Surface Sensitive Spectroscopy Study of Ion Beam Irradiation Induced Structural Modifications in Borosilicate Glasses
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Surface Sensitive Spectroscopy Study of Ion Beam Irradiation Induced Structural Modifications in Borosilicate Glasses Amy S. Gandy1 Martin C. Stennett1 and Neil C. Hyatt1 1 Immobilisation Science Laboratory, Department of Materials Science and Engineering, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK ABSTRACT Fe K edge X-ray absorption (XAS) and Fourier Transform Infra-Red (FT-IR) spectroscopies have been used to study potential structural modifications in sodium borosilicate glasses as a consequence of Kr+ irradiation. Glasses were doped with simulant waste elements and irradiated at room temperature with 450 keV Kr+ ions to a fluence of 2x1015 Kr+ ions cm-1. According to SRIM calculations, a damaged surface region approximately 400nm wide was produced. In order to probe only the damaged surface layer, XAS measurements were taken in total electron yield mode and FT-IR spectroscopy was conducted in reflectance off the glass surface. No change in Fe valence state was detected by XAS following irradiation. Reflectance FT-IR data revealed a shift to higher wavenumbers in the absorption bands located between 850 and 1100 cm-1 in the doped glasses, corresponding to bond stretching in the silicate network. Deconvolution of FT-IR spectra revealed the shift was due to polymerisation of the silicate network. Network connectivity was found to decrease in the un-doped glass, following irradiation. The results suggest an increase in silicate network connectivity by a cation mediated process, and demonstrates the successful application of surface sensitive XAS and FT-IR to the investigation of ion beam induced damage in amorphous materials. INTRODUCTION In the UK, alkali borosilicate glasses are used to vitrify high level waste (HLW) produced by reprocessing spent nuclear fuel. HLW contains fission products and minor actinides which continue to undergo radioactive decay in the wasteform for up to 106 years. Cations such as Fe and Zr are also present in the waste stream and require incorporation into the final wasteform. Actinides undergo -decay with the formation of -particles (He nuclei) and energetic (~100 keV) daughter recoil nuclei. Energy is transferred from the energetic recoil nuclei to other atoms in the glass via elastic interactions, resulting in atomic displacements which form collision cascades. It is hypothesized that accumulation of this ballistic damage can lead to migration of alkali ions, resulting in changes in glass network polymerisation. These changes can affect wasteform durability and since the wasteform acts as the final barrier against radionuclide release into the environment, it is important that the effects of -decay on the structure of the glass are understood. Radiation damage in crystalline materials has been extensively studied using a range of complimentary techniques, including ion beam irradiation [1]; actinide doping of materials [2]; investigation of natural analogues [3]; and computational modeling e.g. MD simulations [1]. As a consequence, crystalline material response t
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