226Ra measurements through gamma spectrometric counting of radon progenies: How significant is the loss of radon?

Most determinations of 226Ra are based on gamma spectrometric measurements of 226Ra progenies 214Pb and 214Bi. This approach assumes radioactive equilibrium between these radionuclides which is normally achieved by sealing the sample containers so that the direct decay product of 226Ra, 222Rn, is not lost. We investigated to what extent 222Rn is lost from various sample containers usually used for gamma spectrometric measurements and which have been sealed applying different techniques. The sample containers containing reference material with known amounts of 226Ra were placed in an air-tight measurement chamber and we measured the 222Rn accumulation over time in the chamber. Taking the leakage of the experimental set-up into account, we related the measured 222Rn to the 226Ra content of the reference material to calculate the 222Rn loss from the sample containers. For most of the reference materials investigated the sealing techniques applied reduced the 222Rn loss to < 6%. We found that BaSO4 sample matrix and plastic containers sealed with epoxy resin most effectively minimize the 222Rn loss (< 1%). For those sample matrixes like phosphogypsum and Mn-fibers, which are characterized by high emanation and diffusion of radon, sealing is very important to reduce the 222Rn loss.