Neutron diffraction analysis of atomic displacements in δ-Pu-Ga alloys upon long-term self-irradiation

The process of self-irradiation of a Pu-Ga alloy has been studied by the neutron-diffraction method with the determination of the crystal structure and root-mean-square atomic displacements 〈 u 2 〉 (from the data on the Debye-Waller factor). The analysis was carried out at room temperature on the sample with an fcc structure prepared on the basis of a Pu 242 isotope feebly absorbing neutrons, in which a quickly decaying Pu 238 isotope (1.4 at %) was added to intensify self-irradiation processes; this accelerated the aging processes by four times and allowed achieving the maximum equivalent self-irradiation time of ∼23.5 years. The fcc structure was preserved during all this time interval. An analysis of the small-angle neutron scattering has demonstrated that the sample also contained precipitates with a size of a few hundreds of microns, which did not change during the aging. A change in 〈 u 2 〉 (due to static displacements) occurs in two stages, i.e., a relatively rapid growth (by about 50%) during the first 5–6 years of self-irradiation, and a slow decrease in the subsequent 6–23 equivalent years to nearly the magnitude that exceeds the initial value by ∼20%. The latter stage can be explained by the sinking of continuously generated point defects to helium bubbles and dislocations loops accumulating with time. The extrapolation of the decrease in 〈 u 2 〉 to large aging times demonstrates that if the mechanism of point-defect accumulation initiated at the first stage of the self-irradiation does not change with time, the growth of 〈 u 2 〉 will disappear by about 50 years of equivalent time of self-irradiation.