The excitation functions for the production of some of the nuclei formed by alpha-particle bombardment of copper and silver have been measured for alpha energies up to 40 Mev. Comparison of these data with theoretical predictions based on (a) the assumption of relatively large radius constants ($r_0=1.5\mathrm{}$ for Zn, $r_0=1.65\mathrm{}$ for Cd) and (b) an energy-dependent odd-odd to even-even level density ratio allows two conclusions: (1) ($\alpha , \mathrm{pn}$) cross sections are in good agreement with predictions based on the statistical theory. This lends some support to the proposal that the large ($p, p$), ($p, \mathrm{pn}$), ($n, p$) etc., cross sections recently reported are primarily due to direct interactions. (2) With an energy level density $\omega =C\exp 2{\mathrm{}\left(\mathrm{aE}\right)\mathrm{}}^{\frac{1}{2}}$, calculations based on the values of $a$ deduced from inelastic scattering experiments do not fit the experimental curves; $a$ derived from measured level spacing agrees with the copper, but not with the silver data. The best fit for the silver gives $a\sim 2$. This result, similar to that obtained by the analysis of other experimental data, suggests that there is a considerable excess of high-energy evaporated particles.

• Received 1 August 1955

DOI:https://doi.org/10.1103/PhysRev.101.225