Source size scaling of fragment production in projectile breakup

Fragment production has been studied as a function of the source mass and excitation energy in peripheral collisions of ${}^{35}$Cl+${}^{197}$Au at 43 MeV/nucleon and ${}^{70}$Ge+${}^{nat}$Ti at 35 MeV/nucleon. The results are compared to the Au+Au data at 600 MeV/nucleon obtained by the ALADIN collaboration. A mass scaling, by $A_{source}\sim$ 35 to 190, strongly correlated to excitation energy per nucleon, is presented, suggesting a thermal fragment production mechanism. Comparisons to a standard sequential decay model and the lattice-gas model are made. Fragment emission from a hot, rotating source is unable to reproduce the experimental source size scaling.