Field test around Fukushima Daiichi nuclear power plant site using improved Ce:Gd3(Al,Ga)5O12 scintillator Compton camera mounted on an unmanned helicopter

An improved light-weight Compton camera exhibiting low power consumption was developed to be mountable on an unmanned helicopter to detect cesium radiation hot spots and confirm the decontamination effect of cesium-affected areas. An increase in the Ce:Gd-3(Al,Ga)(5)O-12 scintillator array from 4x 4 to 8x 8 and expansion of the interlayer distance enhanced the detection efficiency and angular resolution, respectively. Measurements were performed over the Ukedo riverbed in Namie, Fukushima Prefecture (Japan). The helicopter's flight path and speed were pre-programmed to lines interspaced by 5 and 10m intervals and 1 m/s, respectively, facilitating measurements over areas of 65x 60 m(2) and 65x 180 m(2) at a height of 10m for approximately 20 and 30min, respectively. Results provided accurate ambient dose equivalent rate maps at a height of 1m with an angular resolution corresponding to a position resolution of approximately 10m from a height of 10m. Hovering flights were executed over hot-spot areas for 10-20min at a height of 5-20m. Gamma-ray images of these hot spots were obtained using a reconstruction software. Comparison between position-shifted measurement results showed that the angular resolution coincided with that evaluated in the laboratory (approximately 10 degrees).