Monte Carlo Calculations of Nuclear Evaporation Processes. III. Applications to Low-Energy Reactions

Abstract

Monte Carlo calculations of nuclear reactions in the low-energy ($E<\mathrm{}50\mathrm{}$ Mev) region are described. The calculations are based on the nuclear evaporation model of Weisskopf. Continuum theory was used for the calculation of inverse reaction cross sections. In the calculation of the level densities of excited nuclei, pairing and shell energy corrections were used in terms of characteristic level displacements. The accurate equation rather than the approximate Maxwell distribution was used for the selection of the kinetic energy of the evaporated particle. Experimentally determined $Q$-values for the various reactions were used. The calculations are compared with experimental measurements for about 60 excitation functions of nuclear reactions in the mass range ${\mathrm{Cr}}^{50}$-${\mathrm{Se}}^{74}$. Cameron's values for pairing energies were used at the outset; but a new set of pairing and shell energy correction values, which leads to substantially improved agreement with the experimental curves, is presented. The procedure which was used to arrive at this set is described and several features of the set are discussed. The need for a further downward correction of the level density of symmetrical ($A=2Z$) nuclei is indicated. Computed excitation functions are shown for all the reactions studied as well as for several reactions for which experimental data are not yet available. Further experiments on reaction cross sections are suggested which would allow a unique determination of the pairing and shell energy corrections of level densities for any value of $Z$ and $N$ in the region under discussion. The existence of a unique set of these correction terms would provide strong evidence for the validity of evaporation theory for the reactions considered.