold, we find them on different levels, when they come close to a reflecting mirror
for neutrons. These quantum states have been observed
in a collaboration between the ILL (Grenoble), PNPI
(Gatchina), CERN (Geneva), and our group at Heidelberg University [1]. The population of the ground state
and the lowest states follows the quantum mechanical
prediction. An efficient neutron absorber removes the
higher, unwanted states. At the entrance of the experiment, a collimator absorber system cuts down on the
neutrons to a adjustable transversal energy E⊥ in the
pico-eV range. Of course, such bound states with dis-
crete energy levels are expected when the gravitational
potential is larger than the energy of the particle.
A side effect of this experiment is its sensitivity for
gravity-like forces at length scales below 10 µm. In light
of recent theoretical developments in higher dimensional
field theory [8], gauge fields can mediate forces that are
108 to 1012 times stronger than gravity at submillimeter
distances, exactly in the interesting range of this experiment and might give a signal in an improved setup.
Figure 1 shows the schematic view of the setup: Neutrons pass through a mirror absorber system and they
are eventually detected by a 3He-counter. Signatures of
quantum states in the gravitational field of the earth are
observed in the following way: the 3He counter measures
the total neutron transmission T, when neutrons are traversing the mirror absorber-system. The transmission is
measured as a function of the absorber height h and thus
as a function of neutron energy since the height acts as a
selector for the vertical energy component E⊥. Above an
absorber height of about 80 µm, the measured transmission is in close agreement with the classical expectation
but below 50 µm, a deviation is clearly visible. From
quantum mechanics, we easily understand this behavior:
Ideally, we expect a stepwise dependence of T as a function of h. If h is smaller than the spatial width of the lowest quantum state, then T will be zero. When h is equal
to the spatial width of the lowest quantum state then T
will increase sharply. A further increase in h should not
Figure 1. Sketch of the setup: (a) classical view: neutron trajectories; (b) quantum view: plane waves and Airy functions [24].
Vol. 13, No. 2, 2003, Nuclear Physics News
25
feature article
Figure 2. Data and classical expectation vs. quantum expectation.
increase T as long as h is smaller than the spatial width
of the second quantum state. Then again, T should increase stepwise. At sufficiently high slit width one approaches the classical dependence and the stepwise increase is washed out (see Figure 2). The measurement
does agree well with a simple quantum mechanical description of quantum states in the earth’s gravitational
field.
Limits for Non-Newtonian Interaction
below 10 microns
A quantum mechanical consistent theory of gravity
remains a big challenge to physicists. However, new theoretical efforts are underway and superstring theory is a
promising attempt to unify general relativity and quantum theory. Discouraging in the past was the fact, that
the Planck length, the scale of quantum fluctuation of
space time geometry, is twenty orders of magnitude
smaller than a neutron with diameter of 10-15 m. But
new theories on string theory and multidimensions cause
gravity to reach Planck length well above 10-35 m. Corrections to the gravitational inverse square law are due
to compactifed extra dimensions. On the assumption
that the form of the non-Newtonian potential is given by
the Yukawa expression, for masses mi and mj and distance r the modified Newtonian potential V(r) takes the
form
mi • mj
(1 + α • e-r/ λ),
V(r) = –G ––
r
26
(1)
where λ is the Yukawa distance over which the corresponding force acts and α is a strength factor in comparison with Newtonian gravity. G is the gravitational
constant. In theories with submillimeter dimensions, a
number of gravity-like phenomena emerge. For example,
a hypothetical gauge field can naturally have miniscule
gauge couplings, independent of the number of extra dimensions [8]. If these gauge fields couple to a neutron
with mass mn, these gauge fields can result in repulsive
forces of million or trillion times stronger than gravity at
micrometer distances, exactly in the range of interest.
The results of a fit to the measured data (see section
“Quantum States in the Gravitational Field of the
Earth”) yields predictions for 90% confidence level exclusion bounds on α and λ. These limits from neutron
mirror experiments are the best known in the range from
1 µm < λ < 3 µm and exclude for the first time gravitylike short-ranged forces at 1 µm with strength α > 1012
and at 10 µm with strength α > 1011 (Figure 3a) [2]. Previous constraints on both α and λ, adapted from [9], are
shown in Figure 3b.
Figure 3. Limits for non-Newtonian gravity: Strength |α| vs.
Yukawa length scale λ. (a) Experiments with neutrons place
limits for |α | in the range 1 µm < λ < 10 µm. (b) Constraints
from previous experiments are adapted from [9].
Nuclear Physics News, Vol. 13, No. 2, 2003
feature article
The Standard Model, Quark Mixing,
and the CKM Matrix
This subsection is about the interplay between the
Standard Model of elementary particle physics and neutron β-decay experiments. The energy range of the Standard Model extends up to about 1TeV. It is a field-theory of strong and electroweak interactions at these
energies. In view of the Standard Model, matter is built
from two types of fundamental fermion, called quarks
and leptons. Quarks occur in several varieties or flavours
labelled up (u), down (d), charm (c), strange (s), top (t),
and bottom (b). The strong interaction glues the quarks
together and these quarks are considered to be quantum
mechanical mass eigenstates. Neutrons are built from
two d-quarks and one u-quark, whereas protons are
built from two u-quarks and one d-quark.
A free neutron is unstable. The decay process is governed by the weak interaction converting a d-quark into
an u-quark. Thus, the lifetime τ is 885 s and a neutron
decays into a proton, an electron and an electron antineutrino with an energy release of 782 keV. The weak
coupling strength responsible for nuclear or neutron
decay is not identical with the coupling for µ-decay. The
difference is about 2% and this feature of the weak interaction is well understood under the assumption of
quark mixing: A weak decaying quark is a mixture of
different flavours of mass eigenstates. The weak eigenstates (primed) are related to the mass eigenstates (unprimed) as
with unitary CKM matrix V. All mixing is expressed in
terms of V operating on d, s, and b quarks. As a consequence, our d-quark that is responsible for neutron βdecay, is a linear superposition of d-, s-, and b-mass
states: d = Vud • d + Vus • s + Vub • b. The basic idea of
V is that what is perceived to be several independent
couplings are actually components of a single force. Now,
if every quark gives as much as it takes in this mixing,
then the quark-mixing CKM matrix V has to be unitary.
The values of the individual matrix elements are determined from weak decays of the relevant quarks. There
exist several parametrizations of the CKM matrix. A
“standard parametrization” uses three angles and a
phase. The phase breaks CP violation invariance. The
range of matrix elements shown in Table 3 corresponds
to 90% C.L. limits on the angles and the phase assuming unitarity of the CKM matrix. This unitarity constraint has pushed Vud about one to two standard deviations higher than given by the experiments. The
Unitarity condition applied to the first and third
columns of the CKM matrix yields VudV*ub = +VcdV*cb +
VtdV*tb = 0. The so-called unitarity triangle with angles
α, β and γ is a geometrical presentation of this equation.
The angles β and γ are phases of the CKM elements Vtd
and Vub. All processes can be understood by γ = 59° ±
13°. The experimental results from BaBar and Belle,
when averaged, yield β = 26° ± 4° [10].
So far precision tests of unitarity are only possible
for the first row of V [11–13]. Unitarity requires that the
sum of the squares of the matrix elements for each row
and column is unity, namely
Vud 2 + Vus 2 + Vub 2 = 1 – ∆.
