Persistence of octupole correlations in ${}^{231}$Ra

Abstract

The structure of ${}^{231}$Ra, the heaviest odd Ra nucleus currently accessible for detailed spectroscopic investigation, has been studied in the $\text{β}{}^{\mathrm{-}}$ decay of ${}^{231}$Fr. The main purpose has been to verify whether fast $\text{B}$(E1) rates and significant octupole correlations recently established in ${}^{229}$Ra persist in ${}^{231}$Ra. The excited states in ${}^{231}$Ra have been determined for the first time. Spins and parities have been deduced from conversion electron measurements, while level half-lives in the ps and ns ranges have been measured with the Advanced Time Delayed $\text{βγγ}$(t) method. The moderately fast $\text{B}$(E1) rates found for transitions connecting the lowest-lying $\text{K}{}^{\mathrm{\pi }}\text{=5/2}{}^{\mathrm{\pm }}$ and $\text{K}{}^{\mathrm{\pi }}\text{=1/2}{}^{\mathrm{\pm }}$ bands, reveal the persistence of octupole collective effects in ${}^{231}$Ra, although the $\text{B}$(E1) rates are lower than in ${}^{229}$Ra. These findings reinforce the differences in the $\text{B}$(E1) systematics between isotonic Ra and Th nuclei.