Dead-line for
submission :
June 14th, 2010
Day 1 SPIRAL2 Phase 1
Experiment Project
Title: Evolution of the Z=40 sub-shell while approaching the π -dripline /
Laser spectroscopy of neutron-deficient zirconium isotopes with SPIRAL2
Spokespersons (if several, please use capital letters to indicate the name of the contact person):
B. BASTIN
Address of the contact person:
GANIL, bvd Becquerel, BP 55027, 14076 CAEN Cedex 05, France
Phone: +33(0)2.31.45.49.92
Fax: +33(0)2.31.45.47.20
E-mail: [email protected]
Other Participants or Organisations:
CEA-DAM (B. Laurent), CERN (V. Fedosseev, B. Marsh, M. Seliverstov), CIMAP (P. Camy, J.Y. Chesnel, D.
Hennecart), GANIL (M. Amthor, P. Delahaye, N. Lecesne, B. Osmond, M.G Saint-Laurent, H. Savajols, C. Stodel, J.C.
Thomas), IKS-KULeuven (T. Cocolios, I. Darby, R. Ferrer, I. Koudriavtseve, M. Huyse, G. Neyens, M. Rajabali, P. Van
Duppen) IPN Orsay (S. Franchoo, F. Le Blanc), LPCCAEN (X. Fléchard), Schuster laboratory-University of
Manchester (J Billows, P. Campbell)
Brief summary of the physics goal (detailed description and counting rates should be given on separate
pages) max. 1/2 a page:
With SPIRAL2, probing the structural properties of N = Z nuclei from 80Zr to 100Sn will become accessible. In the
mass region A≈80, nuclei with N≈Z exhibit dramatic changes in shape with the addition or subtraction of only few
nucleons. Proton and neutron shell effects act coherently and rich structures arise from the coexistence/competition of
different shapes, providing a stringent test for nuclear models [1]. These nuclei give also the opportunity to study the
effects of the neutron-proton pairing correlations, as the protons and the neutrons are expected to occupy identical valence
orbitals, providing maximum overlap of the neutron and proton wave functions. Moreover, the study of the N=Z nuclei
around mass 80 is motivated by their contribution in the astrophysical rapid proton-capture process : they constitute
waiting points of this process because of their long half-live (a few seconds) and because proton capture is expected to be
hindered, which inhibits its flow [2-3].
Of particular interest is the 80Zr nucleus with N=Z=40. It appears to be one of the most deformed nuclei in this
mass region and even known in nature, with an estimated quadrupole deformation of beta~0.4 (from the Grodzin’s
formula) [4-5]. Observations of such large ground-state deformations for Z(or N)=40 nuclei [6-9] call for a natural
breaking of spherical shell closures at Z=N=40. Therefore, additional experimental data on this nucleus and its isotopic
chain could help to further elucidate the role of the single-particle behaviour in the shell model, and also that of the
effective interactions.
Currently, the refractory nature of zirconium isotopes prevents their efficient production at conventional hightemperature isotope separator facilities [6-7]. With SPIRAL2, and more specifically the S3 spectrometer, laser spectroscopy
measurements on neutron-deficient zirconium fusion evaporation products could be performed for instance by using the
recently developed gas catcher laser ion source system. By coupling the gas cell to the focal plane of S3, the reaction
products can be efficiently thermalized and laser ionized. Furthermore, using an RF structure behind the gas cell would
allow to perform laser spectroscopy measurements in the so-called ‘Laser Ion Source Trap (LIST)’ mode [10]. The latter
would results in a better resolution as no pressure broadening is expected. With the intensities expected to be delivered by
the SPIRAL heavy ion linear accelerator (LINAC), the fusion-evaporation reaction of e.g. an argon beam on a chromium
target should lead to more than thousands ions per second. Thus the nuclear shape investigations using in-source laser
spectroscopy of these nuclei will become feasible. We will be able to complete the existing mean square charge radii
measurements for instance [6-7]. The combination of a gas cell with a laser set-up - that brings together rapidity, efficiency
and selectivity - placed after the S3 spectrometer represents a powerful tool for these studies [11] and might furthermore
result in the production of pure short-lived neutron-deficient beams for decay studies (using the S3 tape station after the
high-resolution mass separator).
