The evaporation step

Evaporation calculations are based on the Weisskopf [100] approach, with a few using the Hauser-Feshbach [81] one. Most actual calculations are based on the code developed by Dresner [84]. The most important ingredients of the codes are the level densities. It is important to account for the influence of shell effects on the level density parameters and of their washing out with nuclear temperature. In this respect important improvements have been made with respect to the original Dresner code. They often resort to the level density formula derived by Ignatyuk et al. [101]. Shell effects also have to be treated carefully in their influence on fission barriers. Most INC calculations use the fission model of Atchison [102]. Atchison gives a differ­ent treatment for the fission of heavy nuclei with Z > 88 and for that of nuclei with 70 < Z < 89. In the first case a fixed fission barrier, Bf = 6 MeV, is used and Tn/Tf is assumed to depend only on the charge of the fissioning nucleus, not on its energy. These approximations are based on the experimental data reviewed by Vandenbosch and Huizenga [103]. For lighter nuclei a statistical model calculation is carried out, using fissility dependent parametrizations of Bf and of af/an. Above a few MeV the results of the calculations are in reasonable agreement with experiments. However, it might be timely to improve the present treatment of fission by including such recently discovered features as the time delay to fission [104], which decreases the fission probability at high excitation energies, the temperature dependence of the fission barriers and symmetry and surface dependence of the level density parameters.

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *