ENERGY DEPENDENCE OF $sup 22$Na AND $sup 24$Na PRODUCTION 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 ($Z=12$) and selenium ($Z=34$) 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 $Z\ensuremath{\simeq}30$ and $N\ensuremath{\simeq}37$. 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 $Z=30$. 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.