The development of radioactivity measurement system in gases

A device for measuring radioactive impurities in gas is developed, consisting of an electrostatic collection type radon detector and an extremely low radioactivity proportional counter. This device measures the radioactive impurities released by materials into the gas and a filtering test for radon removal from gas. The performance of the newly developed proportional counter was evaluated by measuring radon rate. The calibration factor of $^{222}$Rn as 27.4$\pm$0.2~count/day/mBq and the background rate of $^{222}$Rn as 0.84$\pm$0.13~mBq were estimated.