Changes in target fragmentation mechanisms with increasing projectile energy in intermediate energy nuclear collisions

Abstract

We have measured the target fragment production cross sections and angular distributions for the interaction of 16 MeV/nucleon $^{32}\mathrm{S}$, 32 MeV/nucleon $^{40}\mathrm{Ar}$, and 44 MeV/nucleon $^{40}\mathrm{Ar}$ with $^{197}\mathrm{Au}$. We have deduced the fragment isobaric yield distributions and moving frame angular distributions from these data. The fission cross sections decrease with increasing projectile energy and the heavy residue cross sections (which are much larger than previous counter measurements) increase. There is an unusual change in the fragment isobaric yield distributions in the reactions induced by 32 MeV/nucleon $^{40}\mathrm{Ar}$ and 44 MeV/nucleon $^{40}\mathrm{Ar}$. We have used the symmetry properties of the moving frame distributions to show the relative time scale of the reaction mechanisms involved. The fission fragments associated with the peripheral collision peak in the folding angle distribution originate in a normal, slow fission process in which statistical equilibrium has been established. At the two lowest projectile energies, the fission fragments associated with the central collision peak in the folding angle distribution originate in part in fast, nonequilibrium processes. At the highest projectile energies, there are no fission fragments associated with high-momentum-transfer events. The intermediate mass fragments originate primarily in events in which statistical equilibrium has not been established.