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

The level of radioactivity from Rn-222 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-218 and Po-214 daughter ions electrically and detects their alpha decays. Results for the detector calibration with a standard radon source, as well as the dependence of the alpha-particle detection efficiency on the humidity, are presented. The radioactivity of radon at the YangYang underground laboratory is measured and continuously monitored.