Random-walk model of precompound decay: Dynamics and multi-particle emission
It is demonstrated that the dynamics of precompound nuclear decay can be formulated, with regard to emission spectra as well as angular distributions, in terms of a simple random walk of the composite nuclear system. In distinction to other preequilibrium models a discrete time parameter is introduced, corresponding to the number of intranuclear interactions. This random-walk description can be employed to analytically calculate several characteristics of the equilibration process, such as the mean time to emission from an exciton state, the mean first passage time, and the attainment of isotropy. Also a simple and compact description of multi-particle emission is obtained. A necessary and sufficient condition is given for the equivalence of the random-walk and the usual exciton models.As an application some neutron emission spectra of neutron-induced reactions on127I atE=15 to 50 MeV have been calculated. The results show that preequilibrium effects are very important in the first-particle emission. In the emission of secondary particles preequilibrium effects need to be included at incoming energies above about 25 MeV. Preequilibrium effects in tertiary emission spectra are not significant belowE=50 MeV.