Reduced transition strengths of low-lying yrast states in chromium isotopes in the vicinity of N=40

Background: In neutron-rich nuclei around N=40 rapid changes in nuclear structure can be observed. While Ni68 exhibits signatures of a doubly magic nucleus, experimental data along the isotopic chains in even more exotic Fe and Cr isotopes—such as excitation energies and transition strengths—suggest a sudden rise in collectivity toward N=40. Purpose: Reduced quadrupole transition strengths for low-lying transitions in neutron-rich Cr58,60,62 are investigated. This gives quantitative new insights into the evolution of quadrupole collectivity in the neutron-rich region close to N=40. Method: The recoil distance Doppler-shift (RDDS) technique was applied to measure lifetimes of low-lying states in Cr58,60,62. The experiment was carried out at the National Superconducting Cyclotron Laboratory (NSCL) with the SeGA array in a plunger configuration coupled to the S800 magnetic spectrograph. The states of interest were populated by means of one-proton knockout reactions. Results: Data reveal a rapid increase in quadrupole collectivity for Cr58,60,62 toward N=40 and point to stronger quadrupole deformations compared to neighboring Fe isotopes. The experimental B(E2) values are reproduced well with state-of-the-art shell-model calculations using the LNPS effective interaction. A consideration of intrinsic quadrupole moments and B42 ratios suggest an evolution toward a rotational nature of the collective structures in Cr60,62. Compared to Cr58, experimental B42 and B62 values for Cr60 are in better agreement with the E(5) limit. Conclusion: Our results indicate that collective excitations in neutron-rich Cr isotopes saturate at N=38, which is in agreement with theoretical predictions. More detailed experimental data of excited structures and interband transitions are needed for a comprehensive understanding of quadrupole collectivity close to N=40. This calls for additional measurements in neutron-rich Cr and neighboring Ti and Fe nuclei.