Contact

Head of Radon Calibration Service Laboratory

Dr. Volkmar Schmidt Phone: +49 (0)3018 333 4219 Fax: +49 (0)3018 333 4215 Email: vschmidt@bfs.de

Deputy Head Radon Calibration Service Laboratory

Helmut Buchröder Phone: +49 (0)3018 333 4213 Fax: +49 (0)3018 333 4215 Email: hbuchroeder@bfs.de

Quality management representative

Elisabeth Foerster Phone: +49 (0)3018 333 4221 Fax: +49 (0)3018 333 4215 Email: efoerster@bfs.de

Address

Bundesamt für Strahlenschutz Fachgebiet SW 1.3 Köpenicker Allee 120-130 10318 Berlin Germany

The Federal Office for Radiation Protection (BfS) operates a calibration service laboratory to assure the quality of measurements of radon and radon progenies. The corresponding quantities to be measured are traced back to the national standard. The laboratory offers calibrations, type tests and interlaboratory comparisons for measurement devices. Additionally metrological fundamentals are scientifically investigated as well as evaluation and development of measurement methods are carried out.

Integration of the radon calibration laboratory into the organisational structure of the BfS (Click to zoom.)

Figure 1: Radon calibration chamber (11 m3) (Click to zoom.)

Figure 2: Part of the Radon Calibration Service Laboratory with 0.4 m3 stainless-steel containers, computer-controlled measuring and dosage system (Click to zoom.)

Figure 3: Diagram of the equipment for the calibration of the measurand activity concentration of radon-222 in air (T: temperature, p: air pressure, r.H.: relative humidity) (Click to zoom.)

Figure 4: Part of the Calibration Service Laboratory with PAEC chamber for calibration of devices measuring radon progenies (Click to zoom.)

Figure 5: Structure of the calibration chamber for devices measuring radon-222 progenies (Click to zoom.)

Figure 6: Traceability of the measurand radon-222 activity concentration to the national standard (Click to zoom.)

Figure 7: Traceability of the measurand potential alpha-energy concentration (PAEC) of the short-lived radon-222 progenies (Click to zoom.)

Functions and objectives

A comprehensive program for quality assurance is performed beside the calibration of measurement devices to ensure a quality standard for measurement of radon and radon progenies in all German states. The program is focused to manufacturers to evaluate the conformity of the metrological properties of their measurement devices with international stated normative requirements on the one hand and to users with offering a regular calibration service on the other hand. Interlaboratory comparisons are operated for passive radon monitors annually. These intercomparisons are accompanied by proceedings to recognize the organizational, technical, and professional pre-conditions for the metrological monitoring of radon-222-exposures during work activities according to part 3 chapter 2 of the Radiation Protection Ordinance.

The offers for quality assurance of measurements of radon-222 and radon-222-progenies are used by both national and international institutions.

In addition to the programs for quality assurance, scientific investigations on metrological fundamentals and on the evaluation and development of measurement methods are carried out. This research on methodical subjects is aimed to react on prevailing problems in a timely manner. For example, innovative measuring equipment for determining the radon exhalation from building materials has been developed. The measured results will be used to provide reliable information to growing public interest on naturally occurring radiation existing in the living environment of the population and also used for corresponding measuring programs.

Technique and equipment

The BfS Radon Calibration Service Laboratory is accredited according to norm DIN EN ISO/IEC 17025 (by 8 March 2011 at the German Calibration Service DKD, from 9 March 2011 at the German Accreditation Body DAkkS GmbH)for the measurands activity concentration of radon-222 in air (C_Rn) and the potential alpha-energy concentration (PAEC) of the short-lived radon-222 progenies (C_P).

The Radon Calibration Service Laboratory consists of the following areas:

• Laboratory for the measurand activity concentration of radon-222 in air
• Laboratory for the measurand potential alpha-energy concentration of the short-lived radon-222 progenies
• Physical-technical laboratories.

For the purposes of calibration of measurement devices and scientific and technical investigations radon-222 reference atmospheres were generated in several stainless steel containers with volumes of 0.4 m³, 11 m³ ("radon chamber") and 30 m³ ("PAEC chamber"). The produced reference atmospheres are characterized by the measurands C_Rn and C_P respectively and the climate parameters temperature, pressure, and relative humidity. Temperature and relative humidity can be adjusted and controlled in both radon chamber and PAEC chamber by corresponding technical facilities. Additionally, the PAEC chamber has technical equipments for adjusting and controlling air turbulence and aerosol parameters. A special device is used to generate aerosols with known particle concentration and particle size distribution. The particle concentration and the particle size distribution can be kept homogenously and temporally constant to a large extend at constant climatic conditions and with respect to the aerosol-physical processes, like as deposition of particles on surfaces and coagulation of particles.

Calibration of radon measurement devices

The activity concentration of radon-222 in the calibration containers is adjusted to a pre-determined value by a single injection of radon-222 gas. The calibration containers are connected to a flow-through source containing radium-226 via controlled-volume piston pumps, which are computer-controlled. By this means the activity concentration of radon-222 in the calibration containers can be kept sufficiently homogenous and temporally constant during the tests (see figures 1 to 3).

The radon-222 activity concentration and the climate parameters are monitored quasi-continuously. For the purpose of checking the reference atmosphere the monitored data and to secure redundancy every working day measurements are carried out additionally by manual sampling. All measurement values, which are obtained during the tests, are recorded in data bases and can be managed centrally. Thus the repeatability and traceability are guaranteed. The best measurement uncertainty, which can be achieved during calibrations, according to the measurement range of the radon-222 activity concentration is listed in the following table.

