Measurement of fission time scale and excitation energy at scission for 25 MeV/u 40Ar + 209Bi fission reaction

Alpha particles detected in coincidence with two fission fragments in the reaction of 25 MeV/u 40Ar + 209Bi have been measured at the backwards angles of 145 degree and 95 degree, respectively. The spectra at each angle are incremented in two different intervals of linear momentum transfer (LMT). These two intervals of LMT correspond to two considerably different initial excitation energies, = 472 and 595 MeV, in the primordial hot nuclei formed in the reaction. The moving source analysis of the spectra has been used to extract the pre-scission and post-scission alpha particle multiplicities, as well as other relevant qualities such as the temperatures of the target-like residues and fission fragments. The excitation energies at scission, deduced from the post-scission α particle multiplicity are about 166 and 179 MeV in the two LMT intervals, respectively. Compared with the considerable difference of initial excitation energies, the near equality shows that the excitation energy at scission are not explicitly dependant on the initial excited state of the fission nuclei. The fission time scale of hot nuclei with excitation energy of 470-600 MeV, deduced from pre-scission multiplicity, is about 4 x 10-21 sec, which indicates that the so-called fast fission dominates in the fission channel of hot nuclei in this excitation energy range