Measurement of radon and radon progenies at the German radon reference chamber

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Measurement of radon and radon progenies at the German

radon reference chamber

A. Paul

a,

*, A. Honig

a

, S. RoÈttger

b, 1

, Uwe Keyser

a, c aPhysikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany

bCommissariat aÁ l'Energie Atomique (CEA), 91191 Gif-sur-Yvette Cedex, Saclay, France

cInstitut fuÈr Metallphysik und Nukleare FestkoÈrperphysik, Technische UniversitaÈt Carolo-Wilhelmina zu Braunschweig, 38106 Braunschweig, Germany

Accepted 21 June 1999

Abstract

The activity concentration of radon in the environment can vary over ®ve orders of magnitude. Radon and its progenies thus concern all people involved in radiation protection as well as in low-level experiments. In the German radon reference chamber at the PTB, radon and its progenies are measured with di€erent systems fora -and g-spectrometry with the full set of environmental parameters, e.g. temperature, humidity and aerosol concentration being controlled. Control of air pressure is also possible by use of an extention chamber.

The sampling and measuring technique for radon and its short-lived progenies at the German radon reference chamber are the basis for fundamental studies with regard to the understanding of the equilibrium factor and the unattached fraction of progenies. The facility also serves for the calibration of radon progeny detectors. # 2000 Elsevier Science Ltd. All rights reserved.

PACS:29.30.h; 92.60.Mt

Keywords:ag-spectrometry; Radon progenies; Equilibrium factor; Environmental parameters

1. Introduction

Radon or, more precisely, its short-lived progenies account for about 30% of the total human radiation dose. Especially for the respiratory tract and the lungs, the dose predominantly stems from radon progenies deposited via aerosols (Thieme-Stratton, 1980). Due to this fact, studies of the activity concentration of radon and its progenies are performed worldwide either at

workplaces (e.g. mines) or at home. It is therefore necessary to operate calibration facilities in which the activity concentration of radon and its progenies can be measured under well-de®ned conditions.

The equilibrium factor F and the unattached frac-tion fpof the progenies play a central role in the

esti-mation of the lung dose from radon activity concentration C(222Rn) measurements. Since the dose

directly caused by radon is small compared to that from the progenies for almost all practical situations in radiation protection, the study of the airborne radon progeny activity concentration is of great importance. The quantity which de®nes the fraction of short-lived radon progeny activity concentration in air in

compari-0969-8043/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. PII: S0969-8043(99)00180-3

* Corresponding author. Tel. 531-592-8523; fax +49-531-592-8525.

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son with the radon activity concentration is the equili-brium factor F. It can be understood as a relative measure (normalized to one and weighted for each progeny according to its potential a-energy (International Commission on Radiological Protection, 1987), cf. Eq. (1), uncertainties calculated due to nuclear data (Nuclear Data, 1998)) of the fraction of short-lived progenies in air either in equilibrium or not in equilibrium Ceqˆ0:106…2† C…218Po† ‡0:513…20† C…214Pb† ‡0:381…10† C…214Bi† ‡5:2…1† 10ÿ8 C…214Po† ˆ)Fˆ Ceq C…222Rn† with F2 ‰0:1Š: …1†

Since a more sophisticated lung model is widely used now, separation and measurement of the fraction attached to aerosols and of the unattached fraction of the radon progenies is of rising interest. This has led to the de®nition of another quantity, the unattached fraction fp. It is obtained by splitting the

equilibrium-equivalent concentration Ceq into an attached and a

free equilibrium-equivalent concentrationCeqa andCeqf

CeqˆCaeq‡Cfeqˆ)fpˆ C f eq

Ceq withfp2 ‰0:1Š: …2†

The accurate determination ofF andfp is based on

the measurement of the activity concentrations of

218Po, 214Pb and214Bi2, divided into the attached and

unattached fractions, and the measurement of the radon activity concentration. The German radon refer-ence chamber was set up (1) to make available an accurate calibration facility traceable to the national standards for both, radon and radon progeny activity concentration and (2) to allow investigations covering fundamental questions of physics with regard to the behavior of radon and its progenies as a function of the environmental parameters (Paul, 1995; Honig et al., 1998).

