Effects of Angular Momentum and Gamma-Ray Emission on Excitation Functions

Excitation functions for many compound nucleus reactions are strongly dependent on the competition between gamma-ray and particle emission in the final particle-emission step. This competition depends, in turn, mainly on the energies and spins of a relatively few levels in the product nucleus, namely, on the lowest energy level at every angular momentum J (these energies are herein designated by Eⱼ). A method for making approximate calculations is described in which the influence of the competitive gamma-ray emission on excitation functions well above threshold is estimated, using assumed plausible distributions of the Eⱼ's. It is found that larger values of the level density parameter a are required to achieve agreement of calculations with experimental data when the competitive gamma-ray emission is included than when it is neglected (i.e., practically equivalent to setting Eⱼ=0 for all angular momenta). An approximate analysis of experimental excitation functions for the reaction-pair Ag¹⁰⁹(α, n)In¹¹² and Ag¹⁰⁹(α, 2n)In¹¹¹ suggests that $a>12$ MeV^-1, assuming the level density expression ω(E)∝E^-2exp[2(aE)], or $a>7$ MeV^-1 assuming ω(E)∝exp[2(aE)]. Alternatively, assuming a≈16 MeV^-1 (in the first formula), consistent with values calculated by Lang from level spacings observed near neutron binding energies, it appears that the average value of the Eⱼ's for angular momenta of (11/2) to (17/2) in In¹¹¹ is roughly 2 to 2.5 MeV. It is suggested that instead of trying to extract a from excitation functions, it is perhaps more appropriate to try to extract information on the Eⱼ distribution, using for this purpose values of a from other types of experiment.