A primary standard for activity concentration of 220Rn (thoron) in air

Due to the short half-life of Rn-220, a primary standard for activity concentration of Rn-220 (thoron) in air (i.e. a homogeneous reference atmosphere consisting of a certified activity in a certified volume) has been considered unachievable in the past.Traceability of Rn-222 reference atmospheres is achievable using radon gas activity standards (Picolo, 1996; Dersch, 1998) and standard volumes, and is an established method (Paul et al., 2002). For the short-lived radionuclide Rn-220 this procedure is not feasible, since no Rn-220 gas activity standard with a reasonable activity can be produced. This leads to a lack of traceability for measurements of Rn-220 activity concentration: only atmospheres monitored by reference instruments (i.e. secondary standards) are available. The new primary standard for the activity concentration of 220Rn developed by PTB now closes this gap in radon metrology.