A study of thoron exhalation from monazite-rich beach sands of High Background Radiation Areas of Kerala and Odisha, India

Thoron exhalation from earth's surface is the most important source of environmental thoron. The High Background Radiation Areas (HBRAs) of Kerala and Odisha in India have beaches with sands enriched with monazite, an ore of thorium, making it unique for study of thoron exhalation. In the present study, in situ measurement of thoron exhalation was carried out to map the thoron exhalation rate over complete coastline of the known HBRAs. New experimental setups for measurement of thoron surface exhalation and thoron mass emanation were developed and validated. The exhalation measurements were carried out on beaches of Chavara-Neendakara (CN) area of Kerala and Chatrapur (CH) region of Odisha. The beaches of the Kerala region showed an average thoron exhalation rate of 5.95 +/- 2.98 Bq m(-2) s(-1), which for the beaches of Chatrapur area of Odisha was 2.77 +/- 0.96 Bq m(-2) s(-1). The external gamma exposure rate was also measured at these locations, and it was observed to be well correlated with in situ thoron exhalation rate. Sand samples were collected from ten of the measurement locations and analyzed in the laboratory for a further understanding of thoron exhalation process in the beach sand. Measurement of specific thoron mass emanation rate, specific 224Ra content and thoron emanation coefficient was taken for the samples. A good correlation was found between in situ thoron exhalation rate and specific thoron mass emanation rate. The effective thoron diffusion length was determined by using the measured parameters in the UNSCEAR radon exhalation model and was found to be nearly constant with a value of 2.2 +/- 0.36 cm. This study shows that in spite of very high thorium content (500-1000 times) as compared to normal fine soils, the thoron exhalation from the HBRA beach sands was only 4-12 times higher due to very low emanation coefficient, about 1000 times smaller than that for normal soils.