Radioactivity prospecting with plastic scintillator detector

A location of buried fault is of particular concern for the development of groundwater resources in the rocky mountainous areas because groundwater tend to occur around the fault. The gamma-ray radioactivity prospecting with the NaI detector is one of tools to detect the location of buried fault and was successful in some regions. However, few studies on the relationships between the fault, groundwater, and the gamma-ray were reported. The buried fault in the dam-site where the surface soil was stripped showed that many springs were observed along the fissures in granodiorite of CM and CH levels on the both sides of the main fault zone. The radioactivity prospecting across the main fault zone showed that 214Bi and 214Bi/208Tl decrease over the main fault zone. These phenomena are interpreted as the leaching of the uranium. The fluctuation of 222Rn concentration showed the peak at the point near the f1 fault. 222Rn concentration over the areas of springs showed the low concentration. The high 222Rn concentration did not have any influence on 214Bi and 214Bi/208Tl. Our results request the rethinking of the current principle of the gamma-ray radioactivity prospecting that the reason of high 214Bi is caused by upward 222Rn along the open cracks in fault zone. The NaI detector for the radioactivity prospecting is heavy and expensive. PS detector is lightweight and inexpensive compared with NaI detectors. In order to make a large-scale detector with low price, a three-layered PS detector has been developed. Experiments show that the PS detector can discriminate the energies of gamma-rays. (author)