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 〈u2〉 (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 Pu242 isotope feebly absorbing neutrons, in which a quickly decaying Pu238 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 〈u2〉 (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 〈u2〉 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 〈u2〉 will disappear by about 50 years of equivalent time of self-irradiation.