Gamma-ray Irradiation Effects at 77K on Organic Materials Used in Superconducting Magnets for the J-PARC Neutrino Experiment (2 Evolved Gases)

Polyimide films and glass fiber reinforced plastics (GFRPs) are used as insulating material or structural material of superconducting magnets in a neutrino beam line of the Japan Proton Accelerator Research Complex (J-PARC). It is indispensable to evaluate radiation resistance of these materials, because they are irradiated at a high dose of 30kGy/year at low temperature of 4K. In this work, gas evolution from the polyimide films and GFRPs by γ-ray irradiation at 77K was investigated. It was found that the main component of the evolved gas is hydrogen. The amount of the hydrogen evlved from the superconducting magnet system used in the neutrino beam line was estimated to be 0.37mol/year (0.01L/year as the volume of liquid hydrogen).