Browsing by Author "Lenz, C"

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    On the way to a quantification of radiation damage in accessory minerals using REE3+ photoluminescence spectroscopy
    (University of Vienna, 2017-09-13) Lenz, C; Lumpkin, GR; Thorogood, GJ; Nasdala, L
    The long‐term impact of natural radioactivity may cause severe structural damage in minerals. Self‐irradiation‐induced structural damage is created mainly in alpha‐decay events in the 232Th, 235U, and 238U decay chains, by the nuclear interaction (atomic “knock‐ons”) of recoiled heavy daughter nuclei (e.g., Weber et al. 1990). Many accessory minerals incorporate variable amounts of actinides, whose radioactive decay creates structural disorder, in their crystal structure. The generally increased susceptibility of radiationdamaged minerals to chemical alteration or aqueous leaching is of enormous importance, as these processes may for instance bias results of chemical and isotopic age determinations (e.g., Kuiper 2005; Zamyatin et al. 2017). The investigation of gradually radiation damaged and metamict minerals, and their synthetic analogues, has increased appreciably over the past two decades, stimulated by the potential use of mineral‐like ceramics as waste forms for the immobilisation of reprocessed spent nuclear fuel and other radioactive waste (e.g., Lumpkin 2016). Information obtained from studies on radiation‐damage‐assisted alteration in accessory minerals has important implications for the validation of the long‐term performance of analogue nuclear waste forms for the disposal in geological repositories, as radiation damage affects negatively the ability of solids to immobilise radionuclides. In both research fields, however, a fast and inexpensive technique that operates on the micrometre‐scale and provides direct quantitative information on the structural disorder, may open up new opportunities in the characterisation of radiation damage. Recently, confocal photoluminescence (PL) spectroscopy of rare‐earth elements (REE3+) incorporated in natural zircon crystals, has been used as structural probe for the characterisation of radiation damage due to the self‐irradiation by decay of trace U and Th (Lenz and Nasdala 2015).Similar to results from Raman spectroscopy (Nasdala et al. 1995), the width of PL emission lines has been used as a measure of radiation damage accumulated. One major challenge for using linewidths of PL signals for a quantitative measure of radiation effects, is its calibration using reference samples of known amorphous fraction or α‐dose. Attempted calibrations based on the study of naturally radiation‐damaged minerals, however, are often biased. This is because of insufficient knowledge of their thermal and, hence, annealing history(Nasdala et al. 2001). Here, we present a new concept based on the luminescence emission of REE3+, which aims at the direct determination of the amorphous fraction from a single PL measurement using state‐of‐the‐art confocal spectrometers with spatial resolution in the µm‐range. Careful investigation of PL spectra from self‐irradiated zircon samples from Sri Lanka as well as artificially irradiated single crystals and analogous polycrystalline ceramics (heavy‐ion Au irradiation with energies up to 35 MeV) revealed that the detected luminescence emission of e.g., Dy3+ in zircon is basically a superposition of emissions from Dy ions in various, structurally different sites. The latter comprise ions in fully ordered crystallographic environment and/or sites from stressed, but still crystalline remnants and from completely altered sites within the amorphous fraction. We found that the relative integrated area of a fitted model spectra from an amorphous reference sample in relation to the full integrated area of the luminescence emission obtained gives a good estimate of the amorphous fraction present in the probed sample volume (Fig. 1a). The application of the latter approach for the interpretation of point‐by‐point hyperspectral maps opens up the possibility to investigate the accumulation of radiation damage in natural zircon single crystals (Fig.1b) in very detail and give rise to direct comparison with damage accumulation in heavy ion irradiation experiments (Fig. 1c). In addition to the emission of Dy3+ in zircon, we successfully tested this concept for further accessory mineral‐type phases, such as for Nd3+ in xenotime (YPO4) and zirconolite (CaZrTi2O7).
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    Quantification of irradiation induced structural disorder in nuclear waste-form ceramics with μ-luminescence spectroscopy of lanthanides
    (Materials Research Society (MRS), 2017-10-29) Lenz, C; Thorogood, GJ; Lumpkin, GR; Nasdala, L; Ionescu, M
    The investigation of radiation damaged or metamict minerals and their synthetic analogues has increased appreciably over the past two decades, stimulated by the potential use of mineral-like ceramics as waste forms for the immobilisation of reprocessed spent nuclear fuel and other radioactive waste. In this research field, however, a fast and inexpensive technique operating in the micrometre range may open up new opportunities in the characterisation of radiation damage. We present first results of a heavy-ion (Au) irradiation-study of the important nuclear waste-form matrices zircon (ZrSiO4), xenotime-(Y) (YPO4) and zirconolite (CaZrTi2O7). Bulk, poly-crystalline ceramics were irradiated with accelerated heavy ions (Au) with energies up to 35 MeV. Comparably high heavy-ion energies are chosen to ensure irradiation penetration-depths of 4 - 5 µm accessible to the spatial resolution of optical confocal spectrometers. Summary: We use surface-sensitive, grazing-incident X-ray diffraction of irradiated bulk ceramic pellets for the estimation of the amorphous fraction produced and demonstrate how photoluminescence spectroscopy may be used as a tool for the characterisation and quantification of irradiation-induced structural damage in nuclear waste-form materials on a µm-scale. Ln3+ ions are common substitutes on regular lattice sites in respective ceramic hosts. Their luminescence emissions may be used as structural probe and are very sensitive to their local crystal field.
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    The quantification of radiation damage in orthophosphates using confocal μ-luminescence spectroscopy of Nd3+
    (Frontiers Media S.A., 2019-02-05) Lenz, C; Thorogood, GJ; Aughterson, RD; Ionescu, M; Gregg, DJ; Davis, J; Lumpkin, GR
    In this study, we present a new concept based on the steady-state, laser-induced photoluminescence of Nd3+, which aims at a direct determination of the amorphous fraction f a in monazite- and xenotime-type orthophosphates on a micrometer scale. Polycrystalline, cold-pressed, sintered LaPO4, and YPO4 ceramics were exposed to quadruple Au-ion irradiation with ion energies 35 MeV (50% of the respective total fluence), 22 MeV (21%), 14 MeV (16%), and 7 MeV (13%). Total irradiation fluences were varied in the range 1.6 × 1013–6.5 × 1013 ions/cm2. Ion-irradiation resulted in amorphization and damage accumulation unto a depth of ~5 μm below the irradiated surfaces. The amorphous fraction created was quantified by means of surface-sensitive grazing-incidence X-ray diffraction and photoluminescence spectroscopy using state-of-the-art confocal spectrometers with spatial resolution in the μm range. Monazite-type LaPO4 was found to be more susceptible to ion-irradiation induced damage accumulation than xenotime-type YPO4. Transmission electron microscopy of lamella cut from irradiated surfaces with the focused-ion beam technique confirmed damage depth-profiles with those obtained from PL hyperspectral mapping. Potential analytical advantages that arise from an improved characterization and quantification of radiation damage (i.e., f a) on the μm-scale are discussed. © 2019 Lenz, Thorogood, Aughterson, Ionescu, Gregg, Davis and Lumpkin. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY).