Sources of Light-Charged-Particle Emission in the Reaction 480 MeV 56Fe + natAg

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Inclusive and exclusive measurements of light-charged particles (1,2,3H,4He) and heavy fragments have been made for the reaction 480 MeV 56Fe+natAg. The backward hemisphere emission of He4 and H1 in coincidence with a heavy fragment can be well described by evaporative emission from a combination of three sources: the detected fragment, the undetected fragment, and the composite system prior to scission. Multiplicities for each of these sources are determined for two coincident fragment groups: a fusion-fission-like group and a deeply-inelastic-reaction group. These multiplicities have also been studied at two angles for the trigger fragment (θTR=26° and 50°). For θTR=26° the multiplicity of the light-charged particles (1H or He4) emitted from the composite nuclear system (i.e., prior to scission) is ≊(2/3 for the fusion-fission-like fragments, compared to ≊(1/3 for the deeply inelastic reactions. This decrease implies that the deeply inelastic reactions occur in ≊(1/2 the time required for fission reactions. For the deeply inelastic reactions, these multiplicities change with trigger angle. This result also suggests that the lifetime of the composite system exerts some control on the extent of evaporative particle emission. Energy and angular distributions of the coincident light particles suggest extensive emission from a strongly deformed composite system, such as that of a system en route to scission. Low-energy He4 particles at forward angles, which cannot be attributed to evaporation, exhibit a dependence on the angle with respect to the scission axis. This result is attributed to emission that occurs very near to the instant of scission. High-energy He4 particles at forward angles are emitted prior to thermalization; they do not depend on the angle with respect to the scission axis.

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