Development of a high-sensitivity 80 L radon detector for purified gases

In underground particle physics experiments, the radioactive noble gas ${222}$Rn generated from the decay chain of the uranium series could be a serious background source. We have been developing high-sensitivity radon detectors to assay radon in the Kamioka underground laboratory. In order to achieve a further low background level, we developed a new radon detector with better hermeticity. The high-voltage dependence and humidity dependence of the detection efficiencies were obtained through our calibration systems. The background level of the new radon detector was also measured.