(3)
The Standard Model requires ∆ = 0. A violation of
unitarity in the first row of the CKM matrix is a challenge to the three-generation Standard Model. A deviation ∆ has been related to concepts beyond the Standard
Model like supersymmetry, couplings to exotic fermions,
to the existence of an additional Z boson or the existence
of right-handed currents in the weak interaction.
Due to its large size, a determination of Vud (see
Table 3) is most important. It has been derived from a series of experiments on superallowed nuclear β-decay
through determination of phase space and measurements
of partial lifetimes. With the inclusion of nuclear structure effect corrections a value of Vud = 0.9740(5)
emerges in good agreement of different, independent
Table 3. CKM quark mixing matrix Uquark with 90% C.L. The unitarity constraint has pushed
|Vud | about one to two standard deviations higher than given by the experiments.
Vud = 0.9741 to 0.9756
Vus = 0.219 to 0.226
Vub = 0.0025 to 0.0048
Vcd = 0.219 to 0.226
Vcs = 0.9732 to 0.9748
Vcb = 0.038 to 0.044
Vtd = 0.004 to 0.014
Vts = 0.037 to 0.044
Vtb = 0.9990 to 0.9993
Vol. 13, No. 2, 2003, Nuclear Physics News
27
feature article
measurements in nine nuclei. Combined with Vus =
0.2196(23) from kaon-decays and Vub = 0.0036(9)
from B-decays, this lead to ∆ = 0.0032(14), signaling a
deviation from the unitarity condition by 2.3 σ standard
deviation [14], The quoted uncertainty in Vud, however, is dominated by theory due to amount, size and
complexity of theoretical uncertainties. Although the radiative corrections include effects of order Zα2, part of
the nuclear corrections are difficult to calculate. Further,
the change in charge-symmetry-violation for quarks inside nuclei results in an additional change in the predicted decay rate which might lead to a systematic underestimate of Vud. A limit has been reached where new
concepts are needed to progress. Such are offered by
studies with neutron β-decay and with limitations with
pion β-decay. The pion β-decay has been measured recently at the PSI. The pion has a different hadron structure compared with neutron or nucleons and it offers an
other possibility in determining Vud. The preliminary
result is Vud = 0.9971(51) [15]. The somewhat large
error is due to the small branching ratio of 10-8.
The combination of neutron β-decay experiments at
the Institut Laue-Langevin now challenge the Standard
Model of elementary particle physics: A measurement of
the β-asymmetry A and the world average of the neutron
lifetime τ determine the first element Vud of the quarkmixing CKM matrix. With this value and the particle
data group values for Vus and Vub, the unitarity condition for the first row of the CKM matrix deviate from
unity by ∆ = 0.0083(28), which is 3.0 times the stated
error and conflicts the prediction of the Standard Model
of particle physics.
Vud , Neutron β-Decay, and the
Experiment PERKEO
In the Standard Model only two additional parameters describe neutron β-decay, since the Fermi decay constant is known from muon decay. One parameter is the
first entry Vud of the CKM matrix. The other one is λ,
the ratio of the vector coupling constant and the axial
vector constant. The observables for determining Vud are
the neutron lifetime τ and a measurement of one of the
angular correlation coefficients e.g. the β-asymmetry coefficient A. The β-asymmetry A is linked to the probability that an electron is emitted with angle ϑ with respect to the neutron spin polarization P = 〈σz 〉:
W(ϑ) = 1 + v/c P A cos(ϑ),
where v/c is the electron velocity expressed in fractions
of the speed of light. Neglecting order 1% corrections, A
is a simple function of λ.
For a measurement of the β-asymmetry A, the instrument PERKEO II was installed at the PF1 cold neutron
beam position at the High Flux Reactor at the Institut
Laue-Langevin, Grenoble [16]. Figure 4 shows the setup.
The neutrons are polarized by a 3 x 4.5 cm2 supermirror
polarizer. The degree of neutron polarization was measured to be P = 98.9(3)% over the full cross section of the
beam.
The main component of the PERKEO II spectrometer is
a superconducting 1.1 T magnet in a split pair configuration, with a coil diameter of about one meter. Neutrons pass through the spectrometer, whereas decay electrons are guided by the magnetic field to either one of
Figure 4. Experimental setup of the experiment PERKEO at the Institut Laue-Langevin.
28
(4)
Nuclear Physics News, Vol. 13, No. 2, 2003
feature article
Earlier experiments with large corrections [17–19]
gave significant lower values for λ. With the new value,
and the world average for τ = 885.7(7) s, one finds that
Vud = 0.9713(13). With Vus = 0.2196(23) and the
negligibly small Vub = 0.0036(9), one obtains
Vud 2 + Vus 2 + Vub 2 = 1 – ∆ = 0.9917(28).
(6)
This value differs from the Standard Model prediction
by ∆ = 0.0083(28), or 3 times the stated error.
An independent test of CKM unitarity comes from
W physics at LEP where W decay hadronic branching ratios can be used. Since decay into the top quark channel
is forbidden by energy conservation one would expect
ΣVij 2 to be 2 with a three generation unitary CKM matrix. The experimental result is 2.032(32), consistent
with (6) but with considerably lower accuracy.
Figure 5. Fit to the experimental asymmetry Aexp for detector
1 and detector 2. The solid line shows the fit interval, whereas
the dotted line shows an extrapolation to higher and lower
energies.
two scintillation detectors. The detector solid angle of
acceptance is 2 x 2π. The measured electron spectra
Ni↑(Ee ) and Ni↓(Ee ) in the two detectors (i = 1,2) for neutron spin up and down, respectively, define the experimental asymmetry Aiexp(Ee) as a function of electron kinetic energy Ee and are shown in Figure 5. Aiexp(Ee) is
directly related to the asymmetry parameter A. After a
2% correction for small experimental systematic effects
one obtains
A = –0.1189(7) and λ = –1.2739(19) [3].
(5)
The Future
All earlier experiments on A0 made large corrections
in the 15% to 30% range. The main corrections in the
experiment PERKEO are due to neutron beam polarization
(1.1%), background (0.5%) and flipper efficiency
(0.3%). The total correction is 2.04%. With such small
corrections to the data, we start to see a deviation from
the Standard Model already in the uncorrected raw data.
For the future, the plan is further to reduce all corrections. Since this measurement, major improvements both
in neutron flux and degree of neutron polarization has
been made: First, the new ballistic supermirror guide
[40] at the ILL from the University of Heidelberg gives an
increase of a factor of 10 in the cold neutron flux. Second, a new arrangement of two supermirror polarizers
allows to achieve an unprecedented degree of neutron
polarization P of between 99.5% and 100% over the full
cross section of the beam [20]. Third, systematic limitations of polarization measurements have been investigated: The beam polarization can now be measured with
a completely new method using an opaque 3He spin filter with an uncertainty of 0.1% [21, 22]. As a consequence, we are now in the lucky situation to improve on
the main uncertainties in reducing the main correction of
1.1% to less than 0.5% with an error of 0.1%. Thus, a
possible deviation from the Standard Model, if confirmed,
will be seen very pronounced in the uncorrected data.
