Studies of Isomeric Yield Ratios in the Production of Sc44, Mn52, and Y87 by Photonuclear Reactions

Abstract

Isomeric yield ratios for the production of the ${\mathrm{Sc}}^{44}$, ${\mathrm{Mn}}^{52}$, and ${\mathrm{Y}}^{87}$ isomers have been measured in photo-nuclear reactions. Scandium, iron, manganese, cobalt, yttrium, and niobium targets were used, and they were irradiated with bremsstrahlung of maximum energy in the 100- to 300-MeV range. The observed isomeric yield ratios show a general favoring of the isomer whose spin is closer to that of the target nucleus. In those cases in which several different bremsstrahlung energies were used, there were no significant variations in the observed isomeric yield ratios with changes in the bremsstrahlung maximum energy. The observed isomeric yield ratios are compared with theoretical isomer ratios calculated on the basis of a compound-nucleus mechanism using the statistical-model formulation of Huizenga and Vandenbosch. The observed isomeric yield ratios can be reproduced by using the same values for the level-density spin-cutoff parameter that are required to account for the isomer ratios that have been observed for particle-induced reactions producing the same isomeric pairs. The required spin-cutoff parameters are reduced by 10 and 20% from those expected for a rigid body in the cases of ${\mathrm{Sc}}^{44}$ and ${\mathrm{Y}}^{87}$. For ${\mathrm{Mn}}^{52}$ the required value is about 50% larger than the rigid-body value. The result for ${\mathrm{Mn}}^{52}$ is unusual and may be due to shell structure effects which are not included in the statistical model that was used as a basis for the calculations.