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First Primary Standard for Thoron Activity Concentration Developed

04.01.2010

With the setting up of the thoron emanation measurement facility (TEM), it is the first time that a primary standard has been available for the short-lived radon isotope Rn-220 (T½ = 55 s). By means of activity standards, of the on-line measurement of the emanation capacity of the electrolytically deposited area sources of Th-228, and of the measured volume of the activity circuit, it is possible to realise the unit with a relative standard measurement uncertainty of 3 %. The determination of the emanation capacity is achieved with a relative measurement uncertainty of ≤ 0.4 % by means of the newly developed TEM procedure which is based on the counting rate ratio of different sources.

By analysing two energetically close peaks: Ra-224 with 240.986 keV and Pb-212 with 238.632 keV of a gamma-spectrometric measurement, it is possible to determine the share of Rn-220 activity which is transferred into the calibration chamber.

Differences in the efficiency thus become negligible. By comparing the counting rates in these peaks with a rare gas tight source (radioactive equilibrium of Th-228 and all progenies), it is possible to determine the emanation of the open, electrolytically deposited source.

The largest contribution (approx. 70 %) to the measurement uncertainty for the realisation of the unit is caused by the (in)homogeneity of the activity in the volume. The calculations and measurements are therefore being further developed in order to reduce this contribution.