Systematics of Spontaneously Fissioning Isomers

Abstract

Excitation functions have been measured for the production of fission isomers by ($\ensuremath{\alpha},xn$) reactions for bombarding energies of 20-29 MeV and by ($d,2n$), ($d,p$), and ($d,pn$) reactions for bombarding energies of 9-14 MeV. Excitation functions for ($\ensuremath{\alpha},xn$) reactions were measured for targets of $^{233}\mathrm{U}$, $^{234}\mathrm{U}$, $^{235}\mathrm{U}$, $^{236}\mathrm{U}$, $^{238}\mathrm{U}$, $^{237}\mathrm{Np}$, $^{239}\mathrm{Pu}$, $^{240}\mathrm{Pu}$, $^{242}\mathrm{Pu}$, and $^{244}\mathrm{Pu}$. Excitation functions for deuteron reactions were measured for targets of $^{235}\mathrm{U}$, $^{237}\mathrm{Np}$, $^{239}\mathrm{Pu}$, $^{240}\mathrm{Pu}$, $^{242}\mathrm{Pu}$, $^{244}\mathrm{Pu}$, and $^{243}\mathrm{Am}$. New or more accurate half-lives were determined for the following fission isomers: $^{235m}\mathrm{Pu}$, 30 \ifmmode\pm\else\textpm\fi{} 5 nsec; $^{238m}\mathrm{Pu}$, 6.5 \ifmmode\pm\else\textpm\fi{} 1 nsec; $^{240m}\mathrm{Pu}$, 3.8 \ifmmode\pm\else\textpm\fi{} 0.3 nsec; $^{241m}\mathrm{Cm}$, 15.3 \ifmmode\pm\else\textpm\fi{} 1 nsec; and $^{245m}\mathrm{Cm}$, 23 \ifmmode\pm\else\textpm\fi{} 5 nsec. Isomers in $^{242}\mathrm{Cm}$ and $^{244}\mathrm{Cm}$ 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 ($n,2n$), ($p,2n$), and ($\ensuremath{\gamma},n$) 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.