Stastical analysis of heavy-ion-induced fission excitation functions

Abstract

The Bohr-Wheeler model has been used with the rotating liquid drop model to calculate fission excitation functions using a computer code with multiple particle emission competition. Calculations were performed for fission induced by ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ ions with laboratory energies up to 170 MeV on ${}_{}{}^{62}{}_{}{}^{}\mathrm{Ni}$, ${}_{}{}^{116}{}_{}{}^{}\mathrm{Sn}$, and ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pr}$ targets. The dependence of the fission cross sections on angular momentum, barrier heights, ground state and saddle point level density parameters, and for first versus multiple chance fission is illustrated. One set of parameters, not necessarily unique, is shown which gives fair reproduction of the measured ${}_{}{}^{116}{}_{}{}^{}\mathrm{Sn}$ and ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pr}$ + ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ fission excitation functions. It is shown that fission barriers considerably lower than the rotating liquid drop values are required and that variations of other parameters alone cannot reproduce the experimental excitation functions. Asymptotic weaknesses of the statistical treatment at high excitation energies are discussed and qualitatively shown to give errors in the direction observed in the comparisons of experimental and calculated results.