[1] W. Nazarewicz et al., Nucl. Phys. A 435 (1985) 397, “Microscopic study of the high-spin behaviour in selected A≈80 nuclei”
[2] J.J. Ressler et al., Phys. Rev. Lett. 84 (2000) 2104, “Half-Life measurement for the r-Process waiting point nuclide 80Zr”
[3] A.S. Lalleman et al., Hyp. Int. 132 (2001) 315, “Mass measurements of exotic nuclei around N=Z=40 with CSS2”
[4] S.M. Fischer et al., Phys. Rev. Lett. 87 (2001) 132501, “Observation of delayed alignment in N=Z nuclei 72Kr, 76Sr and 80Zr”
[5] C.J. Lister et al., Phys. Rev. Lett. 59 (1987) 1270, “Gamma radiation from N=Z nucleus 80Zr”
[6] P. Campbell et al., Phys. Rev. Lett. 89 (2002) 082501, “Laser spectroscopy of cooled zirconium fission fragments”
[7] D.H. Forest et al., J. Phys. G: Nucl. Part. Phys. 28 (2002) L63, “Laser spectroscopy of neutron deficient zirconium isotopes”
[8] R. Rodriguez-Guzman et al., preprint sumitted to Phys. Lett. B
[9] L. Gaudefroy et al., Phys. Rev. Lett. 84 (2000) 2104, “Collective structure of the N=40 isotones”
[10] T. Sonoda et al. , Nucl. Instr. Methods B 267 (2009) 2918, “The Laser Ion Source Trap (LIST) coupled to a gas cell catcher”
[11] T.E. Cocolios et al., Phys. Rev. Lett. 103 (2009) 102501, “Magnetic dipole moment of 57,59Cu measured by in-gas-cell laser spectroscopy”
Ion(s)
LINAC
Primary
Beam(s) (see
beam parameter
table at the end
of template)
Energy
(MeV/nucl.)
Intensity
(pµA)
Ar12+
120
100
58
190
130
10
10
36
18+
or Ni
28-30 10+
Si
Total estimated number of beam UTs
(1 UT=8hours): 21 UTs
Number of beam Requested time structure
UT (1UT=8hours)
(if different from
per beam
parameters given in the
attached table)
Δ t(ns):
Beam on:
Beam off:
Approximate time for setting up the apparatus:
5 days : setup the gas cell, the identification and implantation
station / connexion with lasers / setup and test the electronics and
acquisition
Approximate time required for off-beam calibration
and dismounting:
When the experiment might be ready
to run (month, year):
2 days : energy and efficiency calibration, dismounting,
packing…
2014
Beam Line (NFS or S3): S3
Detectors to be used (provide a sketch of the setup):
S3 parameters (for the experiments using the S3 beam line) :
Material
Primary target(s)
If 36Ar beam : 50Cr or 46Ti
If 58Ni beam : 24Mg
If 28-30Si beam : 58-60Ni
Thickness
1 mg/cm2
1 mg/cm2
1 mg/cm2
Stripper(s)
Devices needed
Mark with X
Momentum achromat
Mass separator
Low energy branch
X
X
X
Secondary target
Ancillary detectors
(specify)
Implantation decay station
Gas cell
Other devices (specify)
X
S3 tape station
Laser Ion Source
Setup at achromatic
point
Setup at Mass
separator Focal Plane
Schematic layout of the S3 spectrometer/separator
More information on the S3 spectrometer/separator can be found at:
http://www.ganil.fr/research/developments/spiral2/collaborations.html
For further questions on S3 please contact spokesperson of the collaboration:
[email protected]
Acquisition system (present GANIL or specific one if yes specify):
GANIL acquisition system
Electronics system (type of electronics - provide a reference if possible, estimated number of racks,
necessary electric power, other requirements) and its location (ex. located close to the
detector/spectrometer or in a separate room) :
NIM electronics : one rack close to the gas catcher and one rack in the multipurpose experimental room (close to the
S3 Tape Station)
Security, use of hazardous equipment :
(Radioactive target, liquid nitrogen, explosive gas etc.)
