Ejection of Large Fragments in High-Energy Nuclear Reactions

The formation cross section for ${\mathrm{Na}}^{24}$ and ${\mathrm{Mg}}^{28}$ was measured in targets of Cu, Ag, Au, and U bombarded with protons and helium ions over the energy range 320-880 MeV. In addition, the kinetic energy of these products was measured by a thick-target, recoil-range technique at incident proton energies of 0.7 and 3.0 GeV and an incident helium-ion energy of 0.88 GeV. In both sets of properties there was found a close similarity in the results from helium-ion and proton bombardments. In the case of the proton bombardments the results were compared to the predictions of the conventional model of high-energy regions, i.e., a fast nucleonic cascade followed by a slower evaporation cascade. It was found that the conventional model cannot reproduce the results, which suggests that another reaction mechanism, fragmentation, is required. Possible views of a fragmentation mechanism are presented, and the implications of the experimental results for the nature of the fragmentation mechanism are explored.