Intermediate-energy Coulomb excitation of58,60,62Cr: The onset of collectivity towardN=40

Abstract

Intermediate-energy Coulomb excitation measurements were performed on the neutron-rich isotopes ${}^{58,60,62}$Cr. The electric quadrupole excitation strengths, $B$($E$2; 0${}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+}$), of ${}^{60,62}$Cr are determined for the first time. The results quantify the trend of increasing quadrupole collectivity in the Cr isotopes approaching neutron number $N=40$. The results are confronted with large-scale shell-model calculations in the $fpgd$ shell using the state-of-the-art LNPS effective interaction. Different sets of effective charges are discussed that provide an improved and robust description of the $B(E2)$ values of the neutron-rich Fe and Cr isotopes in this region of rapid shell evolution. The ratio of the neutron and proton transition matrix elements, $|{M}_{n}/{M}_{p}|$, is proposed as an effective tool to discriminate between the various choices of effective charges.