Future trends have been presented in the workshop
“Quark-Mixing, CKM Unitarity” in Heidelberg, September 19–20, 2002. Regarding the Unitarity problem,
about half a dozen new instruments are planed or are
under construction to allow for beta-neutrino correla-
Vol. 13, No. 2, 2003, Nuclear Physics News
29
feature article
tion a and beta-correlation A measurements at the sub10-3 level. With next-generation experiments measurements with a decay rate of 1 MHz become feasible [23].
Summary
Gravitational bound quantum states have been seen
for the first time. We conclude that the measurement is
in agreement with a population of quantum mechanical
modes. Further, the spectrometer operates on an energy
scale of pico-eVs and can usefully be employed in measurements of fundamental constants or in a search for nonNewtonian gravity. The present data constrain Yukawalike effects in the range between 1 µm and 10 µm.
Vud, the first element of the CKM matrix, has been
derived from neutron decay experiments in such a way
that a unitarity test of the CKM matrix can be performed based solely on particle physics data. With this
value, we find a 3σ standard deviation from unitarity,
which conflicts with the prediction of the Standard
Model of particle physics.
Acknowledgments
This work has been funded in part by the German
Federal Ministry (BMBF) under contract number 06 HD
854 I and by INTAS under contract number 99-705.
References
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neutrons in the gravitational field and limits for non-Newtonian interaction in the range between 1 µm and 10 µm.
In: Aspects of Quantum Gravity, ed. by C. Laemmerzahl
(Springer, Berlin, Heidelberg, 2003, (Lecture Notes in
Physics)), in press.
3. H. Abele et al., Phys. Rev. Lett. 88 211801 (2002).
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Nuclear Physics News, Vol. 13, No. 2, 2003
feature article
Nuclear Exchange Currents
DAN-OLOF RISKA
Helsinki Institute of Physics and Department of Physical Sciences,
University of Helsinki, Helsinki, Finland
Introduction
Modern numerical methods have made it possible to
calculate, with remarkably satisfactory results, the spectra of nuclei up to A = 10 on the basis of realistic phenomenological models for the nucleon–nucleon interaction, which have been fitted to nucleon–nucleon
scattering data and with inclusion of a weak three-nucleon interaction [1]. Employment of the corresponding
wavefunctions to calculate the electromagnetic and weak
observables and transition strengths of these nuclei calls
for correspondingly realistic models for the electromagnetic and axial current density operators, which are consistent with the interaction Hamiltonian.
Corresponding to the form of the nuclear Hamiltonian as a sum of single nucleon kinematic energy (Ti ) and
two-nucleon (Vij ) and many-nucleon interactions,
H=
T + V
i
i
ij
i<j
+...,
(1)
the nuclear current density operator may be separated
into a sum of single nucleon currents (ji(1)) and two-nucleon (jij(2)) and many-nucleon currents:
j =
j
(1)
i
i
+
j
(2)
ij
i<j
+....
(2)
If j represents the electromagnetic current density,
the continuity equation,
. j + i[H, ρ],
(3)
where ρ is the charge density operator, demands the
presence of two-nucleon currents, when the interaction
operator V does not commute with the charge density
operator, which in the single nucleon approximation is
e
ρ=_
2
j
(1 + τ 3)ρ j (r – rj ),
j
(4)
where ρ j is the charge density of the jth nucleon.
Because of the state dependence of the nucleon–
nucleon interaction, [V, ρ] ≠ 0, and the single approximation for the nuclear current operator breaks the continuity equation. Electronuclear calculations therefore
have to include a two-nucleon—and if three-nucleon interactions are included also, a three-nucleon—“exchange”
current operator in order to satisfy the continuity equation (3) [2].
Pion Exchange and Electromagnetic Observables
The numerically most significant exchange current
operator for electromagnetic observables is the long
range pion exchange current operator, which may be derived by methods similar to those used to derive the pion
exchange potential. The main component of this operator, which is required by the continuity equation, was derived even before the discovery of the pion [3]. The establishment of its numerical significance had, however,
to await the development of realistic wavefunction models for the few-body systems, by means of which it was
found that the pion exchange current could explain both
the missing ~10% of the calculated thermal neutron
cross section for np → dγ [4] and the difference between
the magnetic moments of 3H and 3He [5]. Subsequently
it has been shown by means of effective field theory
methods that more complicated multipion exchange
contributes but an insignificant additional contribution
to the cross section for np → dγ [6].
That reliable estimates of the exchange current contribution obtain already in the pion exchange approximation follows from the inherently long-range nature of
the exchange current operator, which is associated with
the isospin-dependent interaction, ν(r1 – r2)τ 1 . τ 2. The
continuity equation for this current operator is
. j(2)(r; r1,r2)
= 2ν(r1 – r2)[ρ1(r – r1) – ρ2(r – r2)](τ 1 × τ 2)3.
(5)
As the r.h.s. of this equation vanishes as r1 → r2 , independently of the strength of the interaction, it follows
that this isovector operator j(2) is dominated by its longrange components.
While the pion exchange current contributes a modest but phenomenologically required 10% enhancement
of the calculated cross section for radiative capture of
thermal neutrons on protons, it is responsible for about
Vol. 13, No. 2, 2003, Nuclear Physics News
31
feature article
Figure 1. Differential cross section, proton vector analyzing power, and deuteron tensor analyzing power observables for radiative pd capture at Ecm = 3.33 MeV.
The dashed curve represents the impulse approximation,
and the solid curves, the results that include the exchange current contributions as calculated explicitly
(thin lines) and with the Siegert theorem (thick lines) [9].
The data points are from [10].
one-half of the much smaller empirical cross section for
radiative capture on deuterium [7] and the bulk of the
even smaller cross section for radiative capture on 3He
[8]. In the latter two reactions the matrix element of the
single nucleon currents are suppressed by the orthogonality between the main components of the initial and
final states. A particularly impressive illustration of the
significant role of the exchange current contribution in
reactions of this kind is provided by the p-d radiative
capture reaction where the exchange current contribution is crucial for a description of the data on the angular distributions as shown in Figure 1 [9,10].
32
The Nuclear Interaction and the
Exchange Currents
For such nucleon–nucleon interaction models, which
are expressed as sums of single meson exchange interactions, the construction of the associated exchange current operators is straightforward. Modern realistic phenomenological interactions are not that simple, and
therefore some dynamical assumptions are required for
the construction of the corresponding exchange current
operators that satisfy the continuity equation (3), (5).
One solution to this problem is to express the interaction
in a form that has the same operator structure as the
simple boson exchange models and to then replace the
boson exchange Yukawa functions with the corresponding coefficient functions of the realistic interaction
[10–13].
This method has led to very satisfactory description
of such electronuclear observables, that are sensitive to
exchange currents. Best known of these are the cross section for backward electrodisintegration of the deuteron
[14] and the electromagnetic form factors of the few-nucleon systems [15]. In the case of these observables the
relatively large exchange current contribution is a direct
consequence of a destructive interference between the
matrix elements of the single nucleon currents, which
generates an empirically contraindicated minimum. In
the case of the charge form factors the relatively large exchange current contribution is a consequence of the diffraction minimum in the form factor, which makes the
small exchange current contributions anomalously visible. For larger nuclei the exchange current contributions
to charge form factors are less visible, because of the
dominant shell structure effects [16, 17]. Three-nucleon
exchange currents give but minor numerical contributions [18–20].
