β− decay study of the Mn66−Fe66−Co

Abstract

Background: Shell evolution can impact the structure of the nuclei and lead to effects such as shape coexistence. The nuclei around 68 Ni represent an excellent study case, however, spectroscopic information of the neutron-rich, Z < 28 nuclei is limited. Purpose: The goal is to measure γ-ray transitions in 66 Fe, 66 Co, and 66 Ni populated in the β − decay of 66 Mn to determine absolute β feedings and relative γ-decay probabilities and to compare the results with Monte Carlo shell model calculations in order to study the influence of the relevant single neutron and proton orbital occupancies around Z = 28 and N = 40. Method: The low-energy structures of 65,66 Fe, 66 Co, and 66 Ni were studied in the β − decay of 66 Mn produced at ISOLDE, CERN. The beam was purified by means of laser resonance ionization and mass separation. The β and γ events detected by three plastic scintillators and two MiniBall cluster germanium detectors, respectively, were correlated in time to build the low-energy excitation schemes and to determine the β-decay half-lives of the nuclei. Results: The relative small β-decay ground state feeding of 66 Fe obtained in this work is at variant to the earlier studies. Spin and parity 1 + was assigned to the 66 Co ground state based on the strong ground-state feeding in the decay of 66 Fe as well as in the decay of 66 Co. Experimental log(f t) values, γ-ray de-excitation patterns, and energies of excited states were compared to Monte Carlo shell model calculations. Based on this comparison, spin and parity assignments for the selected number of low-lying states in the 66 Mn to 66 Ni chain were proposed. Conclusions: The β-decay chain starting 66 Mn toward 66 Ni, crossing N = 40, evolves from deformed nuclei to sphericity. The β-decay population of a selected number of 0 + and 2 + states in 66 Ni, which is understood within shape coexistence framework of Monte Carlo shell model calculations, reveals the crucial role of the neutron 0g 9/2 shell and proton excitations across the Z = 28 gap.

Shape Coexistence at Zero Spin in Ni64 Driven by the Monopole Tensor Interaction

“…It should be emphasized that this level is not populated in 64 Co β decay [30], in contrast to all other 0 þ states, up to 0 þ 6 , which are fed in this process. This observation already points to a marked difference in structure for this excitation, and is reminiscent of that occurring in 66 Ni [23], where the prolate-deformed 0 þ 4 state at 2974 keV was also the only 0 þ excitation not fed in the β decay of the spherical 66 Co ground state [40]. Further inspection of the ILL data revealed three 2 þ states (firmly established in this work) at 3647.9, 3749.1, and 3798.7 keV, which complement four such excitations, at 1345.8, 2276.6, 2972.1, and 3276.0 keV, reported in Ref.…”

Properties of low-lying states in Co65 from lifetime measurements

“…The region around 68 Ni (Z = 28, N = 40) has motivated many recent experimental and theoretical studies, aimed at the understanding of the nuclear structure in this region with a large neutron excess. The weakening of the N = 40 sub-shell gap just two protons below 68 Ni has been documented extensively by deformed ground states in 66 Fe [1][2][3][4] and 64 Cr [5,6], which was interpreted as the center of the fourth island of inversion N = 40 by shellmodel calculations using the Lenzi-Nowacky-Poves-Sieja (LNPS) interaction [7].…”

Detailed low-spin spectroscopy of Ni65 via neutron capture reaction

“…In neutron-rich Ni isotopes, detailed studies of the low-spin structures are particularly relevant, since they may shed light on the shape coexistence phenomenon. This phenomenon has been predicted, at low spin, in these nuclei by various theoretical calculations [1][2][3][4][5][6] and also clearly observed in the even-even Ni isotopes with mass A = 64, 66, 68 and 70 [7][8][9][10][11][12].…”

“…For the mixing ratio δ 629 = −1.95(6), the resulting reduced transition probability value would be B(E2) ≈ 480 W.u., which is unrealistically large. On the other hand, δ 629 = 0.052 (11) gives B(E2) ≈ 1 W.u., which is a reasonable value. Having this result, we analyzed the angular correlation between the 629-keV and 1228-keV γ rays, which yielded a multipolarity mixing ratio for the 1228-keV transition δ 1228 = −0.09(4), pointing to its E1+M 2 character.…”