- Email: [email protected]
Metrology of the activity concentration of radon and its progenies at the German radon reference chamber
International Congress Series 1225 (2002) 161 – 167
Metrology of the activity concentration of radon and its progenies at the German radon reference chamber A. Paul a,*, A. Honig a, S. Ro¨ttger a, U. Keyser a,b a
Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Brunswick, Germany Institut fu¨r Metallphysik und Nukleare Festko¨rperphysik, Technische Universita¨t Carolo-Wilhelmina zu Braunschweig, 38106 Brunswick, Germany
b
Abstract The German radon reference chamber at the PTB is a research and calibration facility for the development of new accurate techniques for measuring the activity concentration of radon and its short-lived progenies. Systematic studies of the correlation of the activity concentration of radon and its progenies with the environmental parameters (temperature, humidity, pressure and aerosol size distribution) are performed, resulting in the better understanding of the equilibrium factor F and the unattached fraction fp. This work describes some methods used at the German radon reference chamber and the results obtained with regard to uncertainties and traceability. D 2002 Elsevier Science B.V. All rights reserved. PACS: 29.30.h; 92.60.Mt Keywords: ag-spectrometry; Calibration; Equilibrium factor; Unattached fraction
1. Introduction Radon, or more precisely its short-lived progenies, is responsible for about 30% of the whole human radiation exposure. Especially for the respiratory tract and the lungs, the dose is predominantly caused by the deposition of radon progenies via aerosols [1]. 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 facilities where radon and radon progeny devices can be calibrated so that they are traceable to the national standards under well-defined environmental conditions. The
*
Corresponding author. Tel.: +49-531-592-8523; fax: +49-531-592-8525.
0531-5131/02 D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 3 1 - 5 1 3 1 ( 0 1 ) 0 0 5 2 0 - 9
162
A. Paul et al. / International Congress Series 1225 (2002) 161–167
German radon reference chamber offers such a realization of the traceable activity concentrations of radon and radon progenies combined with the ongoing development of new accurate measuring techniques for the reduction of the assigned uncertainties.
2. Design of the German radon reference chamber The radon reference chamber has an inner volume of V = 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 inside of the chamber. The walls are clad with stainless steel and connected to ground. The air is circulated inside the chamber and can be heated, cooled, dried and moistened. The temperature can be varied from 20 to 40 C and the relative humidity between 5% and 95%. The production of aerosols is based on the method of vapour condensation at a well-defined temperature [2], which allows aerosols of different sizes and concentrations to be obtained [3]. The material chosen for the aerosols [4] is the carnauba wax. The measurement and control of the environmental parameters are important not only for the quality of the calibration of the radon activity concentration but are also absolutely necessary for all measurements of activity concentrations of radon progenies and the resulting equilibrium factor F as well as the unattached fraction fp. Therefore, the basic design and construction of the chamber [5], the climate control, the aerosol generation and the air cleaner are properly chosen to provide stable conditions and a wide variation of the parameters.
3. Realization of the radon activity concentration The activity concentration of 222Rn is realized via a radon gas activity standard (the activity stated at a given time for the sealed vessel [6]) and a defined volume. Activity, volume (differential volume: reference volume minus air displaced by the radon monitor under test) and time measurements have to be traceable. Two typical procedures for the calibration of active radon monitors are used at the PTB: (1) a primary method based on a reference activity concentration realized by a primary radon gas standard and a calibration volume (both values are traceable to national standards), and (2) a secondary method based on a calibration via a reference monitor enclosed in the same atmosphere as the system under test. Both methods provide calibration factors as function of the radon activity concentration and instrumental background. In Table 1, an overview of the realization of the 222Rn activity concentration is given. A calibration factor for the radon monitor under test is related to an activity concentration at a specified time C(tc). This activity concentration is either given by a reference atmosphere or determined by a reference radon monitor. For statistical reasons, a decay interval of 24 h for the extrapolation of C(tc) from Cr(t,k*) is mostly sufficient. For a sealed radon gas standard, the decay constant k is always valid. In calibration volumes, deviations outside the assigned uncertainty can occur, thus an experimental decay constant k* must be applied. The ideal case k = k* is desirable to reduce the uncertainties. Since the instrumental background typically increases during the service life of a radon monitor, regular
A. Paul et al. / International Congress Series 1225 (2002) 161–167
163
Table 1 Uncertainty budget with standard uncertainties [7] for the realization of the activity concentration and the calibration of radon monitors Quantity
Type
Relative uncertainty
Reference atmospherea Radon activity, t = 0 Reference volume Displaced volume Calibration volume Radon decay constant Experimental decay constant Reference time Time Reference radon monitorb Activity concentration Background Calibration factor
C(tc) = A(tc,k,t,k*)V 1 A Vr Vt V = Vr Vt(Vt K Vr) k k* tc t C(tc) = kr(Cr(tc,k*) Cr,bg) Cr(t,k*) Cr,bg kr
B B B B B B B B B B A A B
2 10 10 2 10 2 10 1 10 2 2 10 4 1 10 3 7 10 7 10 2 10 2 10 2 10 2 – 10 2 10 2
Radon monitor under test Activity concentration Background Calibration factor
Ct(t,k*) Ct,bg kt
A A B
10 2 10 2 – 10 3 10 2
a b
Primary method; calibration factor: kt = C(tc)/(Ct(tc) Ct,bg). Secondary method; calibration factor: kt = kr(Cr(tc) Cr,bg)/(Ct(tc)
2
4 4
1
1
Ct,bg).
