Band crossings and near-rigid rotation in 76Kr and 78Kr

Abstract

High-spin states in 78Kr and 76Kr were studied via the reactions 58Ni( 24Mg, 4p) 78Kr and 58Ni( 24Mg, α2p) 76Kr at 110 MeV beam energy. An array of fifteen Compton suppressed Ge detectors was used in the POLYTESSA framework to collect γγ-coincidence spectra. Rotational bands were observed up to probable spins of 24 + and 22 − in 76Kr, and 24 + and 21 in 78Kr. While the positive-parity yrast bands show strong variations in the moments of inertia caused by the alignments of pairs of ( g 9 2 ) proton and neutron quasiparticles, the negative-parity bands display a constant moment of inertia throughout most of their frequency range. The band crossings and alignments in both nuclei are discussed in the context of the cranked shell model. From an examination of the systematics of crossing frequencies along the Kr isotopes, the previous suggestion that protons align first is confirmed. The intrinsic structures of the bands are also analyzed with the Woods-Saxon-Strutinsky cranking model. The interplay between neutron and proton excitations, the shape changes induced by quasiparticle alignment, and the possible reduction of the static proton pairing in the negative-parity bands are discussed in detail. Three lifetimes in the 76Kr ground band were remeasured by using the Doppler shift attenuation method, indicating a constant deformation of β 2 = 0.33 up to spin 10 +.