Development of a low-level radon reference atmosphere

doi:10.1016/j.apradiso.2013.03.032

Applied Radiation and Isotopes

Volume 81, November 2013, Pages 208-211

https://doi.org/10.1016/j.apradiso.2013.03.032 Get rights and content

Highlights

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A calibration of radon devices with Rn-222 below 1000 Bq/m³ is possible.
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This atmosphere is a long time constant, homogeneous Rn-222 reference atmosphere.
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The reference atmosphere is generated with a Ra-226 activity standard.
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A new primary standard is realised with a relative uncertainty of 1.1% for k=1.

Abstract

In order to calibrate measurement devices for the activity concentration of Rn-222 (radon) in air below 1000 Bq/m³, a constant for long time (>5 d), homogeneous reference atmosphere is created by a certified activity in a certified volume. The PTB developed this reference atmosphere from 150 Bq/m³ to 2000 Bq/m³ 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%.

Introduction

The noble gas Rn-222 exists worldwide in different activity concentrations in the air. To determine the exposure of humans, measurements of the radon activity concentration are performed in houses and at places of work, because radon is the main part of natural radiation exposure (UNSCEAR, 2000).

The measurement devices for the determination of the radon activity concentration at the moment are only traceable when calibrated in the range above 1000 Bq/m³ (IEC 61577-1, 2006, IEC 61577-2, 2000, IEC 61577-3, 2002). For lower activity concentrations, the calibration factor is extrapolated and the measurement uncertainty of the commercial devices is in addition too large due to the limited statistic of counts in their active volumes in the standard calibration time of 24 h.

But the arithmetic mean of the radon activity concentration in houses in Germany for example is around 50 Bq/m³. So it is obvious that there is a missing link in the metrology at the moment. All measurements in this range were taken by commercial radon devices without a traceable calibration. To close this gap, constant in time reference atmospheres below 1000 Bq/m³ must be generated to perform long-term calibrations (t≥5 d). These long-term calibrations in the constant reference atmospheres reduce the measurement uncertainty of the commercial radon device significantly and lead to a calibration factor with a combined relative standard uncertainty below 2%.

Therefore, a low-level radon reference chamber was developed at PTB. This chamber consists of a traceable volume for the generation of the reference atmosphere, a facility for the transport of the defined activities and a large volume detector (Linzmaier, 2012) for the realization of the unit.

Section snippets

Experimental set-up of the low-level radon reference chamber

A schematic drawing of the low-level radon reference chamber is shown in Fig. 1.

The traceable volume for the generation of the reference atmosphere consists of a standard vacuum chamber with a volume of 511.8±2.5 L.

The volume of the vacuum chamber is certified by the manufacturer. All other components are measured for dimensions and weighed to determine the volume. To estimate the overall uncertainty, a budget is prepared according to the guide to the expression of uncertainty in measurement (

Composition of the emanation source

The traceability of the activity concentration is given by the activity of radium A(²²⁶Ra) in the emanation source, the emanation coefficient χ of the emanation source and the reference volume V.

The geometry of the extended area source is in accordance with PTB standard geometry (Physikalisch-Technische Bundesanstalt (PTB), 2006).

The source is covered with foils, so that nobody can touch the radium.

The emanation source is an extended area source, which consists of a filter paper as the source

Realization of the reference atmospheres

Due to the α-decay of the radium the noble gas radon is generated. The emanation coefficient χ describes the quotient of the activity equilibrium of Rn-222 and Ra-226 in air, so it is $χ = \frac{A (Rn - 222)}{A (Ra - 226)} .$

In general, for the emanation coefficient of emanation sources the equation 0<χ≤1 is fulfilled.

The measurement procedure is based on the comparison of the count rates of a Rn-222 gas-tight closed (χ=0) and a Rn-222 emanating (χ≠0) Ra-226 source of the same geometry. The procedure of the

Conclusion

Currently, the commercial radon devices were only traceable when calibrated above 1000 Bq/m³. With the development of the low-level radon reference chamber, constant activity concentrations from 1900 Bq/m³ down to 150 Bq/m³ are available. For this purpose, several emanation sources have been manufactured. These emanation sources generate a constant reference atmosphere in the low-level radon reference chamber. The emanation coefficients of the sources are measured and analyzed. This is done

Acknowledgments

We would like to thank our colleagues A. Honig and J. Leppelt for supporting this work.

Additionally, we would like to acknowledge the helpful discussion with our colleagues, especially D. Arnold, as well as S. Röttger. For the years of good cooperation in source production, we would like to thank C. Niedergesä, S. Hennig and M. Ehlers. The construction of major parts of the set-up as well as the novel design of the emanation housing (ER) is a development of T. Reich. For the series production

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