L. M. Melnikova

Isomeric ratios in reactions induced by gamma rays and fast neutrons in the isotopes of Re, Ir, and Au

Isomeric ratios were measured in the (γ, n) and (n, 2n) reactions leading to the formation of 184Re, 190Ir, and 196Au odd-odd nuclides. The measurements were performed by the activation method implemented for Re, Ir, and Au samples of natural isotopic composition that were irradiated with 14.7-MeV neutrons and bremsstrahlung photons of endpoint energy 22 MeV. Isomeric-and ground-state nuclei formed in these reactions were identified by their x-or γ-ray spectra and by their half-lives. The isomeric ratios were calculated on the basis of the statistical model, and the results were compared with experimental data, where by it was possible to determine parameters that characterize the dependence of the level density on the excitation energy and angular momentum. The effect of the nuclear structure on these parameters is discussed.

Excitation of isomeric 1h11/2 states in nuclear reactions induced by γ rays and neutrons and in beta decay

Isomeric ratios were measured for N=81 isotones (135Xe, 137Ba, 139Ce, 141Nd, 143Sm). In the experiment reported here, μν≤3±10−Jπ=11/2− isomers were excited in (n, γ) and (γ, n) reactions and in the β+ decay of 139Pr and 141Pm. In order to determine the reaction yields, use was made of the activation method involving measurement of the gamma-ray spectra of reaction products. It is found that, in the same reactions, isomeric ratios are different for isotones characterized by different atomic numbers Z. Isomeric ratios were calculated with the spectra of low-lying levels and radiative-transition probabilities established on the basis of the quasiparticle-phonon model. Good agreement between the experimental and calculated values of isomeric ratios is obtained for all isotopes invesigated here. The dependence of isomeric ratios on the atomic number Z of a nucleus is explained by the difference of reaction energies, which leads to different probabilities of excitation of activation levels through which the isomers being considered are populated.