Surface entropy in statistical emission of massive fragments from equilibrated nuclear systems

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Statistical fragment emission from excited nuclear systems is studied within the framework of a schematic Fermi-gas model combined with Weisskopf's detailed balance approach. The model considers thermal expansion of finite nuclear systems and pays special attention to the role of the diffuse surface region in the decay of hot equilibrated systems. It is found that with increasing excitation energy, effects of surface entropy lead to a systematic and significant reduction of effective emission barriers for fragments and, eventually, to the vanishing of these barriers. The model predicts a maximum (effective) nuclear temperature and the occurrence of negative nuclear heat capacities, effects that have been reported in the literature. It also accounts for the observed linearity of pseudo-Arrhenius plots of the logarithm of the fragment emission probability versus the inverse square root of the excitation energy, but does not predict true Arrhenius behavior of these emission probabilities.