a Study of Light Charged Particle Emission in the Reaction: 480 Mev IRON-56 + Silver

Inclusive and exclusive measurements of light charged particles (^{1,2,3}H and ^4He) and heavy fragments have been made for the reaction 480 MeV ^{56 }Fe + ^{rm nat} Ag. The backward hemisphere emission of ^1H and ^4He in coincidence with a heavy fragment is attributed to one 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 (fission-fragment-like and deeply-inelastic) at two different trigger fragment angles (theta _{rm TR} = 26^ circ and 50^circ). For theta_{rm TR} = 26^circ, the multiplicity of light particles emitted from the composite nuclear system prior to scission is ~2/3 for the fission-like fragments, compared to ~ 1/3 for the deeply-inelastic fragments. This decrease implies a corresponding decrease (~ 1/2) in the composite system lifetime, between the two processes. Energy and angular distributions of the coincident light particles indicate extensive emission from a strongly deformed composite system, such as that of a system en route to scission. Pre-thermalized and near-scission emission of ^4He is also identified at specific laboratory angles in the forward hemisphere. The evaporation residues, which comprise less than 10% of the total reaction cross section, contribute between 30% and 40% of the inclusive light particle production. By requiring a coincidence between one ^4He and a ^1H, or a second ^4He particle, one is able to select predominately these highly-excited evaporation residue emitters. These large-angle particle-particle correlations indicate very long chain lengths (<n _alpha{>} ~ 3, <n _{rm p}{>} ~ 4) for the light particles in this reaction. The energy spectra of these particles also require extremely low emission barriers for both ^4He and ^1H evaporation. These barriers are lower than those from empirical fusion systematics, even after a Fermi gas expansion correction.