Transition strengths in the neutron-rich Ni73,74,75 isotopes

Abstract

Reduced transition probabilities have been measured for the neutron-rich 73,74,75 Ni nuclei with relativistic Coulomb excitation performed at the RIKEN Nishina Center. For the even-even Ni isotope the determined B(E 2; 0 + → 2 + ) value is compatible within errors with the result of a previous intermediate energy Coulomb excitation experiment, although being somewhat larger. For the odd Ni isotopes B(E 2) values have been determined for the first time. In the middle νg 9/2 shell nucleus 73 Ni three peak candidates have been observed and tentatively assigned to the (5/2 + ), (13/2 + ), and (11/2 + ) excitations. Two peak candidates have been observed in 75 Ni and tentatively assigned to the (13/2 + ) and (11/2 + ) levels. The measured B(E 2) values are in line with the large B(E 2) reported in 70 Ni and at variance with the small strength measured in 72 Ni. Excitation

Persistence of the ${Z=28}$ shell gap in ${A=75}$ isobars: Identification of a possible ${(1/2^-)}$ $μ$s isomer in ${^{75}}$Co and $β$ decay to ${^{75}}$Ni

“…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 − ).…”

“…The location of the level at 1044.6 (12) keV is based on two arguments: first, the 1045-keV transition is the most intense one in the set of coincident γ rays formed by the 1045-, 417-, 867-, and 1596-keV peaks (see Table I); and second, its energy could match with the tentative direct ground-state transition at 1100 (20) keV reported by Ref. [30]. The placement of the states at 1461.1(17) keV, 1911.1( 16) keV, and 3057(4) keV, built up on top of the 1044.6(12)-keV level, is proposed according to the coincidence relationships indicated in Table I.…”

“…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].…”

“…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.…”

“…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.…”