Radon detection by activated charcoal needs corrections according to exposure time and soak time（the duration from the end of collection time to the start time of measurement）.The counts must be corrected by exposure-time adjustment factor t

1

0.49

（where t

1

is exposure time） and decay adjustment factor of e

(

-(t

1/2

+t

2

)λRn)（where t

2

is soak time）

which is widely used in China.In order to test the adjustment and check the adjustment errors

the counts ofγ-rays at different exposure hours and soak hours are calculated using a calculation method.The calculation results show that the exposure time difference of（-12h～+ 12 h） between measurement and calibration can cause exposure-time adjustment error of +5.0%～-3.9%and decay adjustment error of 4.4%～-4.6%.The soak time can be adjusted according to exposure time

which does not cause any decay adjustment error.If the exposure time is less than 48 h

the soak time should be more than 3 h.If the exposure time is more than 72 h

the soak time can be less than 3 h.