LABORATORI NAZIONALI DI LEGNARO
Transition strengths in the Ni isotopes between N=40 and N=50 have been recently subject of extensive experimental and theoretical investigations [1-6], aiming to understand whether the tensor force acts to reduce the Z=28 shell closure as the neutron g9/2 orbit is filled towards 78Ni. The effect of the Z=28 shell gap quenching and its evolution from 68Ni towards 78Ni would be reflected as an enhancement in the quadrupole transition strengths, compared with the seniority scheme predictions for the neutron g9/2 subshell. In 70Ni, the large B(E2) value for the first 2+ excited state obtained by Coulomb excitation [1] was interpreted as an evidence of a large neutroninduced polarization of the proton core [1]. Later, this interpretation was reinforced with an inelastic proton scattering experiment on 74Ni [2], in which a large deformation parameter was found, pointing to an enhanced quadrupole collectivity. However, a much lower B(E2) value has been deduced for 74Ni in a Coulomb excitation experiment [3]. In that work, both experimental and shell-model calculations using the residual LNPS interaction, restore the normal core polarization picture in the neutron rich Ni isotopic chain and suggests that the B(E2) strength predominantly corresponds to neutron excitations. The known experimental transition strengths by Coulomb excitation are constrained so far to 70Ni and 74Ni, while it is still unknown for 72Ni. We report on preliminary results from the Coulomb excitation of 72Ni performed at the Radioactive Isotope Beam Factory at RIKEN. The BigRIPS fragment separator [7] was used to select and purify a secondary beam of 72Ni at 183 MeV/u. Coulomb excitation of 72Ni was produced by impinging the beam on a 950 mg/cm2 Au target. In order to identify the reaction products after the target, the ZeroDegree spectrometer [7] was used, while the gamma rays were detected with the DALI2 array consisted of 186 NaI(TI) detectors around the target position [8]. Detailed analysis and preliminary results will be presented during the talk. [1] O. Perru et al, Phys. Rev. Lett. 96 232501 (2006). [2] N. Aoi et al, Phys. Lett. B692 302 (2010). [3] T. Marchi et al. Phys. Rev. Lett. 113, 182501 (2014). [4] K. Kaneko, T. Mizusaki, Y. Sun and S. Tazaki, Phys. Rev. C89, 11302(r) (2014). [5] Y. Tsunoda, T. Otsuka, N. Shimizu, M. Honma and Y. Utsuno, Phys. Rev. C89 31301(r) (2014). [6] K. Sieja and F. Nowacki, Phys. Rev. C85 51301(r) (2012). [7] T. Kubo et al, Prog. Theor. Exp. Phys. 2012 3C003 (2012). [8] S. Takeuchi et al, Nucl. Instr. Meth. A 763, 596 (2014).

DATA: 27-11-2015

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