Charged-Particle Studies of Isomeric Fission

Delayed fission events due to isomeric fission have been produced using 3He and 4He beams from the Max-Planck-Institut MP Tandem Van de Graaff accelerator. The isomeric nuclei which recoil from the target and subsequently fission in flight are detected by a square cone array of plastic foil detectors mounted along the beam direction. From the distribution of fission tracks along the plates, half-lives are calculated on the assumption that the recoil has its full momentum. In the following, we list the bombarded targets, the most probable reactions, the assumed final nucleus in which isomerism occurs, the bombarding energies, and the deduced half-lives: 233U (4He, 2n) 235Pu, 26.1 MeV, 20 ns; 235U (4He, 2n) 237Pu, 26.1 MeV, 50 ns; 236U (4He, 2n) 238Pu, 26.1 MeV, about 500 ns; 238U (4He, 2n) 240Pu, 26.1 MeV, 7 ns; 237Np (3He, pn) 238Pu, 26.1 and 30 MeV, about 500 ns; 237Np (3He, p2n) 237Pu, 26.1 and 30 MeV, 40 ns; 237Np (4He, 2n) 239Am, 26.1 MeV, 110 ns; 239Pu (3He, p2n) 239Am, 30 MeV, 110 ns; 239Pu (4He, 2n) 241Cm, 26.1 MeV, 20 ns; 242Pu + 3He, 30 MeV, half-life of the order of 1 μs ; 241Am + 3He leading to Cm or Bk, 30 MeV, 45 ns. As only few cross bombardments have been made and the assignment of the reaction mode is often not unique, many of our isotopic assignments are somewhat tentative. (author)