The mutable nature of particle-core excitations with spin in the one-valence-proton nucleus 133Sb

TL;DR

This study investigates the gamma-ray decay and particle-core excitations in 133Sb, revealing a rapid change in the nature of these excitations with increasing spin through experimental and theoretical analysis.

Contribution

It provides the first observation of high-spin states above the isomer and introduces a microscopic model explaining the evolving particle-core coupling in 133Sb.

Findings

Observation of high-spin states above the isomer for the first time

Discovery of a significant difference in B(M1) transition strengths

Evidence of rapid change in particle-core excitation nature with spin

Abstract

The gamma-ray decay of excited states of the one-valence-proton nucleus 133Sb has been studied using cold-neutron induced fission of 235U and 241Pu targets, during the EXILL campaign at the ILL reactor in Grenoble. By using a highly efficient HPGe array, coincidences between gamma-rays prompt with the fission event and those delayed up to several tens of microseconds were investigated, allowing to observe, for the first time, high-spin excited states above the 16.6 micros isomer. Lifetimes analysis, performed by fast-timing techniques with LaBr3(Ce) scintillators, reveals a difference of almost two orders of magnitude in B(M1) strength for transitions between positive-parity medium-spin yrast states. The data are interpreted by a newly developed microscopic model which takes into account couplings between core excitations (both collective and non-collective) of the doubly magic nucleus 132Sn and the valence proton, using the Skyrme effective interaction in a consistent way. The results point to a fast change in the nature of particle-core excitations with increasing spin.