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- As a general rule, the annual mean of Radon-222 activity concentration is determined in order to assess the radiation exposure from Radon-222 indoors.
- It is vital for these measurements that the room in question is used in a way representative of its specific function over the period of measurement.
- Measurements should be carried out over a period of one year if possible. Measurements performed over shorter periods of time are possible as an alternative, provided that the annual mean can be derived therefrom with sufficient certainty.
The measurements of radiation exposures can be conducted using either
- electronic measurement devices (providing readout and/or storage of current data during measurement) or
- passive measurement devices (providing a value based on separate analysis of the detector by a measuring institution upon completion of the measurement).
The Federal Office for Radiation Protection (Bundesamt für Strahlenschutz, BfS) has made available a list of measurement institutions providing evaluation of passive measurement devices in Germany.
The principle of measurement is identical for all types of equipment: as a general rule, radon reaches the sensitive measuring volume of the instrument where the radon and its short-lived progeny decay. The alpha radiation emitted is recorded by a suitable detector. When electronic devices are used, radon can also reach the measured volume as a result of flushing using a pump.
A typical passive measurement device is composed of a diffusion chamber that can be invaded by radon via the chamber walls or a filter. The diffusion chamber is set up at the measuring site and is subsequently transferred to an appropriate institution for analysis upon completion of the measurement.
There are different designs of diffusion chambers (see picture on the right), which are used with different detector materials. The detectors are usually composed of little plastic slices of different chemical composition and sensitivity as to the alpha radiation of radon and its decay products.
The alpha particles produce material defects within these plastic slices which are visualised as tracks based on chemical-physical methods and can subsequently be counted. Based on the result, the mean Radon-222 activity concentration can be determined, considering the measurement time.
State of 2017.01.24