Medium-spin states of the neutron-rich nucleus $^{87}$Br

TL;DR

This study reports the first observation of medium-spin states in neutron-rich $^{87}$Br, revealing three band structures and a notable deviation from shell model predictions at high spins, suggesting increased collectivity.

Contribution

First experimental identification of medium-spin states in $^{87}$Br and comparison with shell model calculations, highlighting deviations indicating enhanced collectivity.

Findings

Three band-like structures observed in $^{87}$Br.

Deviation from shell model at high spins in the ${ m g}_{9/2}$ band.

Evidence of increased collectivity influencing high-spin states.

Abstract

Medium-spin excited states of the neutron-rich nucleus $^{87}$Br were observed and studied for the first time. They were populated in fission of $^{235}$U induced by the cold-neutron beam of the PF1B facility of the Institut Laue-Langevin, Grenoble. The measurement of $\gamma$ radiation following fission has been performed using the EXILL array of Ge detectors. The observed level scheme was compared with results of large valence space shell model calculations. The medium-spin level scheme consists of three band-like structures, which can be understood as bands built on the ${\pi}f_{5/2}$, ${\pi}(p_{3/2}+f_{5/2})$ and ${\pi}g_{9/2}$ configurations. The behavior of the observed ${\pi}g_{9/2}$ band at high spins shows a considerable deviation from the shell model predictions. This deviation in this band is probably the result of an increased collectivity, which can be understood assuming that the ${\pi}g_{9/2}$ high-$\it j$ proton polarizes the core.