Pre-equilibrium effect in (n, xn) reactions

A method has been developed to calculate (n, xn) cross-sections (x=2, 3, 4), taking into account pre-equilibrium emission of the first particle. Emission of subsequent particles is considered to be entirely due to the equilibrium mode. Based on this model the cross-sections have been calculated and compared for a number of nuclei in the mass region A=89 to 238 for incident energies from threshold to 28 MeV. Using the squared matrix element of Kalbach we found that the preequilibrium contribution was considerable at incident energy higher than 16 MeV. In general, agreement between the measured and the calculated cross-sections is within 10 to 15%. Cross-section for (n, 4n) reaction is also calculated and compared with the available data. The method developed here may be used to calculate (n, xn) cross sections for unstable nuclei for which a direct measurement is not possible