The Axial Exchange Current
The exchange current contributions to the axial current of a nucleus, which are associated with the nucleon–
nucleon interaction, are smaller by a factor (v/c)2 than
the corresponding electromagnetic exchange current operators, and therefore numerically of less significance
[21]. The main term of the axial exchange current operator is due to excitation of virtual intermediate ∆(1232)
resonances.
Despite its small nuclear matrix elements the axial
exchange current is required for a satisfactory description of the Gamow-Teller transition in β-decay of 3H
[22]. This empirically known matrix element may in fact
be used to restrict the parameter uncertainty in the ex-
Nuclear Physics News, Vol. 13, No. 2, 2003
feature article
pression for the axial current operator, which then may
be employed to calculate astrophysically significant βtransition rates in light nuclei with some confidence [23,
24].
The phenomenologically observed “quenching” of
~20% of the Gamow-Teller transition strength in nuclei
[25] has a simple explanation in terms of two- (and
many-) body axial pion exchange currents, with virtual
intermediate ∆ (1232) resonance states [26, 27]. If reduced to the form of an effective single nucleon axial
current by integration of the coordinates of all but one
nucleon over the nucleus, these exchange currents combine into a screening factor for the axial current of the
single nucleon, which may be expressed as
gA
σ τa.
Aa = – _________
1 + gU(k)
Proceedings: WE-Heraeus
3” wide x 2” high
NEW
(6)
Here g is a parameter, which determines the strength of
the ∆-hole interaction, and U(k) is the pion optical potential. Most, if not all [28], of the phenomenological
quenching can be explained by this screening mechanism.
The Axial Charge Operator
In contrast to the case of the axial current operator,
two-nucleon mechanisms give very significant contributions to the axial charge operators of nuclei, which are
as large or larger than the contribution of the sum of the
matrix elements of single nucleon axial charge operators.
The role of the axial exchange charge operator is visible
throughout the periodic table in the values of first forbidden β-transitions, which are enhanced by up to
~100% over the corresponding single nucleon values
[29–31].
The axial charge operator has a main pion exchange
component, the operator structure of which is fixed by
chiral dynamics and therefore is model independent [32].
This by itself can explain about half of the empirically
found enhancement that is seen in first forbidden β-transition rates. The remaining half of the enhancement is
due to the axial exchange charge operator, which is associated with the short-range part of the nucleon–
nucleon interaction [33, 34].
The main short-range components of the nucleon–
nucleon interaction may be described as scalar and vector exchange mechanisms. The former represents the attractive scalar field that is felt by a nuclear nucleon. The
effect of this may be taken into account as a shift of the
nucleon mass: m → m + <v>, where <v> is the average
scalar field. As the scalar field is attractive <v> < 0, the
nucleon mass in a nucleus is reduced from its free value.
The enhancement of the axial charge due to this scalar
field may be taken into account by implementation of
this mass shift in the denominator of axial charge oper τa/m.
ator of a single nucleon: – gAσ . p
The spontaneously broken approximate chiral symmetry of the strong interaction implies that pions couple
to hadrons and nuclei through their axial currents. Nuclear pion absorption and production reactions are
therefore governed by the matrix elements of the axial
current density operator and require a realistic description of the latter, with inclusion of the exchange current
terms.
The cross section for the reaction pp → ppπ0 near
threshold [35, 36] provides an intriguing example of
this. In this reaction the pion exchange contribution is
suppressed because of the symmetric isospin state of the
two protons. As a consequence, the short-range exchange current contribution to the axial charge operator,
which is due to the short-range component of the nucleon–nucleon interaction, plays an exceptionally significant role in the description of the empirical cross section
[37], which is much larger than what the single nucleon
axial charge operator would suggest [38]. Part of this enhancement of the single nucleon term may be described
as the nucleon mass shift by the effective scalar field
mentioned above.
Nuclear QCD
Among the goals of theoretical nuclear physics is anchoring the phenomenologically fairly satisfactory nuclear force–based description of nuclear structure and re-
Vol. 13, No. 2, 2003, Nuclear Physics News
33
feature article
actions in quantum chromodynamics (QCD). To the several different approaches to this issue belongs the large
color limit of QCD, which allows a series expansion in
1/Nc with only odd powers of 1/Nc (Nc is the number of
“colors”) of hadronic observables. The first few terms in
this series have been shown to describe the key phenomenological features of the structure of the baryons [39].
The main central and tensor force components of the
nucleon–nucleon interaction are of order Nc, while the
smaller spin-orbit interaction components are of order
1/Nc.
The leading terms of such a 1/Nc expansion of the
components of the nucleon–nucleon interaction correspond to the strongly coupled meson exchange terms in
phenomenological boson exchange models for the interaction [40]. If applied to the exchange current operators
that are associated with the interaction, the corresponding series expansion in 1/Nc also reveals that the leading terms correspond well to those exchange current operators, which have been found phenomenologically to
be significant [41].
The large Nc dependence of the nuclear interaction
operators is most readily derived by referring to a quark
model description, with Nc quark colors. The attempt to
create a quark-based description of nuclear interactions
and currents has, in fact, a good pedigree [42], although
the results have hitherto mainly been of qualitative
value. In quantitative calculations of the electromagnetic
and weak transition rates of mesons and baryons, there
is no avoiding the concept of exchange currents, once realistic hyperfine interaction models for quarks are employed [43–45].
Exchange Currents in Mesons
Mesons with one or more heavy flavor quark and
antiquark constituents form bound states with many
analogies to the bound few-nucleon systems. To these
analogies belong that both their M1 transition rates and
pionic transition rates cannot be satisfactorily described
by the matrix elements of single quark currents alone.
A prime example of this is the M1 decay rate of
charmonium, J/ψ → ηc γ, which is overestimated by factors 3–4 in the quark model. By taking into account the
exchange current that is associated with the scalar confinement interaction in addition to relativistic corrections, this overprediction may be avoided [46–48]. The
exchange current term may in this case also be viewed as
a consequence of an effective shift of the charm quark
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34
Nuclear Physics News, Vol. 13, No. 2, 2003
feature article
mass mc , but in this case upwards to mc + cr, where c is
the string tension of the confining interaction (c ~ 1
GeV/fm).
The pion decay of the D1(2420) charm meson, which
decays to D*π, provides a good illustration of the importance of the two-quark contribution to the axial
charge operator of mesons. The empirical width for this
is overestimated by factors 3–4, without inclusion of the
axial exchange charge operator, which is associated with
the scalar confining interaction [49]. This operator may
either be derived as a pair current term that is associated
with the negative energy pole in the nucleon propagator
[50], or more directly by again making the shift mc → mc
+ mcr in the charm quark mass.
