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

The excited states in ⁸⁰Kr have been studied in the reactions ⁷⁷Se(α, n), ⁷⁸Se(α, 2n), ⁸⁰Se(α, 4n) and ⁶⁵Cu(¹⁸O, 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 ⁸⁰Kr. 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 ⁸⁰Kr and its neighbours ^{77, 78, 79}Kr, ⁷⁷Br and ⁸¹Rb have been analysed in terms of the cranked shell model. In ^78,80Kr 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 ⁸¹Rb and ⁷⁷Br 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.