Energy Dependence ofNa22andNa24Production Cross Sections with 100- to 400-MeV Protons

Cross sections for the production of ${}_{}{}^{22}{}_{}{}^{}\mathrm{Na}$ and ${}_{}{}^{24}{}_{}{}^{}\mathrm{Na}$ have been determined from targets between magnesium ($Z=12\mathrm{}$) and selenium ($Z=34\mathrm{}$) with 100-, 200-, and 300-MeV protons. The main feature of the results is the exponential decrease of the cross sections with increasing target atomic number or neutron number for values up to $Z\simeq 30$ and $N\simeq 37$. This feature has been found to be consistent with the cross-section dependence predicted on the basis of a cascade-evaporation mechanism. Maxima are also observed in the cross-section distributions at atomic numbers greater than $Z=30\mathrm{}$. It appears that these maxima are caused by nonconventional cascade-evaporation events, since they have thresholds below 200 MeV. A major breakup process in which one of the fragments has greater stability is suggested as a possible mechanism.