Compound contributions in the anomalous back-angle elastic scattering of alpha particles fromCa40andCa44

Excitation functions for $\alpha +{}_{}{}^{40,44}{}_{}{}^{}\mathrm{Ca}$ elastic scattering have been measured with bombarding energies in the range of 20-27 MeV in 50-keV steps at several angles between 150 and 175°. For ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ excitation functions were also taken in 10-keV steps over a limited interval. Statistical fluctuations were observed in all the excitation functions and were found to be generally stronger for ${}_{}{}^{44}{}_{}{}^{}\mathrm{Ca}$ than for ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$. An analysis of the fluctuations yields a relative compound contribution of about 2% for ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ and greater than about 5% for ${}_{}{}^{44}{}_{}{}^{}\mathrm{Ca}$. The backward enhancement in $\alpha +{}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ scattering, observed by many previous workers, is shown to arise primarily from a direct contribution and not from a fluctuating compound one.