Spin distribution of the compound nucleus in heavy ion reactions at near-barrier energies

Gamma ray multiplicities and the (HI,xn) cross sections for the dominant reaction channel were measured at near-barrier bombarding energies for the systems α${+\mathrm{}}^{154}$Sm and ${}_{}{}^{12}{}_{}{}^{}$C${+\mathrm{}}^{154}$Sm. The mean value of the spin distribution of the compound nucleus was obtained for each of the systems studied. The results of these measurements together with the results of an earlier study on ${}_{}{}^{16}{}_{}{}^{}$O${+\mathrm{}}^{154}$Sm indicate that at bombarding energies near and below the Coulomb barrier the spin distributions of the compound nucleus are broader than the triangular distributions expected from a sharp cutoff model, these deviations being larger for the more massive projectiles. The different behavior of the three systems clearly indicates the importance of the centrifugal barrier penetrability in determining the spin distribution of the compound nucleus. The absolute values of the mean angular momentum can be accounted for if one also includes deformation effects. A one-dimensional barrier-penetration model including the effect of averaging over the orientation of the deformed target can account for the experimental results.