Excitation Functions and Photon Emission in Compound-Nucleus Reactions with High Angular Momentum

Measured excitation functions are compared to detailed evaporation calculations for the reactions ${}_{}{}^{140}{}_{}{}^{}\mathrm{Ce}({}_{}{}^{16}{}_{}{}^{}\mathrm{O}, xn){}_{}{}^{156-x}{}_{}{}^{}\mathrm{Dy}$, ${}_{}{}^{144}{}_{}{}^{}\mathrm{Nd}({}_{}{}^{12}{}_{}{}^{}\mathrm{C}, xn){}_{}{}^{156-x}{}_{}{}^{}\mathrm{Dy}$, ${}_{}{}^{136}{}_{}{}^{}\mathrm{Ba}({}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}, xn){}_{}{}^{156-x}{}_{}{}^{}\mathrm{Dy}$ ($x=5, 6, 7$), ${}_{}{}^{154}{}_{}{}^{}\mathrm{Gd}({}_{}{}^4{}_{}{}^{}\mathrm{He}, xn){}_{}{}^{158-x}{}_{}{}^{}\mathrm{Dy}$ ($x=7, 8, 9$), and ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}({}_{}{}^4{}_{}{}^{}\mathrm{He}, xn){}_{}{}^{185-x}{}_{}{}^{}\mathrm{Re}(x=2, 3, 4)$. The agreement between theory and experiment is found to depend critically on the rate of photon emission within about 10 MeV of the yrast levels. For the first four systems above, good fits are obtained only when the $\gamma$-emission rate is strongly enhanced with respect to the rate indicated by the $\gamma$ width of neutron resonances. $\gamma$ enhancement by the following two methods leads to quite good fits to experiment: (1) a constant (independent of $J$) enhancement of a factor of 100 and (2) enhancement proportional to ($2J+1$). For the system ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ + ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ such pronounced enhancement does not seem to be required. These results are interpreted in terms of the structure of the $\gamma$-cascade band. At sufficiently high angular momenta, the $\gamma$-cascade band is predicted to give way to an $\alpha$ band. Possibilities are discussed for experimental verification of some of the theoretical predictions.