Absolute measurement of radon 222 activity

Several studies have shown that radon 222 is the most important human radiological hazard. Therefore, good metrology of radon 222 and its progeny is necessary to determine the occurrence, behaviour and biological effects of this nuclide accurately. A new system allowing the absolute measurement of radon 222 activity has been developed at LPRI to provide a primary standard. The principle of this system is the counting in a defined solid angle of the alpha particles emitted by radon 222 condensed on a cold point. The design and characterisation of the specifically developed cryogenic system are described. The measurement results and their uncertainties are detailed.