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Vol. 13, No. 2, 2003, Nuclear Physics News
DAN-OLOF RISKA
35
facilities and methods
CYCLONE44 and ARES:
New tools for nuclear astrophysics
Introduction
Nuclear astrophysics was born
several decades ago, “to understand
energy generation in the sun and
other stars at all stages of stellar evolution, and the nuclear processes
which produced the relative abundances of the elements and their isotopes” [1]. Experimental nuclear astrophysics aims at the measurement
of such nuclear processes, using nuclear techniques. Nuclear reactions
in stars are organized in chains (e.g.,
the p-p chain in the sun) or in cycles
(e.g., the CNO cycle in stars heavier
than the sun). Since its origin, in the
1930s, much work has been devoted
to the measurement of reactions occurring in stable and quiet environments, i.e., stars on the main sequence, like the sun, people trying to
reach to the energy region effective
in stars, the so-called Gamow energy.* In the 1980s, the interest of
astrophysicists turned to explosive
stellar environments [2], like novae,
supernovae, and X-ray bursts, in
which reaction cycles are very different from what they are in quiet
environments; in the former, indeed,
reactions involving radioactive nuclides take an active place, the socalled hydrogen or helium explosive
burning. This posed a great challenge
to experimental nuclear astrophysics:
these light radioactive nuclides had
lifetimes too short to become targets,
and as a consequence they had to be
*Only recently was a measurement of
such a nuclear reaction performed in the
Gamow region, in the LUNA facility: the
reaction was 3He(3He,2p)4He.
36
used as beams, reactions on H or He
being performed in inverse kinematics, and detectors being potentially
wrapped in the beam-induced background. To be fair, one should add
that cross sections are larger by several orders of magnitude compared
to the quiet burning situation.
The Cyclotron Research Center
and Nuclear Physics group in Louvain-la-Neuve were involved from
the beginning in developments leading eventually to the obtention of a
first intense and pure beam of 13N
ions in 1989. The presence of twoaccelerators, i.e., two cyclotrons, in
neighboring vaults was an incentive
to use the ISOL-method: a first cyclotron (CYCLONE30) accelerated
a high-intensity (up to 500 µA) lowenergy (30 MeV) proton beam that
was stopped in a 13C block. 13N
atoms from the 13C(p,n)13N reaction
were extracted from the 13C by a nitrogen gas flushing and transferred
to an ECR ion source where they
were ionized to the 1+ state. A second cyclotron (CYCLONE110) accelerated the 13N1+ ions to the requested energy, i.e., 0.6 MeV/amu. A
detailed description of the set-up can
be found in [3]. Three Belgian universities, Brussels, Leuven, and Louvain-la-Neuve, had joined efforts in
this undertaking. Subsequently,
other beams were developed [4] and
several reactions of astrophysical interest were studied in the 1990s in
Louvain-la-Neuve, by groups from
Europe and the U.S. Let us mention
13N(p,γ) 14O [5], 18F(p,α) 15O [6];
19Ne(p,γ) 20Na [7], 19Ne(α,p) 21Na
[8]. Sophisticated detection set-ups
had to be installed accordingly.
Nuclear Physics News, Vol. 13, No. 2, 2003
Some limitations of the present
installation were noticed early: (i)
the 30 MeV proton cyclotron offered little flexibility as a production
machine; other particle–target pairs
would often lead to larger production and/or easier conditions (for example, 14O could be better produced
by the 12C(3He,n) reaction induced
by a high-energy 3He beam from
CYCLONE110 than by the 14N(p,n)
reaction induced by the 30 MeV
proton beam from CYCLONE30);
(ii) in particular for the measurement
of radiative capture reactions, the direct detection of the final heavy ion
products appeared more attractive in
terms of efficiency [9] ; (iii) the performance of CYCLONE110 as a
post-accelerator could be improved
by a more modern and dedicated accelerator. The CYCLONE44–ARES
pair described in the following is a
combined answer to the abovequoted shortcomings.
CYCLONE44
CYCLONE44 is a compact isochronous cyclotron, designed and
built by the Cyclotron Research Center team. Its main characteristics are
summarized in Table 1. Whereas
CYCLONE110, built in the early
1970s, was originally a low-mass
ion accelerator (p,d,α)-modified subsequently to accelerate heavy ions
(from Li to Xe) for nuclear physics
and nuclear astrophysics, CYCLONE44 is dedicated to accelerating low-mass heavy ions in the energy range relevant to nuclear
astrophysics. The energy range accessible in CYCLONE44 (0.2–0.8
MeV/amu) is typical of astrophysical
facilities and methods
Table 1. Main characteristics of CYCLONE44.
Energy constant K (MeV)
Energy range (MeV/amu)
M/Q range
Max average field (T)
Extraction radius (m)
Frequency range (MHz)
Injection
Extraction
Rejection factor at ∆M/M = 2.10-4
processes like the hot p-p chain or
the hot CNO-cycles and the escape
of the latter to the rp-process [10].
In the ISOL or “two-accelerator” scheme, the radioactive ion
beam has to be separated from its
stable isobar and accelerated. Using
a cyclotron as the post-accelerator
allows us to combine the two functions together. The expected major
improvements with respect to CYCLONE110 are a higher acceleration efficiency (typically 3 % in CYCLONE110) and an improved mass
separation ∆M/M. Both parameters
are in fact very difficult to optimize
simultaneously, as they appear antagonist. To obtain a high rejection
of very near isobaric contaminants,
the acceleration in CYCLONE44 is
performed on high harmonic modes
(5, 6, and 8), and low acceleration
voltages are used. This implies a
small energy gain per turn and a
large number of turns. Typically, for
a ∆M/M of 2.10-4 (e.g., 19F—19Ne) a
rejection factor of 105 was achieved.
Figure 1 represents the entire setup including the CYCLONE30 production machine, the ECR source,
the 30-m-long transport line, and finally the CYCLONE44 post-accelerator [11].
The first radioactive beam accelerated in CYCLONE44 was a 19Ne3+
beam of 9.6 MeV. After extraction,
the beam was focused by a quadru-
44
0.2–0.8
4–14
1.54
0.63
13.3–18.5
axial, spiral inflector
electrostatic deflector
105
pole doublet, and an intensity of 5 x
109 pps was measured in a Faraday
cup located two meters beyond the
doublet. The acceleration efficiencies
of CYCLONE44 are of the order of
10%. The presence of the ARES separator has allowed us to measure the
beam purity precisely on-line: the
19Ne3+ beam was in fact transmitted
through ARES, until the ∆E-E detector at the end. A particle identification was performed, yielding a 19Fover-19Ne ratio of 2.5 10-3. The
beam stability was found to be very
satisfactory, over a one-day period.
In addition to the ARES line, another beam line from CYCLONE44
will be installed. A large reaction
chamber was constructed and will be
placed in the new line; large-size
charged-particle detectors of the
LEDA type [12] will allow classical
measurements of the (p,p), (p,α),
(α,α), and (α,p) type, with increased
precision.
ARES
In a (p,γ) or (α,γ) reaction in inverse kinematics, beam ions and ions
produced in the reaction (hereafter
the product ions) are contained in a
narrow cone beyond the target. Due
to the low momentum carried out by
the α-ray, beam and product ions
have the same momentum in first
approximation, and in addition,
both species are found in different
charge states, the most abundant
representing at least 30% of the
total. The above considerations explain the ARES set-up (Figure 2): in
a first step, a dipole magnet selects
the momentum-over-charge ratio of
the product ions corresponding to
the most abundant charge state, part
of the beam ions being transmitted
Figure 1. General layout of the facility.
