Microscopic effects of self-radiation damage in 244 Cm-doped LuPO 4 crystals
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244
Cm-doped
J.S. Luo and G.K. Liu Argonne National Laboratory, Argonne, Illinois 60439 (Received 6 December 1999; accepted 12 October 2000)
Microscopic effects of self-radiation damage in 244Cm-doped LuPO4 crystals were examined with transmission electron microscopy. These LuPO4 crystals had been doped with 1 wt% 244Cm and exposed to a radiation dose as high as 5 × 1016 ␣-decay events/mg over 18 years. The microscopic analysis revealed dense arrays of individual defect clusters and numerous bubbles. Whereas, the defect clusters may be interpreted as residuals of alpha-recoil tracks, the bubbles likely resulted from the ␣-particles generated during the decay events. The bubbles were found to coalesce under electron beam irradiation. Despite the high accumulated dose over the 18 years, the samples exhibited sharp diffraction patterns and periodic lattice spacings. This finding indicated that the samples remained largely crystalline and that the radiation-induced lattice damage was recovered at a rate comparable to that of damage production. This high recoverability is discussed with respect of various annealing processes that may have occurred in the samples.
I. INTRODUCTION
It has been generally recognized that the radiation effects resulting from incorporated radionuclides may cause significant changes in the physical and chemical properties of nuclear waste solids, which may, in turn, affect their long-term performance in a geological repository.1–3 Alpha-decay events associated with the decay of actinides and their daughter products have been found to cause the most detrimental atomic scale structural damage in nuclear waste solids.4–6 An alpha-decay event results in the simultaneous production of two energetic particles: (1) an alpha particle (about 5 MeV) with a range of 10–20 m that dissipates most of its energy by ionization; (2) an alpha-recoil atom (0.1 MeV) with a range of 20–40 nm that may produce up to several thousand atomic displacements, which can create local amorphization in crystalline oxides. For high-level nuclear waste forms over a period of 103–106 years, the projected number of alpha-decay events is on the order of 1014–1016 ␣-decay events mg−1.1,2 To simulate the long-term effects of alpha-decay events in laboratory, highly-active, short-lived actinides, such as 238Pu (half-life of 88 year) and 244Cm (half-life of 18 year) have been incorporated into simulated waste forms samples so that high alpha-decay dose can be reached in a reasonably short time.1,2 Turcotte et al.5 used the 244Cm-doping method to evaluate changes in crystal structure, density, and stored energy in waste forms containing apatite, pollucite, monazite, and other 366
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minor phases. They found that the Cm-containing crystalline phases become x-ray amorphous after doses reach more than 3 × 1015 ␣-decay events mg−1. Local amorphization is generally believed to result from spontaneous local collapse of the crystal structure and disappearance of long-range order.1,7–9 Weber et al.7,8 reporte
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