systematic checks are necessary. In Fig. 1, examples of both methods are given. It stresses how small statistic uncertainties and good time resolution are contrary to each other for commonly used detectors. Since commercial radon monitors typically have quite small active volumes, this can be the dominant limiting factor in their calibration in lower activity concentrations. With the development of the large Archimedean multiwire pulse ionization chamber, the PTB has provided the missing metrological link for radon studies: on-line activity concentration measurement in the range of 1– 103 Bq/m3 [8].
4. Realization of the radon progeny activity concentration CARnP, the unattached fraction fp and the equilibrium factor F In the German radon reference chamber, the short-lived radon progenies are separated by a sample tube according to the attached or unattached fraction, while their respective activity concentrations CARnPf and CARnPa afterwards were measured by simultaneous a- and g-spectrometry. The results are expressed by the equilibrium factor F and the unattached fraction fp [9]. Both F and fp can therefore be studied with regard to the full set of environmental parameters, e.g. temperature, humidity, air pressure and aerosol concentration. Up to now, well-defined and stable equilibrium factors in the interval from 0.1 to 1.0 have been established. In correlation with this, the unattached fraction can be varied from 0.01 to 0.9 [10,11].
164
A. Paul et al. / International Congress Series 1225 (2002) 161–167
Fig. 1. Realization of the activity concentration of 222Rn by primary (left) and secondary methods (right). In the example of the primary method, the calibration point is C(tc) = 86(2) kBq/m3 with a time resolution of 1 h of the monitor under test, while the example of the secondary method has the calibration point at C(tc) = 10.0(3) kBq/m3 with a time resolution of 10 min for the reference monitor and the monitor under test. The uncertainties for the measuring points are suppressed to show the details.
The experimental set-up consists of two main parts: (1) the sample tube for the collection of short-lived radon progenies on one or more targets and (2) the simultaneous ag-spectrometer for the offline measurement of these targets. The sample tube is placed through an air lock in the middle of the radon reference chamber. In a closed cycle, the air is pumped through the targets (the collection times tc being identical), which are placed in the fixture at the top of the sample tube. The air is reinjected into the chamber after traversing the pump. Therefore, the radon activity concentration is undisturbed. The walls of the chamber and the sample tube itself consist of polished stainless steel connected to the ground. This reduces the influence of the sample tube on the radon progeny field inside the chamber, and the disturbance of the activity distribution becomes negligible. The typical targets used for the sampling are screens [12,13] for the unattached progenies and glass fiber or membrane filters for the attached ones. Consequently, in the standard set-up, one screen is placed in front of a filter. The dimensions of these targets are chosen to be identical to those of the area reference sources (/ = 20 mm inner and / = 30 mm outer diameter) of the PTB. Calibration is achieved with small systematic uncertainties because the solid angle is the same either for the calibration source or for the target. The measurement of the targets produced by the sampling process is based on simultaneous ag-spectroscopy. The target is placed between a surface barrier detector and an HP Gedetector which were installed opposite each other. The target and the surface barrier
