Realization and characterization of a 220Rn source for calibration purposes

The recent interest for measuring 220Rn concentrations in air and the following development of the correspondent measurements techniques require the development of standards for the calibration and characterization of the instruments. Due to the short half-life (55.6 s) of the 220Rn, the techniques usually adopted for the production of 222Rn sources and relative reference atmospheres are not always valuable. The development of a metrology for 220Rn needs, therefore, the realization of new methods for the preparation of such standards, whose starting point is the availability of reliable sources that allow to obtain stable over time thoron activity. These conditions can be achieved using a long living solid source containing a known activity of one precursor of the thoron able to release gas continuously. To produce from this stable source a reference atmosphere with a uniform activity concentration other steps are necessary. At our laboratory a program for the development of a methodology to realize, starting from a 232Th source, a known thoron activity concentration is in progress. In this paper the first phase of the procedure, consisting on the study of measurement techniques of the exhaled thoron activity, will be presented and discussed. A source of 220Rn has been realised starting from a sample containing 232Th, placed in a PTFE container that allows to operate the source in sealed or unsealed configuration. The so assembled source has then been measured by gamma ray spectroscopy with a HP-Ge whose efficiency was measured using a certified source in the geometry and density similar to those of the sample. Since it is difficult to evaluate the thoron exhaled fraction without interferences from the not exhaled one, an eligible methodology has been set up to reach this result, which consists of two different phases. The first is carried out on the sealed source to take information on the total amount of the 220Rn in equilibrium with 232Th. The second one is performed with the source in ???open??? status, at least four days after its opening, using a suction system which takes away the released amount of the gas and allows the measurement of the thoron retained fraction. 220Rn exhaled amount is finally obtained by subtracting from the value of the total activity, the partial one. This procedure has been repeated many times at same environmental conditions to verify its reproducibility. Next step will be the characterization of the source at various values of temperature, humidity and pressure. After the characterization, the source will be used to realize thoron atmospheres in the chamber that is actually used at our laboratory to calibrate active and passive radon measuring devices.