Identification of depth location of a radiation source by measurement from only one direction using a Compton camera

In the decommissioning-work environment of a nuclear power plant, accurately identifying the location of radiation sources is necessary to reduce worker doses and develop decontamination plans. In this regard, gamma-ray imagers are promising devices for the visualization of the location of radiation sources. However, they can only estimate the direction where the radiation source is located and not the distance to the source. A previous study has reported a method for identifying the three-dimensional coordinates of a radiation source by measuring from multiple viewpoints using a gamma-ray imager. In some cases, many objects located in an actual decommissioning site limit the accessible area, making it difficult to measure the target area from multiple viewpoints. Thus, the present study devised and demonstrated a method of estimating the distance to the radiation source by measuring from only one direction using a Compton camera, which is a type of gamma-ray imager. The proposed method considered the reconstructed image intensity as proportional to the inverse square of the distance to the radiation source. This approach is useful in accurately locating radiation sources in environments with limited workable areas such as the broken reactor buildings inside the Fukushima Daiichi Nuclear Power Station.