Factors controlling measurements of mass radon exhalation rate

The mass radon exhalation rate of soil samples was measured using a tight emanation chamber of 10 L volume and a Lucas cell. The results showed that the mass of sample, grain size fraction and water content influence the radon mass exhalation rate. For soil with a radium activity of about 2500 Bq/kg, the values of the mass radon exhalation rate for a sample mass of 0.2-0.5 kg are higher than those at other sample masses. The dependence of the measured exhalation rate on sample mass supposedly can be explained by a heterogeneous distribution of radon atoms in the air space of the emanation chamber and an enlarging of sample dimensions due to the mass increasing. The observed radon exhalation rate decreased with the enlarging of grain sizes; it increased intensively at a water content of a few percent by weight, then decreased with moisture growth. Due to these influences, the measured values of the mass radon exhalation rate should be reported together with the data of the physical parameters of the sample material. (author)