The goal of this PhD is to develop an activity standard for use in calibrating monitors used to measure the thoron (220Rn) concentration in air. The device, which has been designed to be the standard, is a volume with a silicon semi-conductor detector and an electric field which allows the charged decay products of thoron to be trapped on the detector surface.
A finite element method has been used for the electric field simulations. This electric field is high enough to catch the decay products of thoron at the detector surface even with the high flow rate inside the volume. Monte-Carlo calculations were used to define the detection efficiency of the system and to optimize the geometry shape and size. The calculated detection efficiencies have been compared with the results obtained for a reference radon (222Rn) atmosphere produced with a new gas dilution setup. These experiments allowed the sensitivity of the system to be evaluated, as a function of the air properties. It has been demonstrated that the measurement system is independent of the pressure, the relative humidity and the flow rate for a large range of values. Through the analysis of measured alpha spectra the experimental gas detection efficiency was found to be consistent with the Monte-Carlo simulations.
This portable system can now be used to evaluate precisely the thoron activity concentration with a well-defined associated uncertainty. Comparison measurements have been performed at the Italian metrological institute. Both systems are consistent within their uncertainties.