Background: In neutron-rich nuclei around N = 40 rapid changes in nuclear structure can be observed. While 68 Ni 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 58,60,62 Cr 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 58,60,62 Cr. The experiment was carried out at the NSCL with the SeGA array in 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 58,60,62 Cr toward N = 40 and point to stronger quadrupole deformations compared to neighbouring 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 60,62 Cr. Compared to 58 Cr, experimental B42 and B62 values for 60 Cr 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 neighbouring Ti, Fe nuclei.