Competition of Oblate and Prolate Deformation in 34 69,70,71,72 Se

Abstract

In the course of our systematic nuclear structure investigations of the A≈70 mass region, our special interest focused on the light Se isotopes, which are very good examples for shape coexistence phenomena at low spins. In these nuclei, the ground state rotational band is crossed at I≈4–6 by a more deformed configuration [1,2]. Due to the large band interaction (of the order of a few hundred keV) both bands are strongly mixed in the 0+, 2+ and 4+ states. Calculations employing the deformed shell model and the shell correction procedure explain shape coexistence with two minima in the potential energy surface, one at small oblate, the other at large prolate deformation [3]. A rotational band built on the excited 0 2 + state was known since long up to spin 10+ in 72Se (see e.g. the contribution of R.Bengtsson in this book). The microscopic reason for the prolate oblate shape coexistence in Se isotopes can be easily understood from the single particle Nilsson diagram (see e.g. refs [3,4]). The large (about 1.5 MeV) Z=34 gaps appear both at prolate and oblate shapes, at N = 38 the gap opens at large prolate shapes while oblate configurations in Se are stabilized by the N = 36 shell gap.