We have observed the $\alpha$ decay of ${}^{217}\mathrm{Th}$ populating the low-lying two excited states in ${}^{213}\mathrm{Ra}.$ The branching ratio to the first and the second excited states in ${}^{213}\mathrm{Ra},$ which have expected $J^{\pi }$ values of ${\frac{5}{2}}^{-}$ and ${\frac{3}{2}}^{-},$ was determined to be ${2.6}_{-1.1}^{+1.6\mathrm{}}\%$ and ${5.1}_{-1.6}^{+2.0\mathrm{}}\%,$ respectively, and the energy of the second excited state in ${}^{213}\mathrm{Ra}$ was determined to be 834 keV for the first time. The systematical trend of the branching ratio for $N=125\mathrm{}$ isotones with even proton numbers near the ${}^{208}\mathrm{Pb}$ core was reasonably explained by the shell model of $\alpha$ decay, where the relative reduced width for specific neutron-orbital and angular momentum has an identical value for the nuclei in the systematics.

• Received 7 September 1999

DOI:https://doi.org/10.1103/PhysRevC.61.034309