Reference fields and calibration techniques for Rn-220 measuring instruments

One of the main concerns of any developer of a thoron reference chamber is to maintain a uniform and homogeneous gas distribution. However, the use of fans and air circulation systems often produces turbulent flows in the chamber, which can instead generate undesired non-uniformities in the atmosphere. As an example, the homogeneity studies at the calibration facility of the Physikalisch-Technische Bundesanstalt (PTB) are presented. The PTB facility consists of a stainless-steel reference chamber with a volume of approximately 0.1 m3 and several electrodeposited 228Th sources. The turbulent flow created by the fan inside the chamber allows homogeneous 220Rn fields to be created only at a distance from the source greater than 15 cm.