We have measured the nonelastic cross section ${\sigma }_R$ for 40-MeV alpha particle for 19 target elements ranging in atomic, weight $A$ between 9 and 232. The results have been compared with the predictions of the optical model ${\sigma }_I$. The ratio $\frac{{\sigma }_R}{{\sigma }_I}$ deviates from unity in a systematic fashion being less than 1 for the lightest elements and greater than 1 for the heaviest. Superimposed on this trend, $\frac{{\sigma }_R}{{\sigma }_I}$ dips by 18% for elements where the nuclear charge is near 28. A similar effect had been noted earlier in proton-reaction cross-section data but not in deuteron-reaction cross-section data.

• Received 31 January 1963

DOI:https://doi.org/10.1103/PhysRev.131.1251