Emission time scales for light charged particles from symmetric and asymmetric fission processes for 40Ar+197Au reactions at 25 MeV/nucleon

Two-particle correlation functions at small relative momenta were measured in the binary fission processes for Ar-40+Au-197 reactions at 25 MeV/nucleon. Compared to trajectory calculations the mean emission times of light charged particles were determined from two-particle correlation functions. The emission times varied weakly with the mass of the particles, but strongly with the kinetic energy of the particles. A slightly shorter emission time was determined for the particles emitted in the fission plane compared to out of the fission plane due to preferential, in-plane preequilibrium emission with shorter emission time. The emission time of light particles was nearly independent of the mass asymmetry of the fission fragments, depending mainly on the degree of equilibration in the emitting nuclei. A very short emission time of less than 100 fm/c was deduced for the preequilibrium emission of high-energy particle pairs with kinetic energy E greater than or equal to 20 MeV/nucleon while a long emission time of about 600-1000 fm/c was deduced for emission of low-energy particles with E less than or equal to 9 MeV/nucleon. Such a long time suggests evaporation for these low-energy particles from thermalized compound nuclei or fission fragments.