Comparison of 222Rn measurement methods in caves

Radon (222Rn) is a naturally inert radioactive gas, originating from the radioactive decay of 226Ra in the 238U radioactive decay chain. 222Rn has a variety of geoscientific applications. 222Rn, however, represents the most significant source of ionizing radiation exposure and can be critical in underground working and living spaces with little or no ventilation. Particularly, caves are recognized as special indoor occupational environments where extremely elevated concentrations of 222Rn may occur during, at least, half-year during its recharged stage.

The measurement of radon activity concentration in air can be performed using different types of equipment and methodologies. However, it is characterized by the dispersion for relatively short exposition times and depends on the radon activity concentration and environmental parameters. This investigation aims to compare different types of equipment and methodologies to measure 222Rn under real cave conditions.

Rull Cave is located in Vall d’Ebo, in the south-east of Spain (Alicante province). The host rock of the cave is composed of Miocene conglomerates lying on Cretaceous limestones. Above the cave, the soil has a discontinuous thickness of approximately 1 m. The investigation is performed in winter where the cave remains discharged. During this period, the gas concentration reaches minimum values and presents low fluctuation of radon activity concentration. Temperatures in Rull Cave range between 17 and 20°C, the mean relative humidity reaches about 87%, and the constant pressure of 975 mBar. 222Rn measurements have been taken continually since 2016, ranging from 645 to 3959 Bq/m3.

We compare, firstly, cave air radon with three devices: AlphaGUARD DF2000, Radim 5WP, and RadonScout Plus. The second method involves the measurement of air radon samples after collecting them in sampling bags. We perform two types of measures: (i) in-situ measures of air samples and (ii) measure of the collected sampling bags 24-hours later (in the laboratory). For this purpose, we use opaque and transparent 1L-gas sampling bags (GSB), and we also evaluate the influence of the air volume (2 or 4 L) on radon activity concentration measurement using AlphaGUARD DF2000 at 0.3 L/min pump flow.

These findings reveal that i) all devices have similar values of radon activity concentration, with a difference between AlphaGUARD DF2000 with Radim 5WP, and RadonScout Plus of -32% and +19 %, respectively; ii) the use of transparent or opaque GSB provide similar 222Rn concentration; iii) 222Rn concentration after 24-hours is nearly the same than samples tested immediately after collecting; and iv) direct data and the one collected in GSB are equivalent, although 4L GSB often register higher values than 2L. Both methodologies highlight the known problem of radon fluctuations at a short scale. We do recommend collecting air samples in 4L-GSB. It presents practical advantages for cave studies. Thus, 222Rn can be measured in cave areas that are nor not easily accessible areas. In addition, this methodology allows increasing the number of measurements, as well as to safety keep the devices at the lab.