### 9. 英文概要

· 研究課題名：

“Development of imaging analysis method for radioactively contaminated objects by semiconductor Compton camera technology”

· 研究代表者名及び所属：Shinji Motomura, RIKEN (The Institute of Physical and Chemical Research)

· 研究分担者名及び所属：Shuichi Enomoto, Okayama University

·要旨：

Various radioactive substances were released in the environment from the Fukushima Daiichi nuclear power plant in the aftermath of the Great East Japan Earthquake, and there are still significant amounts of radioactive substances left even in the ordinary environment near residential areas in Fukushima. The widely spread radioactive substances were washed out by the rainfall, and condensed at the water purification plants, waste incineration plants, sewage treatment plants, and so forth. The condensed radioactively contaminated wastes have been temporally stored at each site. In order to support appropriate and safe treatment of the radioactively contaminated wastes, we have proposed to use the technology of the semiconductor Compton camera GREI that is able to visualize the distribution of the radioactive substances and distinguish the radionuclide at the same time.

In FY2012, we succeeded in carrying out a field experiment for the first time using the GREI originally designed for bio-medical imaging application. We performed an imaging experiment of ash dust collected in drum cans at an incineration plant in Fukushima, and succeeded in demonstrating the imaging analysis of $ {}^{134} $ Cs and $ {}^{137} $ Cs contained in the radioactively contaminated waste. However, it took too long time as more than 2 hours to obtain the images of sufficiently good quality, as we expected from the beginning though.

In FY2013, we succeeded in constructing a GREI imaging system optimized for distant radioactive substances. The GREI imaging head was designed mainly by optimizing the distance between the two germanium semiconductor detectors composing the Compton camera, and achieved more than ten times increased overall imaging sensitivity for distant radioactive substances.

We have performed imaging experiment of various radioactive sources with the GREI system optimized for distant radioactive substances. To demonstrate the imaging system can construct accurate distribution images of the radioactive sources, we measured two $ {}^{137} $ Cs sources of 2.0 MBq positioned 1.0 meter away from the GREI imaging head changing the angular distance between the two sources at 30-, 15-, and 10-degrees for 10 minutes respectively. The spatial dose rate at the GREI imaging head was 0.57 $ \mu $ Sv/h. We have succeeded in obtaining the accurate images for each source arrangement, and demonstrating the angular resolving power of better than 10 degrees for 10 minutes' imaging time. Our proposed GREI imaging system will be able to contribute not only to the recovery from the present situation in Fukushima, but also to the management of radioactive substances at other radioactive material handling facilities including nuclear power plants, and to the provision for future accidents where the radionuclides are unknown.

·キーワード：Compton camera, gamma-ray imaging, radioactively contaminated wastes, gamma-ray spectrometry