Coulomb excitation of 68,70Cu: first use of postaccelerated isomeric beams.
ABSTRACT We report on the first low-energy Coulomb excitation measurements with radioactive Ipi=6- beams of odd-odd nuclei 68,70Cu. The beams were produced at ISOLDE, CERN and were post-accelerated by REX-ISOLDE to 2.83 MeV/nucleon. Gamma rays were detected with the MINIBALL spectrometer. The 6- beam was used to study the multiplet of states (3-, 4-, 5-, 6-) arising from the pi2p3/2 nu 1g9/2 configuration. The 4- state of the multiplet was populated via Coulomb excitation and the B(E2;6--->4-) value was determined in both nuclei. The results obtained illustrate the fragile stability of the Z=28 shell and N=40 subshell closures. A comparison with large-scale shell-model calculations using the 56Ni core shows the importance of the proton excitations across the Z=28 shell gap to the understanding of the nuclear structure in the neutron-rich nuclei with N approximately 40.
- SourceAvailable from: Stéphane Grévy[show abstract] [hide abstract]
ABSTRACT: The reduced transition probabilities B(E2;0(+) --> 2(+)(1)) of the neutron-rich (74)Zn and (70)Ni nuclei have been measured by Coulomb excitation in a (208)Pb target at intermediate energy. These nuclei have been produced at Grand Accélérateur National d'Ions Lourds via interactions of a 60A MeV (76)Ge beam with a Be target. The B(E2) value for (70)Ni(42) is unexpectedly large, which indicates that neutrons added above N=40 strongly polarize the Z=28 proton core. In the Zn isotopic chain, the steep rise of B(E2) values beyond N=40 continues up to (74)Zn(44). The enhanced proton core polarization in (70)Ni is attributed to the monopole interaction between the neutron in the g(9/2) and protons in the f(7/2) and f(5/2) spin-orbit partner orbitals. This interaction could result in a weakening of magicity in (78)Ni(50).Physical Review Letters 07/2006; 96(23):232501. · 7.94 Impact Factor
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ABSTRACT: Using resonant laser ionization, beta-decay studies, and for the first time mass measurements, three beta-decaying states have been unambiguously identified in 70Cu. A mass excess of -62 976.1(1.6) keV and a half-life of 44.5(2) s for the (6-) ground state have been determined. The level energies of the (3-) isomer at 101.1(3) keV with T(1/2)=33(2) s and the 1+ isomer at 242.4(3) keV with T(1/2)=6.6(2) s are confirmed by high-precision mass measurements. The low-lying levels of 70Cu populated in the decay of 70Ni and in transfer reactions compare well with large-scale shell-model calculations, and the wave functions appear to be dominated by one proton-one neutron configurations outside the closed Z=28 shell and N=40 subshell. This does not apply to the 1+ state at 1980 keV which exhibits a particular feeding and deexcitation pattern not reproduced by the shell-model calculations.Physical Review Letters 04/2004; 92(11):112501. · 7.94 Impact Factor
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ABSTRACT: The monopole effect of the tensor force is presented, exhibiting how spherical single-particle energies are shifted as protons or neutrons occupy certain orbits. An analytic relation for such shifts is shown, and their general features are explained intuitively. Single-particle levels are shown to change in a systematic and robust way, by using the pi + rho meson exchange tensor potential, consistently with the chiral perturbation idea. Several examples are compared with experiments.Physical Review Letters 01/2006; 95(23):232502. · 7.94 Impact Factor
Coulomb Excitation of68;70Cu: First Use of Postaccelerated Isomeric Beams
I. Stefanescu,1,2G. Georgiev,3,4F. Ames,5J. A¨ysto ¨,3,6,22D.L. Balabanski,7,8G. Bollen,5P.A. Butler,9J. Cederka ¨ll,3
N. Champault,3T. Davinson,10A. De Maesschalck,11P. Delahaye,3J. Eberth,12D Fedorov,13V.N. Fedosseev,3
L.M. Fraile,3S. Franchoo,14K. Gladnishki,7D. Habs,5K. Heyde,11M. Huyse,1O. Ivanov,1J. Iwanicki,15J. Jolie,12
B. Jonson,16Th. Kro ¨ll,17R.Kru ¨cken,17O.Kester,5UKo ¨ster,3A.Lagoyannis,18L. Liljeby,19G.Lo Bianco,7B.A.Marsh,3
O. Niedermaier,20T. Nilsson,16M. Oinonen,3,22G. Pascovici,12P. Reiter,12A. Saltarelli,7H. Scheit,20D. Schwalm,20