A. Paul et al. / International Congress Series 1225 (2002) 161–167
165
Table 2 Uncertainty budget for the determination of the activity concentration of the attached and unattached radon progenies, the unattached part fp and the equilibrium factor F Quantity Number of counts Decay constant a-Transition probability g-Transition probability a,g-Efficiency Volume flow Screen collection efficiency, unattached Screen collection efficiency, attached Filter collection efficiency, unattached Filter collection efficiency, attached Recoil rate Collection, delay and measuring times Activity concentration of ARnP Normalized coefficientc Unattached part Radon activity concentration Equilibrium factor c
ZARnP(E) kARnP P(Ea) P(Eg) g(Ea), g(Eg) V˙fl Ssf Ssa Sff Sfa r214Pb tc, td and tm CARnPf,a KARnP fp C222Rn F
Type
Relative uncertainty
A B B B B B B B B B B B A B A B A
3 10 5 10 6 10 10 2 1 10 6 10 10 2 3 10 10 2 7 10 4 10 2 10 3 10 8 10 5 10 10 2 6 10
3
– 1 10 – 3 10 6 – 2 10
1
5
2
2
– 2 10
2
4
– 2 10 – 3 10 3 – 2 10 2 – 5 10
2
2
1
2
1
5
3
1
3 1
– 5 10
2 1
Free and attached, respectively.
detector (3-mm distance) are included in a vacuum chamber system operated in the underpressure mode from 102 to 101 Pa under normal conditions. The calibrations are standard procedures and will therefore not be discussed in detail. For the g-spectra, a calibration with an area reference source of 226Ra (radon is tight and therefore in equilibrium with its decay products up to 214Po) offers the possibility of calibrating the system by the short-lived radon progenies 214Pb and 214Bi for which the detection system has been set up. Due to the fact that 214Bi and 214Po are always in equilibrium, the efficiency calibration of the a-spectra is carried out using a target (high activity for good statistics is necessary), and thus, linking the a-efficiency calibration to the g-efficiency calibration. This experimental set-up measures all short-lived radon progenies separated into a fraction of attached and an unattached aerosols. By a specially constructed sample tube, two targets are exposed to the same well-defined calibrated air flow V˙fl. These targets are measured afterwards (one after another in the same detection system) by simultaneous agspectrometry. Since the volume flow V˙fl, collection time tc, delay time td, measuring time tm and the absolute calibration of the detection system are known, this set-up yields traceable and highly accurate results for C a,f(ARnP) as given in Table 2.
5. Conclusions In the radon reference chamber of the PTB, radon and its progenies are measured with different a- and g-spectrometry systems with all environmental parameters, such as temperature, humidity and aerosol concentration, being controlled. The assigned relative
166
A. Paul et al. / International Congress Series 1225 (2002) 161–167
standard uncertainties for the relevant activity concentrations, equilibrium factor and unattached fraction normally are 2.0 –6.0%. The determination of the activity concentration of the short-lived 222Rn progenies as well as the unattached fraction fp have been traceable since 1999. The traceability of the activity concentration of 222Rn and the equilibrium factor F were successfully established in 2000. Moreover, at the end of 2000, a system for quality management will be installed. Aside from calibration, 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 radon and radon progeny measurement systems, as well as rapid but well-defined changes in the environmental parameters to make exhaustive tests possible. Ongoing research work will include different radon isotopes and low activity concentrations.
