Liquid scintillation methods with coincidence techniques have been applied to the low-level radon measurement in order to reduce background. The methods are based on the principle of either coincidence or pile-up effect between β^- rays from RaC and α rays from RaC' with a very short half-life of 162μsec.
Pair pulses of RaC-C', which correlate with the time duration selected by the pulse shaping, can be discriminated from other single pulses, including background.
The counting efficiency calculated from the time duration agreed well with the experimental results, and adsorption of radon daughters in the walls of the counting vial was observed.
The background of the methods caused by the statistical random coincidence or pile-up depends on the total counting rate of the sample and the time duration. A background counting rate of less than 0.01 cpm is attained without any serious decrease in counting efficiency.