Development of a transfer standard for the measurement of low Rn-222 activity concentration in air.

A large volume transfer standard has been developed to calibrate commercial radon measurement devices in a homogeneous Rn-222 reference atmosphere. The transfer standard serves for the realization, maintenance and dissemination of the unit Bq/m(3) below 1 kBq/m(3). The transfer standard consists of a multi-wire impulse ionization chamber, electronic measurement equipment and the corresponding software. The multi-wire impulse ionization chamber is composed of two parallel printed boards. Between the boards, 478 vertical electrode wires are soldered in to include a volume of 10 L. In the dependency of the energy, the α-particles of Rn-222 and the decay products are detected in the active volume of the multi-wire impulse ionization chamber. By means of the measurement software an α-energy spectrum is produced. Via different algorithms, the activity concentration of the Rn-222 reference atmosphere is determined.

Calibration of commercial radon and thoron monitors at stable activtiy concentrations.

The upcoming revision of Council Directive 96/29/Euratom in the form of the proposed basic safety standards for protection against the dangers arising from exposure to ionizing radiation (BSS) evokes new challenges for the metrology institutes. In the case of the two radon isotopes, the corresponding public exposure will be part of legal metrology for the first time. Since the levels of activity concentration that are laid down in the draft of the BSS cover the range from 200 Bq/m(3) to 300 Bq/m(3) in general (with an exceptional top level of 1000 Bq/m(3)), new calibration procedures for existing commercial monitors with their limited counting statistic have to be developed. This paper gives an overview how this metrological challenge can be overcome.

Development of a low-level radon reference atmosphere.

In order to calibrate measurement devices for the activity concentration of Rn-222 (radon) in air below 1,000 Bq/m(3), a constant for long time (>5d), homogeneous reference atmosphere is created by a certified activity in a certified volume. The PTB developed this reference atmosphere from 150 Bq/m(3) to 2,000 Bq/m(3) based on the precisely known emanation of Rn-222 from a Ra-226 activity standard. This set-up reduces uncertainties and increases the range of traceability for commercial radon measurement devices. Thus, a gap in radon metrology is closed. The new primary standard for reference atmospheres is realised with a combined relative standard uncertainty of 1.1%.