Collectivity of light Ge and As isotopes

Abstract

Background: The self-conjugate nuclei of the $A\ensuremath{\sim}70$ mass region display rapid shape evolution over isotopic or isotonic chains. Shape coexistence has been observed in Se and Kr isotopes reflecting the existence of deformed subshell gaps corresponding to different shell configurations. As and Ge isotopes are located halfway between such deformed nuclei and the $Z=28$ shell closure.Purpose: The present work aims at clarifying the low-lying spectroscopy of ${}^{66}$Ge and ${}^{67}$As, and providing a better insight into the evolution of collectivity in light even-even Ge and even-odd As isotopes.Methods: We investigate the low-lying levels and collectivity of the neutron deficient ${}^{67}$As and ${}^{66}$Ge through intermediate-energy Coulomb excitation, inelastic scattering, and proton knockout measurements. The experiment was performed using a cocktail beam of ${}^{68}$Se, ${}^{67}$As, and ${}^{66}$Ge nuclei at an energy of 70--80 MeV/nucleon. Spectroscopic properties of the low-lying states are compared to those calculated via shell model with the JUN45 interaction and beyond-mean-field calculations with the five-dimensional collective Hamiltonian method implemented using the Gogny D1S interaction. The structure evolution of the lower-mass Ge and As isotopes is discussed.Results: Reduced electric quadrupole transition probabilities $B(E2)$ have been extracted from the Coulomb-excitation cross sections measured in ${}^{66}$Ge and ${}^{67}$As. The value obtained for the $B(E2;{0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ in ${}^{66}$Ge is in agreement with a recent measurement, ruling out the existence of a minimum at $N=34$ in the $B(E2)$ systematics as previously observed. New transitions have been found in ${}^{67}$As and were assigned to the decay of low-lying negative-parity states.