Panasonic PT-DZ12000U Operating Instructions Manual page 194

INTERFERENCE DUE TO ENVIRONMENTAL RADON
Time Mostly Radon
Minutes % Remaining
30 50%
120 6.25%
240 0.49%
300 0.098%
Assume your initial measurement right after removing the
sample filter indicates a count rate of 10,000 cpm.
If you have ONLY radon the count rate will decrease by ½
every 30 minutes and the count rate after 300 minutes will be
only 10 cpm.
If you have ONLY thoron the count rate will decrease by ½
every 10.6 hours and therefore the count rate rafter 300
minutes will be 7,200 cpm.
If you have an unknown mixture of radon and thoron then
after 300 minutes there is no longer any radon so the
measurements should follow the 10.6 hour half-life for thoron
only after that initial 300 minute interval.
IF there is a potential for other airborne radioactivity and you
have ONLY radon you may be able to determine the
presence or absence of that other airborne radioactivity after
waiting 7 to 10 half-lives of the radon particulate progeny.
IF there is thoron progeny present you will have a lot of
difficulty in determining the presence or absence of that other
airborne radioactivity. A portable alpha spectrometer is a
useful instrument to have in this case.
Mostly Thoron
% Remaining
96.78%
87.74%
77.0%
72.1%
Equal Mixture
% Remaining
73.4%
47.0%
38.7%
36.1%