- liquid nitrogen used for the cooling of Ge detectors
- risks during the lasers’ alignment and tuning
Remarks :
See end of document for more details
LINAC beams available for the Day 1 SPIRAL2 Phase 1 experiments*)
Ion(s)
Energy Range
(MeV/nucleon)
Maximum
Intensity
(pµA)
Date of
availability***)
Remarks
NFS beam line;
Intensity with fast
chopper 1/100
NFS beam line;
Intensity with fast
chopper 1/100
NFS beam line;
Intensity with fast
chopper 1/100
1
H1+
20-33
2-10
December 2012
2
H1+
10-20
2-10
December 2012
He2+
10-20
2-10
December 2012
C4+
5-7
≥10**)
February 2013
S3 beam line
O6+
5-7
≥10**)
February 2013
S3 beam line
Ne8+
5-7
≥10**)
February 2013
S3 beam line
Ar14+
4-5
≥10**)
February 2013
S3 beam line
Si10+ or
S
5-7
≥10**)
November 2013
S3 beam line
40
Ca14+
5-7
≥10**)
November 2013
S3 beam line
48
Ca16+
5-7
≥10**)
November 2013
S3 beam line
4-14
≥1**)
November 2013
S3 beam line
4
12
18
22
40
28-30
32-36 12+
58
Ni18+
Remarks:
Beam time structure: acceleration (or bunch) frequency 88 MHz, Δt for each bunch typically
1 ns (depends on beam energy and target position)
*) The parameters indicated in this table are the first and the best approximations that can be
done today. They may be different from those available in reality at the beginning of
operation of SPIRAL2. User’s request of different beams and specifications supported by
recommendations of the Scientific Advisory Committee for the Day 1 SPIRAL2 Phase 1
experiments might be taken into account. The SPIRAL2 project will update the list of
parameters periodically.
**) Based on the order of magnitude of the expected best currents extracted from a high
performance, fully operational, 28 GHz ECR Ion source.
***) These dates assume that: installation of equipment in the NFS and S3 areas can start in
July 2011, commissioning of the LINAC can begin in the first quarter of 2012 and
commissioning of the instrumentation in the S3 and/or NFS halls with the LINAC beam(s)
would begin in September 2012.
Detailed description and counting rates
Evolution of the Z=40 sub-shell while approaching the π-dripline
Laser spectroscopy of neutron-deficient zirconium isotopes with SPIRAL2
1) Physics motivations :
With SPIRAL2, probing the structural properties of N = Z nuclei from 80Zr to 100Sn
will become accessible. In the mass region A≈80, nuclei with N≈Z exhibit dramatic changes
in shape with the addition or subtraction of only few nucleons. For instance, 68Se presents the
characteristics of an oblate shape [1], 72Kr displays the properties of shape coexistence [2] and
78
Sr exhibits the properties of a prolate rotor [3]. In Figure 1, the systematic of the yrast band
of the even–even N = Z nuclei is shown. The ratio of the excitation energy of the first and
second excited states, E(4)/E(2), is also given. This ratio provides a test of the axial
assumption. For example, for 80Zr we obtain E(4)/E(2)=2.86, whereas for a perfectly axial
shape the ratio should be 3.33. Clearly, some nonaxial behaviour is involved in this nucleus,
either through triaxiality or softness to triaxial deformation.
Figure 1 : level schemes evolution through the N=Z line
In this mass region, proton and neutron shell effects act coherently and rich structures
arise from the coexistence/competition of different shapes, providing a stringent test for
nuclear models [4]. These nuclei give also the opportunity to study the effects of the neutronproton pairing correlations, as the protons and the neutrons are expected to occupy identical
valence orbitals, providing maximum overlap of the neutron and proton wave functions.
Moreover, the study of the N=Z nuclei around mass 80 is motivated by their contribution in
the astrophysical rapid proton-capture process : they constitute waiting points of this process
because of their long half-live (a few seconds) and because proton capture is expected to be
hindered, which inhibits its flow [5-6]. From the mass measurement, the doubly binding
energy difference have been extracted and compared with theoretical predictions [7]. The
systematic on δVnp, shows an increase of the residual interaction between the last proton and
the last neutron as N=Z=40 is approached.