Vol. 13, No. 2, 2003, Nuclear Physics News
37
facilities and methods
sique Nucléaire in Louvain-la-Neuve,
who have contributed to all stages of
the project reported here. This work
has been partially supported by the
Belgian Programmes P4/18 and
P5/07 on Interuniversity Attraction
Poles of the Belgian State, Federal
Services for Scientific, Technical and
Cultural Affairs. I also thank the
Fonds National de la Recherche Scientifique, Belgium, of which I am a
Research Director.
References
Figure 2. CYCLONE44 and the ARES Spectrometer.
as well. In a second step, a 1-m-long
Wien filter (or velocity filter) selects
product ions and deflects beam ions.
In a third step, a ∆E-E counter performs a particle identification to reject the remaining beam ions.
The 19F(p,γ)20Ne reaction was
chosen for the first test of ARES.
This reaction has a strong resonance
of strength ωγ = 1.6 eV for an energy
ER = 635 keV in the cm. The fraction
of the 19F beam ions surviving the
second step of ARES was typically 3
x 10-7 of the initial intensity, and at
the same time the fraction of the
20Ne product ions reaching the ∆E-E
counter was 4%. It is important to
point out that not only this percentage but also the energy distribution
of the 20Ne ions were reproduced by
a simulation code. More details concerning these and other measurements performed during these tests
(e.g., distribution of charge states,
beam energy, ∆E gas detector performance, . . .) can be found in a recent publication [13].
Some words should be added regarding the type of targets used for
(p,γ) or (α,γ) measurements. For
(p,γ) reactions, solid polyethylene
foils have been produced in our lab
38
in the last decade, from 20 µgr/cm2
to 500 µg/cm2. For (α,γ) reactions,
helium ions from an ECR source
were implanted in thin (50 µg/cm2)
Al foils. The helium bulk density
was deduced from RBS measurements at our van de Graaff accelerator, while the He uniformity versus
thickness was measured by the
ERDA method using 19F beams from
CYCLONE44. Typical areal densities of 2 x 1017 at/cm2 were obtained. This work is described in a
recent publication [14].
Conclusion
CYCLONE44 is now ready for
operation in nuclear physics and nuclear astrophysics. With respect to
CYCLONE 110, increased intensities of radioactive beams will be obtained on target, and an improved
mass separation will result in a still
better beam purity. The ARES separator was coupled to CYCLONE44.
Extensive measurements were performed on a test reaction with stable
beam before reactions induced by radioactive ions will be studied.
I wish to thank all collaborators
from the Centre de Recherches du
Cyclotron and the Institut de Phy-
Nuclear Physics News, Vol. 13, No. 2, 2003
1. W. A. Fowler, Rev. Mod. Phys. 56
(1984) 149.
2. J. Truran, Ann. Rev. Nuc. Part. 34
(1984) 53.
3. J. Vervier, Progr. Part. Nucl. Phys.
37 (1996) 435.
4. See the present list of the available
beams in http://www.cyc.ucl.ac.be
5. P. Decrock et al., Phys. Rev. Lett. 67
(1991) 808.
6. R. Coszach et al., Phys. Lett. B353
(1995) 184.
7. R. Page et al., Phys. Rev. Lett. 73
(1994) 3066.
8. D. Groombridge et al., Phys. Rev.
C66 (2002) 055802.
9. P. Leleux, in Nuclear Astrophysics,
Int. Workshop on Gross Properties
of Nuclei and Nuclear Excitations,
M. Buballa et al., editors (1998)
356.
10. M. Wiescher et al., J. Phys. G: Nucl.
Part. Phys. 25 (1998) R133.
11. G. Ryckewaert et al., Nucl. Phys.
A701 (2002) 323c.
12. T. Davinson et al., Nucl. Instr. Meth.
Phys. Res. A454 (2000) 350.
13. M. Couder et al., submitted to Nucl.
Instr. Meth. Phys. Res.
14. F. Vanderbist et al., Nucl. Instr.
Meth. B197 (2002) 165.
PIERRE LELEUX
Université Catholique de Louvain
Louvain-la-Neuve, Belgium
news from NuPECC
NuPECC Town Meeting
European nuclear physicists united to discuss their future. (Photo: G. Otto, GSI Darmstadt)
The NuPECC town meeting
(January 30 to February 1) was well
attended with about 300 participants. The meeting was organized in
several sections, namely “Facilities,”
“Nuclear Structure,” “Phases of Nuclear Matter,” “QCD,” “Nuclei in
the Universe,” “Fundamental Interactions,” and “Applications,” where
the corresponding NuPECC report
drafts were presented and discussed.
At the end of the meeting a two-hour
discussion session was held, aimed
at setting NuPECC priorities.
Facilities
In the “Facilities” section, Henning (GSI) gave an overview of the
GSI plans for the coming 10 years.
Both the physics motivation and the
proposed facility were laid out. He
explained that the facility can run
with a 300% duty cycle by operating
the different accelerating structures
in parallel. The EURISOL project (a
next generation ISOL facility) was
discussed by Vervier. This project is
now in its design stage. The projected 1 GeV high-intensity proton
driver would also be of interest for
other applications such as transmutation of nuclear waste and for studies under the chapter of “Fundamental Interactions.” The SPIRAL
facility was presented by Mittig
(GANIL) including the plans for SPIRAL II, which should be in operation by 2008. The REX facility at
Isolde (CERN) was discussed by
Butler (CERN). In the near future
(<~5y) the experimental hall will be
extended and the beam energy increased to 4.3 MeV/u. For the longer
term future, a further increase in energy is foreseen and possibly the use
of anti-protons. Pisent (Legnaro) discussed the developments at Legnaro,
in particular the construction of superconducting LINAC modules for a
proton driver and the research on a
converter target for intense neutron
beams. The upgrade (to 1.5 GeV) for
MAMI-C was discussed by Beck
(Mainz). The physics goals range
from polarizability of the nucleon
to open strangeness production.
Habs (Munich) suggested a completely new design for a radioactive
beam facility based on laser acceleration.
Vol. 13, No. 2, 2003, Nuclear Physics News
39
news from NuPECC
In the discussion session Henning emphasized that the new GSI
facility will be a European facility,
open to participation from other
countries.
Nuclear Structure
The theory part of the draft report on nuclear structure was presented by Dobaczewski (Warsaw).
Due to the enormous advance in
computer capabilities, much progress
has been made in recent years in spite
of a lack of manpower. The results of
mean-field and shell-model calculations are now approaching each
other. Van Duppen (Leuven) discussed the experimental priorities of
the nuclear structure community.
ISOL and in-flight (IF) facilities offer
a nice complementarity. The recommendation of this working group
was therefore to give full support to
GSI (as an IF facility) and to EURISOL.
The plans for the AGATA detector were discussed by Krücken (Munich), where the high angular resolution allows for superior corrections
for Doppler shifts, thus allowing for
high-resolution spectroscopy. In the
other short contributions, Azaiez
(Strassbourg) expressed the continued need for stable beam facilities,
Schmidt (GSI) showed that interesting nuclear structure information is
contained in level densities, and
Pochodzalla (Heidelberg) indicated
the interesting aspects of the structure of hyper nuclei. Mavrommatis
(Athens) showed the use of neural
networks in applications of multiparameter models.
In the lively discussion session
several additional aspects of nuclear
structure were brought to the attention of the writing committee, in
particular ab initio approaches.
