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Precise measurement of radioactive thoron

Especially interesting for

  • geologists
  • epidemiologists
  • radiation protection

A primary standard for the measurement of the short-lived radioactive thoron developed at PTB is used for the calibration of thoron measuring instruments from the whole world. It provides the basis for accurate measurements of the radioactive gas which occurs naturally in the ground, can collect in living areas and whose progenies are considered carcinogenic.

The emanation container developed at PTB contains an electro-deposited thorium-228 source for the thoron (radon-220) needed.

Just like its sister isotope radon (Rn-222), the radioactive gas thoron (Rn-220) can also cause lung cancer through its progenies if these are breathed in over long periods of time and in high concentrations. The degree of the radiation exposure of living areas varies greatly and is dependent on the construction of the house and the way it is ventilated. Whereas radon has been measurable with high precision for a long time already, this has not yet been the case with thoron. The exact measurement of thoron is, however, important for the estimation of risks, as at the same activity concentration, a 14-fold higher radiation level results from the thoron progenies than from the progenies of radon.

The development of a primary standard like the one that already exists for radon was considered impossible for a long time, because thoron has a relatively short half-life of only 55 seconds. PTB finally succeeded, though. The core of the primary standard is a test container which is filled with a defined amount of thoron. Since a closed container, like the one used in the case of radon, was out of the question due to the short half-life of thoron, a circuit system was developed that constantly introduces newly produced thoron with a high flow air stream and keeps the activity in the container constant.

The production and the accurate measurement of the activity which was fed in, was a technical challenge. In the case of the novel measurement set-up – the thoron emanation measuring arrangement (TEM) – the activity comes from a thorium (Th-228) source which produces thoron continually. A constant, strong air stream transports the thoron into the test container. The quantity of the thoron removed and of that remaining in the source can be measured to the nearest per mil. Parallel to operating the TEM, the measuring instrument to be calibrated can be connected to the test container.

As more and more geological and epidemiological studies in Asia, Europe and Latin America are focussed on thoron (to which little attention has been paid to date), the newly set-up measuring arrangement at PTB is constantly fully booked by international customers.

Scientific publication

Röttger, A.; Honig, A.: Recent developments in radon metrology: New aspects in the calibration of radon, thoron and progeny devices. Radiation Protection Dosimetry 145 (2011) 260–266