Acknowledgements We would like to express our thanks to all those involved for their support to this work as regards with the design, set-up and installation of the radon reference chamber of the PTB within the scope of several EU, BMWi and BMU projects, and especially, the successful cooperation under Contract No. 6108 between PTB and BfS (St. Sch. 4008/36). Finally, we would like to thank the Braunschweig Technical University for years of close cooperation and to all our PTB colleagues involved for their valuable support. References [1] J. Porstendo¨rfer, T.T. Mercer, Adsorption probability of atoms and ions on particle surfaces in submicrometer size range, Journal of Aerosol Science 9 (1978) 469 – 474. [2] Kommission Reinhaltung der Luft VDI, Herstellungsverfahren fu¨r Pru¨faerosole, VDI-Richtlinien VDI 3491,Verein Deutscher Ingenieure, 1980. [3] A. Paul, U. Keyser, A reference aerosol for a radon reference chamber, Nuclear Instruments & Methods in Physics Research 368 (1996) 819 – 824. [4] A. Paul, U. Keyser, Adsorption of radioactive ions on carnauba wax aerosols, European Physical Journal A A2 (1998) 417 – 423. [5] A. Honig, A. Paul, S. Ro¨ttger, U. Keyser, Environmental control of the German radon reference chamber, Nuclear Instruments & Methods in Physics Research, A 416 (1998) 525 – 530. [6] R. Dersch, U. Scho¨tzig, Production and measurement of 222Rn standards, Applied Radiation and Isotopes 49 (1998) 1171 – 1174. [7] International Organization for Standardization, Guide to the Expression of Uncertainty in Measurement, 1st edn., corrected and reprinted, 1995 (Geneva, Switzerland). [8] S. Ro¨ttger, A. Paul, A. Honig, U. Keyser, Vieldraht-Impuls-Ionisaionskammern zur Pra¨zisionsmessung der Radon-Aktivita¨tskonzentration in Luft, PTB-Report PTB-PG-10,Physikalisch-Technische Bundesanstalt, 1998. [9] International Commission on Radiological Protection, Annals of the ICRP 17 (1987) 1 – 60, ICRP Publication 50. [10] A. Paul, S. Ro¨ttger, A. Honig, T. Sulima, A. Buchholz, U. Keyser, Measurement of short-lived radon progenies by ag-spectrometry at the radon reference chamber, Nuclear Instruments & Methods in Physics Research, A 434 (1999) 303 – 312.
A. Paul et al. / International Congress Series 1225 (2002) 161–167
167
[11] A. Paul, A. Honig, S. Ro¨ttger, U. Keyser, Measurement of radon and radon progenies at the German radon reference chamber, Applied Radiation and Isotopes 52 (3 – 4) (2000) 369 – 375. [12] Y.S. Cheng, Y.A. Keating, G.M. Kanapilly, Theory and calibration of a screen-type diffusion battery, Journal of Aerosol Science 11 (1980) 549 – 556. [13] Y.S. Cheng, H.C. Yeh, Theory of a screen-type diffusion battery, Journal of Aerosol Science 11 (1980) 313 – 320.
Metrology of the activity concentration of radon and its progenies at the German radon reference chamber A. Paul a,*, A. Honig a, S. Ro¨ttger a, U. Keyser a,b a
Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Brunswick, Germany Institut fu¨r Metallphysik und Nukleare Festko¨rperphysik, Technische Universita¨t Carolo-Wilhelmina zu Braunschweig, 38106 Brunswick, Germany
b
Abstract The German radon reference chamber at the PTB is a research and calibration facility for the development of new accurate techniques for measuring the activity concentration of radon and its short-lived progenies. Systematic studies of the correlation of the activity concentration of radon and its progenies with the environmental parameters (temperature, humidity, pressure and aerosol size distribution) are performed, resulting in the better understanding of the equilibrium factor F and the unattached fraction fp. This work describes some methods used at the German radon reference chamber and the results obtained with regard to uncertainties and traceability. D 2002 Elsevier Science B.V. All rights reserved. PACS: 29.30.h; 92.60.Mt Keywords: ag-spectrometry; Calibration; Equilibrium factor; Unattached fraction
1. Introduction Radon, or more precisely its short-lived progenies, is responsible for about 30% of the whole human radiation exposure. Especially for the respiratory tract and the lungs, the dose is predominantly caused by the deposition of radon progenies via aerosols [1]. 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 facilities where radon and radon progeny devices can be calibrated so that they are traceable to the national standards under well-defined environmental conditions. The
*
Corresponding author. Tel.: +49-531-592-8523; fax: +49-531-592-8525.
0531-5131/02 D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 3 1 - 5 1 3 1 ( 0 1 ) 0 0 5 2 0 - 9
162
A. Paul et al. / International Congress Series 1225 (2002) 161–167
German radon reference chamber offers such a realization of the traceable activity concentrations of radon and radon progenies combined with the ongoing development of new accurate measuring techniques for the reduction of the assigned uncertainties.
