Evolution of fragmentation momentum distributions with mass

Parallel momentum distributions of fragmentation products as a function of fragment mass have been used extensively to understand the fragmentation mechanism. Mass dependencies of the perpendicular momentum distributions, however, are much less well-understood. Complete momentum distributions of projectile-like fragments produced in $^{76}$Ge+$^{9}$Be and $^{76}$Ge+$^{197}$Au reactions have been measured using a 130 MeV/nucleon beam. Parallel distributions of all fragments follow established mass systematics, regardless of target species. However, the perpendicular distributions of fragments produced with the $^{197}$Au target that are near the projectile mass contain a clear peak near the grazing momentum that diminishes in significance as fragment mass decreases, a deviation from predictions. In addition, proton-pickup fragments were also observed to peak away from zero degrees. The origin of this peak and its systematic variation will be discussed in the context of fragmentation reaction mechanisms.