Monte Carlo algorithms for simulating particle emissions from preequilibrium states during nuclear spallation reactions

Investigations have been made on the method how to incorporate the process of particle emissions from the preequilibrium states into the simulation calculations of nuclear spallation reactions. The spallation reactions have been treated usually as the two step processes. The first step is the intra-nuclear cascade of nucleons initiated by a sufficiently energetic particle, such as a proton with the energy of hundreds to thousands MeV. The second step is the competing decay of the residual nucleus in highly excited states by fissions and/or particle evaporations from the compound state. The two step model is generalized to a three step model, in which the preequilibrium decay step is inserted in between the two steps mentioned above. The treatment of preequilibrium decay is based on the Griffin's exciton model. A proposal is made of the conditions for making transitions from Step 1 to Step 2 and Step 2 to Step 3 in simulation calculations of the entire process of spallation. The first condition is defined by the number of particles trapped in the nucleus during the intra-nuclear cascade and the second one by the number of excitons reaching the effectively equilibrium state. Monte Carlo algorithms have been formulated for simulating the spallation reaction on the basis of the three step model. (author)