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Setting-up of a Low-Level Radon Reference Chamber

31.01.2011

In order to calibrate measurement devices for the activity concentration of Rn-222 in air below 1000 Bq/m3, a low-level radon reference chamber is currently being set up: It covers a traceable volume (0.5 m3) for the generation of the reference atmosphere, a device for the transfer of defined activities and the development of a high-sensitivity transfer standard.

The transfer standard for the spectrometry of α-particles consists of a multi-wire impulse ionization chamber, with a volume of 10 litres. The α-particles ionize the air and generate charge carrier pairs which must be separated, detected and amplified in the multi-wire impulse ionization chamber. A decay in the chamber causes a voltage drop of 70 µV. With the aid of the pre-amplifiers developed, these small voltages can be measured and the data evaluated. For this purpose, a Lab View program has been designed which determines the impulses and transfers them into an energy spectrum. In first measurements, it has already been possible to determine the activity concentration of radon in air from the energy spectra.

First tests with different pre-amplifiers, which worked in accordance with the current measuring principle or with the voltage measuring principle, have shown that the impulses generated by the radon are superimposed by a 50 Hz noise. To suppress the noise, a shielding and a vibration damping are adapted to the multi-wire impulse ionization chamber. In addition, the pre-amplifiers are improved with respect to noise filtering or noise suppression.

The establishment of a radon emanation measuring set-up for the transfer of defined activities has been started. Ra-226 serves as the source material. It is dropped onto a filter paper and then clamped between two 2 PE foils in an aluminium ring. In this way, the enclosed Ra-226 is protected against abrasion and contact. The radon diffuses through the foils and is then transferred via a noble gas leak-proof circuit into the reference chamber.

The emanation ability of the Ra-226 source is determined by γ-spectrometry. For this purpose, the counting rates of Pb-214 and Ra-226 of a Ra-226 activity standard are compared at the radon emanation measuring set-up with the counting rates of an open source (Ra-226 with PE foil), analogue to the procedure developed for the thoron emanation measuring set-up. At present, there are two stable emanation sources of different activity available with the aid of which atmospheres of 1 kBq/m3 and 60 Bq/m3 can be generated.