Extensions to the pre-equilibrium statistical model and a study of complex particle emission

Abstract

Griffin's pre-equilibrium statistical model for nuclear reactions has been further extended to take account of the distinguishability between protons and neutrons as well as of the competition of particle emission with particle-hole pair creation and destruction. In addition, an empirical factor equal to the factorial of the mass number of the emitted particle has been introduced into the rate expressions for particle emission. In this extended form, the model has been used to analyze the spectra of protons, deuterons, tritons, 3He and 4He ions emitted in protoninduced reactions. For each reaction system, the model parameters were varied to reproduce the proton spectrum, and the spectra for the remaining four particle types were calculated with no added adjustable parameters. For systems lighter than gold, the relative charged particle yields are well reproduced by the model, while for 197Au and 209Bi too few tritons are predicted. The spectral shapes for the complex particles are approximately reproduced at bombarding energies around 30 MeV, but for higher energies the data show more high-energy tritons, 3He and 4He ions than can be accounted for with the model. It is thought that these high-energy particles may be produced in direct two and three nucleon pick-up processes which are not included in the model. The initial exciton numbers obtained from the analysis appear to be consistent with earlier values if strong collective excitations are removed from the proton spectra. The fraction of pre-equilibrium emission is found to increase with increasing bombarding energy.