1Instituut voor Kern- en Stralingsfysica, K. U. Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
2Horia-Hulubei National Institute for Physics and Nuclear Engineering, PO-Box MG-6, Bucharest, Romania
3ISOLDE, CERN, CH-1211 Geneva 23, Switzerland
4CSNSM, CNRS/IN2P3; Universite ´ Paris-Sud, UMR8609, F-91405 ORSAY-Campus, France
5Ludwig Maximilians Universita ¨t Mu ¨nchen, Am Coulombwall 1, D-85748 Garching, Germany
6Department of Physics, University of Jyva ¨skyla ¨, P.O. Box 35, FIN-40014, Finland
7Dipartimento di Fisica, Universita di Camerino, I-62032 Camerino, Italy
8INRNE, Bulgarian Academy of Science, BG-1784 Sofia, Bulgaria
9Oliver Lodge Laboratory, University of Liverpool, United Kingdom
10Department of Physics and Astronomy, University of Edinburgh, United Kingdom
11Vakgroep Subatomaire en Stralingsfysica Universiteit Gent, Belgium
12IKP, University of Cologne, D-50937 Cologne, Germany
13Petersburg Nuclear Physics Institute, 188300 Gatchina, Russia
14IPN Orsay, F-91406 Orsay Cedex, France
15Heavy Ion Laboratory, Warsaw University, Pasteura 5A, 02-093 Warsaw, Poland
16Fundamental Fysik, Chalmers Tekniska, Ho ¨gskola, S-412 96, Go ¨teborg, Sweden
17Technische Universita ¨t Mu ¨nchen, James-Franck-Strasse, D-85748 Garching, Germany
18National Research Center Demokritos, Greece
19Manne Siegbahn Laboratory, Frascativa ¨gen 24, S-10405 Stockholm, Sweden
20Max-Planck-Institut fu ¨r Kernphysik, Heidelberg, Germany
21Joint Institute for Nuclear Research, 141980, Dubna, Moscow Region, Russia
22Helsinki Institute of Physics, University of Helsinki, P.O. Box 64, FIN-00014, Finland
(Received 5 December 2006; published 23 March 2007)
We report on the first low-energy Coulomb excitation measurements with radioactive I?? 6?beams
of odd-odd nuclei68;70Cu. The beams were produced at ISOLDE, CERN and were post-accelerated by
REX-ISOLDE to 2:83 MeV=nucleon. ? rays were detected with the MINIBALL spectrometer. The 6?
beam was used to study the multiplet of states (3?, 4?, 5?, 6?) arising from the ?2p3=2?1g9=2
configuration. The 4?state of the multiplet was populated via Coulomb excitation and the B?E2;6?!
4?? value was determined in both nuclei. The results obtained illustrate the fragile stability of the Z ? 28
shell and N ? 40 subshell closures. A comparison with large-scale shell-model calculations using the56Ni
core shows the importance of the proton excitations across the Z ? 28 shell gap to the understanding of
the nuclear structure in the neutron-rich nuclei with N ? 40.
DOI: 10.1103/PhysRevLett.98.122701PACS numbers: 25.70.De, 21.10.Ky, 25.60.?t, 27.50.+e
Radioactive beams provide great opportunities for in-
vestigating the nuclear structure away from the stable
nuclei. One of the regions of the nuclear chart that has
attracted a considerable interest in the past years is the one
close to68Ni [1–8]. Coulomb excitation experiments with
radioactive beams of even-even isotopes showed that the
coupling of a few extra particles to the68Ni core induces
large polarization effects [2,3]. These effects were associ-
ated with a weakening of the Z ? 28 and N ? 40 gaps
when neutrons start filling the 1g9=2orbital . Beyond
N ? 40, results of ?-decay measurements in the neutron-
rich69–73Cu isotopes revealed a dramatic and sudden low-
ering of the ?1f5=2orbital with the increased occupancy of
the ?1g9=2orbital . Referred to as monopole migration,
this energy shift was interpreted as originating from the
residual proton-neutron interaction and it is expected to
have profound implications on the structure of the doubly
magic nucleus78Ni [4,5].
nucleon-nucleon interactions were used in order to under-
stand the observed properties in the nuclei around68Ni and
predict the evolution of the shell structure towards78Ni [5–
8]. The calculations indicated that the values of the Z ? 28
and N ? 40, 50 energy gaps strongly depend on the effec-
tive interaction used. A consistent understanding of the
evolution of the nuclear structure in these regions requires
also experimental information such as excitation energies
and transition rates in the odd-A and odd-odd nuclei.
