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Transition probabilities in neutron-rich 84Se, 86Se

Reduced quadrupole transition probabilities for low-lying transitions in neutron-rich Se-84, Se-86 are investigated with a recoil distance Doppler shift (RDDS) experiment. The experiment was performed at the Istituto Nazionale di Fisica Nucleare (INFN) Laboratori Nazionali di Legnaro using the Cologne Plunger device for the RDDS technique and the AGATA Demonstrator array for the gamma-ray detection coupled to the PRISMA magnetic spectrometer for an event- by-event particle identification. In Se-86 the level lifetime of the yrast 2(1)(+) state and an upper limit for the lifetime of the 4(1)(+) state are determined for the first time. The results of 86Se are in agreement with previously reported predictions of large-scale shell-model calculations using 78Ni-I and 78Ni- II effective interactions. In addition, intrinsic shape parameters of lowest yrast states in 86Se are calculated. In semi-magic 84Se level lifetimes of the yrast 4(1)(+) and 6(1)(+) states are determined for the first time. Large-scale shell-model calculations using effective interactions 78Ni-II, JUN45, jj4b, and jj4pna are performed. The calculations describe B(E2 ; 2(1)(+) -> 0(1) (+)) and B(E2 ; 6(1)(+) -> 4(1)(+)) fairly well and point out problems in reproducing the experimental B(E2 ; 4(1)(+) -> 2(1)(+)).

SHELL-MODEL ; LIFETIME MEASUREMENTS ; EXCITED-STATES ; EXCITATIONS

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