Schutz (Nantes/CERN), representing the writing committee on
40
“Phases of Nuclear Matter,” showed
the progress made in investigating
the liquid–gas phase transition at intermediate energies as well as the
chiral and the hadron–quark–matter
phase transition at ultra-relativistic
energies. In subsequent presentations some of the topics were highlighted. Senger (GSI) presented the
importance of vector mesons as
probes of the early high-density
phase. Trautman (GSI) emphasized
that to improve our understanding
of the liquid–gas phase transition,
quantitative comparisons should be
made with molecular-dynamics
models. Bougault (Caen), Rivet
(Orsay), and Alba (Catania) showed
different ways in which level densities as accessed in fragmentation reactions yield information on the
shell structure at lower energies and
the liquid–gas phase transition.
The community expressed as top
priorities:
• exploiting existing facilities,
• timely completion and operation of ALICE,
• construction of CBM.
QCD
Weise (Trento/Munich) presented the recommendations and
physics of “QCD” in a very exciting
presentation. The important open
questions in the field concern: (i) the
“characteristic” mass gap, 1 GeV,
separating the pion and other excitations; (ii) color confinement; (iii)
spontaneous symmetry breaking.
Theoretically three approaches
to QCD can be distinguished: (i) perturbative QCD which applies at high
momenta where probes sense physics
at a length scale of a tenth fermi; (ii)
effective field theory approaches
which apply at extremely low energies where the physics is dominated
by Goldstone bosons and chiral
Nuclear Physics News, Vol. 13, No. 2, 2003
symmetry and quantitative predictions can be made; (iii) lattice QCD,
which is a rapidly developing field.
Experimentally, the important
observables are: (i) Generalized parton distribution (GPD) functions,
which measure quark–quark correlations. These GPDs are fundamental to the understanding of the quark
structure of the nucleon. For example, one may extract from these the
orbital and spin contributions to the
total angular momentum of the nucleon. (ii) Glue-ball and hybrid
states, which are closely related to
physics of confinement. Probably
pure glue-ball states do not exist
_
since they will be mixed with qq states. Hybrid states, i.e., states with
exotic JCP, may form a more clean
probe. (iii) Charmonium states, in
particular pseudo-scalar eta-like
states, are also related to the physics
of confinement. (iv) Chiral dynamics
and spontaneous symmetry breaking
are fundamental to the field. Quantitative predictions should be tested.
(v) Propagation of mesons in nuclear
matter and transparency which is related to questions of how hadrons
are formed in QCD.
It will be important to use the
experience in nuclear many-body
theory. The spin crisis, for example,
stating that only 30% of spin is carried by the quarks, comes as no surprise to strong-interaction manybody physics.
Recommendations are:
• Maintain and expand, where
necessary, adequate support
for theoretical work, including
lattice QCD calculations.
• Fully exploit present facilities.
• Strongly support HESR at GSI.
• Develop a high luminosity facility (ELFE).
• Engage in worldwide collaborations.
news from NuPECC
Metag (Giessen) presented the
physics accessed by the PANDA
project at HESR. Explicitly, the relation was shown with several of the
topics (glue balls, hybrids, charmonium spectroscopy, GPD) mentioned
by Weise. At DEAR (DaΦne Exotic
Atom Research), exotic atoms such as
kaonic deuterium will be measured.
This yields direct information on
strangeness content of the nucleon as
discussed by Guaraldo (Frascati).
Marton (Vienna) presented the pionic-atom programme at PSI. The
high precision data on pion scattering lengths form a stringent test of
chiral symmetry. Similar physics can
also be accessed through deeply
bound pionic and kaonic states as
discussed by Kienle (Munich).
In the discussion on the physics
issues, questions were asked ranging
from the model (in)dependence of
GPDs to the structure of the confining potential. In the discussion on
recommendations, it was emphasized that the priorities have not
been ordered.
Nuclei in the Universe
Langanke (Aarhus) presented
the case for the astrophysics aspect
of nuclear physics and named a few
characteristic examples. (i) The recent results from the Sudbury neutrino observatory (SNO) confirm
solar model predictions but are,
however, sensitive to nuclear reaction rates. (ii) The life time of the
isomeric state in 180Ta depends on
the temperature of the ambient stellar medium and can thus be used to
learn about stellar environment. (iii)
Neutrino capture of nuclei plays a
role beside the capture of protons in
the dynamics of supernova explosions. (iv) In supernova explosions
the equation-of-state of the core is
important. (v) The r-process may
occur in different stellar environments resulting in different relative
abundances. To determine the stellar
history from the abundances, one
needs masses, half-lives, and reaction rates. (vi) In type II supernovae,
electron capture after the explosive
burning occurs, depleting the electron density and thus the electron
pressure which affects flame propagation. (vii) Far away from the valley of stability, models show increasing discrepancies with data on
masses.
Recommendations for facilities
are:
• need GSI,
• need small facilities,
• need 5 ~ MeV underground
machine, and
• need meeting place (ECT*) for
theorists and experimentalists.
The astrophysics perspective was
presented by Diehl (Garching). It is,
for example, assumed that we know
supernova explosions and can use
them as standard candles for determining distances. For this, more details of flame evolution are necessary. Present models do not account
for the evolution of known supernovas. New probes are also investigated, such as emitted positrons.
Several space missions are planned
and one needs the nuclear physics
backing for proper interpretation of
the data.
In the following short contributions, Rolfs (Bochum) presented the
case for the LUNAR underground
laboratory. Angulo (Louvain la
Neuve) presented the need for lowenergy radioactive nuclear beams.
Kratz (Mainz) stressed that ISOL facilities are essential to obtain masses
and β-decay probabilities, while
Sümmerer (GSI) showed that storage
rings also are suited for this purpose.
At the n-TOF facility at CERN, neutron capture rates, important for the
s-process, are measured as discussed
by Mengoni (CERN). Aliotta (Edinburgh) stressed that electron-screening effects are important in extrapolating astrophysical cross sections to
very low energies.
Fundamental Interactions
A spirited presentation of the report on “Fundamental symmetries”
was made by Jungmann (KVI). Several different projects fall under this
topic. (i) Neutrino disappearance
has been observed which can be interpreted as neutrino oscillations.
This has put the question regarding
the structure of neutrinos (Majorana
vs. Dirac) center stage. Another aspect concerns the unitarity of the
CKM mass matrix, which seems to
be violated at the level of two standard deviations. This calls for further measurements of neutrino-less
double-beta decay and the investigation of rare decay branches of
mesons and baryons. (ii) CP-violating static moments (electric dipole
moments) are searched for. Parityviolating moments (anapole moments) have been determined. CP- or
T-violation can also be accessed in
certain triple-correlation functions
in beta decay. These measurements
can best be done in traps. CPT-violation has been addressed with a new
interaction-based approach. These
symmetry violations can be tested in
several high-precision experiments
including experiments with anti-hydrogen. (iii) It has been suggested
that the fine-structure constant is
time dependent. This can be accessed
through the Re/Os ratio in meteorites.
Recommendations for facilities
are:
Vol. 13, No. 2, 2003, Nuclear Physics News
41
news from NuPECC
• intense neutrino beam,
• (ultra) cold neutrons (both
could be driven by an intense 1
GeV proton beam),
• radioactive beams and improved trapping facilities,
• underground facility.
