Mapping of fragmented νf5/2→πf7/2 transitions in the Co

Persistence of the Z=28 shell gap in A=75 isobars: Identification of a possible (1/2Escrig1, Morales2, Nishimura3 et al. 2021Phys. Rev. C3020View full textAdd to dashboardCite

“…Meanwhile, the main contribution to the ground-state wave function in the daughter nucleus 75 Ni is expected from the unpaired neutron in the νg 9/2 shell, resulting in J π = (9/2 + ). The assignment is supported by the systematics of lighter neutron-rich νg 9/2 even-odd Ni isotopes [21,68] and the recent experimental studies of 75 Ni [30,32]. The strong β feeding to the excited state at 1864.4(8) keV, I β = 38(7)%, and the corresponding logft = 4.7(2), provide a robust proof for the occurrence of an allowed Gamow-Teller (GT) decay from the πf −1 7/2 ground state of 75 Co. As the most energetic single-particle GT transition occurring in the region of 78 Ni transforms a neutron in the νf 5/2 orbital into a proton in the πf 7/2 shell, the associated wave function in the final 1864.4(8)-keV state of 75 Ni is expected to have a large νf −1 5/2 contribution, resulting in a tentative spin and parity J π = (5/2 − ).…”

“…With these goals in mind, the isomeric and β decay of 75 Co, with one proton and two neutron holes relative to the 78 Ni doubly-magic core, were investigated following the in-flight fission of a relativistic 238 U beam on a thin natural Be target at the Radioactive Isotope Beam Factory (RIBF) at RIKEN, Japan. Despite being pinpointed as one of the A ∼ 78 nuclei with a significant impact on rprocess estimates [53], the only experimental information reported hitherto in the literature for 75 Co is limited to the half-life t 1/2 and an upper limit for the β-delayed oneneutron emission probability P 1n [2,32,54]. Here we provide an additional lower P 1n limit, of help to extend the experimental databases used by nuclear astrophysicists.…”

“…The robustness of the Z = 28 closed shell and the coexistence of deformed and spherical shapes in the neutronrich νg 9/2 Ni isotopes have been a matter of debate in a number of recent experimental and theoretical works [11,[19][20][21][22][23][24][25][26][27][28][29][30][31][32]. Particularly important is the conservation of the seniority quantum number υ -the number of protons or neutrons that are not coupled in pairs to J = 0 [33,34]-as it is a good indicator of gap stability.…”

“…Here we provide an additional lower P 1n limit, of help to extend the experimental databases used by nuclear astrophysicists. For 75 Ni, four γ rays at 232 keV, 893 keV, 950 (20) keV and 1100 (20) keV have recently been reported [30,32]. While the first two were observed following β decay of 75 Co in an in-flight fragmentation experiment in NSCL [32], the latter two were reported in an intermediate Coulomb excitation experiment carried out at RIKEN [30].…”

“…For 75 Ni, four γ rays at 232 keV, 893 keV, 950 (20) keV and 1100 (20) keV have recently been reported [30,32]. While the first two were observed following β decay of 75 Co in an in-flight fragmentation experiment in NSCL [32], the latter two were reported in an intermediate Coulomb excitation experiment carried out at RIKEN [30]. Of them, only the 232-keV and 950(20)-keV transitions have tentatively been placed in the level scheme of 75 Ni, decaying directly to the ground state from levels with proposed spins and parities J π = (7/2 + ) and (13/2 + ), respectively.…”