Recoil Range Evidence for the Compound-Nucleus Mechanism in Reactions between Complex Nuclei

We have measured the recoil-range distributions of 4.1-h ${\mathrm{Tb}}^{149}$ produced in a variety of complex nuclear reactions. The targets were ${\mathrm{Pr}}^{141}$, ${\mathrm{Ce}}^{140}$, ${\mathrm{La}}^{139}$, and ${\mathrm{Ba}}^{138}$; projectiles were ${\mathrm{C}}^{12}$, ${\mathrm{N}}^{14}$, ${\mathrm{N}}^{15}$, ${\mathrm{O}}^{16}$, ${\mathrm{O}}^{18}$, ${\mathrm{F}}^{19}$, and ${\mathrm{Ne}}^{20}$. In every case the range distributions could be fitted to a Gaussian function. The average range values give evidence for total momentum transfer, and the range straggling is consistent with nuclear evaporation. We conclude that these reactions are pure compound-nucleus reactions. Excitation energies were approximately 50 to approximately 150 MeV, corresponding to incident energies of 5 to 10.4 MeV per nucleon.