TEM investigation of 14-MeV neutron damage

Using transmission electron microscopy, 14-MeV damage has been investigated in gold. The sites of energetic displacement cascades are visible because of the presence of vacancy clusters formed by the collapse or rearrangement of vacancies within the depleted zones. A strong tendency toward subcascade formation has been found in the 14-MeV neutron-induced cascades. On the average, 1.8 clusters are formed per cascade. Individual cascades with as many as six subcascades have been found. The number densities of clusters and cascades are proportional to the fluence. The cross section for the formation of visible cascades is σc = 3.3 × 10-24 cm2. It can be shown that recoils from elastic neutron-scattering events can account for <20% of the visible cascades. The cross section corresponding to the balance of the observed cascades is, within experimental error, equal to the nonelastic neutron-scattering cross section. This indicates that all nonelastic scattering events lead to the formation of a visible cascade. We find quantitative agreement with what is expected from heavy-ion bombardments regarding the cross sections involved; however, estimates of the average cascade energy in the 14-MeV neutron bombardments are somewhat higher than expected.