Development of a hotspot detector with an acrylic filter and dose rate survey meters

Fukushima and adjacent regions still have a large number of high dose rate areas called hotspots. It is necessary to know these hotspots for efficient decontamination of radioactive substances such as ¹³⁷Cs and for relief of residents coming home. To find the hotspots rapidly, we have to specify the direction of the area where the dose rate is at least 1μSv/h higher than those of surroundings. We have developed a detector that consists of an acrylic filter and three NaI(Tl) scintillation survey meters, and the detector can be expected to indicate the direction of the hotspot in the short time. A basic performance of the detector was examined by using acrylic filters of 10, 15, 20 and 25cm diameter and a tiny sealed ¹³⁷Cs source of 3 MBq as the alternative of a hotspot. It demonstrated the possibility of identifying the direction of γ-rays emitted from the source in 90 seconds. (author)