Investigation of octupole collectivity near the $A =72$ shape-transitional point

TL;DR

This study investigates octupole collectivity in neutron-deficient isotopes near A=72, revealing a sudden increase in octupole strength at the shape transitional point, challenging simple theoretical explanations.

Contribution

It provides new experimental data on octupole strength in Se and Kr isotopes near A=72, highlighting the complexity of shape coexistence effects.

Findings

Enhanced octupole strength observed in $^{72}$Se (~32 W.u.)

Lower octupole strength in $^{74,76}$Kr (~15 W.u.)

Identification of two regions with distinct octupole strengths separated by a transitional point.

Abstract

Enhanced octupole collectivity is expected in the neutron-deficient Ge, Se and Kr isotopes with neutron number $N \approx 40$ and has indeed been observed for $^{70,72}$Ge. Shape coexistence and configuration mixing are, however, a notorious challenge for theoretical models trying to reliably predict octupole collectivity in this mass region, which is known to feature rapid shape changes with changing nucleon number and spin of the system. To further investigate the microscopic configurations causing the prolate-oblate-triaxial shape transition at $A \approx 72$ and their influence on octupole collectivity, the rare isotopes $^{72}$Se and $^{74,76}$Kr were studied via inelastic proton scattering in inverse kinematics. While significantly enhanced octupole strength of $\sim 32$ Weisskopf units (W.u.) was observed for $^{72}$Se, only strengths of $\sim 15$ W.u. were observed for $^{74,76}$Kr. In combination with existing data, the new data clearly question a simple origin of enhanced octupole strengths around $N = 40$. The present work establishes two regions of distinct octupole strengths with a sudden strength increase around the $A=72$ shape transitional point.