THE EXTENT OF RADIOACTIVE EQUILIBRIUM BETWEEN RADON AND ITS SHORT-LIVED DAUGHTER PRODUCTS IN THE ATMOSPHERE.

A set of experimental conditions has been derived which optimizes the accuracy with which the radon content of the atmosphere and its apparent residence time there can be determined from measurement of the filterable gross Beta activity during two separate 10-minute periods following the end of a 20-minute collection period. The 20-minute collection is made on a filter of essentially 100 retentivity glass fiber filter and counted for beta activity through a 10-mil 70.mgsq cm aluminum absorber on equipment of known efficiency for the beta particles emitted by RaB and RaC. The ratio of measured counts during the 61-through-71-minute decay period to that during the 1-through-11-minute period is employed to obtain the atom ratio of RaC to RaB rho in the air sampled, the apparent age tau of the conglomerate of radon and its daughters, and the total counts to be expected from any given radon concentration in the air. Extensive examples of atmospheric measurements are given which suggest that under most conditions in the free atmosphere radon is essentially at secular equilibrium with its daughter products. Other collections made following addition of radon to a restricted volume show the expected increase in rho with time, which confirms the basic validity of the procedure. Author