Systematics of Spontaneously Fissioning Isomers

Excitation functions have been measured for the production of fission isomers by (α,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 (α,xn) reactions were measured for targets of ²³³U, ²³⁴U, ²³⁵U, ²³⁶U, ²³⁸U, ²³⁷Np, ²³⁹Pu, ²⁴⁰Pu, ²⁴²Pu, and ²⁴⁴Pu. Excitation functions for deuteron reactions were measured for targets of ²³⁵U, ²³⁷Np, ²³⁹Pu, ²⁴⁰Pu, ²⁴²Pu, ²⁴⁴Pu, and ²⁴³Am. New or more accurate half-lives were determined for the following fission isomers: ²³⁵ᵐPu, 30 ±{} 5 nsec; ²³⁸ᵐPu, 6.5 ±{} 1 nsec; ²⁴⁰ᵐPu, 3.8 ±{} 0.3 nsec; ²⁴¹ᵐCm, 15.3 ±{} 1 nsec; and ²⁴⁵ᵐCm, 23 ±{} 5 nsec. Isomers in ²⁴²Cm and ²⁴⁴Cm 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 (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.