Elastic moduli of $\delta $-Pu$^{\mathrm{239}}$ reveal changes in aging in real time

We study the time evolution (aging) of the elastic moduli of an eight-year-old polycrystalline $\delta $-Pu 2.0 at {\%} Ga alloy ($\delta $-Pu:Ga ) from 295K to nearly 500K in real time using Resonant Ultrasound Spectroscopy (RUS). After 8 years of aging at 295K, the bulk and shear moduli increase at a normalized rate of 0.2{\%}/year and 0.6{\%}/year respectively. As the temperature is raised, two time dependences are observed, an exponential one of about a week, followed by a linear one (constant rate). The linear rate is thermally activated with an activation energy of 0.33$+$0.06 eV. Above 420K a qualitative change in the time evolution is observed; the bulk modulus decreases with time while the shear modulus continues to stiffen. No change is observed as the $\alpha -\beta $ transition temperature is crossed as would be expected if a decomposition of $\delta $-Pu:Ga to $\alpha $-Pu and Pu$_{\mathrm{3}}$Ga 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.