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

Abstract

AbstractEffective radium-226 concentration, ECRa, is the product of radium activity concentration, CRa, 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 ECRa values obtained from more than 11 800 such experiments. Rocks (n=1351) have a mean ECRa value of 1.9±0.1 Bq kg−1 (90% of data in the range 0.11–35 Bq kg−1), while soils (n=1524) have a mean ECRa value of 7.5±0.2 Bq kg−1 (90% of data between 1.4 and 28 Bq kg−1). Using this large dataset, we establish that the spatial structure of ECRa is meaningful in geology or sedimentology. For plants (n=85), ECRa is generally <1 Bq kg−1, but values of larger than 10 Bq kg−1 are also observed. Dedicated experiments were performed to measure emanation, E, in plants, and we obtained values of 0.86±0.04 compared with 0.24±0.04 for sands, which leads to estimates of the radium-226 soil-to-plant transfer ratio. For most measured animal bones (n=26), ECRa is >1 Bq kg−1. Therefore, ECRa appears essential for radon modelling, health hazard assessment and also in evaluating the transfer of radium-226 to the biosphere.