Low radon concentrations accurately measurable for the first time

Radon (Rn-222) is a radioactive noble gas that occurs especially in the ground of mountainous regions due to the decay of various, naturally occurring radioactive substances. Despite relatively low activity concentrations from approx. 50 Bq/m³ to 200 Bq/³, radon represents such a strong contribution to the total exposure rate of the German population that the main share in the dose can be attributed to radon according to the ICRP's re-evaluation. Exposure due to medical examinations is approximatively as strong. Hence with its new European Radiation Protection Ordinance, the EU is aiming to reduce radon levels in buildings. However, the measuring instruments used to date, which were calibrated at 1000 Bq/m³ and more – thus a value much higher than the typically encountered concentrations – did not allow compliance with the new reference values to be checked reliably.

Thus, a novel facility for the calibration of radon measuring instruments has been developed at PTB. The low-level radon reference chamber is now available as an additional standard facility for the realization of the unit of activity concentration in Bq/m³. It consists of three parts: a traceable volume to generate the reference atmosphere, a facility to generate and transport defined radon activities, and a transfer standard to maintain and disseminate the unit. In order to generate constant reference atmospheres, the radon emanated from a radium-226 activity standard is transferred, via a noblegas- tight circuit, into a reference volume. With the aid of the known quantities ”radium activity“, ”emanation degree“, and ”volume“, measuring instruments can be calibrated with this chamber to determine the Rn-222 activity concentration in air in the range below 1000 Bq/m3 in a traceable way.

In order to determine the emanation power of the radium-226 activity standard, an emanation measuring facility based on γ spectrometry was developed. Within the scope of a doctoral dissertation, several emanation sources were produced and characterized. The eight existing emanation sources of different activity now allow stable reference atmospheres from 150 Bq/m3 to 1900 Bq/m3 to be generated so that now, also commercially available measuring instruments can be used to carry out a long-term calibration over more than 5 days.

The new facility extends PTB's calibration offer to a previously not available enbut socially extremely relevant range: the calibration procedure considerably increases the measurement accuracy of the collected data relating to the exposure rate of people. It is expected that this will have influences on future radon studies dealing with the re-evaluation of the lung cancer risk due to radon.