Of particular interest is the 80Zr nucleus with N=Z=40. It appears to be one of the
most deformed nuclei in this mass region and even known in nature, with an estimated
quadrupole deformation of beta~0.4 (from the Grodzin’s formula) [8-9]. Observations of such
large ground-state deformations for Z(or N)=40 nuclei [10-13] call for a natural breaking of
spherical shell closures at Z=N=40. As show by Figure 2, the zirconium isotopes provide a
good opportunity to study the microscopic causes of collective motion in nuclei, where all
types of nuclear excitation are found : sphericity persists between N=50 and N=56 and
beyond N=56, an abrupt transition to deformation occurs. Therefore, additional experimental
data on the 80Zr nucleus and the neutron-deficient part of the zirconium isotopic chain could
help to further elucidate the role of the single-particle behaviour in the shell model, and also
that of the effective interactions.
Figure 2 : [left] The experimental zirconium mean-square charge radii as function of neutron
number from ref. [10]. [right] From ref. [12], calculated change in the mean square charge radii
(a), two neutron separation energy (b) and one neutron separation energy (c) as function of the
neutron number for Zr isotopes. Results for prolate, oblate, and spherical minima are displayed
with different symbols (see legend). Open circles correspond to ground-state results.
2) Production and counting rate estimates :
The experimental access to very neutron-deficient zirconium isotopes, such as 80Zr, is
difficult :
- The refractory nature of zirconium isotopes prevents currently their efficient production at
conventional high-temperature isotope separator facilities. Research and Development studies
are required.
- The fusion of stable nuclides - to form the lightest possible compound nucleus - requires
further neutron evaporation to reach the N=Z line. Since the proton separations energies are
smaller than the neutron ones, mainly charged particles are evaporated. However, as the
evaporation process is statistical, a small fraction of neutron-deficient exotic nuclei are still
produced.
Therefore, the production of the nuclei of interest via fusion-evaporation is possible but they
are typically produced in the midst of a copious background of other undesired reaction
channels products that are orders of magnitude more intense. An experimental limitation
comes also from the very limited number of stable beam and target combinations available for
which these N=Z nuclei can be produced.
In the past, the inverse kinematics 24Mg(58Ni,2n) reaction at a beam energy of about
200 MeV was mainly used to populate excited states in 80Zr, with an extremely low cross
section of σ=10±5 µb [5-6][8-9][14-16]. States up to spin (12+) have been observed in these
studies [8]. At this energy, the strongest reaction channels are the ones that lead to the
following evaporation residues : 79Rb (σ=190 mb), 79Sr (σ=77 mb), 80Sr (σ=52 mb),…(see
table 1 of reference [14]).
The LINAC of SPIRAL2 should deliver at the beginning heavy ions beams (A/Q ratio
of 3) up to krypton, with beam intensities for gaseous elements going from 1mAe for oxygen
to few 10 µAe for krypton. As for the energy, it can vary from 0.75 to 14.5 MeV/A and can
thus be suitable for fusion-evaporation reactions. L.C. Penescu studied during his master
thesis other possible reactions to produce 80Zr [17] and compared them with the 58Ni + 24Mg
reaction used in the past (see table 7.2 page 43) :
(a) 36Ar + 50Cr
(b) 40Ca + 46Ti
(c) 36Ar + 46Ti
(d) 58Ni + 24Mg
He concluded that reaction (a) is the best choice from production rate point of view and
target fusion point. However, one shall note that the other reactions could still be considered
(only a factor 5 of difference in the production rate is expected).
If we do consider 50% transmission through the S3 separator that we combine with an
assumed 10% efficiency for the gas catcher, the following 80Zr production rate are estimated :
Ions
36
58
40
12+
Ar
18+
Ni
14+
Ca
Energy
(MeV)
Intensity
(pµA)
120
100
190
10
130
10
Target
50
46
Cr / Ti
24
Mg
46
Ti
Thickness
(mg/cm )
Rate (1+)
(pps)
1
3,76E+03
1
7,83E+02
1
1,00E+03
2
Concerning the other even-mass neutron-deficient zirconium isotopes, they will be
produced using the same reaction as 80Zr but with a heavier Ar beam and/or a heavier Cr/Ti
target. Note that in the past :
- 82Zr was produced via the 58Ni(28Si,2p2n) reaction @ 120MeV [18],
- 84Zr was produced via the 58Ni(29Si,2pn) or 58Ni(32S,α2p) reactions @ 110MeV [19],
- 86Zr was produced via the 60Ni(30Si,2p2n) reaction @ 135MeV [20].
