Development of a radioactive gas monitor using plastic scintillation fibers

Nozaki, Tatsuo ; Torii, Tatsuo  ; Takada, Chie ; Tobita, Kazunori; Ando, Hideki

The basic research on large electron linear accelerator is under current investigation at Quantum Technology Development Facility in O-arai Engineering Center, Power Reactor and Nuclear Fuel Development Corporation. The result of this, the transmutation of fission products would be incorporated In the future demonstrative stage of the transmission of fission products, Nitrogen-13 and Oxygen-15 will be produced by the interaction between high energy bremsstrahlung and air during operation of the linac. The detection limits of conventional monitors are not sufficient for the limits of the concentration of Nitrogen-13 (610 Bq/cm) and Oxygen-15 (610 Bq/cm) in exhaust air. Therefore we have developed a new type of gas monitor detector that is packed with plastic scintillation fibers as the detection elements to detect the limits of concentration of Nitrogen-13 and Oxygen-15 in exhaust air. On designing the gas monitor detectors, the sensitivities of the detectors were simulated to optimize the shape and the volume of detector by using EGS4, the Monte-Carlo code for electrons and photons. Based on the results of the simulations, the detectors were composed and the characteristic examinations of sensitivities were carried out by using beta-ray checking sources and Krypton-85 gas. Main points are as follows : (1) The detection limit of the developed torus-type detector for Krypton-85 is 5.410 Bq/cm based on the results of the experiments. (2) The counting efficiency of a detector for Nitrogen-13 is 40 percent higher than the counting efficiency for Krypton-85 based on the results of simulations. This indicates that the detection limit of the detector for Nitrogen-13 is about 3.9 10 Bq/cm. (3) The beta-ray energy of Oxygen-15 is higher than the beta-ray energy of Nitrogen-13. This theoretically reaches the conclusion that the detection limit of the detector for Oxygen-15 is lower than the ...