Applied Radiation and Isotopes 52 (2000) 369±375
www.elsevier.com/locate/apradiso
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
a
Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany
b
Commissariat aÁ l'Energie Atomique (CEA), 91191 Gif-sur-Yvette Cedex, Saclay, France
c
Institut 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 for aand 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
* Corresponding author. Tel. +49-531-592-8523; fax +49531-592-8525.
1
Tel. +33-169-08-81-68; fax +33-169-08-75-84
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 fp of the progenies play a central role in the estimation 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: S 0 9 6 9 - 8 0 4 3 ( 9 9 ) 0 0 1 8 0 - 3
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
Ceq ˆ 0:106…2† C…218 Po† ‡ 0:513…20† C…214 Pb†
‡ 0:381…10† C…214 Bi † ‡ 5:2…1† 10ÿ8
The accurate determination of F and fp is based on
the measurement of the activity concentrations of
218
Po, 214Pb and 214Bi2, 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).
C…214 Po†
ˆ)F ˆ
Ceq
with F 2 ‰0:1Š:
C…222 Rn†
…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 equilibriumequivalent concentration Ceq into an attached and a
free equilibrium-equivalent concentration Caeq and Cfeq
Ceq ˆ Caeq ‡ Cfeq ˆ)fp ˆ
Cfeq
with fp 2 ‰0:1Š :
Ceq
…2†
2
Since 214Po is in activity equilibrium with 214Bi for all
practical purposes due to its very short half-life of 164 ms.
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 aand 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
A. Paul et al. / Applied Radiation and Isotopes 52 (2000) 369±375
371
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 21:035…30† m3 , the outer dimensions being
(4800 2190 2500) mm3. An air lock of (1270 1210
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 temperature can be varied from ÿ20 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 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 106 mÿ3.
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
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.
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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 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 …222 Rn† ˆ 5489:5…3† keV† from those
of its short-lived progenies, especially from Ea …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 (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 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 Wmÿ2 Kÿ1 : 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.
A. Paul et al. / Applied Radiation and Isotopes 52 (2000) 369±375
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 (102 to 101 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
373
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
214
Bi for which the detection system has been set up.
As 214Bi and 214Po 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).
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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 fp 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† ˆ 12:3…6† kBq mÿ3 ). By introducing a high
aerosol concentration into the chamber within 1 h, the
unattached fraction is drastically reduced to
fp 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 fp 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 fp
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 fp 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 fp ˆ 0:91…4†, when the air cleaner is run for
1 h.
Nevertheless, the aerosol concentration is not the
only environmental parameter de®ning the values of F
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€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 103 Bq mÿ3 and
for higher activity concentrations with di€erent commercial systems. Control of the equilibrium factor F
and the unattached fraction fp 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. 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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