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

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1 Applied Radiation and Isotopes 52 (2000) 369±375 Measurement of radon and radon progenies at the German radon reference chamber A. Paul a, *, A. Honig a,s.roè ttger b, 1, Uwe Keyser a, c a Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, Braunschweig, Germany b Commissariat aá l'energie Atomique (CEA), Gif-sur-Yvette Cedex, Saclay, France c Institut fuèr Metallphysik und Nukleare FestkoÈrperphysik, Technische UniversitaÈt Carolo-Wilhelmina zu Braunschweig, 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 for a- 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: h; 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 * Corresponding author. Tel ; fax Tel ; fax 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 fraction f p of the progenies play a central role in the estimation of the lung dose from radon activity concentration C( 222 Rn) 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 /00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S (99)

2 370 A. Paul et al. / Applied Radiation and Isotopes 52 (2000) 369±375 Fig. 1. Aerosol production and reduction in the radon reference chamber. son with the radon activity concentration is the equilibrium 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 C eq ˆ 0:106 2 C 218 Po 0: C 214 Pb 0: C 214 Bi 5: C 214 Po ˆ)F ˆ Ceq C 222 with F 2 0:1Š: 1 Rn 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 f p. It is obtained by splitting the equilibriumequivalent concentration C eq into an attached and a free equilibrium-equivalent concentration C a f eq and C eq C eq ˆ C a eq Cf eqˆ)f p ˆ Cf eq C eq with f p 2 0:1Š : 2 Since 214 Po is in activity equilibrium with 214 Bi for all practical purposes due to its very short half-life of 164 ms. 2 The accurate determination of F and f p is based on the measurement of the activity concentrations of 218 Po, 214 Pb and 214 Bi 2, divided into the attached and unattached fractions, and the measurement of the radon activity concentration. The German radon reference 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 for a- and g-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 parameters (Honig et al., 1998) is not only important for the quality of the calibration of radon activity concentration, but it is necessary for all measurements of activity concentrations of radon progenies and the resulting equilibrium factor. The basic design and construction of the chamber, the climate control, the aerosol 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

3 A. Paul et al. / Applied Radiation and Isotopes 52 (2000) 369± Fig. 2. Low-level measurement of the radon activity concentration (with one standard deviation) in the PTB underground laboratory 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. of V ˆ 21: m 3, the outer dimensions being ( ) mm 3. An air lock of ( ) mm 3 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 temperature can be varied from 20 to 408C and the humidity between 5 and 95%. The production of aerosols 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 carbon hydrogen. Carnauba wax makes it possible to produce particles of a wide range of sizes (starting at about 10 nm aerodynamic diameter up to 1 mm) 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 distributions with an integral aerosol concentration ranging from 10 8 to m 3 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 10 6 m 3. The large number of environmental parameters of the radon reference chamber that can be combined allows the conclusion to be drawn that almost all conditions 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 realistic 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 institutes. A de ned activity concentration is achieved by the use of a radon gas activity standard (Dersch and SchoÈ tzig, 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 m 3.

4 372 A. Paul et al. / Applied Radiation and Isotopes 52 (2000) 369±375 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 optimized. 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 (temperature, humidity, pressure and aerosol size distribution). The on-line determination of low and medium radon activity concentrations in the region from 1 to 10 3 Bq m 3 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 E a 222 Rn ˆ5489:5 3 kev from those of its short-lived progenies, especially from E a 218 Po ˆ 6002:35 9 kev: It is achieved by a-spectrometry in air, under atmospheric pressure, without counting gas, in volumes ranging from 5 l to 13 l (RoÈ ttger 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 negligible. The plate-out e ect is a basic wall e ect and has to be controlled independently of the chamber dimensions. 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 polyurethane 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 k ˆ 0:2 Wm 2 K 1 : The inner wall is polished and connected 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 produced by the aerosol generator which is run either in the di usion or in the ow mode. In the case of di usion, 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 Fig. 3. Aerosol production and air sampling to control and determine the progeny activity concentrations.

5 A. Paul et al. / Applied Radiation and Isotopes 52 (2000) 369± aerosol generator, thus providing two control parameters for the aerosol size distribution: temperature and air ow. To avoid condensation at nuclei (which are present in the chamber in varying sizes and concentrations), the air has to be thoroughly cleaned before being injected into the aerosol generator. The air sampling cycle includes the sampling tube accommodating two targets for the collection of radon progenies. 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 target, 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-spectrometry. The target is placed between a surface barrier detector and an HP Ge-detector which are installed opposite each other. The target and the surface barrier detector (3 mm distance) are enclosed in a vacuum chamber system usually operated in a lowpressure mode (10 2 to 10 1 Pa). Tailing of the a-peaks is thus reduced, without leading to signi cant contamination of the surface barrier detector as a result of vacuum-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 226 Ra (in equilibrium) o ers the possibility of calibrating the system for the short-lived radon progenies 214 Pb and 214 Bi for which the detection system has been set up. As 214 Bi and 214 Po 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 factor F is basically Fig. 4. Rapid change of the equilibrium factor F and 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).

6 374 A. Paul et al. / Applied Radiation and Isotopes 52 (2000) 369±375 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 equilibrium factor and the unattached fraction is compared with the progeny sampling and measuring set-up developed 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, F ˆ 0:08 2 and f p ˆ 0:83 4, due to low aerosol concentration in the radon reference chamber, while all environmental parameters are kept constant. The radon activity concentration provided by an exhalation source is also constant during the entire experiment C 222 Rn ˆ12:3 6 kbq m 3 ). By introducing a high aerosol concentration into the chamber within 1 h, the unattached fraction is drastically reduced to f p ˆ 0:08 2, while the equilibrium factor reaches F ˆ 0:97 5 due to the increase of activity in air via aerosols. 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 of F and an increase in f p are observed as well. This is caused by a slight decrease of the aerosol concentration 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 in F and/or f p to higher values is enforced. This e ect takes place although the calibration factors are optimized under stable conditions. Obviously, these non-stable conditions are a good test for the quality of any progeny measuring system, beyond the calibration itself. Vice versa, a jump in F and f p can also be enforced quickly by reducing the aerosol concentration. The example shows a decrease to F ˆ 0:03 2 and an increase to f p ˆ 0:91 4, when the air cleaner is run for 1h. Nevertheless, the aerosol concentration is not the only environmental parameter de ning the values of F and f p 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 erent a- and g-spectrometry 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 ionization chamber in the range from 1 to 10 3 Bq m 3 and for higher activity concentrations with di erent commercial systems. Control of the equilibrium factor F and the unattached fraction f p is 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 atmospheres for the calibration of radon and radon progeny measurement systems, as well as rapid but wellde 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 successful cooperation under contract No 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. References Cheng, Y.S., Keating, J.A., M, G., Anapilly, K., Theory and calibration of a screen-type di usion battery. J. Aerosol Sci. 11, 549±556. Cheng, Y.S., Yeh, H.C., Theory of a screen-type di usion battery. J. Aerosol Sci. 11, 313±320. Dersch, R., SchoÈ tzig, U., Production and measurement of 222 Rn standards. Appl. Radiat. Isot. 49, 1171±1174. Gunkel, F Anwendung der MolekuÈ ldynamiksimulation auf Carnauba-Wachs und Phospholide. Ph.D. thesis, Technische UniversitaÈ t Carolo-Wilhelmina zu Braunschweig, 1997.

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