Population of isomers in decay of the giant dipole resonance

The value of an isomeric ratio (IR) in N=81 isotones ($^{137}$Ba, $^{139}$Ce, $^{141}$Nd and $^{143}$Sm) is studied by means of the ($\gamma, n)$ reaction. This quantity measures a probability to populate the isomeric state in respect to the ground state population. In ($\gamma, n)$ reactions, the giant dipole resonance (GDR) is excited and after its decay by a neutron emission, the nucleus has an excitation energy of a few MeV. The forthcoming $\gamma$ decay by direct or cascade transitions deexcites the nucleus into an isomeric or ground state. It has been observed experimentally that the IR for $^{137}$Ba and $^{139}$Ce equals about 0.13 while in two heavier isotones it is even less than half the size. To explain this effect, the structure of the excited states in the energy region up to 6.5 MeV has been calculated within the Quasiparticle Phonon Model. Many states are found connected to the ground and isomeric states by $E1$, $E2$ and $M1$ transitions. The single-particle component of the wave function is responsible for the large values of the transitions. The calculated value of the isomeric ratio is in very good agreement with the experimental data for all isotones. A slightly different value of maximum energy with which the nuclei rest after neutron decay of the GDR is responsible for the reported effect of the A-dependence of the IR.Comment: 16 pages, 4 Fig

Production of Spontaneously Fissioning Isomers in Th, U, Np. Pu and Am Isotopes in Reactions Induced By 14.7 Mev Neutrons

The cross-sections for‘ the formation of spontaneously fissioning isomers have been measured in (n, 2n) reactions on 239Pu, 241Pu, 240Am and 242Am and in (n, n') reactions on 230Pu,243Am induced by 14.7 MeV neutrons. Fission fragments were detected by means of a multiwire spark chamber, It was found that the spontaneously fissioning isomers are formed with considerably lower cross-section in (n, n') reactions than in (n, 2n) reaction