Field testing of a portable two-filter dual-flow-loop 222Rn detector

An overlapping need exists between the climate science, air quality and radiological protection communities for a robust, portable and direct monitor of atmospheric ²²²Rn concentrations typical of the ambient outdoor atmosphere. To reliably characterise afternoon radon concentrations, or resolve daytime vertical radon gradients in the atmospheric boundary layer (requirements for radon measurements to be used to evaluate the performance of chemical transport models), detection limits of ≤0.2 Bq m^-3 at an hourly temporal resolution are required. Commercial portable radon detectors are mainly designed for indoor use, and the best of these has a detection limit of ≥2 Bq m^-3 for hourly sampling, with an approximate uncertainty of 60% at typical outdoor daytime radon concentrations.  Here we introduce a portable (200 L) version of the two-filter dual-flow-loop radon detector, designed and built by ANSTO in collaboration with the EMPIR 19ENV01 traceRadon project. While not as compact as commercial monitors (standing 1.6 m tall, and 0.48 m wide), its longest component is 1.2 m, enabling transportation in a standard utility vehicle or 4x4 (and can fit inside a 19” instrument rack). Constructed of marine grade stainless steel, it is weather resistant, robust, and suitable for long-term, continuous, autonomous deployment; in fact it is fully remotely controllable if a networked computer is available. The estimated lower limit of detection is 0.17 Bq m^-3 for hourly observations, and the counting uncertainty at typical ambient outdoor radon concentrations is around 7%. Additional uncertainty associated with current calibration techniques, which inject calibration gas on top of ambient sampled air, varies from 2-6%. Some objectives of the traceRadon project include establishing direct calibration traceability to the SI and developing an improved closed-loop calibration technique, using a new, low activity Radium-226 source. If successful, the absolute accuracy of the 200 L radon detector at typical ambient outdoor concentrations could be kept well below 15% for hourly observations. This project has received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. 19ENV01 traceRadon denotes the EMPIR project reference.