Luminescence study of radiation damage in accessory minerals

The safe and long-term disposition of high-level radioactive waste is an ongoing challenging task, whatever this waste is produced by using nuclear energy, dismantling nuclear weapons or medical and research applications. The immobilisation of radioactive nuclides in crystalline ceramics as nuclear waste forms is discussed by material scientists since years as potential alternative to an encapsulation in glasses (vitrification). The former strategy is advantageous in so far, as crystalline ceramics are more resistant to alteration with respect to geological time-scales and conditions. Most of the discussed ceramic phases are synthetic analogues of natural-occurring accessory minerals, which incorporate radioactive actinides like uranium or thorium into the crystal structure. Radioactive decay is accompanied by ballistic knock-ons at atomic scale which cause a breakdown of the nearby crystalline environment. The accumulation of radiation damage over geologic periods of time therefore may results in complete or partial loss of their crystallinity. This process, which is called metamictisation, is an ongoing subject of matter in mineralogical research. Mineralogical studies on natural analogues such as zircon, monazite, titanite, pyrochlore or zirconolite therefore provide important clues to the applicability of such mineral phases as nuclear waste forms. This project aims at developing a complementary analytical tool for the quantitative characterisation of structural disorder (crystallinity) in minerals and mineral-like ceramics, as the quantification of radiation damage in materials is challenging on a micrometre scale. The luminescence of rare earth elements (REEs) may be a fast, low-cost, imaging technique for this purpose. Typically, minor amounts of REEs are substituted in the crystal structure of accessory minerals. Their luminescence is characterised by narrow spectroscopic bands which are dependent on the local structural environment and, hence, may be used as structural probe. It has been observed that luminescence band-widths broaden appreciably with increasing structural disorder as caused e.g., by the accumulation of radiation damage. One major goal of this project is to establish the latter as analytical tool for the quantification of radiation damage of important mineral phases potentially being used as nuclear waste forms. It is planned to calibrate the REE-luminescence band-width with the respective radiation damage created artificially by experimental heavy-ion irradiation. The latter will be done at a research facility operating an adequate ion accelerator. Results of this study may be important for Earth scientists also, as the quantification of radiation damage in accessory minerals may provide information on their geothermal history. Comparison of the expected radiation damage, which can be estimated from the concentration of the actinides and the geologic age of the sample, with the radiation damage actually present gives an idea of potential thermal events in the geologic history, which typically cause a complete or partial annealing of the radiation-damaged mineral phase.

The safe and long-term deposition of high-level radioactive waste is an ongoing challenging task, whether this waste is produced by nuclear power plants, dismantling nuclear weapons or by medical and research applications. The immobilisation of radioactive nuclides in crystalline ceramics (so-called nuclear-waste form materials) have been discussed by scientists since the late '70s as potential alternative to an encapsulation in glasses (vitrification). The former strategy is advantageous in so far, as crystalline ceramics are generally more resistant to alteration with respect to geological time-scales and conditions. Most of the discussed ceramic phases considered are synthetic analogues of accessory minerals found in nature. These minerals naturally incorporate radioactive actinides like uranium or thorium into the crystal structure and, hence, are important ores of the latter elements. Radioactive decay is accompanied by ballistic "knock-ons" at atomic scale which cause a breakdown and damage of the nearby crystalline environment. The accumulation of radiation damage over geologic periods of time therefore may results in complete or partial loss of their crystallinity and its resistance. This process, which is called metamictisation, is an ongoing subject of matter in mineralogical and materials science research. Mineralogical studies on actinide-bearing minerals such as zircon, monazite, titanite, pyrochlore or zirconolite therefore provide important clues to the applicability of such mineral phases as nuclear-waste form-materials. In cooperation with one of the leading nuclear research institution based in Sydney, Australia (ANSTO: Australian Nuclear Science and Technology Organization), we have developed a new, complementary analytical tool to quantitatively characterize the structural disorder (crystallinity) in minerals and mineral-like ceramics. Such structural characterisation is a high analytical challenge, especially if analyses need to be performed in-situ on the micrometre-length scale. In this project, we developed a methodology for laser-based luminescence spectroscopy as a fast, low-cost, imaging technique applicable for this purpose. The luminescence of Rare-earth elements (REEs) was found to be a suited structural probe as the luminescence spectral characteristics are highly sensitive to the local structural environment at the atomic scale. Results based on the new methodology developed in this project, are of utmost importance not only for material scientists that need advanced analytical tools to characterise waste-form materials, but also for Earth scientists that are interested in the stability of uranium- and thorium-containing accessory minerals. These minerals are commonly used as the "geologist's clock" as the radioactive decay of uranium- and thorium isotopes provide important information on the age of the rocks they are found in.