For research of high energy radiation effects of some materials as superconductors at cryogenic temperature, a cryostat was constructed. In the cryostat, the samples immersed in liquid He are irradiated by electrons and X-rays of about 30meV from a linear accelerator. The irradiated samples and the surrounding materials can be activated and some part of the radioisotopes may contaminate the evaporated He gas. The contamination interferes reuse of He gas. In this paper, possibility of the activation was estimated and the contamination of the He gas was experimentally confirmed. A radiation safety system was installed in association with the cryostat. The system monitors radiation dose in the He gas evaporated from the cryostat in real time, and exhausts the contaminated He gas and transports only the clean He gas to a gas holder for the reuse.
The present paper describes monitors for the detection of β-ray and γ-ray radionuclides in He gas recovered from the cryogenic irradiation equipment reported in the preceding publication (I). The γ-ray monitor consists of a 2″×2″φ Nal (Tl) scintillation counter in the monitor vessel and the β-ray monitor is composed of thin box-type ionization chambers piled in three layers which can measure separately the radioactive concentrations of the groups consisting of low, medium and high energy β-emitters. The performance tests of these detectors revealed a typical sensitivity of 3.5×10-3Bq/cm3 for a low energy β-emitter such as tritium in He gas. In addition, satisfactory selection of the three energy groups of β-emitters was demonstrated using 133Xe in the β-ray monitor.