The reduced nuclear density in the surface layer of a nucleus gives rise to an increase in the level density parameter and also makes the fission barrier of the nucleus temperature dependent. The manner in which these effects can be consistently incorporated in the transition state formalism for statistical model calculations is discussed. The naive replacement of zero temperature with temperature-dependent fission barriers in the standard formula to obtain the level density at the saddle-point configuration is incorrect.
Exclusive measurements of two-and three-body events were performed for the system '^^SnH-'^Mo at 19.1 MeV/nucleon. Most ternary events are consistent with sequential processes in which one of the two deep-inelastic fragments fissions. For such events large differences are found between the fission probabilities of projectilelike and targetlike fragments of a given mass, this probability being larger for the nucleus which gained nucleons. This behavior demonstrates that there is a lack of equilibrium at the end of the deep-inelastic collision.
The particle decay ensuing from the reactions 86.0 MeV ' 0 + ' Sm and 239.1 MeV Ni + 'Mo was studied. These reactions each form ' Yb compound nuclei excited to = 54 MeV. Particle decay from compound nucleus producing reactions was selected by gating on the gamma-ray fold and the angular region of the particle emission. While there are no discernable differences in the dominant decay channels between the two reactions, there are fewer deuterons from the more symmetric system. This difference can be interpreted two ways: as a suppression of the emission of energetically expensive clusters during the time required for shape equilibration (which is predicted to be longer for the more symmetric entrance channel), or as an enhancement of the emission of energetically expensive clusters from the more asymmetric system at the very early stage of the collision when the initial energy deposited is only available to a reduced number of nucleons. The first explanation is identical to that used in recent high energy photon work while the second could be identified as the result of the emission of clusters on the multistep compound branch leading to the fusion of the low energy heavy ions. If the first explanation is adopted, the observed suppression is larger than predicted by a standard statistical decay model coupled to a dynamical fusion model, but consistent with work using high energy photons as a probe of fusion dynamics.
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