Establishment of the activity equilibrium between 226Ra-222Rn and its application to determining activities by γ spectrometry technique

In determining the activity of 226Ra in environmental samples by γ spectrometry, many analysts usually think that, after the samples of 222Rn are hermetically sealed by 4∼5 half-life (15∼20 d), the activity of 226Ra can be determined by the activity of 214Pb which is daughter product of 222Rn. It was theoretically revealed from strictly deducted formulas that the 15∼20 d equilibrium time is far from enough for some samples, such as water samples, where serious destruction of the equilibrium was conducted in the process of sample preparation. A systematic error must be introduced by using this method. The author theoretically deduced the computing and measuring methods which can be used by γ spectrometry in determining the activity of 226Ra in the samples where the equilibrium has not yet fully established. The report described the advantages of the three time method in determining the activity of 226Ra in the sample. The report also discussed the innegligible systematic error which were tested and verified in most of such measurement with water samples in the past. The shortest possible equilibrium time is recommended by the author. The formulas and the determine method can also be referenced or used for determining activities of other parent nuclei with similar decay cascade