Interlaboratory comparisons for passive radon measuring devices at BfS.

Since 2003, the German Federal Office for Radiation Protection (BfS) has conducted annual interlaboratory comparisons for passive radon measuring devices in order to ensure the quality of these measurements. Passive radon devices which use solid state nuclear track detectors, electrets or activated charcoal can be tested. The exposures of radon devices are carried out in the radon calibration laboratory at BfS. Radon activity concentrations are traced back to the national standard, being established at the National Institute of Physics and Metrology (PTB). According to the national guideline, radon services which offer radon monitoring at workplaces have to participate in the intercomparisons and prove the suitability of their radon devices for the measurements.

Radon activity concentration--a Euromet and BIPM supplementary comparison.

For the first time, a comparison of radon activity concentration in air has been performed within the scope of Euromet. In the project 657, 'Comparison of calibration facilities for the radon activity concentration,' 12 participants from 9 countries compared different radon reference atmospheres at 1, 3 and 10 k Bq m-3 via a transfer standard. The comparison was listed as BIPM supplementary comparison EUROMET.RI(II)-S1. The results of most participants are correlated due to common traceability to one single radon gas standard producer. This makes a careful correlation analysis necessary to achieve an appropriate comparison reference value. The results of the comparison as well as the complex analysis of the correlated set of data is presented and discussed.

Reference instruments based on spectrometric measurement with Lucas Cells.

The Bundesamt für Strahlenschutz (Berlin, Germany) and the Paul Scherrer Institute (Villigen, Switzerland) both operate accredited calibration laboratories for radon gas activity concentration. Both the institutions use Lucas Cells as detector in their reference instrumentation due to the low dependence of this detector type on variations in environmental conditions. As a further measure to improve the quality of the reference activity concentration, a spectrometric method of data evaluation has been applied. The electric pulses from the photomultiplier tube coupled to the Lucas Cells are subjected to a pulse height analysis. The stored pulse height spectra are analysed retrospectively to compensate for fluctuations in the electric parameters of the instrumentation during a measurement. The reference instrumentation of both the laboratories is described with the respective spectrum evaluation procedures. The methods of obtaining traceability to the primary calibration laboratories of Germany and Switzerland and data of performance tests are presented.

Intercomparison exercise of the PTB, BfS, MPA and PSI calibration facilities for radon gas concentration.

The traceability chain of one national reference laboratory (PTB) and three accredited radon calibration laboratories (BfS MPA and PSI) to internationally acknowledged radon gas standards is specified. As an additional tool for quality assurance, interchange of an electronic radon measuring instrument was used as a means for a relative comparison of the radon gas reference atmospheres. The instrument was exposed to radon gas activity concentrations between 500 Bq.m(-3) and 15 kBq.m(-3). Measured sensitivities of the participants agree well inside the range of specified calibration uncertainties.

The measurement accuracy of passive radon instruments.

This paper analyses the data having been gathered from interlaboratory comparisons of passive radon instruments over 10 y with respect to the measurement accuracy. The measurement accuracy is discussed in terms of the systematic and the random measurement error. The analysis shows that the systematic measurement error of the most instruments issued by professional laboratory services can be within a range of ±10 % from the true value. A single radon measurement has an additional random measurement error, which is in the range of up to ±15 % for high exposures to radon (>2000 kBq h m(-3)). The random measurement error increases for lower exposures. The analysis especially applies to instruments with solid-state nuclear track detectors and results in proposing criteria for testing the measurement accuracy. Instruments with electrets and charcoal have also been considered, but the low stock of data enables only a qualitative discussion.

Performance tests for instruments measuring radon activity concentration.

Performance tests of electronic instruments measuring the activity concentration of (222)Rn have been carried out with respect to the standard IEC 61577-2. In total, 9 types of instrument operating with ionization chambers or electrostatic collection have been tested for the influence of different climatic and radiological factors on the measurement characteristics. It is concluded that all types of instrument, which are commercially available, are suitable for indoor radon measurements. Because of the dependence on climatic conditions, the outdoor use is partly limited.

Measurements of radioactivity in environmental samples from the southern Urals.

A region between Chelyabinsk and Ekaterinburg in the Southern Urals has been heavily contaminated due to operational and accidental releases from the first Soviet plutonium production facility Mayak. In 1992 and 1993, the German Federal Office for Radiation Protection organized a measuring campaign involving two Russian institutes to assist with the validation of former Soviet measurement data. The results of this measuring campaign are reported here. Environmental samples were collected from areas affected by significant radioactive releases into the Techa river, which started in 1948, and by fallout from the explosion of a fission product storage tank in 1957. Soil, sediment, water, milk and food samples were independently analysed for 90Sr, 137Cs and plutonium by the three institutes involved. This paper presents data on the present levels of environmental radioactivity. The highest contamination of areas accessible to the local population was found in the vicinity of the Techa river around Muslumovo. Activity concentration of floodplain samples reached up to 37,000 Bq.kg-1 137Cs, 5,600 Bq.kg-1 90Sr and 9.9 Bq.kg-1 Pu. Milk and potatoes from private farms in Muslumovo showed low activity in the range from 0.7 Bq.kg-1 to 25 Bq.kg-1 90Sr. The results of the three independent measurement teams showed sufficient agreement. One Russian laboratory obtained plutonium activities that exceeded the results of the other laboratories by about 20%. Contrary to the International Chernobyl Project, there was no overestimation of 90Sr activities in the Russian analyses. Therefore, the validity of earlier data sets acquired with same methodology and quality control can be considered a valuable basis for further assessments and for dose reconstruction in epidemiological projects.