Development of a calibration facility for measurements of the thoron activity concentration

A new calibration facility for measuring the thoron (220Rn) activity concentration in air is currently being developed at the Physikalisch-Technische Bundesanstalt (PTB). The system consists of a reference chamber of well-defined volume and several electrodeposited 228Th sources. In order to produce and maintain a homogeneous and constant activity concentration, a flow field is created inside the reference chamber and monitored. The 220Rn activity concentration is measured with a multi-wire ionisation chamber (MWIC) operating in pulse mode and externally connected to the reference chamber. With this method thoron fields with activity concentrations ranging from 1 to 10 kBq/m3 are presently produced with an overall relative standard uncertainty of less than 5%.