Elastic moduli of δ-Pu239 reveal aging in real time

We study the time evolution (aging) of the elastic moduli of an eight-year-old polycrystalline δ-Pu 2.0 at. % Ga alloy (δ-Pu:Ga) at different fixed temperatures from 295 K to nearly 500 K in real time using Resonant Ultrasound Spectroscopy. After 8 years of aging at 295 K, the bulk and shear moduli increase at a normalized rate of 0.2%/year and 0.6%/year, respectively. As the temperature is raised, two time dependences are observed, an exponential one of about a week, followed by a linear one (constant rate). The linear rate is thermally activated with an activation energy of 0.33 + 0.06 eV. Above 420 K a qualitative change in the time evolution is observed; the bulk modulus decreases with time while the shear modulus continues to stiffen. No change in the behavior of the time evolution is observed as the α−β transition temperature is crossed as would be expected if a decomposition of δ-Pu:Ga to α-Pu and Pu3Ga occurred over the temperature range studied. Our results indicate that the main mechanism of aging is creation of defects that are partially annealed starting at T = 420 K.