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Prototype for a Transportable Measuring Instrument for Radon Progenies Now Available

04.01.2010

Radon (Rn-222) and thoron (Rn-220) progenies are responsible for the main contribution to the dose to which the lung is exposed. In order to achieve a true estimation of the lung dose, it is therefore important to know the progenies’ concentrations. Previously in houses and dwellings, under real conditions, this was achieved only to an insufficient extent, since there is a lack of traceable, portable measuring instruments.

In order to understand how important this is for radiation protection, it must be pointed out that at the same activity concentration of radon and thoron, the alpha energy concentration - and thus the dose contribution - of the thoron progenies have a value which is 14 times as high as that of radon progenies. Therefore, a new calibration facility was completed at PTB in 2007 - the thoron progeny chamber.

The concept of the thoron progeny chamber includes the establishment of reference atmospheres, the sampling of progenies, as well as procedures for the measurement of radon and thoron progenies. The production of different aerosol concentrations and aerosol particle size distributions enables the setting of equilibrium factors in the range from 0.1 to 1 for Rn-220 and from 0.1 to 0.8 for Rn-222. Typical relative measurement uncertainties for the measurement of the accumulated and unaccumulated activity concentrations of the progenies lie between 7 % and 8 % (k=2).

The new progeny measuring instrument can differentiate between the relevant α lines of the α,β spectra of the nuclides of the natural decay chains. Furthermore, the measuring instrument can reproducibly measure the respective activity concentration of the unattached fraction as well as the fraction attached to aerosols in a large range (from 100 Bq/m3 to 50 kBq/m3). First measurements of the Radon Decay Product Spectrometer (RDS) were performed in the thoron progeny chamber and compared with PTB's standard method for the measurement of progenies. The system is currently being further optimised.