Viscosity In The Fission Of Hot Nuclei

Although the fission process has been investigated for almost sixty years, it is only recently that the dynamical aspects of fission have been studied in detail. For many years these studies were hampered by the lack of suitable experimental observables against which theoretical calculations could be tested. For example, it was found that the total kinetic energy release in fission can be described equally well by very different dissipation mechanisms. Also the success of the transition state model(1) in describing fission cross sections at lower excitation energies masked the need for a dynamical description of the fission process. Recently, however, Gavron ct al(2) showed that the transition state model was unable to account for the number of neutrons emitted prior to scission at high excitation energy in reactions of O-16+Nd-142. It appeared that a fission delay was required to explain the observations. Subsequent experimental work using different methods to measure the fission dissipation/viscosity has confirmed these initial observations. This paper reviews the recent advances in the study of fission dynamics obtained from a range of experimental observables.