2. The radon reference chamber and its environmental control

In the radon reference chamber of PTB, radon and its progenies are measured with di€erent systems fora -andg-spectrometry, with the full set of environmental parameters, such as temperature, humidity, air pressure and aerosol size distribution being controlled. Measurement and control of the environmental par-ameters (Honig et al., 1998) is not only important for the quality of the calibration of radon activity concen-tration, but it is necessary for all measurements of ac-tivity concentrations of radon progenies and the resulting equilibrium factor. The basic design and con-struction of the chamber, the climate control, the aero-sol generation and the air cleaner (cf. Fig. 1) have therefore been chosen to provide stable conditions and a wide variability of the parameters.

The radon reference chamber has an inner volume

Fig. 1. Aerosol production and reduction in the radon reference chamber.

2Since 214Po is in activity equilibrium with 214Bi for all

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of Vˆ21:035…30†m3, the outer dimensions being

(480021902500) mm3. An air lock of (12701210

2440) mm3 has to be traversed to reach the inner

part of the chamber. The walls are connected to the ground. The air is circulated inside the chamber and can be heated, cooled, dried and moistened. The tem-perature can be varied from ÿ20 to 408C and the humidity between 5 and 95%. The production of aero-sols is based on the method of vapor condensation at a well-de®ned temperature (Kommission Reinhaltung der Luft VDI, 1980), by which aerosols of di€erent size and concentration can be obtained (Paul and Keyser, 1996). The material chosen for the aerosols is carnauba wax. It consists mainly of long chains of car-bon hydrogen. Carnauba wax makes it possible to pro-duce particles of a wide range of sizes (starting at about 10 nm aerodynamic diameter up to 1mm) that are almost spherical as has been proved by molecular dynamics simulation (Gunkel, 1997) and pictures taken with an electron microscope. Carnauba wax (Tu, 1981) is put into a sample boat of elliptical shape, connected to a condensation volume and heated by an insulated wire cord by which it is surrounded. The critical nuclei and the aerosols are formed by the vapor. Both, the concentration and the distribution of the aerosols can be individually regulated by varying temperature and air ¯ow (Paul et al., 1997). Well-de®ned aerosol distri-butions with an integral aerosol concentration ranging

from 108 to 1013 mÿ3 and a mean diameter ranging

from 30 to 300 nm can thus be produced. The lowest integral aerosol concentration achievable with the air cleaner system is of the order of 106mÿ3.

The large number of environmental parameters of the radon reference chamber that can be combined allows the conclusion to be drawn that almost all con-ditions under which people either live or work can be simulated: clean room conditions, highly polluted air as well as tropical or cold climate. Radon and radon progenies can therefore be studied systematically for varying environmental parameters, and the calibration of active and passive monitors is performed under rea-listic conditions.

3. Experimental set-up for the measurement of radon

The calibration of active or passive radon devices is now a standard procedure for several metrology insti-tutes. A de®ned activity concentration is achieved by the use of a radon gas activity standard (Dersch and SchoÈtzig, 1998) in a known and sealed calibration volume, e.g. the radon reference chamber. Di€erent types of commercial active radon monitors are in use in the radon reference chamber, covering the calibrated range of radon activity concentrations from 1 to 100 kBq mÿ3.

Fig. 2. Low-level measurement of the radon activity concentration (with one standard deviation) in the PTB underground labora-tory UDO. Areas A and C indicate a measurement with a commercial radon monitor, while data in B and D were obtained using the large volume ionization chamber developed at the PTB.

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When the German radon reference chamber at the PTB was set up, an active detection system for the accurate and precise determination of radon using a-spectrometry in air was developed and opti-mized. Especially for low-level experiments, the radon and radon progeny activity concentration has to be known precisely on-line because it rapidly varies with the environmental parameters (tempera-ture, humidity, pressure and aerosol size distri-bution).

The on-line determination of low and medium radon activity concentrations in the region from 1 to 103 Bq

mÿ3 is not possible with commercial devices.

Therefore, a large-volume multiwire pulse ionization chamber with an optimized electrode structure has been developed at the PTB: the layout of the electrode wires has the shape of two archimedic spirals lying one inside the other. This provides an optimization of the electric ®eld, resulting in an improvement in the energy resolution. Thus it is possible to separate the events in the radon decay…Ea…222Rn†ˆ5489:5…3†keV†from those of its short-lived progenies, especially fromEa…218Po† ˆ 6002:35…9†keV:It is achieved bya-spectrometry in air, under atmospheric pressure, without counting gas, in volumes ranging from 5 l to 13 l (RoÈttger et al., 1998). In Fig. 2 a measurement using such a large multiwire pulse ionization chamber and a commercial radon monitor is compared in the underground laboratory for dosimetry and spectrometry (UDO) of the PTB in the Asse salt mine.

