Measuring effective radium concentration with less than 5 g of rock or soil

Abstract

Radon generation in natural systems and building materials is controlled by the effective radium concentration EC(Ra), product of the radium concentration C(Ra) and the emanation factor E. An experimental method is proposed to measure EC(Ra) in the laboratory by radon accumulation experiments using less than 5 g of sample inserted in 125 mL scintillation flasks. Accumulation curves with fine temporal resolution can be obtained, allowing the simultaneous determination of the effective leakage rate. The detection limit, defined as the EC(Ra) value giving a probability larger than 90% for a determination with a one-sigma uncertainty better than 50%, is moderate, varying from 2 to 5 Bq kg(-1) depending on the conditions. Obtained punctual uncertainties on EC(Ra) vary from about 10 to 20% at 10 Bq kg(-1) to less than 3% for EC(Ra) larger than 500 Bq kg(-1). The representativity of small samples to estimate meaningful values at site or system level is, however, a definite limitation of the method, and the sample dispersion needs to be considered carefully in every case. Nevertheless, the value obtained with 5 g or less differs on average by 9 ± 13% from the value given by standard methods using 100 g or more, thus is sufficiently reliable for most applications. When EC(Ra) is sufficiently large, the temperature sensitivity of EC(Ra) can be measured reliably with this method, with obtained mean values ranging from 0.39 ± 0.05% °C(-1) for Compreignac granite, to 2.8 ± 0.2% °C(-1) for La Crouzille pitchblende, both from the centre of France. This method is useful to study dedicated problems, such as the small scale variability of EC(Ra), and in circumstances when only a small amount of sample is available, for example from remote areas or from precious materials such as historical building stones.