Radioactivity of the Cerium-137 Isomers

Abstract

A cerium isotope with an 8.7-hr half-life has been assigned to ${\mathrm{Ce}}^{137}$ and shown to decay by electron capture with about 3% of the captures to a 445-kev level in ${\mathrm{La}}^{137}$. The 34.5-hr decay period previously characterized as electron capture of ${\mathrm{Ce}}^{137}$ is an isomeric transition from a 255-kev level in ${\mathrm{Ce}}^{137}$. Spin assignments consistent with the experimental data indicate that the transition between ground states of ${\mathrm{La}}^{137}$ should be allowed. However, the $K$ or $L$ electron capture half-life of ${\mathrm{La}}^{137}$ was found to be greater than ${10}^{5}$ years.