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Morishita, Yuki; Kurosawa, Shunsuke*; Yamaji, Akihiro*; Hayashi, Masateru*; Sasano, Makoto*; Makita, Taisuke*; Azuma, Tetsushi*
Scientific Reports (Internet), 11(1), p.5948_1 - 5948_11, 2021/03
Times Cited Count：2 Percentile：31.78(Multidisciplinary Sciences)The internal exposure of workers who inhale plutonium dioxide particles in nuclear facilities is a crucial matter for human protection from radiation. To determine the activity median aerodynamic diameter values at the working sites of nuclear facilities in real time, we developed a high-resolution alpha imager using a ZnS(Ag) scintillator sheet, an optical microscope, and an electron-multiplying charge-coupled device camera. Then, we designed and applied a setup to measure a plutonium dioxide particle and identify the locations of the individual alpha particles in real time. Employing a Gaussian fitting, we evaluated the average spatial resolution of the multiple alpha particles was evaluated to be 16.2 umFWHM with a zoom range of 5 x. Also, the spatial resolution for the plutonium dioxide particle was 302.7 umFWHM due to the distance between the plutonium dioxide particle and the ZnS(Ag) scintillator. The influence of beta particles was negligible, and alpha particles were discernible in the alpha-beta particle contamination. The equivalent volume diameter of the plutonium dioxide particle was calculated from the measured count rate. These results indicate that the developed alpha imager is effective in the plutonium dioxide particle measurements at the working sites of nuclear facilities for internal exposure dose evaluation.
Morishita, Yuki; Kurosawa, Shunsuke*; Yamaji, Akihiro*; Hayashi, Masateru*; Sasano, Makoto*; Makita, Taisuke*; Azuma, Tetsushi*
no journal, ,
It is crucial in considering internal exposure when workers inside nuclear facilities inhale plutonium particles. The internal exposure dose is strongly affected by the particle size distribution defined as activity median aerodynamic diameter (AMAD). To acquire the AMAD value at the working site of the nuclear facility, we developed an alpha imaging detector using an optical camera and an optical microscope. Then, we applied it for plutonium particle measurements. The ZnS(Ag) scintillator sheet and a plutonium particle were close to each other. Alpha particles were absorbed in the ZnS(Ag) scintillator and were converted to scintillation light. An Electron Multiplying (EM) CCD camera was mounted on top of the optical microscope to capture scintillation light. The zoom range was adjustable from 5x - 20x. When using the zoom range of 20 x, the resolution and Field of View were 0.81 um/pixel and 412.9 um 
412.9 um, respectively. The Full width at half maximum (FWHM) of an alpha particle was evaluated to be 17.9 um. Locations of individual alpha particles from a plutonium particle can be identified in real-time. The number of alpha counts was agreed with those measured using a commercial ZnS(Ag) scintillation counter. The measured alpha counts will be able to convert to radioactivities and AMAD. Thus, the ultra-high-resolution alpha imager will be promising plutonium particle measurements at the working sites of nuclear facilities.
Yoshida, Hiroko*; Hayashi, Masateru*; Makita, Taisuke*; Azuma, Tetsushi*; Sasano, Makoto*; Tsuda, Shuichi
no journal, ,
Investigation on distribution of radioactive substances in Fukushima within former evacuation areas has been ongoing since 2012. Reduction rates of air dose rate indoor and outdoor were compared for 54 residential houses in Iidate-mura and Minami-Soma. Just after the decontamination, the averaged value of the outdoor reduction rates was higher than those indoor, while the averaged outdoor reduction rate in 2019 and 2020 was almost identical to the indoor one within uncertainties. This means that indoor air dose rate has decreased less than outdoors. One of the reasons would be the existence of additional radionuclides coming from outside of the decontamination area around the houses, according to spectra analyses of air dose measurement and deposition of radionuclides on the roof.
Hayashi, Masateru*; Makita, Taisuke*; Azuma, Tetsushi*; Sasano, Makoto*; Yoshida, Hiroko*; Tsuda, Shuichi
no journal, ,
Evaluation of dose rates of artificial nuclides is of importance to make a decision on return of residents to their hometown contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. This study applies an unfolding method to measured data by a NaI(Tl) detector in/outside houses, and obtained gamma-ray energy spectra. We calculated ratios of dose rate from natural nuclides such as 40K and Uran-Thorium series to those of artificial nuclides. It was found that the ratios are almost constant, given that no artificial nuclide exists. This means a possibility that dose rates of artificial nuclides can be precisely estimated by subtracting dose rates of natural nuclides.
Morishita, Yuki; Sagawa, Naoki; Fujisawa, Makoto; Kurosawa, Shunsuke*; Sasano, Makoto*; Hayashi, Masateru*; Tanaka, Hiroki*
no journal, ,
This study reports the effects of different types of radiation on a high-resolution alpha imager, developed using an Electron Multiplying Charge-Coupled Device (EMCCD) camera. The imager was originally designed to visualize Pu oxide particles in decommissioning sites, where other types of radiation such as beta particles, gamma-rays, and neutrons are also present. The measurement of alpha particles, beta particles, gamma-rays, and neutrons were performed, and it was found that the EMCCD camera's sensor detected gamma-rays and neutrons. The discrimination method was based on the characteristics of the image distribution, where the image values were binarized and a Gaussian filter was applied to count the number of alpha particle spots. The results show that it is possible to discriminate between alpha and gamma (neutron) rays using the difference in intensity. The study provides important information for the development of radiation detection techniques in decommissioning sites.
Morishita, Yuki; Sagawa, Naoki; Fujisawa, Makoto; Kurosawa, Shunsuke*; Sasano, Makoto*; Hayashi, Masateru*; Tanaka, Hiroki*
no journal, ,
This study reports the effects of different types of radiation on a high-resolution alpha imager, developed using an Electron Multiplying Charge-Coupled Device (EMCCD) camera. The imager was originally designed to visualize Pu oxide particles in decommissioning sites, where other types of radiation such as beta particles, gamma-rays, and neutrons are also present. The study aims to confirm the effects of these radiations on the imager and to develop a discrimination method between alpha particles and other radiations. The measurement of alpha particles, beta particles, gamma-rays, and neutrons were performed. The discrimination method was based on the characteristics of the image distribution, where the image values were binarized and a Gaussian filter was applied to count the number of alpha particle spots. The results show that it is possible to discriminate between alpha and gamma (neutron) rays using the difference in intensity. The study provides important information for the development of radiation detection techniques in decommissioning sites.
