Radiation damage in biotite mica by accelerated α-particles: A synchrotron microfocus X-ray diffraction and X-ray absorption spectroscopy studyk

Abstract

A critical radiation damage assessment of the materials that will be present in a Geological Disposal Facility (GDF) for radioactive waste is a priority for building a safety case. Detailed analysis of the effects of high-energy a-particle damage in phyllosilicates such as mica is a necessity, as these are model structures for both the clay-based backfill material and the highly sorbent components of a crystalline host rock. The a-radiation stability of biotite mica [general formula: K(Mg,Fe) 3 (Al,Si 3 O 10) (F,OH) 2 ] has been investigated using the 5 MV tandem pelletron at the University of Manchester's Dalton Cumbrian Facility (DCF) and both the microfocus spectroscopy (I18) and core X-ray absorption spectroscopy (B18) beamlines at Diamond Light Source (U.K.). Microfocus X-ray diffraction mapping has demonstrated extensive structural aberrations in the mica resulting from controlled exposure to the focused 4 He 2+ ion (a-particle) beam. Delivered doses were comparable to a-particle fluences expected in the highly active, near-field of a GDF. At doses up to 6.77 displacements per atom (dpa) in the region of highest particle fluence, biotite mica displays a heterogeneous structural response to irradiation on a micrometer scale, with sequential dilation and contraction of regions of the structure perpendicular to the sheets, as well as a general overall contraction of the phyllosilicate layer spacing. At the peak of ion fluence, the structure collapses under a high point defect density and amorphous areas are pervasive among altered domains of the original lattice. Such structural alterations are likely to affect the material's capacity to sorb and retain escaped radionuclides over long timescales; increased edge site availability may favor increased sorption while interlayer uptake will likely be reduced due to collapse. Radiation-induced reduction of structural iron at the region of highest structural damage across an a-particle's track has been demonstrated by Fe K-edge X-ray absorption near edge spectroscopy (XANES) and local structural disorder has been confirmed by analysis of both potassium K-edge XANES and Fe K-edge extended X-ray absorption fine structure analysis. An infrared absorption study of deformations in the OHstretching region, along with electron probe microanalysis complements the synchrotron data presented here.

Radiation Damage Effects in Chlorite Investigated Using Microfocus Synchrotron Techniques

“…The data allow several assertions to be made regarding the structural and chemical responses of chlorite to α-irradiation at varying doses. Collapse of the interlayer spacing is consistent with previous studies of radiation damaged phyllosilicates . At the point of maximum dose, a contraction of up to 0.5 Å of the basal (003) plane was observed in both chlorite samples.…”

“…Spectra collected from the low-dose samples, (Figure D–F; Sample 2-B), share commonalities with those from sample 1-A (Figure A–C). Chiefly, a minor loss in oscillation amplitude beyond the edge, as well as a slight reduction in the edge step, which is consistent with increased defect densities resulting in local disorder . Most notably, a “shoulder” appears on the low energy side of the edge, at ∼7122 eV (arrow 3, Figure D).…”

“…Samples were irradiated under vacuum with a focused 5 MeV 4 He 2+ ion beam, using the Dalton Cumbrian Facility’s 5 MV tandem pelletron via an established method. − The beam was focused into a pseudo-Gaussian profile, such that the center of the beam delivered a much higher dose than the perimeter. In this way, a single sample was able to accumulate a range of doses over a circular beam spot (area = 1 cm 2 ) suitable for synchrotron microfocus analysis.…”

Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 3: Aluminosilicates

“…The Si dissolution rate of muscovite is about 10 −13 to 10 −12 moles m −2 s −1 at neutral conditions (Brady and Walther, 1989;Lammers et al, 2017), which is sufficient to lead to surface alterations within the timescale of our experiments. The initial dissolution of Al is higher than the one of Si (Pachana et al, 2012), confirming that edges dissolve more readily than the basal faces.…”

Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions. Part 3 – Aluminosilicates

“…Feldspars have been reported to be the most IN active minerals although the individual members of the feldspar group exhibit very different IN efficiencies (Atkinson et al, 2013;Zolles et al, 2015;Harrison et al, 2016;Kaufmann et al, 2016). During long range transport in the atmosphere, mineral dust particles can acquire organic and inorganic coatings (Usher et al, 2003;Sullivan et al, 2007), which may change their IN efficiencies (Zuberi 55 et al, 2002;Zobrist et al, 2008;Eastwood et al, 2009;Augustin-Bauditz et al, 2014;2016;Kumar et al, 2018a). When the INPs are fully covered by a water-soluble coating, IN may occur by an immersion freezing mechanism, typically after the aerosol particles experience increasing humidity.…”