According to the principle of radon detection in electric field

ThA as well as RaA are both ionized particles with positive charge in its original state

so that they are able to collect within the electric field

Radon meter type FDC-80 was used to measure thorium content. Theoretical calculation and the condition in practical use were studied. The experimental results indicate the method has a greater rapidity and a higher accuracy as compared with both static meter and balloon method.