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In the focus of attention: thoron

31.01.2011

Whether in Asia, Europe or Latin America: An ever increasing number of geological and epidemiological studies are turning their attention to the short-lived radon isotope Rn-220 (half life T½ = 55 s) - also called "thoron" - which has so far hardly received any consideration. At present, the newly developed detector systems used can be traceably calibrated only at PTB: Since 2010, the primary standard for the thoron activity concentration - the thoron emanation measuring set-up (TEM) - has been available to customers and is almost continuously booked up.

For the measurement of the fraction of the Rn-220 activity transferred into the volume, the procedure makes use of γ-spectrometry by means of a differential measurement: The ratio of Ra-224 to Pb-212 is analyzed in different sources of identical geometry: Thereby, an emanation source (Th-228) is measured relative to a closed activity standard. After the thoron had been transferred into the reference volume, the measurement uncertainty for the activity concentration could be reduced to approx. 5 % (k=2) by an improvement of the homogeneity of the atmosphere.

Figure : Emanation vessel according to T. Reich with open Th-228 source in PTB area geometry. The distance of the source to the detector of the TEM is 8 mm.