Se p 19 99 Population of isomers in decay of giant dipole resonance

The value of an isomer ratio (IR) in N=81 isotones (137Ba, 139Ce, 141Nd and 143Sm) is studied by means of the (γ, n) reaction. This quantity measures a probability to populate the isomer state in respect to the ground state population. In (γ, n) reaction giant dipole resonance (GDR) is excited and after its decay by neutron emission the nucleus has an excitation energy of a few MeV. The forthcoming γ decay by direct or cascade transitions deexcites the nucleus into isomer or ground state. It has been observed experimentally that the IR for 137Ba and 139Ce equals to about 0.13 while in two heavier isotones it is more than twice smaller. To explain this effect the structure of the excited states in the energy domain up to 6.5 MeV has been calculated within Quasiparticle Phonon Model. A lot of states connected to the ground and isomer states by E1, E2 andM1 transitions are found. The single-particle component of the wave function is responsible for the large values of the transitions. The calculated value of isomer ratio is in a very good agreement with experimental data for all isotones. It is concluded that a slightly different value of maximum energy with which nuclei rest after neutron decay of the GDR is responsible for the reported effect of A-dependence of the IR. PACS numbers: 21.60.-n, 23.20.-g, 27.60.+j Typeset using REVTEX