Priority will be given to realize the spectroscopy of the most exotic even-mass
neutron-deficient Zr isotopes during the experiment, to complete the existing measurements
of the mean square charge radii [10-11]. As all the previous isotope shift measurements, 90Zr
is planned to be taken as reference. Zirconium has a high melting point (Tf=2125K) and
therefore requires an efficient device for its vaporization. We will try to include in the gaz cell
a system based on the electrical discharge of an electrode coated with natural zirconium, for
instance, to produce the stable 90Zr beam. This aspect is still under study.
Considering the expected resolution of the hyperfine spectra (not better than 1.5 GHz)
with the in-source laser spectroscopy technique, we do not look forward to extract the
quadrupole moment during these runs. Such measurements could be done in the future with
the LUMIERE setup of the DESIR facility [21], if the counting rates appear to be sufficient,
or using the novel technique of in-flight resonant ionization spectroscopy [22]. In this letter,
we are requesting 21 UTs. The ultimate number of shifts required will mostly depend on the
efficiencies of the Zr ionisation scheme and detection setup. More accurate values will be
given in the future.
3) Laser ionization scheme :
By coupling a gas cell to the focal plane of S3, the reaction products can be efficiently
thermalized and laser ionized. Furthermore, using an RF structure behind the gas cell would
allow to perform laser spectroscopy measurements in the so-called ‘Laser Ion Source Trap
(LIST)’ mode [23]. The latter would results in a better resolution as no pressure broadening is
expected. Thus the nuclear shape investigations using in-source laser spectroscopy of these
nuclei become feasible.
The laser ionization spectroscopy is based on the resonant photo-ionization (ionization by the
absorption of one or several photons) of atoms via laser beams. In two or more steps, the
electron is promoted from its ground state to beyond the ionization continuum of atoms.
The measurement of the optical resonance in the neutron-deficient Zr isotopes will allow to
extract the isotope shifts while the study of the hyperfine structure of the odd-A Zr isotopes
will also bring information on the nuclear moments.
Several databases that compile lines and energy levels of neutral zirconium (Zr-I) and
singly ionized zirconium (Zr-II) exist [24-27]. We are currently studying the most efficient
three-photon-ionization scheme that could be used, taking into account former experimental
[28] and theoretical [29] studies. The development of Zr ionization scheme will be realized at
the CERN LARIS lab with a laser photo-ionization spectrometer equipped with a laser
ablation source of atomic vapor which is suitable for the spectroscopy of refractory atoms like
Zr [30].
4) Detection setup :
The combination of a gas cell with a laser set-up - that brings together rapidity,
efficiency and selectivity - placed after the S3 spectrometer represents a powerful tool for
these studies and might furthermore result in the production of pure short-lived neutrondeficient beams for decay studies (using the S3 tape station after the high resolution mass
separator). We do expect at the end rather pure monocharged +1 Zr beams with an energy of
50-60 keV.
A new setup for implantation, identification and decay studies of radioactive beams
produced via S3 is under construction in collaboration with IPHC-Strasbourg and IKS-Leuven
laboratories. The “cahier de charge” of the so-called “S3 tape station” is under construction.
The apparatus should be ready for first beams delivered by S3.
