Radon Environment in the Korea Invisible Mass Search Experiment and Its Measurement

The level of radioactivity from Rn 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 Po and Po 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.