$\gamma$-ray linear polarization measurements and $(g_{9/2})^{-3}$ neutron alignment in $^{91}$Ru

TL;DR

This study measured gamma-ray linear polarization in $^{91}$Ru to determine high-spin state properties and confirmed the neutron alignment in the $ u(g_{9/2})^{-3}$ configuration using experimental data and shell model calculations.

Contribution

First-time measurements of gamma-ray polarization in $^{91}$Ru enabled precise spin and parity assignments, supporting the neutron alignment interpretation with shell model analysis.

Findings

Several new gamma-ray transitions observed.

Spin and parity of high-spin states firmly assigned.

Shell model calculations agree with experimental data.

Abstract

Linear polarization measurements have been performed for $\gamma$-rays in $^{91}$Ru produced with the $^{58}$Ni($^{36}$Ar, $2p1n$$\gamma$)$^{91}$Ru reaction at a beam energy of 111 MeV. The EXOGAM Ge clover array has been used to measure the $\gamma$-$\gamma$ coincidences, $\gamma$-ray linear polarization and $\gamma$-ray angular distributions. The polarization sensitivity of the EXOGAM clover detectors acting as Compton polarimeters has been determined in the energy range 0.3$-$1.3 MeV. Several transitions have been observed for the first time. Measurements of linear polarization and angular distribution have led to the firm assignments of spin differences and parity of high-spin states in $^{91}$Ru. More specifically, calculations using a semi-empirical shell model were performed to understand the structures of the first and second (21/2$^{+}$) and (17/2$^{+}$) levels. The results are in good agreement with the experimental data, supporting the interpretation of the non yrast (21/2$^{+}$) and (17/2$^{+}$) states in terms of the $J_{\rm max}$ and $J_{\rm max}-2$ members of the seniority-three $\nu(g_{9/2})^{-3}$ multiplet.