Development of a high-resolution alpha-particle imaging system for detection of plutonium particles from the Fukushima Daiichi nuclear power plant

For the detection of plutonium particles released from the Fukushima Daiichi nuclear power plant, we developed a high-resolution alpha-particle imaging system. The detector of the alpha-particle imaging system consists of a thin ZnS(Ag) sheet, a light guide, and a high quantum efficiency 1-inch square position-sensitive photomultiplier tube (PSPMT). The ZnS(Ag) sheet was optically coupled to the PSPMT with a 1.5 mm thick light guide between them. The Anger principle was used for the position calculation of the alpha particles. The spatial resolution of the alpha-particle imaging detector was 0.45 mm full width at half maximum for 5.5 MeV alpha particles. The uniformity of the imaging detector in the central part of the field of view was +/- 7%. The detection efficiency was 76% for 5.5 MeV alpha particles. Although the background count rate was 8.1 counts per minute because of the collection and detection of alpha particles from radon daughters in the air, it could be decreased to 0.1 counts per minute if the detector surface was covered with paper. There was no increase in the background count rates of Cs-137 gamma photons and Ca-45 beta particles. We obtained high-resolution variable phantom images and particles of alpha emitters with the system. We conclude that the alpha-particle imaging system developed is promising for the detection of plutonium particles in samples collected near the Fukushima Daiichi nuclear power plant.