2. Design of the German radon reference chamber The radon reference chamber has an inner volume of V = 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 inside of the chamber. The walls are clad with stainless steel and connected to ground. The air is circulated inside the chamber and can be heated, cooled, dried and moistened. The temperature can be varied from 20 to 40 C and the relative humidity between 5% and 95%. The production of aerosols is based on the method of vapour condensation at a well-defined temperature [2], which allows aerosols of different sizes and concentrations to be obtained [3]. The material chosen for the aerosols [4] is the carnauba wax. The measurement and control of the environmental parameters are important not only for the quality of the calibration of the radon activity concentration but are also absolutely necessary for all measurements of activity concentrations of radon progenies and the resulting equilibrium factor F as well as the unattached fraction fp. Therefore, the basic design and construction of the chamber [5], the climate control, the aerosol generation and the air cleaner are properly chosen to provide stable conditions and a wide variation of the parameters.
3. Realization of the radon activity concentration The activity concentration of 222Rn is realized via a radon gas activity standard (the activity stated at a given time for the sealed vessel [6]) and a defined volume. Activity, volume (differential volume: reference volume minus air displaced by the radon monitor under test) and time measurements have to be traceable. Two typical procedures for the calibration of active radon monitors are used at the PTB: (1) a primary method based on a reference activity concentration realized by a primary radon gas standard and a calibration volume (both values are traceable to national standards), and (2) a secondary method based on a calibration via a reference monitor enclosed in the same atmosphere as the system under test. Both methods provide calibration factors as function of the radon activity concentration and instrumental background. In Table 1, an overview of the realization of the 222Rn activity concentration is given. A calibration factor for the radon monitor under test is related to an activity concentration at a specified time C(tc). This activity concentration is either given by a reference atmosphere or determined by a reference radon monitor. For statistical reasons, a decay interval of 24 h for the extrapolation of C(tc) from Cr(t,k*) is mostly sufficient. For a sealed radon gas standard, the decay constant k is always valid. In calibration volumes, deviations outside the assigned uncertainty can occur, thus an experimental decay constant k* must be applied. The ideal case k = k* is desirable to reduce the uncertainties. Since the instrumental background typically increases during the service life of a radon monitor, regular
A. Paul et al. / International Congress Series 1225 (2002) 161–167
163
Table 1 Uncertainty budget with standard uncertainties [7] for the realization of the activity concentration and the calibration of radon monitors Quantity
Type
Relative uncertainty
Reference atmospherea Radon activity, t = 0 Reference volume Displaced volume Calibration volume Radon decay constant Experimental decay constant Reference time Time Reference radon monitorb Activity concentration Background Calibration factor
C(tc) = A(tc,k,t,k*)V 1 A Vr Vt V = Vr Vt(Vt K Vr) k k* tc t C(tc) = kr(Cr(tc,k*) Cr,bg) Cr(t,k*) Cr,bg kr
B B B B B B B B B B A A B
2 10 10 2 10 2 10 1 10 2 2 10 4 1 10 3 7 10 7 10 2 10 2 10 2 10 2 – 10 2 10 2
Radon monitor under test Activity concentration Background Calibration factor
Ct(t,k*) Ct,bg kt
A A B
10 2 10 2 – 10 3 10 2
a b
Primary method; calibration factor: kt = C(tc)/(Ct(tc) Ct,bg). Secondary method; calibration factor: kt = kr(Cr(tc) Cr,bg)/(Ct(tc)
2
4 4
1
1
Ct,bg).
systematic checks are necessary. In Fig. 1, examples of both methods are given. It stresses how small statistic uncertainties and good time resolution are contrary to each other for commonly used detectors. Since commercial radon monitors typically have quite small active volumes, this can be the dominant limiting factor in their calibration in lower activity concentrations. With the development of the large Archimedean multiwire pulse ionization chamber, the PTB has provided the missing metrological link for radon studies: on-line activity concentration measurement in the range of 1– 103 Bq/m3 [8].
4. Realization of the radon progeny activity concentration CARnP, the unattached fraction fp and the equilibrium factor F In the German radon reference chamber, the short-lived radon progenies are separated by a sample tube according to the attached or unattached fraction, while their respective activity concentrations CARnPf and CARnPa afterwards were measured by simultaneous a- and g-spectrometry. The results are expressed by the equilibrium factor F and the unattached fraction fp [9]. Both F and fp can therefore be studied with regard to the full set of environmental parameters, e.g. temperature, humidity, air pressure and aerosol concentration. Up to now, well-defined and stable equilibrium factors in the interval from 0.1 to 1.0 have been established. In correlation with this, the unattached fraction can be varied from 0.01 to 0.9 [10,11].
