Radon environment in the Korea invisible mass search experiment and its measurement

The level of radioactivity from ²²²Rn in the air has to be monitored to control systematic effects on the background level at the KIMS (Korea Invisible Mass Search) dark matter search experiment. We designed and constructed a radon detector using a silicon photodiode that collects ²¹⁸Po and ²¹⁴Po daughter ions electrically and detects their α decays. Results for the detector calibration with a standard radon source, as well as the dependence of the α-particle detection efficiency on the humidity, are presented. The radioactivity of radon at the YangYang underground laboratory is measured and continuously monitored.