Volpe (Orsay) showed that nuclear structure is crucial for the interpretation of the experiments. Heil
(Mainz) showed the need for an intense source of cold neutrons for
measuring permanent dipole moments. Maas (Mainz) has drawn attention to different interesting aspects of parity-violating electron
scattering. Pachucki (Warsaw) discussed precise measurements in
QED.
In the discussion session it was
argued that QED effects should be
treated non-perturbatively because
42
of the high accuracy needed for the
interpretation of parity violation experiments on nuclei.
Applications
Beautiful examples of applications of nuclear physics techniques
such as PIXE and RBS were shown
by Mandò (Firenze). Calligaro
(Paris) even took us on an on-line
visit to the facility at the Louvre during his presentation. Other examples
such as applications of radiotherapy
and AMS were discussed in the contributions of Cantone (Milano).
Leray (Saclay) gave an overview of
the status and future plans of the
HINDAS project which aims at
forming a comprehensive database
of nuclear cross sections which are
important for applications such as
nuclear-waste transmutation. Patelli
Nuclear Physics News, Vol. 13, No. 2, 2003
(Legnaro) discussed the mutual benefits of interactions between nuclear
physicists and material scientists.
Cinausero reported on the status of
human de-mining based on nuclear
techniques.
General
James Symons gave an eloquent
overview of the NSAC long-range
plan, both its historical background
and the present recommendations.
The meeting was concluded by a
lively and constructive discussion regarding the priorities of the different
projects. The assigned two hours
were not enough to complete the discussion, but consensus was met on
certain priorities.
OLAF SCHOLTEN
KVI Groningen
news from NuPECC
Integrated Infrastructure Initiatives
The coordinators for the various activities within EURONS at their meeting at GSI Darmstadt on March 31, 2003.
(Photo: A. Zschau, GSI Darmstadt)
Under broad participation of the
community, two proposals of Integrated Infrastructure Initiatives are
being prepared for the Sixth Framework Programme of the European
Commission. The two proposals,
EURONS (European Nuclear Structure Integrated Infrastructure Initiative) and I3HP (Integrated Infrastructure Initative in Hadron Physics),
cover the main fields of research
using Major Research Infrastructures in nuclear physics.
General information can be
found on the NuPECC website,
• http://www.nupecc.org/iii
And more information on I3HP
can be found on
• http://www.infn.it/eu/i3hp/
More detailed information on
EURONS can be found on
• http://www-new.gsi.de/
informationen/EURONS/
index_e.html
Vol. 13, No. 2, 2003, Nuclear Physics News
43
calendar
2003
June 9–21
Seville, Spain, Exotic Nuclear
Physics (VIII Hispalensis International
Summer School). Contact: J. M. Arias
and M. Lozano, University of Sevilla,
Spain. E-mail: [email protected]
Web: www.cica.es/aliens/
dfamnus/oromana/
June 10–13
Paris France. International Conference on Collective Motion in Nuclei
under Extreme Conditions (COMEX
1). Contact: Valerie Frois, Secretary of
COMEX 1, Institut de Physique Nucleaire, 91406 Orsay Cedex, France.
E-mail: [email protected]. Fax:
1-6915-4475. Tel.: 1-6915-7749.
Web: http://ipnweb.in2p3.fr/
comex1/comex1.html
June 17–21
Moscow, Russia. VIII International Conference on Nucleus-Nucleus
Collisions. Contact: Yu. Ts. Oganessian or R. Kalpakchieva, Flerov Laboratory of Nuclear Reactions, JINR,
141980 Dubna, Moscow region, Russia. E-mail: [email protected]. Tel: 709621-62151. Fax: 7-09621-65083.
Web: http://www.nn2003.ru/
June 25–28
McGill University, Montreal,
Canada. Topics in Heavy Ion Collisions: An international conference on
the physics of hot and dense strongly
interacting matter. Contact: HIC03@
physics.mcgill.ca
Web: http://www.
physics.mcgill.ca/HIC03/
July 13–19
August 25–29
October 7–12
Vrnjacka Banja, Yugoslavia. Fifth
General Conference of the Balkan
Physical Union. Contact: E-mail:
[email protected]. Fax: +381 11
3162190. Tel. +381 11 31620099, +381
11 3160598.
Web: http://www.phy.bg.ac.yu
Santorini, Greece. Electromagnetic
Interactions with Nucleons and Nuclei.
EuroConference on Hadron Structure
Viewed with Electromagnetic Probes.
Web: http://www.esf.org/
euresco/03/pc03117
October 14–17
August 31–September 7
Krzyze, Poland, XXVIII Mazurian
Lakes Conference on Physics. Atomic
Nucleus as a Laboratory for Fundamental Processes. Contact: Katarzyna
Delegacz, The Andrzej Soltan Institute
for Nuclear Studies, Hoza 69, 00-681
Warsaw, Poland. Fax: (48-22) 7793481.
Tel.: (48-22) 7180583.
Web: http://zfjavs.fuw.edu.pl/
mazurian/mazurian.html
September 2–6
Dubna, Russia. Nuclear Structure
and Related Topics. Contact: R. Jolos,
V. Voronov, Bogoliubov Laboratory of
Theoretical Physics, JINR, 141980
Dubna, Moscow region, Russia. Email: [email protected]. Fax: (709621) 65084.
Web: http://thsun1.jinr.ru/~nsrt03
September 22–26
Riviera, Zlatny Piasatsi (Golden
Sand), Bulgaria. Perspectives of Life
Sciences Research at Nuclear Centers.
Contact: Dr. M. V. Frontasyeva, Email: [email protected]
September 24–27
Pavia, Italy. Sixth Workshop on
“Electromagnetically Induced TwoHadron Emission.” Contact: 2hconf@
pv.infn.it
Web: http://isnwww.
pv.infn.it/~2hconf/
Crete, Greece. International
Conference on The Labyrinth in Nuclear Structure. Contact: kalfas@inp.
demokritos.gr
Web: http://www.INP.demokritos.
gr/~kalfas/CLNS/bulletin.html
44
Nuclear Physics News, Vol. 13, No. 2, 2003
Grenoble, France. International
Workshop on Probing Nuclei via the
(e,e’p) Reaction. Contact: EEP03@isn.
in2p3.fr
Web: http://isnwww.in2p3.fr/
EEP03
November 16–20
Napa Valley, California, USA. 2nd
International Conference on the
Chemistry and Physics of the Transactinide Elements (TAN 03). Contact:
Dianna Jacobs, e-mail: djacobs@
lbl.gov. Fax: +1-510-486-7444. Tel.:
+1-510-486-7535.
Web: http://tan03.lbl.gov
November 19–23
Kurokawa Village, Niigata, Japan.
“A New Era of Nuclear Structure
Physics” (NENS03). Contact: Yasuyuki Suzuki or Susumu Ohya, nens03@
nt.sc.niigata-u.ac.jp
Web: http://ntweb.
sc.niigata-u.ac.jp/nens03/
November 24–29
Nara, Japan. The 8th International
Conference on Clustering Aspects of
Nuclear Structure and Dynamics. Email: [email protected]
Web: http://ribfwww.riken.go.jp/
cluster8/