PRL 98, 122701 (2007)
23 MARCH 2007
© 2007 The American Physical Society
Coulomb excitation experiments provide information on
the energies, spins, and parities of the excited levels and
reduced transition probabilities. Such experiments are ex-
tremely challenging due to the low intensity of the radio-
active beams and the possible low collectivity of the
transitions involved. Moreover, the very complex level
structure of the odd-A and odd-odd nuclei can lead to
isomeric states. Apart from being an important probe for
nuclear structure, Coulomb excitation might cause an in-
duced isomeric deexcitation, as suggested in , leading to
triggered ?-ray emission—a field of great current interest.
In this Letter we report on pioneering studies, opening
the field for Coulomb excitation and other reaction experi-
ments with odd-odd nuclei, on the structure of68;70Cu
through selective laser ionization. In a simple shell-model
picture, the low-lying level structures of these two nuclei
can be regarded as one 2p3=2proton plus one neutron
particle or hole occupying either the 2p1=2or 1g9=2orbi-
tals, coupled to the68Ni core, giving rise to the multiplet of
states I?? ?1?;2?? and I?? ?3?;4?;5?;6??, respec-
tively. The spins 1?and 6?were assigned to the ground
states of68Cu and70Cu, respectively. The 6?state in68Cu
and (1?, 3?) levels in70Cu were found to be ?-decaying
isomers [10–13]. Prior to this study, the experimental
information on the remaining members of the negative-
parity multiplet was rather incomplete. Candidates for
these states were suggested at 778, 956, and 1350 keV, in
68Cu, and 226 and 506 keV, respectively, in
In the present work we made use of Coulomb excitation
of post-accelerated 6?beams of68;70Cu to characterize the
states of the ?2p3=2?1g9=2multiplet. The beams were
produced in a similar way as in [12,13] where narrow
band laser scans provided the optimum values of the laser
frequency that maximize the ionization of the different
The 6?beams of68;70Cu, post-accelerated by REX-
ISOLDE  up to 2:83 MeV=nucleon, were used to
bombard a 2:3 mg=cm2 120Sn target. Typical beam inten-
sities at the detection setup were 3 ? 105pps [68Cu?6??]
and 5 ? 104pps [70Cu?6??]. Scattered projectile and re-
coiling target nuclei were detected by a DSSSD detector
, covering the forward angles between 16.4?and 53.3?
in the laboratory system.
The detection of the ? rays was performed with the
MINIBALL array  consisting of 8 clusters each com-
bining three sixfold segmented HPGe crystals. While the
?-ray energy was extracted from the core signal of the
individual crystals, the segment with the highest energy
deposition determined the emission angle of the ? ray.
Doppler correction was applied by combining this infor-
mation with the direction and velocity of the coincident
scattered particle detected in the DSSSD detector.
Experiments with radioactive beams often suffer from
the contamination ofthe beam ofinterest with otherisobars
and, in this particular experiment, isomers. The isobaric
contamination was investigated by performing measure-
ments with and without laser radiation (laser ON/OFF) at
regular time intervals. The amount of Ga contaminant was
determined by comparing the yield of elastically scattered
particles in the DSSSD detector in the periods with the
lasers on (both Ga and Cu present in the beam) and the
periods with the lasers off (only Ga present in the beam).
Values of 74(2)% and 70(5)% were obtained for the purity
of the68;70Cu beams, respectively.
The isomeric beam contamination stemmed from the
broadening of the hyperfine-split resonances of each iso-
mer [12,19]. This introduced a small contamination of the
6?beam with contributions from the lower spin isomers.
The characteristic ? rays produced in their ? decay al-
lowed to determine the isomeric content of the beam. The
analysis showed that when the laser was tuned to the
maximum production of the 6?beam, 86(3)% and
85(5)% of the total68;70Cu ion yield was produced in this
spin state, respectively. In70Cu, the (3?, 1?) isomers were
found to contribute with almost equal amounts (?7%) to
the total Cu yield.
The upper part of Fig. 1 shows the particle–?-ray coin-
cidence spectrum observed after 12.3 h of data taking with
the 6?isomeric beam of68Cu. No Doppler correction was
applied to this spectrum. The three peaks at low energies,
namely, 84,178, and 693 keV,were identified as transitions
depopulating excited levels in68Cu . The prompt ? ray
of 178 keV deexcites the state at 956 keV, populated in our
work by Coulomb excitation. It feeds the 3?state at
778 keV, which further deexcites via the 693 and 84 keV
transitions defining the 3?! 2?! 1?sequence. A spin
with the 6?beam of68Cu. The partial level scheme and deexci-
tation ? rays shown in the upper right corner are based on
Refs. [15,21] and this work. Energies are given in keV. Levels
drawn with thick lines represent the ?-decaying states. Bottom:
particle–?-ray coincidence spectrum acquired with the 1?
beam. No Doppler correction was applied to these spectra.
