SPONTANEOUSLY FISSIONING ISOMERS IN U, NP, PU AND AM ISOTOPES

Spontaneously fissioning isomers have been discovered and assigned to 236U, 239Np, 236, 237, 240, 241, 242, 243Pu and 239, 241Am. They were produced by irradiation of thin targets with 9-14 MeV protons or deuterons. Fission-in-flight of recoiling reaction products was observed with plastic foils as fission track detectors. The half-lives range from 9 ns to 1.5 μs and the formation cross sections from 12 μb to 0.13 μb. The proton excitation functions for the 239mAm, 241mAm and 236mPu isomeric states show threshold behaviour typical of a (p, 2n) reaction. The differences EII in thresholds for the ground state and the isomeric state, respectively, are: EII(239mAm) = 2.9 ± 0.1 MeV, EII(241mAm) = 2.5 ± 0.1 MeV, EII(236mPu) = 4.1 ± 0.2 MeV. An isomer ratio of 2.2 × 10-5 for 236mPu was obtained from the measured yield of the isomer and of the 326Pu ground state. Other isomer ratios were deduced using calculated ground-state yields. The variations in half-lives and excitation energies with Z and N are discussed and compared to theoretical calculations of the barrier shapes. © 1969.