Spectroscopy of ${}^{76}\mathrm{Se}$: Prolate-to-oblate shape transition

The spectroscopy of ${}^{76}\mathrm{Se}$ has been studied using the ${}^{70}\mathrm{Zn}({}^{12}\mathrm{C}, \alpha 2{\mathrm{n})}^{76}\mathrm{Se}$ fusion evaporation reaction. The yrast band of ${}^{76}\mathrm{Se}$ has been extended to substantially higher spin, allowing observation of the second band crossing. The much-delayed $g_{9/2}$ proton-pair alignment is discussed in terms of the cranked shell model and most likely is caused by a shape transition from prolate to oblate along the yrast line occurring in ${}^{76}\mathrm{Se}$. Based on the systematic investigation of the band crossings associated with the $g_{9/2}$ quasiparticle alignments and their relationships with the shape evolutions in the even-$A$ Se and Kr isotopes, a comprehensive picture of shape evolution along with spin and isospin in these nuclei is obtained.

Figure 1

Coincidence $\gamma$-ray spectra generated from (a) the sum of gates on 1007.1-, 1029.6-, and 1131.2-keV transitions, (b) the sum of gates on 942.8- and 962.0-keV transitions. The peaks marked with stars are known contaminants.

Figure 2

Partial level scheme of ${}^{76}\mathrm{Se}$. Energies are in keV. New observed transitions are indicated by red lines.

Figure 3

The kinematic $J^{\left(1\right)}$ and dynamic $J^{\left(2\right)}$ moments of inertia as functions of rotational frequency for the yrast bands in ${}^{74}\mathrm{Se}, {}^{76}\mathrm{Se}$, and ${}^{78}\mathrm{Kr}$. New data of ${}^{76}\mathrm{Se}$ are indicated by solid red triangles.

Figure 4

Experimental and calculated moments of inertia for the yrast band of ${}^{76}\mathrm{Se}$.

Figure 5

Calculated crossing frequency, $\hslash {\omega }_c$, for the yrast band as a function of the triaxiality parameter $\gamma$ for the fixed quadrupole and hexadecapole deformation parameters ${\beta }_2$ = 0.267 and ${\beta }_4$ = 0.014.

Figure 6

Systematics of experimental crossing frequencies for the $g_{9/2}$ proton-pair and $g_{9/2}$ neutron-pair alignments in the (a) Se and (b) Kr isotopes. The arrow indicates that only the lower limit can be deduced from the present data. Experimental data are taken from Refs. [24, 26, 31, 35, 53, 54, 55, 56, 57, 58, 59].