In-situ TEM study of radiation-induced amorphization and recrystallization of hydroxyapatite

Abstract

The radiation response of hydroxyapatite, a nuclear waste form candidate, was investigated using 1 MeV Kr2+ and 200 keV electron irradiations combined with in-situ TEM observation. The results showed a remarkable difference in the behavior of nano-crystalline hydroxyapatite under the two different radiation conditions. A crystalline-to-amorphous transformation occurred under the 1 MeV Kr2+ irradiation due to the accumulation of displacement damage, with a critical amorphization dose (Dc) of 2 × 1014 ions/cm2 (0.05 dpa) at room temperature. The Dc increased dramatically with temperature due to effective dynamic annealing of radiation-induced defects, such that no amorphization occurred above a critical temperature (Tc) of 545 K. In contrast, under 200 keV electron irradiation, which mainly dissipates its energy through inelastic ionization processes, the pre-amorphized hydroxyapatite exhibited rapid recrystallization, likely due to the bond breaking and reforming at the amorphous-crystalline interface assisted by the ionization. The electron-irradiation induced recrystallization in amorphous hydroxyapatite was extremely sensitive, with a critical electron dose several orders of magnitude lower than most previously reported ceramics. Under concurrent ion and electron irradiations, the materials retained crystalline even at the critical dose of 0.05 dpa, and remained so until the dose reached 0.15 dpa, lending further proof to the enhanced defect recovery by ionizing electrons. The radiation data of hydroxyapatite highlights the material's sensitivity to radiation conditions and its excellent dynamic defect annealing ability.