Low energy levels in 72Ni

Abstract

The yrast Jπ=8+ states in neutron-rich 70,72,74,76Ni nuclei are predicted to be isomeric. The present paper describes two GANIL experiments. In the first of them a search was made for the 8+ isomeric states in 72,74Ni nuclei via fragmentation of 76Ge using the ion γ-decay correlation technique. Although these states were not observed, limits for their lifetimes were determined. In the second experiment the decay spectroscopy of 70,72Co nuclei was performed using fragmentation of a 86Kr36- beam and the new LISE2000 spectrometer. The β delayed γ rays from the decay of 70,72Co to 70,72Ni were observed using the EXOGAM germanium detectors. The half life of 72Co was measured to be 62(3) ms and the level sequence of the lowest excited states in 72Ni was suggested, with the 2+ state at 1096 keV. An attempt to reproduce the level scheme in terms of shell-model calculations was undertaken. The reasons for the disappearance of the 8+ isomer in 72Ni are discussed.