α decay of ²¹⁷Th populating excited states in ²¹³Ra

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