Methods Used in Evaluating Data for the Interaction of Neutrons with Light Elements (A

摘要

In the interaction of neutrons with light nuclei, many anomalies are observed. In particular, the probability for gamma-ray production is generally small over most of the neutron energy range. On the other hand, exp 6 Li, exp 3 He, exp 10 B, and exp 7 Be have thermal absorption cross sections which range from 940 to 48,000 barns. exp 10 B is the only isotope that has a positive Q for a 3-body reaction, the (n,t2 alpha ). As the neutron energy increases, however, 3- and 4-particle direct breakup and sequential formation cross sections dominate the nonelastic for D, T, exp 6 Li, exp 7 Be, exp 10 B, and exp 12 C above a few MeV. For higher-mass isotopes, particle emission (protons and alpha 's) are often the preferred mode for deexcitation of levels excited via (n,n') reactions, where energetically possible. Very few of these partial cross sections have been measured with the necessary precision. Problems are particularly inherent in experiments on negative-Q reactions near the 3-body threshold. The many-body problem must be treated as several two-body sequential steps in a theoretical analysis; the emitted particle angular distribution is required as input, but is rarely known. Precise knowledge about individual partial cross sections is often important, especially when neutron multiplication, breeding of fusion fuel, radioactive contamination, depletion or buildup of the target, energy transfer, or time-dependent parameters are required. Specific examples are described for the evaluation of neutron interactions with light elements which employ isotopic spin, inverse reactions, charge-conjugate reactions, and the elastic scattering of charged particles (with Wick's Limit). 18 figures, 1 table. (ERA citation 06:003143)