Top: particle–?-ray coincidence spectrum acquired
PRL 98, 122701 (2007)
23 MARCH 2007
I?? 4?was suggested in Refs. [14,15] for the state at
956 keV, based on the parabolic rule for proton-neutron
multiplets proposed by Paar . Such a spin assignment
would imply a M1-E2 multipolarity for the 178 keV ? ray.
The fact that this ? ray is Doppler broadened in the
Coulomb excitation spectrum of Fig. 1 indicates a lifetime
of the order of picoseconds for the level at 956 keV. This
fixes the spin of the level to I?? 4?, in agreement with
Refs. [14,15], as the partial decay lifetimes of the M1
transitions are indeed in the picoseconds range, while E2
transitions would have a partial lifetime 4 orders of mag-
nitude higher, based on Weisskopf estimates. Furthermore,
the fact that in the same spectrum, the transitions of 84 and
693 keVare not Doppler broadened indicates that these ?
rays were emitted after the68Cu ions were implanted in the
particle detector. A half-life of T1=2? 7:84 ns was mea-
sured in Ref.  for the I?? 2?state, whereas a value
between 0.7 and 4 ns was reported in Ref.  for the half-
life of the I?? 3?level.
It is interesting to note that by the Coulomb excitation of
the 6?isomer in
instantaneous depopulation of a nuclear isomer. The trig-
gered ? emission of an isomer was extensively investigated
in K isomers via Coulomb excitation and photoabsorption
experiments (see  and references therein). These experi-
ments are extremely challenging since the isomer depopu-
lation can only proceed through weak transitions arising
from K mixing [22,23]. In this work, an alternative scheme
is revealed, based on the multiplet structure of an odd-odd
nucleus. The E2 Coulomb excitation feeds a member of the
multiplet which deexcites faster through M1 than E2 tran-
sitions, eventually bypassing the isomer. In the present
experiment, an isomer depopulation cross section of 42 ?
4 mb was observed. A higher cross section can be obtained
by, e.g., increasing the beam energy.
There isa possiblecontributiontothe spectrum shownin
Fig. 1 from Coulomb excitation arising from the contami-
nation of the 6?isomeric beam with the 1?ground state.
This was checked by setting the laser frequency to the
value found to produce the maximum ionization of the
ground state [12,13]. The spectrum acquired after 5 h of
68Cu?1?? beam on target is shown in the bottom part of
Fig. 1. Apart from the Coulomb excitation peak of
1171 keV of the120Sn target, the only ? ray present in
the spectrum is the transition 2?! 1?of 84 keV.
The particle–?-ray coincidence spectrum acquired after
28 h of70Cu?6?? beam on target is presented in Fig. 2. The
spectrum was Doppler corrected for mass A ? 70. The
prompt peak at E?? 127 keV was identified as the tran-
sition between the state at 228 keV populated by Coulomb
excitation and the isomeric state I?? 3?in70Cu. A spin
I?? 4?for the state at 228keV wasproposedin Ref. ,
based on the observed ?-decay pattern. The observation of
the fast 127 keV decaying transition implies a M1 charac-
ter for this ? ray, thus confirming the I?? 4?spin assign-
ment for the 228-keV level , see Fig. 2. In this case, as
68Cu, we demonstrated the induced
well as in the case of68Cu, population of the 5?state was
not observed and therefore will not be considered in the
The experimental Coulomb excitation cross section
?CE?6?! 4?? was determined in both nuclei relative to
the known cross section for exciting the 2?state in the
120Sn target. For the fit of the experimental cross sections
and corresponding B?E2? values, the Coulomb excitation
code GOSIA  was used. The code calculates the experi-
mental ? yields integrated over the scattering angle of the
detected particle and corrected for angular distributions,
internal conversion coefficients, and energy loss of the
beam in the target. In the GOSIA fitting code, the unknown
h6?jjE2jj4?i reduced matrix element was varied so as to
reproduce the measured yield of the observed deexcitation
4?! 3?. The B?E2;6?! 4?? value extracted for68Cu is
68?6?e2fm4[4.1(4) W.u.] (Weisskopf unit). The quoted
error is dominated by the statistical errors arising from
the determination of the peak area and purity of the
beam. The systematic error introduced by the reorientation
matrix elements was found to be below 1% when assuming
the values for the quadrupole moments predicted by the
shell model. For the estimation of the B?E2;6?! 4??
value in70Cu, the population of the I?? 4?level through
an E2=M1 excitation with the 3?isomeric contaminant
needs to be considered. From the measured magnetic mo-
ment for the 3?isomeric state in70Cu , a B?M1;3?!
