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

Radon gas is the largest source of public exposure 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 important for supporting successful greenhouse gas (GHG) mitigation 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 subject to metrological research so far. This evokes new challenges 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 traceRadon, stable atmospheres with low-level activity concentrations of radon have been produced to enable calibration of radon detectors capable of measuring these environmental activity concentrations. The required traceability of the calibration at very low activity concentrations, was not possible in the past. To achieve this goal, low activity sources of radium have been produced with different methods and different characteristics. Sources down to few Bq of 226 Ra have been developed and characterized leading to uncertainties as low as 2 % ( k = 1), even in case of the lowest activity sources. Additionally, sources with medium and high activities were produced, with advanced production methods like ion implantation of mass separated 226 Ra in different target materials. As an outcome, for the first-time traceable methods for measuring low-level atmospheric radon activity concentrations 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 performance of the sources in application, e.g., to establish a reference atmosphere, a calibration exercise with two highly sensitive, large volume, low radon activity concentration detectors 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 conclusion new developments in the field of radon metrology are presented.