Calibration of commercial radon and thoron monitors at stable activtiy concentrations

•New calibration procedures for commercial radon and thoron monitors have been developed.•Calibration in a constant level: emanation power of sources determined on-line.•Activity concentrations of 200 Bq/m3, with corresponding calibration factors.

The upcoming revision of Council Directive 96/29/Euratom in the form of the proposed basic safety standards for protection against the dangers arising from exposure to ionizing radiation (BSS) evokes new challenges for the metrology institutes. In the case of the two radon isotopes, the corresponding public exposure will be part of legal metrology for the first time. Since the levels of activity concentration that are laid down in the draft of the BSS cover the range from 200 Bq/m3 to 300 Bq/m3 in general (with an exceptional top level of 1000 Bq/m3), new calibration procedures for existing commercial monitors with their limited counting statistic have to be developed. This paper gives an overview how this metrological challenge can be overcome.