164
A. Paul et al. / International Congress Series 1225 (2002) 161–167
Fig. 1. Realization of the activity concentration of 222Rn by primary (left) and secondary methods (right). In the example of the primary method, the calibration point is C(tc) = 86(2) kBq/m3 with a time resolution of 1 h of the monitor under test, while the example of the secondary method has the calibration point at C(tc) = 10.0(3) kBq/m3 with a time resolution of 10 min for the reference monitor and the monitor under test. The uncertainties for the measuring points are suppressed to show the details.
The experimental set-up consists of two main parts: (1) the sample tube for the collection of short-lived radon progenies on one or more targets and (2) the simultaneous ag-spectrometer for the offline measurement of these targets. The sample tube is placed through an air lock in the middle of the radon reference chamber. In a closed cycle, the air is pumped through the targets (the collection times tc being identical), which are placed in the fixture at the top of the sample tube. The air is reinjected into the chamber after traversing the pump. Therefore, the radon activity concentration is undisturbed. The walls of the chamber and the sample tube itself consist of polished stainless steel connected to the ground. This reduces the influence of the sample tube on the radon progeny field inside the chamber, and the disturbance of the activity distribution becomes negligible. The typical targets used for the sampling are screens [12,13] for the unattached progenies and glass fiber or membrane filters for the attached ones. Consequently, in the standard set-up, one screen is placed in front of a filter. The dimensions of these targets are chosen to be identical to those of the area reference sources (/ = 20 mm inner and / = 30 mm outer diameter) of the PTB. Calibration is achieved with small systematic uncertainties because the solid angle is the same either for the calibration source or for the target. The measurement of the targets produced by the sampling process is based on simultaneous ag-spectroscopy. The target is placed between a surface barrier detector and an HP Gedetector which were installed opposite each other. The target and the surface barrier
A. Paul et al. / International Congress Series 1225 (2002) 161–167
165
Table 2 Uncertainty budget for the determination of the activity concentration of the attached and unattached radon progenies, the unattached part fp and the equilibrium factor F Quantity Number of counts Decay constant a-Transition probability g-Transition probability a,g-Efficiency Volume flow Screen collection efficiency, unattached Screen collection efficiency, attached Filter collection efficiency, unattached Filter collection efficiency, attached Recoil rate Collection, delay and measuring times Activity concentration of ARnP Normalized coefficientc Unattached part Radon activity concentration Equilibrium factor c
ZARnP(E) kARnP P(Ea) P(Eg) g(Ea), g(Eg) V˙fl Ssf Ssa Sff Sfa r214Pb tc, td and tm CARnPf,a KARnP fp C222Rn F
Type
Relative uncertainty
A B B B B B B B B B B B A B A B A
3 10 5 10 6 10 10 2 1 10 6 10 10 2 3 10 10 2 7 10 4 10 2 10 3 10 8 10 5 10 10 2 6 10
3
– 1 10 – 3 10 6 – 2 10
1
5
2
2
– 2 10
2
4
– 2 10 – 3 10 3 – 2 10 2 – 5 10
2
2
1
2
1
5
3
1
3 1
– 5 10
2 1
Free and attached, respectively.
detector (3-mm distance) are included in a vacuum chamber system operated in the underpressure mode from 102 to 101 Pa under normal conditions. The calibrations are standard procedures and will therefore not be discussed in detail. For the g-spectra, a calibration with an area reference source of 226Ra (radon is tight and therefore in equilibrium with its decay products up to 214Po) offers the possibility of calibrating the system by the short-lived radon progenies 214Pb and 214Bi for which the detection system has been set up. Due to the fact that 214Bi and 214Po are always in equilibrium, the efficiency calibration of the a-spectra is carried out using a target (high activity for good statistics is necessary), and thus, linking the a-efficiency calibration to the g-efficiency calibration. This experimental set-up measures all short-lived radon progenies separated into a fraction of attached and an unattached aerosols. By a specially constructed sample tube, two targets are exposed to the same well-defined calibrated air flow V˙fl. These targets are measured afterwards (one after another in the same detection system) by simultaneous agspectrometry. Since the volume flow V˙fl, collection time tc, delay time td, measuring time tm and the absolute calibration of the detection system are known, this set-up yields traceable and highly accurate results for C a,f(ARnP) as given in Table 2.
