A new technique for the accurate measurement of 226Ra by gamma spectroscopy in voluminous samples

Accuracy in the measurements of 226Ra concentrations by gamma spectroscopy depends on an appropriate determination of the concentrations of 222Rn decay products (214Pb and 214Bi) in the sample. As an inert gas, radon can leak from the sample vessel or accumulate in the void upper part of it. In the latter case, radon decay products can be attached on the inner surface of the sample container, and therefore the gamma rays have another geometry and self-absorption factor which differs from the one assumed (similar to the calibration sample). The influence of the above procedures on the accuracy of the measurements depends mainly on the emanation factor of radon in the sample, the geometry used in the measurement and the structure of the sample container. Results of the tests that have been performed with a new technique developed in order to eliminate radon diffusion through the sample are presented. Two materials with different effective 226Ra concentrations were used for the tests: soil with 5 Bq kg−1 effective 226Ra (total 226Ra concentration 20 Bq kg−1 and emanation factor 25%) and phosphate fertilizer with 22 Bq kg−1 effective 226Ra (total 226Ra concentration 306 Bq kg−1 and emanation factor 7%). From gamma-spectroscopy measurements it was found out that the increase in 226Ra concentration, when charcoal was added to the samples, was about 35% of the effective 226Ra of the sample. This could lead to an underestimate of about 10% of the 226Ra concentrations in voluminous samples with high emanation factor (25%).