Application of micro-ion-beam analysis to investigate the distribution of Cs in silt particles for environmental remediation of Fukushima

(1) Research Center for Remediation Engineering of Environments Contaminated with Radioisotopes, Graduate School of Engineering, Tohoku University, 6-6-01-2 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8579, Japan
(2) Department of Quantum Science and Energy Engineering, Tohoku University, 6-6-01-2,  Aramaki Aza-Aoba, Aoba-ku, Sendai, 980-8579, Japan

After the great east Japan earthquake on 11 March 2011,the big tsunami caused Fukushima Dai-ichi nuclear power plant accident.  The northeastern region of Japan (mainly Fukushima prefecture) was contaminated with the radioisotopes of 131I, 132I, 132Te, 134Cs and 137Cs. Among these radioisotopes, 131I, 132I and 132Te disappeared soon due to their short half lives. The half life of 134Cs, and of 137Cs are 2 years and 30 years, respectively. The contaminated soil of 5 cm in  thickness from the ground surface has been removed and its total volume was estimated about 2  107 m3. It is a burden to Fukushima residents to reserve a very large place to secure such a huge quantity of contaminated soil. Therefore,it is needed to reduce the volume of contaminated soil. It was known that Radioactive cesium atoms have been adsorbed with the silt particles in the soil. We developed a methods by which the decontaminated soil volume / the contaminated soil volume was ~ 0.9 ,namely, the volume reduction rate was ~ 0.1 [1]. By classifying soil particles, we found  that the specific activity of silt particle depended on diameter[2] and proposed that, in the case of the silt particles with the diameter of less than 10~30 mm, radioactive cesium atoms were distributed over the whole and, in the case of larger than 10~30 mm, over the husks [3], respectively.
The use of PIXE-CT is proposed to investigate the distribution of Cs in the silt particle. In consideration of decrease of X-ray production cross sections and self-absortion of X-ray in the silt particle in the case of 3 MeV proton bombardment, PIXE-CT can be applied to the silt particles with the diameter of less than about 10 mm. We extracted silt particles from the muddy water obtained by washing the soil and then we sprayed the solution of cesium carbonate (Cs2CO3). We used the micro-beam formation system of Tohoku university to PIXE-CT and obtained 2D-projection data of 0° ~ 348° at intervals of 18°. On the basis of the filtered backprojection method, we reconstructed cross-sectional images reflecting the distribution of Cs in the silt particle of about 10 mm diameter and found that Cs atoms were uniformly distibuted in the silt particle. This supported the previous result of the classification method of soil particles. To investigate the interior structure of the silt particles with the diameter of larger than 10mm, we apply an X-ray CT using a micron X-ray source produced by micro-ion-beam bombardment[4]. The existence of Cs is expected to be emphasized in the difference image between a V-K-X-ray (4.94 keV) image and a Cr-K-X-ray (5.41keV) image owing to the strong absorption of Cs-L3-absorption edge (5.01keV).

[1] K.Ishii et al. IJPIXE, 22(2012)13-19.
[2] K.Ishii et al., Nuclear Inst. and Methods in Physics Research, B 318(2014)70-75.
[3] K.Ishii et al., to be published in IJPIXE.
[4] K.Ishii et al., Nuclear Inst. and Methods in Physics Research, B 249(2006)726-729.