Properties of the excited nuclei and neutron induced reactions

The results of experimental investigations of the (α, nγ) reactions on 113 In, 130 Te and 134 Ba at the energies of 10-30 MeV are analyzed using the statistical model in terms of the Hauser-Feshbach formalism with consideration of the probability of preequilibrium single particle emission. The influence of the angular momentum distribution of compound nucleus on isomeric ratios is discussed. In particular, the change of average angular momenta of compound nucleus as a result of the angular momenta carried away by preequilibrium particles is considered with fixed standard level density parameters. The process of nuclear reactions is divided into the following two steps: preequilibrium emission and compound nuclear formation, consecutive evaporation of particles in competition with γ-quanta emission. The total reaction cross section is determined in terms of the optical model. The preequilibrium fraction in the total reaction cross section is estimated by exciton model. The conclusion on the possibility of estimation of the angular momentum carried away in preequilibrium processes basing on the analysis of isomeric ratio is made. The set of experimental data on neutron spectra and isomeric ratios allows an unambiguous determination of both the angular momenta and the exciton model parameter k. 9 refs., 4 figs