Mass exhalation rates, emanation coefficients and enrichment pattern of radon, thoron in various grain size fractions of monazite rich beach placers

A study on exhalation rates of 222Rn and 220Rn and their enrichment pattern in different grain size fractions was carried out. Four different size fractions of the sand samples (1000-500 μm, 500-250 μm, 250-125 μm, and <125 μm) collected from the beach placers of south west coast of India were analysed for 226Ra and 232Th activity concentrations (Bq kg−1) by gamma spectrometry. Automated continuous measurement technique using scintillation cell based monitors were used to measure the 222Rn and 220Rn concentrations. These measured values were then fitted to appropriate model to extract the mass exhalation and emanation coefficients. The 222Rn mass exhalation rate varied from 0.7 ± 0.2 mBq kg−1 h−1 to 11.0 ± 0.9 mBq kg−1 h−1 while that of 220Rn varied from 88.5 ± 4.8 Bq kg−1 h−1 to 3066 ± 14 Bq kg−1 h−1. Higher mass exhalation rates, for both radioactive gases, were observed in finer grain size fractions. However, this was in contrast to the emanation coefficient values. Interestingly, higher radioactive monazite rich beach samples exhibited low emanation coefficient values compared to the low radioactive soil and sand samples reported in the literature. Statistical analysis showed a positive correlation, with correlation coefficient R = 0.84 and R = 0.79, respectively, between mass exhalation rates of 222Rn, 220Rn with their respective radioactive predecessors for different grain size fractions.