4. Experimental set-up for the measurement of radon progenies

The radon reference chamber has deliberately been so dimensioned that (1) a large volume for parallel measurements is provided and (2) wall e€ects are negli-gible. The plate-out e€ect is a basic wall e€ect and has to be controlled independently of the chamber dimen-sions. In order to reduce the in¯uence of the chamber walls their design is of primary importance. The walls of the chamber, therefore, consist of 100-mm poly-urethane foam, clad inside and outside with stainless steel 0.6 mm in thickness. Due to this construction, the heat transmission coecient is smaller than kˆ0:2Wmÿ2Kÿ1:The inner wall is polished and

con-nected to the ground, thus providing a homogeneous radon progeny ®eld. By these means, high temperature stability is achieved; a temperature change can be brought about rapidly and gradients of temperature and electric ®eld are negligible.

Fig. 3 shows the two closed air cycles attached to the radon reference chamber for measurement and control of the radon progeny activity concentration in air. The cycles are independent of each other. In the aerosol generation cycle, well-de®ned aerosols are pro-duced by the aerosol generator which is run either in the di€usion or in the ¯ow mode. In the case of di€u-sion, no air ¯ow passes through the aerosol generator: it is totally temperature-controlled. In the ¯ow mode, air from inside the chamber is pumped through the

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aerosol generator, thus providing two control par-ameters for the aerosol size distribution: temperature and air ¯ow. To avoid condensation at nuclei (which are present in the chamber in varying sizes and concen-trations), the air has to be thoroughly cleaned before being injected into the aerosol generator. The air sampling cycle includes the sampling tube accommo-dating two targets for the collection of radon proge-nies. The unattached fraction of radon progenies is collected on the ®rst target, a screen (Cheng et al., 1980; Cheng and Yeh, 1980), while the progenies attached to aerosols are deposited on the second tar-get, a glass ®ber ®lter. For a systematic investigation of the collection eciencies of the targets two screens (or two ®lters) are often used instead of the standard set-up (screen in front of ®lter).

The measurement of the targets produced by the sampling process is based on simultaneous ag -spec-trometry. The target is placed between a surface bar-rier detector and an HP Ge-detector which are installed opposite each other. The target and the sur-face barrier detector (3 mm distance) are enclosed in a vacuum chamber system usually operated in a low-pressure mode (102 to 101 Pa). Tailing of the a-peaks

is thus reduced, without leading to signi®cant contami-nation of the surface barrier detector as a result of vac-uum-supported recoil e€ects. The target enters the vacuum chamber through an air lock; as a result, the delay time (end of the sampling process until start of

the measurement) is only 60 s. For the g-spectra, a calibration with a large-area reference source of 226Ra

(in equilibrium) o€ers the possibility of calibrating the system for the short-lived radon progenies 214Pb and 214Bi for which the detection system has been set up.

As214Bi and214Po are always in equilibrium, the

e-ciency calibration of the a-spectra is carried out using a target (high activity necessary for good statistics), thus linking the a-eciency calibration to the g -e-ciency calibration.

This experimental set-up enables to measure all short-lived radon progenies separated into a fraction attached to aerosols and an unattached fraction: a specially constructed sample tube allows two targets to be exposed to the same well-de®ned, calibrated air ¯ow. These targets are measured afterwards (one after another in the same detection system) by simultaneous

ag-spectrometry. As the air ¯ow, the collection time, the measuring time, the delay time between collection and measurement and the absolute calibration of the detection system are known, this set-up yields traceable and highly accurate results: 3% of the progeny activity concentrations is one standard deviation.

5. Control of the equilibrium factor by environmental parameters

The control of the equilibrium factorF is basically

Fig. 4. Rapid change of the equilibrium factorFand the unattached fraction due to fast variations of the aerosol concentration. The e€ect is measured by the o‚ine radon progeny measuring system developed at PTB (points given with one standard deviation) and a calibrated commercial system (lines with uncertainty area).

