Lifetimes of the 41+ states in Fe62,64 and the 11/21- states in Co61,63 and Mn59 were measured at the Grand Accélérateur National d'Ions Lourds (GANIL) facility by using the Advanced Gamma Tracking Array (AGATA) and the large-acceptance variable mode spectrometer (VAMOS++). The states were populated through multinucleon transfer reactions with a U238 beam impinging on a Ni64 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 ν2p1/2 and ν1g9/2 orbitals leads to an increased population of the quasi-SU(3) pair (ν1g9/2,ν2d5/2), 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.

The authors would also like to thank M. Loriggiola, Laboratori Nazionali di Legnaro, and G. Fremont, GANIL, for preparing the target and degrader foils. This project received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement n°654002. Support from the Norwegian Research Council, project grant 213442, is acknowledged. The project has received funding from the Swedish Research Council. This work is supported by the Spanish Ministerio de Economía y Competitividad under contract FPA2014-57196-C5-4-P. One of the authors, A. Gadea, was partially supported byMINECO and Generalitat Valenciana, Spain, under the grants FPA2014- 57196-C5 and PROMETEOII/2014/019, and by EU FEDER

M. Klintefjord et al. -- 11 pags., 8 figs., 1 tab. -- Open Access funded by Creative Commons Atribution Licence 4.0

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