Abstract
Scission-related emission of light particles has been studied in the fusion-fission reactions 200, 254 MeV37Cl+124Sn→161Ho*(f) (A, B; E x =100, 140 MeV) and 318 MeV28Si +141Pr→169Ta*(f) (C; E x =207 MeV). Light-particle spectra measured in coincidence with fission fragments at 200 angles in and out of the reaction plane are dominated by sequential emission: (i) from the composite nuclei, prior to scission (α multiplicitiesM α ×100 ≃-1.1, 8.7, 38 forA, B, C) and (ii) from fully accelerated fragments (M α ×100 ≃-0.5, 4.5, 8). At forward angles pre-thermalization emission is observed (M α × 1000≃-0.31, 11.7, 33). While absent in the proton spectra, near-scission emission (NSE) is disclosed in theα spectra by marked deviations from sequential emission: a deficit in direction of (M α ×1000≃-0.14, 10, 16) and a surplus yield perpendicular to the scission axes (M α ×1000≃-0.8, 13, 17) reminiscent of the signature of ternary fission. However, NSE is identified as an evaporative process by the observed scaling of the multiplicities vs.E x with sequential multiplicities thus discarding the ternary fission picture. A new description invokes Coulomb-field induced modulation of the evaporation barriers while fragments are in close contact, and a simple static model is presented that consistently describes the data.
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