Light Charged Particle Emission in 485 MeV56Fe + 197Au Reactions: Correlations with Heavy Fragments and Relationships to Spin and Lifetime

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Emission of 4He and 1H has been studied in reactions of 485 MeV 56Fe + 197Au, using low-threshold detector arrays for light charged particles, and two heavy-fragment trigger detectors placed at symmetrically opposite angles with respect to the beam direction. The light charged particles were measured both in singles and in coincidence with heavy products of deeply inelastic and fission reactions detected near the grazing angle. Statistical model analyses of the data show that most of the 4He/1H intensity is due to evaporation from energy equilibrated emitters both in deeply inelastic and fission reactions. In deeply inelastic reactions, the observed 4He/1H emission can be attributed to evaporation from the post-scission reactant-like products, while in fusion-like fission reactions evaporation from pre- and post-scission sources are observed in comparable amounts. Angle-integrated multiplicities for 4He and 1H are deduced for each source of emission, and are compared with results from similar systems. The experimental 4He/1H multiplicity ratios indicate roughly equal probability for 4He and 1H evaporation from a given excited source. The observation of pre-scission 4He/1H in fusion-like fission reactions supports the notion that thermal equilibration and subsequent particle evaporation proceed more rapidly than the collective motions that drive the system toward fission. Conversely, the lack of appreciable 4He/1H emission from pre-scission sources in deeply inelastic reactions implies that the interaction times are short compared to the time required for particle evaporation.

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