5. Conclusions In the radon reference chamber of the PTB, radon and its progenies are measured with different a- and g-spectrometry systems with all environmental parameters, such as temperature, humidity and aerosol concentration, being controlled. The assigned relative
166
A. Paul et al. / International Congress Series 1225 (2002) 161–167
standard uncertainties for the relevant activity concentrations, equilibrium factor and unattached fraction normally are 2.0 –6.0%. The determination of the activity concentration of the short-lived 222Rn progenies as well as the unattached fraction fp have been traceable since 1999. The traceability of the activity concentration of 222Rn and the equilibrium factor F were successfully established in 2000. Moreover, at the end of 2000, a system for quality management will be installed. Aside from calibration, 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 radon and radon progeny measurement systems, as well as rapid but well-defined changes in the environmental parameters to make exhaustive tests possible. Ongoing research work will include different radon isotopes and low activity concentrations.
Acknowledgements We would like to express our thanks to all those involved for their support to this work as regards with the design, set-up and installation of the radon reference chamber of the PTB within the scope of several EU, BMWi and BMU projects, and especially, the successful cooperation under Contract No. 6108 between PTB and BfS (St. Sch. 4008/36). Finally, we would like to thank the Braunschweig Technical University for years of close cooperation and to all our PTB colleagues involved for their valuable support. References [1] J. Porstendo¨rfer, T.T. Mercer, Adsorption probability of atoms and ions on particle surfaces in submicrometer size range, Journal of Aerosol Science 9 (1978) 469 – 474. [2] Kommission Reinhaltung der Luft VDI, Herstellungsverfahren fu¨r Pru¨faerosole, VDI-Richtlinien VDI 3491,Verein Deutscher Ingenieure, 1980. [3] A. Paul, U. Keyser, A reference aerosol for a radon reference chamber, Nuclear Instruments & Methods in Physics Research 368 (1996) 819 – 824. [4] A. Paul, U. Keyser, Adsorption of radioactive ions on carnauba wax aerosols, European Physical Journal A A2 (1998) 417 – 423. [5] A. Honig, A. Paul, S. Ro¨ttger, U. Keyser, Environmental control of the German radon reference chamber, Nuclear Instruments & Methods in Physics Research, A 416 (1998) 525 – 530. [6] R. Dersch, U. Scho¨tzig, Production and measurement of 222Rn standards, Applied Radiation and Isotopes 49 (1998) 1171 – 1174. [7] International Organization for Standardization, Guide to the Expression of Uncertainty in Measurement, 1st edn., corrected and reprinted, 1995 (Geneva, Switzerland). [8] S. Ro¨ttger, A. Paul, A. Honig, U. Keyser, Vieldraht-Impuls-Ionisaionskammern zur Pra¨zisionsmessung der Radon-Aktivita¨tskonzentration in Luft, PTB-Report PTB-PG-10,Physikalisch-Technische Bundesanstalt, 1998. [9] International Commission on Radiological Protection, Annals of the ICRP 17 (1987) 1 – 60, ICRP Publication 50. [10] A. Paul, S. Ro¨ttger, A. Honig, T. Sulima, A. Buchholz, U. Keyser, Measurement of short-lived radon progenies by ag-spectrometry at the radon reference chamber, Nuclear Instruments & Methods in Physics Research, A 434 (1999) 303 – 312.
A. Paul et al. / International Congress Series 1225 (2002) 161–167
167
[11] A. Paul, A. Honig, S. Ro¨ttger, U. Keyser, Measurement of radon and radon progenies at the German radon reference chamber, Applied Radiation and Isotopes 52 (3 – 4) (2000) 369 – 375. [12] Y.S. Cheng, Y.A. Keating, G.M. Kanapilly, Theory and calibration of a screen-type diffusion battery, Journal of Aerosol Science 11 (1980) 549 – 556. [13] Y.S. Cheng, H.C. Yeh, Theory of a screen-type diffusion battery, Journal of Aerosol Science 11 (1980) 313 – 320.