The effect of gamma-ray competition in statistical-model calculations of isomer yield ratios

Abstract

The distributions in angular momentum in the formation of compound nuclei in 4He ion bombardments and in their de-excitation by particle or gamma-ray emission were investigated in a compound-statistical model calculation of excitation functions and particle evaporation spectra. The calculation was performed by the Monte Carlo technique and from the distributions in angular momentum in the reaction products, isomer yield ratios were calculated and compared with experiment. The importance of gamma-ray emission in competition with particle evaporation is discussed in detail. It is shown that the effect of the preferred population of the higher angular momentum states in the (α, n) reaction product over that for the (α, 2n), termed “spin fractionation”, depends critically on the strength on the gamma-ray competition. Agreement with experimental isomer ratios is obtained with model parameters which are consistent with the predictions of the Fermi gas model and which give agreement between calculated and experimental excitation functions.