Spin Isomers of the Shape IsomerPu237m

Abstract

Two components in the spontaneous-fission decay of ${}_{}{}^{237m}{}_{}{}^{}\mathrm{Pu}$ have been investigated. The half-lives of the two isomers are 82 ± 8 nsec and 1120 ± 80 nsec. The production ratio of the two components was determined for three different bombarding conditions: ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ + 24-MeV $\alpha$ particles, ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ + 27-MeV $\alpha$ particles, and ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$ + 12.1-MeV deuterons. The dependence of the production ratio on angular momentum deposition for the various reactions shows that the two shape isomers differ in spin, with the short-lived component corresponding to the lower-spin state. The experimental isomer ratios are compared with statistical-model calculations. Assuming the validity of the statistical description of isomer population, and the absence of a significant $\gamma$-decay branch for the high-spin isomer, the deformation associated with shape isomerism can be deduced from isomer-spin information extracted from the present experiment and reference to the Nilsson diagram at large deformation. The deformation indicated is approximately $\epsilon =0.6\mathrm{}$, in good agreement with theoretical estimates.