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Setting-up of a low-level radon reference chamber

29.12.2011

To traceably calibrate measuring instrument for the determination of the Rn-222 activity concentration in air in the range below 1000 Bq/m3, a Low-Level Radon Reference Chamber is at present being set-up: this comprises a traceable volume for the generation of the reference atmosphere, a facility for the generation and transfer of defined activity and a highly sensitive transfer standard.

A radon emanation measuring set-up for the transfer of defined activity has been concluded and is now available as an additional primary procedure for the realization of the activity concentration.

In the case of this measurement procedure, constant reference atmospheres are generated. For that purpose, the radon emanated from a radium 226 activity standard is transferred over a noble gas tight circuit into a reference volume. Traceability of the activity concentration is thus realized via the radium activity A(Ra-226), the emanation degree χ and the volume V. To determine the emanation power of the radium 226 activity standard, an emanation measuring set-up was developed on the basis of γ-spectrometry. Analogue to the procedure developed for the thoron measuring set-up, the count rate ratios of closed and emanating Ra-226 source of identical geometry are observed.

The available emanation sources of different activity allow stable reference atmospheres below 1200 Bq/m3 to be generated.

Low-Level Radon Reference Chamber (on the left) and radon emanation measuring set-up (on the right).

For the transfer standard for spectrometry of the α-particles, two multi-wire impulse ionization chambers of different geometry are at present being set up. Both chambers have an active volume of 10 litres and consist of a ceramic base material. By the discharge of the α-particles, signals are generated in the multi-wire impulse ionization chamber which must be detected and amplified. A decay in the chamber causes a voltage depression of approx. 70 µV. These small voltage changes are evaluated with the aid of the developed pre-amplifiers.

For the detection and transfer of the impulses into an energy spectrum, a LabView programme has been developed. From the energy spectra, several measurements of the activity concentration of radon in air were carried out. Tests with the preamplifiers which work in accordance with the current measuring principle "JFET with Gain" showed that the impulses generated by the radon are superposed with a 50 Hz noise.

To improve the energy resolution and maintaining, at the same time, the linearity, the time constants were adapted for digital processing, different processing algorithms were tested and test signals successfully generated and fed-in.