* The extended relative measurement uncertainty is indicated, which is resulting from the standard uncertainty of measurement multiplied with a coverage factor k=2. It has been determined in accordance with "Guide to the Expression of Uncertainty in Measurement"(ISO, 1995). The conventional true value of the measurement quantity lies in the dedicated range of values with 95 per cent probability.

Calibration of devices measuring radon progenies

The calibration of measurement devices for radon-222 progenies is technical more sophisticated than those of radon measurement devices, because a multitude of further parameters influencing the measurement of short-lived progenies have to be accounted for and monitored, including:

• Particle concentration and particle size distribution of the aerosol,
• Relation between the aerosol-attached part and the unattached part of the short-lived progenies,
• Temperature, relative humidity, and air pressure,
• Flow velocity and degree of turbulence of the air in the chamber, and
• Measuring position within the chamber.

For the measurements of radon progenies and the calibration of corresponding measurement devices a chamber with a volume of 30 m³ is used to assure the homogeneity of the reference conditions within the chamber volume during the measurements and to reduce the influence of interfering parameters (figure 4). All relevant parameters are detected by sensors and logged automatically for calibration and quality assurance purposes. Furthermore the parameters of the reference atmosphere can be adjusted and/or kept constant during the calibration process. For this purpose the following technical facilities are used:

• Aerosol generators with different principles of operation and particle size distribution,
• Facility for continually dosing of aerosols,
• Flow-through source containing radium-226 for continually compensation of the loss of radon-222 due to radioactive decay,
• Clean-up circuit to reduce the aerosol concentration,
• In-floor heating and cooling elements, and
• Ventilators controllable in direction and intensity.

Radon progeny measurement devices are calibrated for the measurand potential alpha-energy concentration of the short-lived radon-222 progenies. For this purpose the air of the reference atmosphere is conveyed with a constant volume flow rate through a combined mesh-filter probe. The fraction of the short-lived radon-222 progenies, which are attached to aerosols, are deposited to the filter and the free (for example unattached) fraction to the wire mesh. Immediately after sampling the decay curves of the alpha activity on the filter and the screen are acquired by α-spectrometry. By means of an algorithm the potential alpha-energy concentration of the short-lived radon-222 progenies and the free fraction are deduced. In figure 5 the structure of the calibration chamber for radon progenies and the applied peripheral measurement facilities is depicted.

The best measurement uncertainty of the measurand potential alpha-energy concentration of the short-lived radon-222 progenies, which can be achieved during calibrations is given in in the following table.

* The extended relative measurement uncertainty is indicated, which is resulting from the standard uncertainty of measurement multiplied with a coverage factor k=2. It has been determined in accordance with "Guide to the Expression of Uncertainty in Measurement" (ISO, 1995). The conventional true value of the measurement quantity lies in the dedicated range of values with 95 per cent probability.

Representation of measurands and their traceability to national standards

The measurand activity concentration of radon-222 is traceable to the national standard of the German National Metrology Institute Physikalisch-Technische Bundesanstalt (PTB) via a radon measurement device, which is the reference normal. Scintillation cells and a radon measurement device of the same type as the reference normal are used as laboratory’s working standards and are internally calibrated with the reference standard (see figure 6).

Quality assurance in the calibration laboratory

The BfS Radon Calibration Service Laboratory operates a quality management system according to norm DIN EN ISO/IEC 17025 and is periodically audited by the accreditation body (until 8 March 2011: German Calibration Service DKD, starting of 9 March 2011: German Accreditation Body DAkkS).

Service

Publications

Schmidt, V.; Feddersen, C.; Ullmann, W.
Untersuchungen zur Aussagefähigkeit von passiven Messsystemen zur Bestimmung der Strahlenexposition durch Radon und kurzlebige Radonfolgeprodukte, Bundesamt für Strahlenschutz, BfS-ST-6/95

Hamel, P.; Schmidt, V.
The Calibration Laboratories for the Measurement of Radon and short-lived Radon Decay Products at the Federal Office for Radiation Protection (BfS), In: Kerntechnik 66 (4), 202-205, (2001)

Schmidt, V.; Hamel, P.
Measurements of Deposition Velocity of Radon Decay Products for Examination of the Correlation between Air Activity Concentration of Radon and the accumulated Po-210-Activity, The Science of the Total Environment, 272, 189-194, (2001)

Röttger, A.; Honig, A.; Schmidt, V.; Buchröder, H.; Rox, A.; Butterweck, G.; Schuler, Ch.; Maringer, F.-J.; Michielsen, N.; Howarth, C.; Miles, J.C.H.; Vargas, A.; Ortega, X.; Burian, I.; Turtiainen, T.; Jachs, P.; Edelmaier, R.; Hagberg, N.
Comparison of Calibration Facilities for Radon Activity Concentration: EUROMET Project 657, In: Metrologia 42 (2005), Technical Supplement, doi: 10.1088/0026-1394/42/1A/06003

Beck, T. R.; Buchröder, H.; Foerster, E.; Schmidt, V.
Interlaboratory Comparisons for Passive Radon Measuring Devices at BfS, In: Radiation Protection Dosimetry, 125 (2007), 572-575, doi: 10.1093/rpd/ncm201

Beck, T.R.; Buchröder, H.; Schmidt, V.
Performance Tests for Instruments Measuring Radon Activity Concentration, In: Applied Radiation and Isotopes 67 (5), 876-880, (2009), doi: 10.1016/j.apradiso.2009.01.049

Beck, T.R.; Buchröder, H.; Döring, J.; Foerster, E.; Schmidt, V.
Messgeräte zur Bestimmung der Radon-Aktivitätskonzentration oder der Radonexposition - Vergleichsprüfung 2010, URN: urn:nbn:de:0221-201008113016)