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Abstract：
Alpha particles detected in coincidence with two fission fragments in the reaction of 25MeV/u 40Ar+209Bi have been measured at the backwards angles of 145° and 95°, 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 considesably diffesent initial excitation energies, =472 and 595MeV, in the primordial hot nuclei formed in the reaction. The moving source analysis of the spectate has been used to extract the prescission and postScission alpha particlemultiplicities, 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 postscission α pasticle multiplicity are about 166 and 179 MeV in the two LMT intervals, respectively. Compared with the considesable 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 bine scale of hot nuclei with excitation energy of 470-600MeV, deduced from prescission multiplicity, is about 4×10－21 sec, which indicates that the so-called fast fission dominates in the fission channel of hot nuclei in this excitation energy range.
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References
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