## 1 Introduction  
Plutonium (Pu) metal is considered one of the most complex metals in the periodic table due to 5f electronic behavior of being delocalized or localized depending on the atomic environment. Elemental plutonium undergoes five solid-state phase transformations starting with its complicated room temperature 16-atom simple monoclinic \(\alpha\)-phase, to the 34-atom body-centered-monoclinic \(\beta\)-phase, and then the eight-atom face-centered orthorhombic \(\gamma\)-phase to the highest temperature highly symmetric face-centered-cubic (fcc) \(\delta\)-phase [1]. Beyond this phase is the body-centered-tetragonal \(\delta^{\prime}\)-phase and the body-centered-cubic \(\varepsilon\)-phase, right before the low melting temperature of 913 K [2], and the \(\delta\) and \(\delta^{\prime}\) phases undergo a negative thermal expansion. This complicated phase diagram is attributed to the variable nature of the Pu 5f electrons and its atomic environments. Since the \(\alpha\)-phase is brittle, like cast iron, the ductile \(\delta\)-phase is most often used for technological applications. For ductile \(\delta\)-phase production, Pu is stabilized usually with a Group IIIA element such as aluminum, gallium (Ga), and indium. The stabilization of the \(\delta\)-phase to room temperature is dependent on alloying solute concentration, with as little as one atomic percent (at. %) Ga may stabilize the fcc phase [3], but upon cooling or mechanical shear, this alloy will undergo a martensitic phase transformation to an expanded \(\alpha\)-phase lattice (identified as \(\alpha^{\prime}\)-phase) retained to ambient conditions. These stimuli are no longer effective at producing retained martensitic phase beyond \(\sim\)3 at. % Ga and the \(\delta\)-phase negative thermal expansion changes to a positive thermal expansion [4; 5; 6; 7; 8]. The metastability of Ga-stabilized \(\delta\)-Pu at room temperature is dependent on the solute alloy concentration but is also dependent on defects and their influence on the local atomic environment and electronic structure. On this subject, many fundamental questions remain unanswered. One source of defect production in Pu is self-irradiation, which occurs via alpha emission by a decay into daughter products of uranium and helium nuclei causing lattice damage in the form of Frenkel pairs (interstitial and mono-vacancy) [9; 10]. Interest in the self-irradiation of Pu has led to insightful work showing unconventional defect-induced fcc stabilization of \(\delta\)-Pu-Ga alloys against martensitic phase transformation [11], providing theoretical analysis into the influence of daughter product ingrowth and why there has been no detected void swelling in fcc Ga stabilized \(\delta\)-Pu [12]. There are conflicting studies of the Ga influence on the lattice swelling observed under self-irradiation, and it remains unclear if increasing Ga concentrations either increase or have no influence on the relative lattice swelling [13; 14]. However, recent extended x-ray absorption fine structure (EXAFS) studies show that during isochronal annealing, the cryogenically accumulated local defect environment is different around the Pu than around the Ga [15]. Understanding defect to damage evolution is extremely important to our understanding of Ga-stabilized \(\delta\)-Pu alloy phase metastability. This is most evident for the two Pu-Ga phase diagrams--one founded in metastability (US) and the other in thermodynamic equilibrium (Russian)--in which the kinetically favorable means of progression from one to the other is by damage-induced eutectoid decomposition of \(\delta\)-Pu to \(\alpha\)+Pu\({}_{3}\)Ga in less than \(10^{4}\) h [16; 17]. Otherwise, computational coupling of phase diagrams and thermochemistry (CALPHAD) calculations have concluded that the kinetics for decomposition will not occur until after \(1.5\times 10^{6}\) years [18].