Speciation of cesium in a radiocesium-bearing microparticle emitted from Unit 1 during the Fukushima nuclear accident by XANES spectroscopy using transition edge sensor.

The chemical state of radiocesium (RCs) was determined using X-ray absorption near-edge structure (XANES) in fluorescence mode and microbeam X-ray fluorescence (μ-XRF) mapping for the cesium (Cs) incorporated in a radiocesium-bearing silica microparticle (CsMP) that was released during the Fukushima Dai-ichi Nuclear Power Plant accident in 2011. The sample investigated here was classified as a CsMP emitted from Unit 1 (diameter: approximately 500 µm; Cs weight concentration: approximately 50 µg/g). The Cs Lα emission from this Type-B CsMP with a much lower Cs concentration than those of calcium and titanium was difficult to detect using a silicon drift detector, severely decreasing the signal-to-background ratio of the Cs Lα emission. In this study, a high-energy-resolution energy-dispersive transition-edge sensor was used for X-ray spectroscopic analysis, which enabled the measurement of Cs L-edge XANES for RCs in the Type-B CsMP. The results revealed the presence of two distinct RCs species in CsMPs. Bulk XANES analysis indicated that one species consists of RCs dissolved in the silicate matrix. Additionally, μ-XRF-XANES revealed that the RCs species were heterogeneously distributed within the CsMP with enrichment observed on the surfaces of internal voids, suggesting that the gaseous RCs species, such as CsCl, adhered to these surfaces during the cooling process of the molten silicate. The speciation and μm-scale mapping of RCs provide further insights into (i) the formation process of spherical Type-B CsMP and (ii) the incorporation and internal distribution processes of RCs within the particle. (239 words).