Traceable low activity concentration calibration of radon detectors for climate change observation networks

Radon gas is the largest source of public expo-sure to naturally occurring radioactivity, and concentration maps based on atmospheric measurements aid developers in complying with EU Safety Standard Regulations. But radon can also be used as a tracer to evaluate dispersal models im-portant for supporting successful greenhouse gas (GHG) mit-igation strategies. That is why reliable measurements of low-level radon activity concentrations, such as those found in the environment (< 20 Bq∙m-3), are important for both radiation protection and climate research. Despite the enormous changes in radon metrology that have occurred in recent years activity concentrations below 100 Bq∙m-3 had not been sub-ject to metrological research so far. This evokes new chal-lenges which are the development of traceable methods and robust technology for measurements of environmental low-level radon activity concentrations and radon fluxes from the soil. Both are important to derive information on greenhouse gas fluxes in the environment and therefore are important for planning the reduction strategy. In the framework of the EMPIR project 19ENV01 trac-eRadon [1], stable atmospheres with low-level activity con-centrations of radon have been produced to enable calibration of radon detectors capable of measuring these environmental activity concentrations. The required traceability of the cali-bration at very low activity concentrations, was not possible in the past. To achieve this goal, low activity sources of radium have been produced [2,3] with different methods and different characteristics. Sources down to few Bq of 226Ra have been developed and characterized leading to uncertainties as low as 2 % (k=1), even in case of the lowest activity sources. Ad-ditionally, sources with medium and high activities were pro-duced, with advanced production methods like ion implanta-tion of mass separated 226Ra in different target materials. As an outcome, for the first-time traceable methods for measuring low-level atmospheric radon activity concentra-tions in the range of 1 Bq·m-3 to 50 Bq·m-3 with uncertainties below 5 % (k=1) are now available. To compare the perfor-mance of the sources in application, e.g., to establish a refer-ence atmosphere, a calibration exercise with two highly sen-sitive, large volume, low radon activity concentration detec-tors of different design and principle was carried out at the PTB. The details of the calibration procedure of these unique prototypes of detectors are presented. The results of the characterization of the detectors are discussed and as a con-clusion new developments in the field of radon metrology are presented. The project 19ENV01 traceRadon received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme.