Cascade gamma decay of the 176,177 Lu compound nuclei

In order to search for the most general peculiarities of the cascade γ-decay of the compound states the two-step γ-cascades following thermal neutron capture in 176 Lu and 177 Lu and terminating respectively at 16 and 7 low-laying levels of the 176 Lu and 177 Lu nuclei are studied using the sum coincidence technique from two Ge detectors. The dynamics of the complication of the nuclear structure at the transition from simple low-laying levels to compound states is investigated, as well. The experiment is performed at the thermal neutron beam of the REZ reactor. The method of analysis of the accumulated γ - γ coincidence is described in details. The experimental data are compared with the results of three versions of calculations. In order to decrease the level density the first variant is based on using the model of non-interacting Fermi particles; the same model is applied in the second variant but accounts sre used for a redetermination of the nuclear energy between the quasiparticle and vibrational excitations; and one of the versions of the generalized superfluid model of the nucleus is utilized in the third variant. All variants of the calculations are realized using the same model for the energy scaling of the γ-transition widths which is the Giant Electric Dipole Resonance model accounting for the resonance width dependence on nuclear temperature and quanta energy for E1 transitions, and the single-particle Weisskopf model for M1 and E2 transitions. It is concluded that the cascade γ-decay process of the 176,177 Lu compound nuclei following thermal neutron capture demonstrates the same peculiarities as those revealed for the heavy nuclei studied earlier. These are the following: a lower density of the excited states observed experimentally than that predicted by the models; the rather sharp change in the structure of the excited states at the energy of about excitation energy half; the probable dominant influence of vibrational excitations below this energy on the γ-decay process in comparison with quasiparticle excitations