A simple gamma ray direction finder.

One of the simplest gamma spectrometry systems that could provide the directional information of incident gamma rays has been developed. The system consists of a 3'' x 3'' phi NaI (Tl) scintillator, a specially shaped lead shield, and software. The measurement was carried out four times by rotating the shield position along the axis of the detector to obtain four energy spectra at one location. Four count rates at a special region of interest in the spectra were fed into the software for determining incident directions of gamma rays. Experiments using (137)Cs and (54)Mn at the same time demonstrated that the direction of gamma rays from several dominant sources from any direction could be identified with good precision by the total measurement time of 10 to 20 min. The system could be used to identify the locations of missing radioactive sources or the cause of elevation in ambient radiation dose rates. The disadvantages of the present system are follows: (1) It requires four time measurements at one location; (2) It can provide one pseudo incident angle when several contamination sources exist around the detector system and emit the same energy gamma rays; and (3) It can scan only one plane geometry that is usually chosen as the horizontal plane.