Basic study on simultaneous measurement of coming direction and energy of gamma-ray using a tandem detector

A tandem detector, which positively raises directional dependence for coming gamma-rays, has been produced experimentally to measure directions and energies of coming gamma-rays simultaneously. In the tandem detector, a cylindrical NaI(Tl), the same sized BGO scintillator and a fitted photomultiplier tube are combined optically at this order. Since the lengths of crossing each scintillator are changed according to coming directions of gamma-rays, the directions can be recognized by counting photopeaks on a spectrum made by the NaI(Tl) and the BGO scintillator and by obtaining the ratio of photopeak counts. Using the tandem detector, the following experiments were carried out for purposes of confirmation of the measurement principle and verification of the performance. A 137Cs source of 3.7 MBq was put 20 cm in front of the detector. Coming gamma-rays were counted for 60 S and the counting ratio was calculated from the spectrum. The source was moved in each 10 degrees for the side of the detector until it becomes 90 degrees. Then the similar experiments were repeated. Next, the experiments were kept taking the distances between the source and the detector as 30 cm, 100 cm and 200 cm. From the results of the experiments, it was proven that the counting ratio changed approximately from 1.7 to 3.0 when the direction changes from 0 to 90 degrees. This means that the coming direction can be decided when the counting ratio is known. At the same time, it was confirmed that the energy characteristics of the tandem detector were the same as those of a NaI(Tl) and a BGO detector. As a result, it has been shown that the tandem detector has a possibility of measuring the coming direction and the energy of gamma-rays simultaneously. (author)