Excitation of isomeric 1h _11/2 states in nuclear reactions induced by γ rays and neutrons and in beta decay

Abstract

Isomeric ratios were measured for N=81 isotones (¹³⁵Xe, ¹³⁷Ba, ¹³⁹Ce, ¹⁴¹Nd, ¹⁴³Sm). In the experiment reported here, μ _ν ≤3±10⁻ J ^π=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.