Determination of small quantities of radon and thoron

In the measurement, by the alpha ray method, of less than 2x10^(-12) curies of radon, the limit of precision is imposed by the small statistical variations in the background of residual ionization due to cosmic and local radiation. This in the excellent work of Paneth and Koeck(1) a mean deviation of 3 percent in the background would introduce an uncertainty of 50 percent in the measurement of 10^(-13) curies of radon.