Development of a low-level radon reference atmosphere

•A calibration of radon devices with Rn-222 below 1000 Bq/m3 is possible.•This atmosphere is a long time constant, homogeneous Rn-222 reference atmosphere.•The reference atmosphere is generated with a Ra-226 activity standard.•A new primary standard is realised with a relative uncertainty of 1.1% for k=1.

In order to calibrate measurement devices for the activity concentration of Rn-222 (radon) in air below 1000 Bq/m3, a constant for long time (>5 d), homogeneous reference atmosphere is created by a certified activity in a certified volume. The PTB developed this reference atmosphere from 150 Bq/m3 to 2000 Bq/m3 based on the precisely known emanation of Rn-222 from a Ra-226 activity standard. This set-up reduces uncertainties and increases the range of traceability for commercial radon measurement devices. Thus, a gap in radon metrology is closed. The new primary standard for reference atmospheres is realised with a combined relative standard uncertainty of 1.1%.