Reaction cross-section calculations and level structure

In the course of carrying out nuclear reaction cross-section calculations of the statistical-model, Hauser-Feshbach type we continue to observe the necessity of describing nuclei in the first few MeV above their ground states with discrete levels in order to obtain accurate results. There is no adequate way in which one can use a level density expression to represent or substitute for the level structure. Furthermore, all levels must be included; the level information obtained from experiment must be supplemented with that known to be present theoretically. This is true both for spherical nuclei, where typically tens of levels are found in the first 1 to 4 MeV of nuclear excitation, and for deformed nuclei, where perhaps a thousand levels are present in the first 1.5 MeV. We have found that discrete level sets are not only important in the description of nuclei at low excitation energies but also are very valuable in the prediction of a number of calculational parameters, particularly for nuclei off the line of stability. Discrete level information can be used to obtain improvements on global parameterizations of level densities; to predict unknown values of D/sub ob/, the average s-wave neutron resonance spacing; to compute correct radiation widths; and to deduce absolute E1 and M1 gamma-ray strength functions. If correct gamma-ray branchings among the discrete levels are provided, reliable isomer ratios can be computed. In addition, complete discrete level sets allow one to analyze primary gamma-ray spectra. 27 refs., 5 figs