Can citizen science ambient dose rate data (Safecast) be used for predicting indoor radon?

Radium-226, part of the ²³⁸U decay chain, which is ubiquitous in the ground, generates a terrestrial gamma ray field which can be detected above ground, through its strongly gamma radiating progeny ²¹⁴Bi and ²¹⁴Pb and to minor degree through ²²⁶Ra itself. The measurand is ambient dose equivalent rate, ADER, nSv/h, that also includes contribution from cosmic rays and other terrestrial radionuclides (i.e. ⁴⁰K and ²³²Th decay chain). On the other hand, its decay produces ²²²Rn (here shortly Rn) which can migrate through the ground and lead to measurable Rn concentration (Bq/m³) in ambient media, namely soil, ground water and the indoor and outdoor atmosphere. One can therefore expect that originating from the same source, ADER and Rn are statistically related and ADER may serve as predictor of Rn related quantities, such as mean Rn concentration over an area, its probability to exceed a level or the status of an area as radon priority area. However, as the pathway from Ra in the ground to ambient Rn is complex, and as measured ADER has also other contributions than Ra, the relation must be expected to be blurred by nuisance factors, which pose a challenge to analysis.

A large and ever increasing dataset of ADER is freely available from the Citizen Science project Safecast [1], founded in Japan after the Fukushima accident 2011. It has since spread over the entire world (with measurements in regionally very different density, though) and by late 2022, the dataset comprised 180M measurements, of which about 50M in Europe. The measurements were performed with a standard instrument called bGeigie nano, of which several 1000 circulate around the globe, used by voluntary citizen scientists who send their data to Safecast. On the other hand, in Europe a good indoor Rn concentration (IRC) database is available, based on about 1.2M individual measurements [2], as well as an interpolated European IRC map [3].

Thus, we relate ADER (Safecast) with IRC and derived quantities, both aggregated on a common 10 km × 10 km grid. Raw ADER is reduced by cosmic dose rate (related to altitude a.s.l., accessible from digital elevation database) and mean internal detector background. Since it can be assumed that ADER on a point depends on its urbanization status (due to the influence of building materials which also contain gamma radiating nuclides), this factor is also investigated. 

First results are promising and will be shown in the presentation.

 

[1] https://safecast.org/

[2] European Commission, Joint Research Centre – Cinelli, G., De Cort, M. & Tollefsen, T. (Eds.), European Atlas of Natural Radiation, https://remon.jrc.ec.europa.eu/About/Atlas-of-Natural-Radiation/Download-page

[3] Elío J., et al. (2019): The first version of the Pan-European Indoor Radon Map. Nat. Hazards Earth Syst. Sci., 19, 2451–2464, https://doi.org/10.5194/nhess-19-2451-2019