Time scales for binary processes from light heavy-ion reactions

Binary decay excitation functions have been measured for the ${}^{16}\mathrm{O}{+}^{10}\mathrm{B}$ system at 22 MeV $<E_{\mathrm{c}.\mathrm{m}.\mathrm{}}<24.5\mathrm{}\hspace{0.5em}\mathrm{MeV}$ $\left(\delta E_{\mathrm{c}.\mathrm{m}.\mathrm{}}\sim 20\hspace{0.5em}\mathrm{keV}\right)$ and 17 $\mathrm{MeV}<E_{\mathrm{c}.\mathrm{m}.\mathrm{}}<25\mathrm{}\hspace{0.5em}\mathrm{MeV}$ $\left(\delta E_{\mathrm{c}.\mathrm{m}.\mathrm{}}\sim 190\hspace{0.5em}\mathrm{keV}\right)$ and for the ${}^{19}\mathrm{F}{+}^{12}\mathrm{C}$ system at 22 $\mathrm{MeV}<E_{\mathrm{c}.\mathrm{m}.\mathrm{}}<24.5\mathrm{}\hspace{0.5em}\mathrm{MeV}$ $\left(\delta E_{\mathrm{c}.\mathrm{m}.\mathrm{}}\sim 20\hspace{0.5em}\mathrm{keV}\right).\mathrm{}$ The binary fragments were identified by the kinematic coincidence technique. The excitation functions for these systems show strong fluctuations and average decay widths were obtained through the analysis of energy correlation functions. The small widths observed for the ${}^{16}\mathrm{O}{+}^{10}\mathrm{B}$ (about 20 keV) and ${}^{19}\mathrm{F}{+}^{12}\mathrm{C}$ (about 40 keV) systems show a constant behavior with the fragments’ excitation energy and scattering angle. These widths correspond to an intermediate system time scale of about $2\times {10}^{-20}\hspace{0.5em}\mathrm{s},$ which is larger than its revolution time. These features suggest the presence of the fusion-fission mechanism, where the compound nucleus takes a long time to acquire a relaxed shape and thermal equilibrium. For the ${}^{16}\mathrm{O}{+}^{10}\mathrm{B}$ system, larger widths (about 350 keV) were also observed, which are related to a faster process. These widths show a clear dependence with the scattering angle in the elastic scattering channel. These aspects are expected for a process where the time scale is comparable to the intermediate system revolution time, like an orbiting mechanism. Theoretical predictions for fusion-fission and orbiting mechanisms were compared to the experimental results and a good agreement was observed.