Radiation Medicine and Protection (Dec 2024)
An update on soil gas harvesting technique as a non-depleting source of 222Rn for a large volume calibration chamber for long-term exposure experiments
Abstract
Objective: To demonstrate the versatility of the natural soil gas harvesting technique, coupled with a semi-dynamic injection algorithm, as an efficient and reliable source for continuous 222Rn delivery to a large-volume calibration chamber across different soil conditions and climatic conditions. Methods: Long-term experiments were performed during three seasons of a year in a tropical monsoonal climatic region with high rainfall on the Southwest Coast of India. Soil gas extraction was performed using soil-gas probes, inserted to 1 m deep into the ground. Soil gas was harvested at a flow rate of 60 L/min, passed through a moisture trap, a 222Rn progeny filter, and a delay volume and pumped into a 222Rn calibration chamber of volume 22.7 m3. Continuous monitoring of 222Rn concentration in the chamber was performed using ionization chamber-based reference monitors. To achieve and maintain the desired 222Rn concentration values in the calibration chamber, the semi-dynamic injection method was adopted, in which the 222Rn concentration in the soil gas was monitored periodically, and injection into the chamber was controlled using a well-defined pumping algorithm. Three ranges of 222Rn concentration values were chosen as the target values to be maintained in the calibration chamber for long-term experiments (15 d): low concentration (500–1,000 Bq/m3), medium concentration (1,000–10,000 Bq/m3), and high concentration (10,000–25,000 Bq/m3). Results: The achieved 222Rn concentration values were in good agreement with the target values, with deviations of 12%, 8% and 5% for the low, medium and high concentration exposures, respectively, during the summer season. During the monsoon season, the deviations between the target and the achieved concentration values were 12%, 10% and 5% for low, medium and high concentration exposures, respectively, and the corresponding deviations were 12%, 4%, and 5% for the winter season. These deviations are well within the cumulative uncertainty associated with the measurements. Conclusions: The results of this study conclusively demonstrate that the soil gas harvesting method, when coupled with the semi-dynamic injection approach, is a reliable method for generating and maintaining the desired 222Rn concentration in the large volume calibration chamber during different seasons and environmental conditions.
