Measurement of lifetimes in ^62,64Fe, ^61,63Co, and ⁵⁹Mn

Lifetimes of the 4₁⁺ states in ^62,64Fe and the 11/2₁^- states in ^61,63Co and ⁵⁹Mn were measured at the Grand Accelerateur National d'Ions Lourds (GANIL) facility by using the Advanced γ Tracking Array (AGATA) and the large-acceptance variable mode spectrometer (VAMOS++). The states were populated through multinucleon transfer reactions with a ²³⁸U beam impinging on a ⁶⁴Ni target, and lifetimes in the picosecond range were measured by using the recoil distance Doppler shift method. The data show an increase of collectivity in the iron isotopes approaching N = 40. The reduction of the subshell gap between the ν2p_1/2 and ν1g_9/2 orbitals leads to an increased population of the quasi-SU(3) pair (ν1g_9/2), ν2d_5/2), which causes an increase in quadrupole collectivity. This is not observed for the cobalt isotopes with N < 40 for which the neutron subshell gap is larger due to the repulsive monopole component of the tensor nucleon-nucleon interaction. The extracted experimental B(E2) values are compared with large-scale shell-model calculations and with beyond-mean-field calculations with the Gogny D1S interaction. A good agreement between calculations and experimental values is found, and the results demonstrate in particular the spectroscopic quality of the Lenzi, Nowacki, Poves, and Sieja (LNPS) shell-model interaction. (authors)