In-beam study of 80Kr; Quasiparticle excitations in nuclei around mass 80

Abstract

Abstract The excited states in 80 Kr have been studied in the reactions 77 Se(α, n), 78 Se(α, 2n), 80 Se(α, 4n) and 65 Cu( 18 O, p2n) by using in-beam γ-ray spectroscopy. In addition to γγ-coincidences, excitation functions and angular distributions, linear polarization of γ-rays and conversion electrons were also measured. All together, 32 levels have been identified up to spin 14 at an excitation energy of 6.7 MeV in 80 Kr. For 21 of these levels the mean lifetime could be determined by Doppler shift methods and by the pulsed-beam γ-timing method. The B (E2) values of 30–60 W.u., derived for many transitions, indicate strong collectivity and the existence of several band structures is suggested. Above 2.5 MeV 2-quasiparticle (qp) excitations become important. The excitation energies of 80 Kr and its neighbours 77, 78, 79 Kr, 77 Br and 81 Rb have been analysed in terms of the cranked shell model. In 78,80 Kr two-proton excitations have been found to be responsible for the observed band crossing. Quasiparticle excitations strongly influence the pairing and stabilize the deformation. The anomalies in the negative-parity bands of 81 Rb and 77 Br are interpreted as a crossing of a 3qp and a 1qp band and the relatively low frequency of the crossing point is ascribed to the blocking effect.