Remote Sensing of Nuclear Radiation Leakage by a Direction Finding Dtector

There is a considerable interest in measuring an incident direction of gamma rays for searching radioactive materials and contaminations from the viewpoint of radiation protection and safety, or in finding out a nuclear accident for a quick response. . In order to meet this demand, a new gamma ray detector, which positively raises directional sensitivity against incoming gamma rays, has been developed. This detector is composed of three kinds of different scintillators and a photomultiplier tube. The types of scintillators are NaI(Tl), CsI(Tl), and BGO. The section shape of each scintillator is the sector of 120 degrees. The one cylindrical scintillator is made by optically connecting three sectorial scintillators. Gamma rays penetrate along a straight path through each scintillator of the detector, and the length of each path is dependent on the incident direction. Therefore the probability of photoelectric effect in each scintillator is changed, according to incident directions of gamma rays. As a result, the numbers of gamma ray counts of three peaks observed in a spectrum obtained by a multi-channel analyzer (MCA) have a strong dependency with the direction. The response curve of each scintillator with respect to a peak count and an incident direction was obtained by experiments using a gamma ray source of Cs-137. It has been recognized that these response curves have sensitivity in the total range from 0 to 360 degrees. Therefore the proposed detector has the possibility that the direction can be identified and will be used as a radiation monitoring post or a radiation survey meter