Measurement method for Cs in environment by use of the coincidence method

Hirade, Tetsuya  ; Katayama, Atsushi ; Masaki, Nobuyuki 

The quantity of Cesium radioisotopes is estimated with full absorption peaks on the energy spectra measured by -ray detectors such as Ge semiconductor detector. The heavy shield box made of Lead is needed to obtain reliable data, because the detection efficiency is in inverse proportion to square of the distance between the detectors and the radioactive materials. We have noticed that Cs emits two -rays, 605 keV and 796 keV, with high efficiency, and that there are almost no radio isotopes which emit two -rays as Cs does in environment. In the case of coincidence measurements of two -rays, the detection efficiency is in inverse proportion to the fourth power of the distance between the detectors and the radioactive materials. Therefore the contamination with radioisotopes around the detectors do not affect to the count of the coincidence events. Hence, it is possible to estimate the quantity of Cs by in-situ and nondestructive measurement.