To guarantee the quality of measurements

the radon-thoron discriminative detector improved by our laboratory was calibrated at Radon Laboratory of University of South China

and participated in the international in- tercomparison organized by NIRS(National Institute of Radiological Sciences)in Japan. The calibration result demon- strated that correlation between the exposure level and the track density was fairly good for both radon and thoron

with the correlation coefficient of 0.9894 and 0.9885

and the calibration factors of 4.27 ± 0.40 and 2.33 ± 0.39 tracks·cm–2·(kBq·m–3·h)–1 for radon and thoron

respectively. According to the results of inter-comparison

in pure ra- don

relative percent difference from the reference value (RPD) was 12% and coefficient of variation (COV) was 7.7%

ranking category B of NRPB

and was in the control level of EPA; while in high 220Rn condition

RPD and COV for thoron were 22% and 4.8%

respectively. Concerning the higher deviation for thoron measurement

the cause should be further discussed.