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

Alpha particles detected in coincidence with,two fission fragments in the r eaction of 25MeV/u Ar-40 + Bi-209 have been measured-at the backwards angle s of 145 degrees and 95 degrees, respectively. The spectra at each angle ar e:incremented in two different intervals of linear momentum transfer (LMT). These two intervals of LMT correspond to:two considesably diffesent initia l excitation energies, [E*] = 472 and 595MeV, in the primordial hot nuclei formed in the reaction. The moving source analysis of the spectra has been used to extract the prescission-and postscission alpha particle multiplicit ies, as well as other relevant qualities: such as the temperatures of the t arget-like residues and fission fragments. The excitation energies at sciss ion, deduced from the postscission alpha pasticle multiplicity are about ,1 66 and 179 MeV in the two LMT intervals, respectively. Compared with the co nsidesable difference of initial excitation energies, the near equality sho ws that the excitation energy at scission are not explicitly dependant on t he initial excited state of the fission nuclei. The fission time scale of h ot nuclei with excitation energy of 470-600MeV, deduced from prescission mu ltiplicity, is about 4 x 10(-21) sec, which indicates that the so-called fa st fission dominates in the fission channel of hot nuclei in this excitatio n energy range.