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The thoron activity concentration


For radon-220, the realization of the unit of the activity concentration is a special challenge, because this radioactive noble gas has a half-life of only 55 s. Since 2010, the primary standard for thoron activity concentration – the thoron emanation measuring set-up (TEM) – has been available for customers and is almost continuously fully booked. A comparison of 4 devices shows the metrological gaps still to be filled: the deviations of the calibration factors reach a factor of up to 4. The tests were carried out on standard devices which are usually used for measurements and whose data generally enter into the research work without calibration!

0.5 kBq/m3
1.7 kBq/m3
4.3 kBq/m3
16.4 kBq/m3
Device A12.03(14)1.50(11)1.63(5)1.91(5)
Device A24.03(37)4.35(18)3.05(9)3.61(11)
Device B1.25(04)1.16(4)0.97(5)1.05(3)
Device C2.55(09)2.13(14)1.95(6)2.11(6)

Table 1 : The table shows the calibration factors with the associated standard uncertainty of four devices (A1 and A2 are of the same construction) for different activity concentrations of radon-220. The deviation from linearity lies at 25 %, the absolute deviation may amount to a factor of up to 4.

The results of this comparison clearly show the need for reliable calibrations and the necessity of all research institutions to make use of this offer in spite of the costs involved. This is valid in particular as measurements are to an ever increasing extent carried out by persons without any background in the respective field.