4?? value of 6:74?2
M1 Coulomb excitation cross section of 0.16 mb, which is
a factor of 16 less than the cross section for an E2 excita-
tion corresponding to a B?E2? value of 1 W.u.; therefore,
the M1 contribution can be considered negligible. Shell-
model calculation for a pure ?2p3=2?1g9=2configuration
predicts that the two reduced matrix elements are con-
nected by the relation h6?jjE2jj4?i ? 0:94h3?jjE2jj4?i.
Taking into account also the isomeric ratio 6?:3?? 85:7
determined experimentally, we estimated that ?14% from
the observed deexcitation yield 4?! 3?is due to the
population of the 4?state by an E2 excitation with the
3?isomer. From the remaining number of counts, a value
of B?E2;6?! 4?? ? 41?5?e2fm4[2.4(3) W.u.] was deter-
Ncould be deduced. This would imply a
50100150 200250300350 400
beam of70Cu. The spectrum is Doppler corrected for mass A ?
70. Upper right corner: partial level scheme of70Cu taken from
Particle–?-ray coincidence spectrum obtained with 6?
PRL 98, 122701 (2007)
23 MARCH 2007
The B?E2? values measured in the present work are
summarized in Table I and compared to the predictions
of the shell model. The value deduced from the assumed
?6?? ! ?4?? transition in72Cu [25–28] is also included in
the table. The shell-model calculations were performed
with the ANTOINE code  using the realistic force de-
termined in Refs. [30,31], also used for the calculation of
the levels in70–78Cu [12,25,32]. The valence space con-
sidered for both protons and neutrons consist of the fpg
orbitals outside the56Ni inert core, without any restriction
on their occupation. The effective proton and neutron
charges used in the calculations are e?? 1:5e and e??
0:5e. The calculation predicts that while the main contri-
bution to the E2 transition rates is due to protons, the
neutron contribution becomes important with the increased
occupancy of the 1g9=2orbital. However, the B?E2? values
calculated with the effective charges e?? 1:5e and e??
0:5e do not vary significantly as a function of the neutron
number, in disagreement with the experimental observa-
tions. The values in68;72Cu are underestimated by the
theory pointing to the importance of proton excitations
across the Z ? 28 shell gap that are not included in the
used model space. The lower B?E2? value in70Cu, as well
as its agreement with the calculation with a56Ni core,
indicates a stabilizing effect at N ? 40 and Z ? 28. This
effect appears to be very delicate since the coupling of
three quasiparticles to68Ni induces significant core polar-
ization, as observed in the case of68;72Cu. Indeed, an
increased neutron effective charge e?? 1:0e, keeping
e?? 1:5e, leads to a better agreement with the experiment
[Bth?E2? ? 55, 48, and 70e2fm4for
tively] implicitly implying proton p-h excitations across
Z ? 28 through a polarization of the neutrons.
In conclusion, we have determined the B?E2;6?! 4??
values of the ?2p3=2?1g9=2multiplet in
Coulomb excitation of post-accelerated I?? 6?beam.
The experimental results were compared with large-scale
shell-model calculations using56Ni as a core. The low
B?E2? value in70Cu indicates weak polarization effects
induced by the extra-proton and neutron coupled to the
68Ni.However,the results showthat the coupling of at least
two like quasiparticles to68Ni weakens the stabilization
effects of the N ? 40 subshell and Z ? 28 shell gaps, in
agreement with [2,3]. The availability of post-accelerated
radioactive beams in combination with isomer selective
resonant laser ionization opens new possibilities for further
investigations as it allows probing the underlying structure
of the different configurations present in the same nucleus
through Coulomb excitation or transfer reactions. Also, the
triggered depopulation of the nuclear isomer in68Cu using
Coulomb excitation with isomeric beams calls for a de-
tailed study of other multiplets in odd-odd nuclei for
evaluating the possibility of triggered energy release.
This work was supported by EU Sixth Framework
through No. EURONS-506065, BMBF under Contracts
No. 06KY205I and No. 06MT238, IAP Research
Program No. P5/07, and FWO-Vlaanderen (Belgium).
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ues in68;70;72Cu. The last three columns give the calculated total
matrix element ME ? M?e?? M?e?, M?? h6?jjE2jj4?i?for
protons and M?? h6?jjE2jj4?i?for neutrons. Effective charges
e?? 1:5e, e?? 0:5e were used in the calculations.
Experimental and calculated B?E2;6?! 4?? val-
PRL 98, 122701 (2007)
PHYSICAL REVIEW LETTERS
23 MARCH 2007