Defect studies of stainless steel via positron annihilation energy spectroscopy

High Energy proton (up to 800 MeV) and spallation neutron irradiated samples of stainless steel 316L and Mod 9Cr1Mo were studied using positron annihilation energy spectroscopy. Doses delivered to 316L were up to 10 displacements per atom (dpa) and doses to 9Cr1Mo were up to 2.5dpa. We studied the change of T-parameter, which is calculated as the ratio of the number of counts in the wings of the Doppler-broadened 511 keV peak to the number of counts in the center of the peak. T-parameter is related to the density of defects in the sample of interest. Higher defect densities induce, generally, smaller T-parameter, although this is complicated by additional effects that include the size, nature and other properties of defects that may lead to saturation of T-parameter. For the large doses studied, positron annihilation energy spectroscopy showed that the T-parameter dropped sharply from 0 to 3 dpa, and continued dropping up to 10 dpa. In 9Cr1Mo, similarly, T-parameter dropped sharply from 0 dpa to 1dpa, but from 1 dpa to 2.5 dpa it remained constant, indicating that the density of defects or T-parameter saturated with dose above 1 dpa in 9Cr1Mo. These results, where the change in T-parameter from zero dose to 1 or more dpa, is much larger than the effect that we see from one irradiated specimen to another, led us in both cases to investigate lower doses. We measured energy spectra in 316L and 9Cr1Mo that were irradiated under the similar conditions as the above samples, but with doses less than 0.1dpa. These results fill in the gap between 0 and 1 dpa and suggest that most of the change in T-parameter occurs below 0.05 dpa.