Three-quasiparticle excitations in 79Kr

Abstract

Excited states in 79Kr have been investigated up to spin 27 2 using techniques of in-beam γ-ray spectroscopy. Mean lifetimes of 33 levels have been determined applying Doppler-shift and γ-RF methods. The bands built on the g 9 2 , [303] 5 2 , [301] 1 2 and [301] 3 2 neutron configurations have been interpreted in terms of the cranking model. The g 9 2 band is a rotation-aligned band. The band crossing at spin 21 2 is attributed to the alignment of a pair of g 9 2 protons. The 3qp band above the second 21 2 + state includes M1 transitions with high transition rates up to 0.6 W.u. The negative-parity bands show a regular structure at low spins. Irregularities in these bands above spin 15 2 are interpreted as due to the alignment of two g 9 2 protons as well. A ΔI = 1 sequence starting at a ( 17 2 − ) state includes M1 transitions of rates of about 0.3 W.u. This structure has been proposed to involve a non-collective 3qp excitation with two g 9 2 nucleons. Deformed-shell-model cranking calculations with the Woods-Saxon potential predict the coexistence of prolate and oblate shapes at low spin due to the polarization of the core by the valence neutron, and a change towards prolate shapes after the alignment of two g 9 2 protons.