Fine structure in the $\alpha$ decay of ${}_{90}{}^{221}\mathrm{Th}$, populating excited states in ${}_{88}{}^{217}\mathrm{Ra}$, was studied using $\alpha \gamma$-coincidence spectroscopy. Two $\alpha$-decay branches from ${}^{221}\mathrm{Th}$ have been newly observed, with $E_{\alpha }\left(\mathrm{keV}\right)\left[b_{\alpha }(\%)\right]=7951\left(8\right)\left[0.14\left(3\right)\right]$ and 8247(3)[1.51(12)], together with three previously known branches. Also, two new states in ${}^{217}\mathrm{Ra}$ were identified at $E$ = 177 and 227 keV. The ground-state configurations of the odd-$A$, $N$ = 131 transitional isotones above ${}^{208}\mathrm{Pb}$ are interpreted from their $\alpha$-decay fine structure systematics and considered in terms of predictions using spherical shell and reflection-asymmetric models.

• Received 5 June 2020
• Revised 30 September 2020
• Accepted 10 November 2020

DOI:https://doi.org/10.1103/PhysRevC.102.054335