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ensured by the control of the aerosol size distribution by means of an aerosol generator and an air cleaner system, cf. Fig. 1. An example is given in Fig. 4. In this experiment, a commercial monitor for the equili-brium factor and the unattached fraction is compared with the progeny sampling and measuring set-up devel-oped at the PTB and described above. It should be noted that the good consistency of the data is due to several calibrations of the commercial monitor at the PTB, before.

The measurement starts with stable values, Fˆ 0:08…2† and fpˆ0:83…4†, due to low aerosol

conctration in the radon reference chamber, while all en-vironmental parameters are kept constant. The radon activity concentration provided by an exhalation source is also constant during the entire experiment …C…222Rn† ˆ12:3…6†kBq mÿ3). By introducing a high

aerosol concentration into the chamber within 1 h, the unattached fraction is drastically reduced to fpˆ0:08…2†, while the equilibrium factor reaches Fˆ

0:97…5† due to the increase of activity in air via aero-sols. These values are mean values on the assumption that the conditions are stable, which is valid within the stated uncertainty, though a slight systematic decrease ofFand an increase in fpare observed as well. This is

caused by a slight decrease of the aerosol concen-tration due to the settling of aerosols. A systematic e€ect on measurements with commercial systems tested in the radon reference chamber is an overestimation of the activity concentration while a jump inFand/orfp

to higher values is enforced. This e€ect takes place although the calibration factors are optimized under stable conditions. Obviously, these non-stable con-ditions are a good test for the quality of any progeny measuring system, beyond the calibration itself.

Vice versa, a jump inF andfpcan also be enforced

quickly by reducing the aerosol concentration. The example shows a decrease to Fˆ0:03…2† and an increase tofpˆ0:91…4†, when the air cleaner is run for

1 h.

Nevertheless, the aerosol concentration is not the only environmental parameter de®ning the values ofF and fp even for stable conditions. The parameters:

inner surface (in this case the radon reference chamber and everything inside it), humidity, temperature and aerosol size also in¯uence the results.

6. Conclusions

In the radon reference chamber of PTB, radon and its progenies are measured with di€erenta- andg -spec-trometry systems with all environmental parameters, such as temperature, humidity, air pressure and aerosol concentration being controlled. The uncertainties (one

standard deviation) for these values normally range from 0.5 to 7.0%.

Measurement and monitoring of the radon activity concentration is achieved by a large multiwire ioniz-ation chamber in the range from 1 to 103Bq mÿ3and

for higher activity concentrations with di€erent com-mercial systems. Control of the equilibrium factor F and the unattached fractionfpis ensured mainly by an

e€ective control of the aerosol concentration using an aerosol generator and an air cleaner system. A precise facility for measuring these values is set up, including a newly developed sample tube for the separation of the attached and unattached fractions and a simultaneous

ag-spectrometry system for the measurement of these fractions.

Thus the radon reference chamber provides the opportunity for systematic studies of the equilibrium factor and the unattached part, which are fundamental for all kinds of dose estimations concerning radon. Moreover, it is possible to provide stable reference at-mospheres for the calibration of radon and radon pro-geny measurement systems, as well as rapid but well-de®ned changes in the environmental parameters to make exhaustive tests possible.

Acknowledgements

We would like to express our thanks to all those involved, for the support of this work in connection with the design, set-up and installation of the radon reference chamber of PTB within the scope of several EU, BMWi and BMU projects; especially the success-ful cooperation under contract No. 6108 between PTB and BfS (St. Sch. 4008/3-6) is gratefully acknowledged. Moreover, we would like to thank Torsten Sulima and Andreas Bucholz from the Braunschweig Technical University, who continue our close cooperation with this University.

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Figure

Fig. 2. Low-level measurement of the radon activity concentration (with one standard deviation) in the PTB underground labora- labora-tory UDO
Fig. 2. Low-level measurement of the radon activity concentration (with one standard deviation) in the PTB underground labora- labora-tory UDO p.3
Fig. 3 shows the two closed air cycles attached to the radon reference chamber for measurement and control of the radon progeny activity concentration in air
Fig. 3 shows the two closed air cycles attached to the radon reference chamber for measurement and control of the radon progeny activity concentration in air p.4
Fig. 4. Rapid change of the equilibrium factor F and the unattached fraction due to fast variations of the aerosol concentration.
Fig. 4. Rapid change of the equilibrium factor F and the unattached fraction due to fast variations of the aerosol concentration. p.5

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