Shape coexistence and high-Kstates in74Se populated following theβdecay of

Abstract

Excited states in74 Se were populated following the ǫ/β + decay of 74 Br, mainly from the J=K=4, 13.8 keV β-decaying isomer. Off-beam γ-ray spectroscopy was performed with the array of Clover HPGe detectors at WNSL Yale. Many new transitions were observed and rigorous spin assignments were made based on γ-γ angular correlations. The β-decay strength was found to proceed almost entirely to a few high lying states near 4500 keV, which are consistent with deformed two-quasi neutron high-K configurations. These doorway states then provide an effective tool for populating and identifying the lower prolate structures. Once categorized, the low-lying states of 74 Se can be described as a set of near-spherical vibrational levels mixing strongly with a spectrum of prolate states which have an unperturbed band head near 1350 keV.

Systematical shell-model calculation in the pairing-plus-multipole Hamiltonian with a monopole interaction for thepf5/2g9/2shell

“…It was reported that in 70 Se, the oblate shape near the ground state evolves quickly to a prolate shape at higher spins [10]. Further experiments suggested that in 72 Se, there is no well-defined shape for the lowest levels due to shape mixings [11], and the low-lying states in 74 Se show coexistence of spherical and prolate shapes [12].…”

“…It has been shown that this contradiction can be understood by the parity change across the N = 40 shell gap [3][4][5][6]. Moving to heavier isotones of N = 40 with increasing proton number, there have been many experimental evidences suggesting an increasing collectivity, for example those in the Ge [7,8] and Se isotopes [9][10][11][12]. With Z approaching 40, the N = Z isotone 80 Zr is known to be strongly deformed [13], and signs of a shell closure disappear completely and the nucleus can be regarded as a good rotor.…”

“…Another interesting property to be mentioned for the p f 5/2 g 9/2 shell is the shape evolution in the mass-70 region [10][11][12][14][15][16]. In particular, the nuclei around the N = Z line exhibit a variety of nuclear shapes [17,18].…”

“…For 68 Se, it was found that the oblate ground-state band coexists with a prolate excited band, forming a shape coexistence phenomenon [16,18]. Shape coexistence was also investigated for 70,72,74 Se [10][11][12]. It was reported that in 70 Se, the oblate shape near the ground state evolves quickly to a prolate shape at higher spins [10].…”

“…It has been known that the conventional magic numbers disappear in some cases of the neutron-rich region, but new magic numbers may emerge. For example, the neutron-rich nuclei 12 Be, 32 Mg, and 42 Si were found to exhibit large collectivity in spite of the corresponding neutron magic numbers N = 8, 20, and 28. The monopole interaction is the key ingredient for explaining the binding energies, the emergent magic numbers, and the shell evolution in the neutron-rich region.…”