β-delayed spectroscopy of neutron-rich-tantalum nuclei: Shape evolution in neutro-rich tungsten isotopes

Abstract

The low-lying structure of ^188,190,192W has been studied following β decays of the neutron-rich mother nuclei ^188,190,192Ta produced following the projectile fragmentation of a 1-GeV-per-nucleon ²⁰⁸Pb primary beam on a natural beryllium target at the GSI Fragmentation Separator. The β-decay half-lives of ¹⁸⁸Ta, ¹⁹⁰Ta and ¹⁹²Ta have been measured, with γ-ray decays of low-lying states in their respective W daughter nuclei, using heavy-ion β-γ correlations and a position-sensitive silicon detector setup. The data provide information on the low-lying excited states in ¹⁸⁸W, ¹⁹⁰W and ¹⁹²W, which highlight a change in nuclear shape at ¹⁹⁰W compared with that of lighter W isotopes. This evolution of ground-state structure along the W isotopic chain is discussed as evidence for a possible proton subshell effect for the A~190 region and is consistent with maximisation of the γ-softness of the nuclear potential around N~116