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Air monitoring system for the surveillance of radioactive reference atmospheres

09.08.2007

When measuring systems are calibrated in the thoron progenies chamber it must be ensured that the generated reference atmosphere is homogeneous with respect to the individual activity concentrations. However, a homogeneous distribution of 220Rn is not that easy to ensure as this isotope has a half-life of only 55 s. This is why 220Rn is supplied to the atmosphere via the source control system in a spatially distributed way. Proof of a uniformly distributed activity concentration specific to the nuclide is furnished online by means of the air monitoring system (4 so-called Cam-detectors, i. e. airtight surface junction detectors with an active area of 1700 mm2) which allows - among other things - a local measurement of 216Po. As 216Po is - due to its half-life of 0.15 s - always in equilibrium with 220Rn, proof of the homogeneity of the atmosphere with respect to 220Rn is also furnished.

For the calibration of measuring systems in the thoron progenies chamber it must be ensured that the generated reference atmosphere is homogeneous with respect to the individual activity concentrations. Due to the comparably long half-life of 222Rn, this is no problem for this isotope. By careful earthing and the avoidance of temperature gradients, a homogeneous distribution can also be achieved for the usually charged progenies of the two radon isotopes. A homogeneous distribution of 220Rn is not, however, that easy to ensure as this isotope has a half-life of only 55 s. This is why 220Rn is supplied to the atmosphere via the source control system in a spatially distributed way. Slight ventilation ensures additional homogenization.

Proof of a uniformly distributed activity concentration specific to the nuclide can be furnished online by means of the air monitoring system (4 so-called Cam-detectors, i. e. airtight surface junction detectors with an active area of 1700 mm2) which allows - among other things - a local measurement of 216Po. As 216Po is - due to its half-life of 0.15 s - always in equilibrium with 220Rn, proof of the homogeneity of the atmosphere with respect to 220Rn is also furnished.

Figure : α-spectrum measured in a 220Rn atmosphere on the thoron progenies chamber. For testing of the homogeneity, spectra are recorded and evaluated with four PIPS detectors (1700 mm2 in size) at different places of the thoron progenies chamber.