Alpha-recoil damage in titanite (CaTiSiO 5 ): Direct observation and annealing study using high resolution transmission
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. INTRODUCTION
In minerals, heavy particle interactions with crystalline structures include the effects of fission fragments, alpha-recoil nuclei, and alpha particles. Only the first two are thought to create significant damage volumes within the crystalline matrix. Fission tracks were directly observed for the first time in mica thirty years ago by Silk and Barnes,1 using the then relatively new technique of transmission electron microscopy (TEM). After the discovery of fission tracks, they were observed by chemical etching techniques and optical microscopy.2 Measurement of fission track densities by these and other methods forms the basis for an important age dating technique. Yada and coworkers3 have recorded impressive images of fission tracks in zircon using phase contrast, high resolution transmission electron microscopy (HRTEM). Widespread interest in the development of new methods of age dating led to a landmark investigation of zircon by Hurley and Fairbairn in 1953.4 Hurley and Fairbairn were among the first to recognize the importance of the alpha-recoil nucleus as a cause for the transition from the crystalline to the metamict (aperiodic) state. Furthermore, they considered annealing effects on structural damage by including an annealing term in their "expression of metamictization." In 1967, alpha-recoil tracks were directly observed, again in mica, using the technique of chemical etching and phase contrast, optical microscopy.5 "'Current address: Australian Nuclear Science & Technology Organization, Lucas Heights Research Laboratories, New Illawarra Road, Lucas Heights, New South Wales, Australia. 560 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 3, Mar 1991 Downloaded: 13 Mar 2015
The phenomenon of metamictization (amorphization) in minerals is the result of the accumulation and overlap of alpha-recoil tracks.6 Recent investigations have focused on the use of HRTEM to characterize the transition from the crystalline to the metamict state in pyrochlore,6 zirconolite,7 and zircon.8 The subject of this investigation is titanite, CaTiSiO5, a phase that has provided some of the best HRTEM images of isolated alpha-recoil tracks. Titanite is a useful phase to examine because the U and Th contents are low (less than 0.1 wt. %). Isolated alpha-recoil tracks in the ciystalline matrix are common, as track overlap has not obscured the individual tracks. The characteristics of these tracks and their stability over long periods of time, and at different temperatures, is directly applicable to the evaluation of the long-term stability of titanite as an actinide-bearing phase in nuclear waste forms. Titanite (= sphene) is a principal component in the glass ceramic being developed by the Canadian Nuclear Fuel Waste Management Program as an alternative to nuclear waste from borosilicate glass.9 II. EXPERIMENTAL TECHNIQUES
Crushed grains of titanite were dispersed in acetone and deposited on holey-carbon filmed copper grids. Electron microscopy was carried out using a JEOL 2000 FX transmission electron mi
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