[1] S.M. Fischer et al., Phys. Rev. Lett. 84 (2000) 4064, “Evidence for Collective Oblate Rotation in N=Z 68Se”
[2] B.J. Varley et al., Phys. Lett. B 194 (1987) 463, “Evidence for shape coexistence in the N=Z nucleus 72Kr”
[3] C.J. Lister et al., Phys. Rev. Lett. 49 (1982) 308, “Extreme Prolate Deformation in Light Strontium Isotopes”
[4] W. Nazarewicz et al., Nucl. Phys. A 435 (1985) 397, “Microscopic study of the high-spin behaviour in
selected A≈80 nuclei”
[5] J.J. Ressler et al., Phys. Rev. Lett. 84 (2000) 2104, “Half-Life measurement for the r-Process waiting point
nuclide 80Zr”
[6] A.S. Lalleman et al., Hyp. Int. 132 (2001) 315, “Mass measurements of exotic nuclei around N=Z=40 with
CSS2”
[7] P. Van Isacker et al., Phys. Rev. Lett. 74 (1995) 4607, “Test of Wigner's spin-isospin symmetry from double
binding energy differences”
[8] S.M. Fischer et al., Phys. Rev. Lett. 87 (2001) 132501, “Observation of delayed alignment in N=Z nuclei
72
Kr, 76Sr and 80Zr”
[9] C.J. Lister et al., Phys. Rev. Lett. 59 (1987) 1270, “Gamma radiation from N=Z nucleus 80Zr”
[10] P. Campbell et al., Phys. Rev. Lett. 89 (2002) 082501, “Laser spectroscopy of cooled zirconium fission
fragments”
[11] D.H. Forest et al., J. Phys. G: Nucl. Part. Phys. 28 (2002) L63, “Laser spectroscopy of neutron deficient
zirconium isotopes”
[12] R. Rodriguez-Guzman et al., preprint sumitted to Phys. Lett. B
[13] L. Gaudefroy et al., Phys. Rev. Lett. 84 (2000) 2104, “Collective structure of the N=40 isotones”
[14] K.E.G. Löbner et al., Nucl. Instrum. Methods B 26 (1987) 301, “Münich high-frequency recoil
spectrometer”
[15] C.J. Lister et al., Phys. Rev. C 42 (1990) R1191, “Shape changes in N=Z nuclei from germanium to
zirconium”
[16] S.M. Fischer et al., Nucl. Phys. A 682 (2001) 35c, “Observation of delayed alignment in N=Z nuclei 72Kr,
76
Sr and 80Zr”
[17] L.C. Penescu, GANIL R 05 02, “Evaluation de la méthode ISOL par fusion-évaporation des faisceaux
d’ions lourds stables de LINAG”
[18] C.J. Lister et al., International Conference on In-beam Nuclear Spectroscopy, Debrecen, Hungary, 1984,
edited by Z.S. Dombradi and T. Fenyes.
[19] A.A. Chishti et al., Phys. Rev. C 48 (1993) 2607, “Collectivity in light zirconium isotopes : evolution with
neutron number and angular momentum”
[20] P. Chowdhury et al., Phys. Rev. Lett. 67 (1991) 2950, “Large B(M1) staggering at high spins in 86Zr:
Broken boson pairs in the four-quasiparticle regime”
[21] http://www.cenbg.in2p3.fr/desir/spip.php?rubrique39
[22] K. Flanagan, contribution to the ISOLDE newsletter, spring 2009.
[23] T. Sonoda et al. , Nucl. Instr. Methods B 267 (2009) 2918, “The Laser Ion Source Trap (LIST) coupled to a
gas cell catcher”
[24] http://www.pmp.uni-hannover.de/cgi-bin/ssi/test/kurucz/sekur.html
[25] http://www.nist.gov/physlab/data/asd.cfm
[26] W.F. Meggers, C.H. Corliss and B.F. Scribner, Tables of Spectral Line Intensities, Part I—arranged by
Elements, NBS Monograph 145, U.S. Government Printing Office, Washington, DC (1975).
[27] Charlotte E. Moore, Atomic Energy Levels, Vol. II (Chromium through Niobium), Circular of the National
Bureau of Standards 467, U.S. Government Printing Office, Washington, DC (1952).
[28] R.H. Page, et al., 6th international symposium on resonance ionization spectroscopy (RIS) and its
applications, Santa Fe, NM (United States), 24-29 May 1992.
[29] P.V. Kiran Kumar et al., J. Opt. Soc. Am. B 20 (2003) 1807, “Calculation of 91Zr optical selectivities in
two-color resonant three-photon ionization schemes”
[30] V. Fedoseev, on the behalf of the CERN RILIS group, private communication.