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New reference atmospheres at PTB


The main component of the natural exposure of humans to ionizing radiation is the inhalation of ephemeral progenies of the radioactive rare gas radon which occurs in the earth's atmosphere in the form of the two nuclides 222Rn and 220Rn. In epidemiological studies it is assumed that in the Federal Republic of Germany approx. 5% of all lung cancer cases must be attributed to the inhalation of 222Rn progenies.

Little is known about the influence of thoron (220Rn) whose metrological determination is difficult due to its short half-life. This is problematic as - due to the different energies and half-lives in this decay chain - a dose which is higher by a factor of 14 (!) than in the case of a comparable 222Rn concentration, results for an increased 220Rn concentration.

This is why PTB has introduced the project "Manufacture and investigation of defined thoron decay product reference fields for the calibration of thoron decay product measuring instruments" into the Environment Research Plan (BMU) for the realization of the unit Bq/m3, so that radiation protection and epidemiological research can in future resort to quality-assured data from 222Rn and 220Rn measuring systems. The project was welcome and approved. It was performed from 2005 to 2007 and had a total volume of 328 500 euro.

After conclusion of the project, PTB has operated a thoron decay product chamber in accordance with the latest state of the art (see figure) in which 220Rn, 222Rn and its progenies are made available under all environmental conditions. This chamber allows all active (i.e. directly indicating) measuring systems and all passive (i.e. not directly indicating) measuring systems for radon and radon progenies to be calibrated under realistic climatic conditions and with a globally unique accuracy. This is the basis of each assessment of the radon situation in a building. With its measuring technology, PTB ensures reliability of the results of radon measurements in Germany (and in many other countries). Thanks to several projects promoted by the BMU, PTB could establish itself as a leading competence centre for radon metrology at the international level.

Figure : Thoron progenies chamber with source control system, air monitoring system and sensor for surveillance of the environmental parameters