Onset of multifragmentation in intermediate energy light asymmetrical collisions

The multifragment emission for central collisions of $^{40}\mathrm{Ar}$ (25--150 MeV/nucleon) on $^{27}\mathrm{Al}$ was studied via the quantum molecular dynamics model. The distribution of intermediate mass fragments (IMFs) can be described by a power-law function with the parameter \ensuremath{\tau} or an exponential function with the parameter \ensuremath{\lambda}. By increasing the beam energy, the mean multiplicity of IMFs and relative cross sections emitting multifragment first increase to their maximum values and then decrease; while the extracted \ensuremath{\tau}, \ensuremath{\lambda} parameters and relative cross section for single IMF emission show the converse tendency. The calculation and relevant experiment suggest that the reactions have a critical energy point where the onset of multifragmentation takes place.