Alpha-ray imaging with alkali copper halide scintillator

Internal exposure for decommissioning workers at the Fukushima Daiichi Nuclear Power Plant must be prevented, and we have developed a monitoring system for alpha-ray emitting dust (alpha dust). When the dust size is less than several tens of micrometers, a dust protection mask does not work effectively to prevent internal exposure. Since no devices have been operated to observe alpha-dust images in real-time up to now, we have developed an alpha-ray imaging detector consisting of a scintillation material and imaging detector. Scintillators are required to have high light output and chemical stability. Cs3Cu2I5 (CCI) scintillator was found to be one of the candidates, and an imaging test was operated with this material. The CCI crystal was grown by the Bridgman-Stockberger method, and a scintillation sheet for alpha-ray imaging was prepared by CCI microcrystals with a thickness of approximately 70 μm on a 200-μm thickness transparent polyethylene terephthalate film as a first imaging test. The sheet was irradiated with 5.5-MeV alpha rays from an 241Am source, and scintillation photons were detected with a CMOS camera through an optical lens. We succeeded in visualizing alpha rays, and a position resolution of our system achieved approximately 16.2 ± 2.6 μm (10–90 %). This resolution was similar value to other imaging devices with Ag:ZnS powder scintillator, and a CCI single crystal sheet was expected to reach better resolution.