Quadrupole collectivity in the Ni isotopes: relativistic Coulex of 73,74

Quadrupole collectivity in the Ni isotopes: relativistic Coulex of 73,74,75Ni
A. Gottardo1, G. de Angelis1, P. Doornenbal2, G. Benzoni3, J. Lee2, H. Liu2, M. Matsushita4, D. Mengoni5,
V. Modamio-Hoybjor1, S. Momiyama6, T. Motobayashi2, D. Napoli1, M. Niikura6, E. Sahin7, Y. Shiga2,8, H. Sakurai2,6,
R. Taniuchi6, S.Takeuchi2, H. Wang2,12, J.J. Valiente-Dobón1, R. Avigo3,11, H. Baba2, N. Blasi3, F.L. Bello Garrote7,
F. Browne2,9, F.C.L. Crespi3,11, S. Ceruti3,11, R. Daido12, M.-C. Delattre13, D. Fang12, Zs. Dombradi14, T. Isobe2,
I. Kuti14, G. Lorusso2, K. Matsui5, B. Melon15, T. Miyazaki5, S.Nishimura2, R. Orlandi16, Z. Patel2,17, S. Rice2,17,
L. Sinclair2,18, P.A. Söderström2, D. Sohler14, T. Sumikama19, R. Taniuchi5, J. Taprogge20,21, Zs. Vajta14,
H. Watanabe2,22, O. Wieland3, J. Wu2,19, Z. Y. Xu5, M. Yalcinkaya23, R.Yokoyama4
1INFN,
3INFN
Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy. 2RIKEN, Nishina Center,Tokyo, Japan.
sezione di Milano, Milano Italy. 4CNS, University of Tokyo, Tokyo,Japan. 5Università degli Studi di Padova, Padova, Italy.
6University of Tokyo, Tokyo, Japan. 7University of Oslo, Oslo, Norway. 8Rikkyo University, Tokyo, Japan.
9Peking University, Beijing, China. 10Brighton University, Brighton, UK.
11Università degli Studi di Milano, Milano, Italy. 12Osaka University, Osaka, Japan. 13IPN Orsay, Orsay, France.
14MTA Atomki, Debrecen, Hungary. 15INFN sezione di Firenze, Firenze, Italy.
16Instituut voor Kern- en StralingsFysica/K.U., Leuven, Belgium. 17University of Surrey, Guildford, UK.
18University of York, York, UK. 19Tohoku University, Tokyo, Japan.
20Consejo Superior de Investigaciones Cientificas, Madrid, Espagna.
21Universidad Autónoma de Madrid, Madrid, Espagna.
22Beihang University, Beijing, China. 23University of Instambul, Instambul, Turkey.
INTRODUCTION
Doubly-magic nuclei located in very exotic regions of
the nuclear chart are key elements in nuclear structure
studies. The appearance or disappearance of the shell gaps
associated with magic numbers in very exotic nuclei is
strongly related to the single-particle energies of nucleon
orbitals and to the residual interactions among valence
nucleons. The 100Sn and 78Ni regions are fundamental in
this regard and they are the focus of the efforts of many
research laboratories in the world. Indeed from the
N=Z=50 100Sn, located at the proton drip line, to the
neutron rich N=50 78Ni, with N/Z=1.78, it is nowadays
possible to have access to the shell structure of these
isotones including the residual interactions, spin-isospin
and shape correlations. This very large excursion in isospin
also allows magnifying and probing the isovector part of
the nuclear mean field. In particular, the tensor part of the
spin-isospin term of the residual interaction has been
predicted to modify the single-particle structure, inducing a
collective behavior in this region [1]. For the Ni isotopic
chain the filling up of the g9/2 neutron orbit is expected to
induce a strong core polarization due to the spin-isospin
interaction which enhances the B(E2:2+ → 0+), a measure
of the quadrupole collectivity. In the specific case of 68-78Ni
isotopes, it is predicted that the Z=28 gap for protons in the
pf-shell becomes smaller moving from 68Ni to 78Ni as a
result of the attraction between the proton f5/2 and the
neutron g9/2 orbits and repulsion between the proton f7/2
and neutron g9/2 configurations, thus modifying or even
inverting the effective single-particle states [2]. This
change in the nuclear structure should induce significant
quadrupole collectivity in the 2+ excited states. Indeed, the
cause of the large B(E2:2+→0+) value observed in 70Ni has
been attributed to these effects [2]
EXPERIMENTAL SETUP
We have performed an intermediate energy Coulomb
excitation study of the 73,74,75Ni isotopes, in order to fix the
seniority-scheme pattern of the B(E2) strength.
The measurement has been performed during the
experimental campaign of EURICA [3] in conjunction
with a decay experiment focused on the 78Ni region.
Neutron rich Ni isotopes were produced by fission of a
238
U beam at a bombarding energy of 345 MeV/u, with an
average intensity of 10 pnA. The resulting fragments were
analyzed using the BigRIPS separator [4] and transported
to a secondary natural Pb target for Coulex reactions. After
this target, the ions were again analyzed in the ZeroDegree
spectrometer and delivered down to the final focal plane.
Figure 1 shows the typical identification plot for this
spectrometer: in the ordinate the measured atomic number,
while in the abscissa the measured A/q (mass over charge)
ratio. The gamma rays from Coulex have been detected by
the DALI2 spectrometer [5] in coincidence with the
recoiling ions identified at the focal plane of the
ZeroDegree spectrometer.
74
Ni
Fig. 1. Identification plot for the ions after the secondary target in
the Zero-Degree spectrometer.
PRELIMINARY RESULTS
The analysis has been carried out at LNL. A preliminary
gamma spectrum, after selection on 74Ni ions, is reported
in figure 2. The gamma peak corresponding to the 2+ →0+
de-excitation at 1024 keV of 74Ni is clearly visible.
Fig. 2. The gamma-ray spectrum detected in DALI2 after
selection on 74Ni. The 2+→0+ transition at 1024 keV is clearly
visible.
Fig. 3. The angular distribution of the 74Ni recoils after elastic
and inelastic scattering on the secondary Pb target.
The angular distribution for the 74Ni recoils has also been
measured, as shown in figure 3. This is fundamental for
thecalculation of the Coulex cross section and the
validation of the optical potential which will be employed
in the analysis.
Decay-spectroscopy measurements [6] with EURICA were
performed at the final focal plane in conjunction with the
in-beam part.
A complete data cross check is still to be performed and
the analysis is in progress.
[1] T. Otsuka et al., Phys. Rev. Lett. 87, 082502 (2001)
[2] O. Perru et al., Phys. Rev. Lett. 16, 232501 (2006)
[3] P.-A. Söderström et al., Nucl. Instr. and Meth. B317, 649
(2013).
[4] T. Kubo: Nucl. Instr. Methods Phys. Res. Sect. B204, 97
(2003).
[5] S. Takeuchi et al.: RIKEN Acc. Prog. Rep. 36, 148 (2003).
[6] G. Benzoni, Riken annual report 2013.

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