Speaker
Description
D. Bazzacco, S. Lenzi, S. Lunardi, D. Mengoni, A. Montaner-Piza’,
A. Goasduff, R. Menegazzo, G. Pasqualato, F. Recchia, D. Testov
Dipartimento di Fisica, Universita’ di Padova and INFN (Italy)
P. Doornenbal, K. Wimmer
RIKEN Nishina Center (Japan)
T. Bayram, D. Brugnara, L. Cortes, G. de Angelis,
A. Gottardo, E. Gregor, A. Illana, D.R. Napoli, I. Zanon, J.J. Valiente Dobon
INFN Laboratori Nazionali di Legnaro (Italy)
In the vicinity of $^{78}$Ni the quadrupole collectivity is expected to be large because the orbits close to the Fermi level allow the realization of a quasi-SU(3) symmetry, having orbits with Δj =2 Δl = 2 quasi-degenerated. As a consequence the quadrupole interaction produces a shape transition in which highly correlated many-particles -holes configurations gain binding energy and become as bound as the spherical states. These intruder deformed bands often appear at low excitation energy in the magic nuclei and also $^{78}$Ni is expected to show such features.
Potential energy surface calculated for $^{78}$Ni [1] shows a spherical minimum which is very flat. This interesting pattern is predicted also for the first 2$^+$ state, reflecting a particular fluctuation. This fluctuation is much narrower in the lighter Nickel isotopes such as $^{68}$Ni, where the E2 excitation from the ground state goes to very high $2^+$ states. The overlap probabilities with the closed shell are predicted to be 60%, 53%, and 75% for $^{56,68,78}$Ni, respectively in [1].
On the other hand, recent large scale shell model calculations [2] predict a ground state of doubly magic 65% character, but the first $2^+$ excited state at 2.88 MeV belongs to the (prolate) deformed band based in the intruder second 0$^+$ state at 2.65 MeV. The B(E2) for the decay to the g.s. is B(E2: $2_2^+$--> $0^+$) = 32 e2 fm4 These calculations give a second 2$^+$ of 1p-1h nature at 3.15 MeV, connected to the ground state with B(E2) = 110 e$^2$ fm$^4$.
The neighboring odd-even nuclei are expected to present a relatively simple structure, at least for the lowest lying states, and as consequence such nuclei constitute a unique test bench for the effective interactions and the nuclear degrees of freedom. However, as a consequence of the presence of intruder configurations in even-even nuclei, it is of fundamental importance to characterize the states of the odd-even nuclei in terms of their single particle character versus a core-excited character.
The nucleus $^{79}$Cu can be exploited to probe the proton excitations outside $^{78}$Ni, provided that the character of the states is understood. Calculations reported in [3] show that indeed the lowest lying states are predicted to have very different microscopic structure. The 5/2$^-$ g.s and the 3/2$^-$ first excited correspond mainly to a proton in the $f_{5/2}$ and $p_{3/2}$, respectively, while the first 1/2$^-$ state has a more mixed character, with an occupancy near 50% for both $p_{1/2}$ (single particle configuration) and $f_{5/2}$ (core coupled configuration). We propose here to study the structure of the excited states via lifetime measurement. Most of the states of interest are predicted to have a lifetime in the ps range that is accessible with plunger techniques.
It would be very interesting to study the nucleus $^{79}$Ni to probe the neutron excitations along the N=51, however this nucleus is not at reach. Instead $^{81}$Zn will be the most exotic N=51 isotope that will be accessible for in-beam experiments. This nucleus [4] is expected to present different states corresponding to a 1/2$^+$ states of dominating single particle $s_{1/2}$ character, two 5/2$^+$ states, one corresponds to a core-coupled configuration and the other having a neutron $d_{5/2}$ single-particle character. Lifetime measurement will be able to disentangle the character of such states.
[1] Y. Tsunoda et al. PRC 89, 031301(R) (2014)
[2] F. Nowacki et al. PRL 117, 272501 (2016)
[3] L. Olivier et al. PRL 119, 192501 (2017)
[4] C.M. Shand et al. PLB 773 492–497 (2017)
