Abstract
We measured alpha emitters obtained from a reactor building in the Fukushima Daiichi Nuclear Power Plant (FDNPP) by using an alpha particle imaging detector. For developing the detector, we used a very thin (0.05-mm-thick) a cerium-doped Gd3(Ga,Al)5O12 (Ce:GAGG) scintillator and silicon photomultiplier (SiPM) arrays as the photodetector. The floor of the reactor building in FDNPP was wiped off by using smear papers, and the radioactivity of these papers was measured by the alpha particle imaging detector. In addition, we measured a Plutonium (Pu) sample (mainly 5.5 MeV alpha particles from 238Pu) obtained from a nuclear fuel facility by using of the same detector for comparison with the smear papers. The alpha spectrum was in the energy range of 5–6 MeV, which corresponds to the alpha particle energy of 238Pu (5.5 MeV). The correlation coefficient of the alpha spectra of the smear papers and the Pu sample had a strong positive linear relation. Moreover, the peak of 241Am was identified by gamma spectrum measurement. Based on these results, we report actual findings of alpha emitters in the FDNPP reactor buildings originating from nuclear fuels. The surface contamination level of alpha emitters exceeded 4 Bq/cm2.
Highlights
After the Great East Japan Earthquake hit the Fukushima Daiichi Nuclear Power Plant (FDNPP), large quantities of radioactive materials were released inside and outside of the FDNPP1–6
Measuring alpha activity directly at the FDNPP site is extremely difficult because the dose-rate of gamma rays there is higher than 10 mSv/ h13, which makes it impossible for workers to stay in the field for any significant duration
It was confirmed that the ZnS(Ag) survey meter was not influenced by beta and gamma rays by comparing the count rates obtained with and without paper
Summary
After the Great East Japan Earthquake hit the Fukushima Daiichi Nuclear Power Plant (FDNPP), large quantities of radioactive materials were released inside and outside of the FDNPP1–6. To identify a radionuclide of an alpha emitter, an alpha particle detector with high energy resolution is required[22] Another aim of measuring alpha emitters is obtaining information about the two-dimensional distribution of alpha emitters. An imaging plate (IP) has been widely used to obtain the two-dimensional distribution of alpha emitters[25,26], but this plate is sensitive to beta particles and gamma rays as well. We have developed an alpha particle imaging detector that can measure the energy spectrum and the two-dimensional distribution of alpha particles by combining a cerium-doped Gd3(Ga,Al)5O12 (Ce: GAGG) scintillator with a silicon photomultiplier (SiPM). We measured alpha emitters in the reactor building of FDNPP for the first time by using the developed alpha particle imaging detector. The features of radionuclides of alpha emitters in the reactor building of FDNPP were clarified by these measurements
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