Effective radium-226 concentration in rocks, soils, plants and bones

Effective radium-226 concentration, EC (sub Ra) , is the product of radium activity concentration, C (sub Ra) , multiplied by the emanation coefficient, E, which is probability of producing a radon-222 atom in the pore spaces. It is measured by accumulation experiments in the laboratory, achieved routinely for a sample mass >50 g using scintillation flasks to measure the radon concentration. We report on 3370 EC (sub Ra) values obtained from more than 11 800 such experiments. Rocks (n=1351) have a mean EC (sub Ra) value of 1.9+ or -0.1 Bq kg (super -1) (90% of data in the range 0.11-35 Bq kg (super -1) ), while soils (n=1524) have a mean EC (sub Ra) value of 7.5+ or -0.2 Bq kg (super -1) (90% of data between 1.4 and 28 Bq kg (super -1) ). Using this large dataset, we establish that the spatial structure of EC (sub Ra) is meaningful in geology or sedimentology. For plants (n=85), EC (sub Ra) is generally <1 Bq kg (super -1) , but values of larger than 10 Bq kg (super -1) are also observed. Dedicated experiments were performed to measure emanation, E, in plants, and we obtained values of 0.86+ or -0.04 compared with 0.24+ or -0.04 for sands, which leads to estimates of the radium-226 soil-to-plant transfer ratio. For most measured animal bones (n=26), EC (sub Ra) is >1 Bq kg (super -1) Therefore, EC (sub Ra) appears essential for radon modelling, health hazard assessment and also in evaluating the transfer of radium-226 to the biosphere.