Plutonium particle imaging using an ultra-high-resolution alpha imager

Morishita, Yuki   ; Kurosawa, Shunsuke*; Yamaji, Akihiro*; Hayashi, Masateru*; Sasano, Makoto*; Makita, Taisuke*; Azuma, Tetsushi*

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.