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First proof of principle for the calibration of a radon monitor for climate observations

23.12.2021

With this first calibration, PTB has successfully demonstrated within the frame of the project EMPIR 19ENV01 traceRadon that traceable calibrations of radon monitors for climate research within radon activity concentration ranges below 20 Bq·m-3 are possible with uncertainties on the order of 5 %. Such measurements are metrologically necessary to enable the use of the radon tracer method (RTM) in establishing a correlation with greenhouse gas fluxes. In a proof of principle, a calibration of a high‑volume radon monitor at radon activity concentrations of less than 20 Bq m-3 was carried out for the first time in PTB’s climate chamber with radon emanation sources. This work was performed within the frame of the  EMPIR 19ENV01 traceRadon project.

Measuring radon “correctly” in the outdoor air means providing the metrological traceability of the activity concentration of radon in the outdoor air from about 1 Bq·m-3 to 20 Bq·m-3 and of the radon flux from soil in the range of about 20 mBq·m-2·s-1. Because of the small event numbers, this poses a great challenge to the dissemination via transfer standards (in general, measuring instruments) [Linzmaier 2013]. This becomes obvious through the following arithmetic example: Let’s assume that a radon measuring instrument has an active volume of 1 L. This results in count rates of 36 h-1 at 10 Bq·m-3, which, in statistical terms, leads to an uncertainty (with Poisson statistics) of 17 % for a measurement time of one hour. As it is necessary to additionally take into account the uncertainty of the intrinsic zero effect, the linearity, and the calibration in such a system [Röttger 2013, Röttger 2011], the total measurement uncertainty as well as the detection limit and decision threshold of such a system are not very suitable for detecting dynamic processes in the activity concentration in outdoor air. This is even the case under ideal calibration conditions. Increasing the active volume or measuring the progenies with an increased flux rate provides a solution to the statistical problem. In this way, the count rates can be significantly increased. ANSTO’s monitors (active volume between 200 L and 1500 L), the Atmospheric Radon Monitor (ARMON) and the Heidelberg Radon Monitor (HRM) are such detectors.

The calibration of a 200 L measuring system under non‑dynamic conditions could, for the first time, be realized in PTB’s climatic chamber with the aid of ion‑implanted emanation sources [Röttger 2021]. The progress of the registered count rates depending on time is shown in Figure 1. A mathematic model of the generated radon activity concentrations derived from knowing the Ra‑226 activity and the emanation from the sources used as well as the volume of the chamber was developed. This mathematical model was furthermore determined by matching the measured count rates, the calibration factor, and the intrinsic background of the detector. The fields of the different sources are indicated by using different colors. The residuum between the measured quantity values and the model is represented in the lower field. The numerical values reflect the maximum activity concentration of the sources calculated for radioactive equilibrium. The quality of the calibration and the correctness of the model used are shown by Figure 2. Here, the deviation between the model and the measured count rate is shown as a distribution. All distributions were standardized to an area of 1 for better comparability.

diagramme (calibration procedure)

Figure 1: Time-dependent development of a calibration procedure of a new, portable detector prototype of the ANSTO company with 200 L active volume. Different ion‑implanted emanation sources were used. The equilibrium activity concentrations determined for these sources are noted in the graph. The different fields of application of the sources are distinguished by color. An overall model over the entire period was used to determine the calibration factor and the intrinsic background of the detector.

diagramme (distribution of the deviation)

Figure 2: The distribution of the deviation from Figure 1 with identical color coding is shown. The identical, symmetrical, and very narrow distributions, except for statistical fluctuations, speak for the quality of the calibration and the correctness of the model. For better comparability of the distributions, the areas of all distributions were normalized to 1.

This calibration is an important milestone in the development of new services in the field of climate observations, to which the consortium of  traceRadon is committed. The very first customers within the frame of the project are the radon monitor operators of ICOS, the climate observation network. With radon measurement, ICOS can systematically improve the detection of greenhouse gas fluxes. This is urgently necessary since the increase of greenhouse gases in the atmosphere is regarded as a major cause of climate change. This increase is caused by direct anthropogenic emissions from fossil fuel combustion and changes in land use. In addition, there may be strong feedbacks between the climate and the natural sources and sinks of greenhouse gases. However, these effects have only been partially understood to date, and they are therefore poorly represented in global climate models [ICOS: https://www.icos-cp.eu/ last accessed 2021-11-16, ICOS-D: https://www.thuenen.de/de/fachinstitute/agrarklimaschutz/projekte/icos-d last accessed 2021-11-16].

Literature

[Linzmaier 2013] Linzmaier D., Röttger, A. 2013. Development of a low-level radon reference atmosphere. Applied Radiation and Isotopes 2013, 81, 208 - 211. https://doi.org/10.1016/j.apradiso.2013.03.032

[Röttger 2011] Röttger, A., Honig, A.: Recent developments in radon metrology: New aspects in the calibration of radon, thoron and progeny devices. Radiation Protection Dosimetry, 145, 2-3, p. 260 – 266. https://doi.org/10.1093/rpd/ncr047  

[Röttger 2013] Röttger, A., Linzmaier, D., Honig, A.: Calibration of comercial radon and thoron monitors at stable activity concentrations. Applied Radiation and Isotopes 2013, 87, 44 - 47. https://doi.org/10.1016/j.apradiso.2013.11.111

[Röttger 2021] Röttger, A. et al: New metrology for radon at the environmental level 2021 Meas. Sci. Technol. 32, 124008, https://doi.org/10.1088/1361-6501/ac298d

Contacts

Opens local program for sending emailS. Röttger, Department 6.1, Working Group 6.13

Opens local program for sending emailF. Mertes, Department 6.1, Working Group 6.13

Opens local program for sending emailA. Honig, Department 6.1, Working Group 6.13

Opens local program for sending emailV. Morosh, Department 6.3, Working Group 6.32

Opens local program for sending emailA. Röttger, Division 6