Spectroscopic study of ion-irradiated minerals

The sufficiently intense (or sufficiently long) impact of radioactivity may cause severe changes and damages to crystalline minerals, up to their complete amorphisation. The structural damage to the mineral lattice results in significant property changes, such as generally lowered chemical resistance and enhanced susceptibility to alteration, or the creation of new and/or the suppression of existing luminescence emission centres. A more precise quantification of radiation effects is needed for a better understanding of the basic damage-generation processes. For this, irradiation experiments with MeV heavy ions are planned. Corresponding to penetration depths of these ions in the micrometre range, correspondingly thin FIB (focused ion beam) lamellae, prepared from single crystals of minerals, will be used. One Ph.D. project will address the stress that results from the corpuscular irradiation. This includes nanometre-scale stress as caused by point effects and atomic distortions, as well as the micrometre- range stress among heterogeneously expanded zones in zoned crystals. Resulting effects on the short-range order around anionic molecular units (such as PO 4 groups in phosphate minerals) will be evaluated, with the objective to calibrate a vibrational spectroscopic quantification of the degree of the structural radiation damage in unknowns. This Ph.D. project will be conducted in the framework of doctorate school IK052 "Deformation of geological materials" of the University of Vienna. A second Ph.D. project will be focused on analogous radiation-damage effects on the short-range order of lanthanide-related centres and their luminescence behaviour. Besides an examination of as to which degree lanthanide centres can be used to probe quantitatively the degree of radiation damage, lanthanide emissions will also be used to study phase transitions, such as in shock-compressed samples and DAC (diamond anvil cell) experiments. Both Ph.D. projects have the joint objective to contribute to a better quantification of radiation effects and the process of radiation-damage accumulation in minerals.

Spectroscopic study of ion-irradiated minerals was dedicated to the study of structural irradiation effects in minerals. This was driven by the need to understand the structural changes, and related property changes, in minerals when being exposed to intense and/or long-term corpuscular irradiation.Radiation damage in the minerals zircon (ZrSiO4) and monazite(Ce) (CePO4), and their synthetic analogues, was created under controlled laboratory conditions by irradiation with Au ions. Experimental irradiation of minerals with heavy ions is considered as rough equivalent of natural alpha recoils, which is the process that produces the vast majority of bulk damage in self-irradiated, natural minerals. Our irradiation experiments were done with ion energies in the range 110 MeV, which are significantly higher than energies of alpha recoils. This was necessary because alpha recoils (60160 keV) have short travel distances on the order of tens of nanometres, whereas MeV ion irradiation creates damaged volumesthat can be analysed using modern confocal spectroscopic systems. Samples used were thin lamellae with thicknesses of ca. 1.5 m, prepared using the FIB (focused ion beam) technique. In the course of the project, an improved and more time efficient protocol for FIB- lamellae preparation was developed. The experimental work was accompanied by extensive Monte Carlo simulations, done to predict penetration patterns and defect concentrations created in the targets irradiated.Results have contributed to an improved quantitative estimation of irradiation effects in minerals, that is, how many corpuscles of which energy produce which concentration of defects. It is now better possible to relate progressive changes in long-range- and short- range-order parameters as obtained by analyses, with degrees of damage. In particular, the irradiation-induced broadening of signals in Raman and laser-induced photoluminescence (PL) spectra can be related to defect concentrations expressed in dpa (displacements per atom). In the case of monazite(Ce), a mineral that, to the best of our knowledge, has never been observed severely radiation-damaged in spite of its high Th content, our experiments resulted in elevated levels of structural damage up to amorphisation. The ability of monazite (Ce) to accumulate structural damage at low irradiation temperatures, verifies continuous thermal annealing of monazite(Ce) in nature, and helps to understand the vastly different irradiation responses of different mineral species.In addition, the potential of laser-induced PL spectroscopy of rare earth element (REE) related emissions was demonstrated as a means for the study of minute amounts of (radiation damaged or undamaged) accessory species in geological samples. The specific crystal field of each mineral causes specific splitting of electronic transitions. The resulting fingerprints of split emissions can be used for identification purposes, provided an appropriate database is available. Extensive syntheses of REE-doped minerals were therefore done, and reference spectra were produced and made available to the public. The REE-doped synthetic materials were also included in the irradiation experiments. Based on the results, confocal REEPL is proposed as promising tool, complementary to confocal Raman spectroscopy, to estimate the degree of radiation damage and its internal heterogeneity within single crystals of accessory minerals. This technique will prove most useful especially if Raman cannot be used, for instance when Raman bands show too minor parameter changes or are obscured by luminescence.