Systematics of Spontaneously Fissioning Isomers

H. C. Britt; S. C. Burnett; B. H. Erkkila; J. E. Lynn; W. E. Stein

doi:10.1103/PhysRevC.4.1444

Abstract

Excitation functions have been measured for the production of fission isomers by ( $α, x n$ ) reactions for bombarding energies of 20-29 MeV and by ( $d, 2 n$ ), ( $d, p$ ), and ( $d, p n$ ) reactions for bombarding energies of 9-14 MeV. Excitation functions for ( $α, x n$ ) reactions were measured for targets of ${}^{233}U$ , ${}^{234}U$ , ${}^{235}U$ , ${}^{236}U$ , ${}^{238}U$ , ${}^{237}{Np}$ , ${}^{239}{Pu}$ , ${}^{240}{Pu}$ , ${}^{242}{Pu}$ , and ${}^{244}{Pu}$ . Excitation functions for deuteron reactions were measured for targets of ${}^{235}U$ , ${}^{237}{Np}$ , ${}^{239}{Pu}$ , ${}^{240}{Pu}$ , ${}^{242}{Pu}$ , ${}^{244}{Pu}$ , and ${}^{243}{Am}$ . New or more accurate half-lives were determined for the following fission isomers: ${}^{235 m}{Pu}$ , 30 ± 5 nsec; ${}^{238 m}{Pu}$ , 6.5 ± 1 nsec; ${}^{240 m}{Pu}$ , 3.8 ± 0.3 nsec; ${}^{241 m}{Cm}$ , 15.3 ± 1 nsec; and ${}^{245 m}{Cm}$ , 23 ± 5 nsec. Isomers in ${}^{242}{Cm}$ and ${}^{244}{Cm}$ were identified but their half-lives were too long for measurement with the present techniques. The results are analyzed with a statistical model using realistic level-density expressions with many parameters fixed by comparison with experimental measurements of neutron-fission cross sections, neutron-to-fission decay widths, spallation cross sections, and fission lifetimes. This model is applied to the presently measured excitation functions and the results obtained previously by other groups for ( $n, 2 n$ ), ( $p, 2 n$ ), and ( $γ, n$ ) reactions. With this model fission-barrier parameters are determined for Pu, Am, and Cm isotopes. Inconsistencies in the results obtained by applying this model to experimental isomer excitation functions indicate the directions for future refinements.

Received 17 May 1971

DOI:https://doi.org/10.1103/PhysRevC.4.1444

Physical Review C

covering nuclear physics