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Environmental control of the German radon reference chamber
Nuclear Instruments and Methods in Physics Research A 416 (1998) 525—530
Environmental control of the German radon reference chamber A. Honig, A. Paul, S. Ro¨ttger, U. Keyser* Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany Received 6 April 1998; accepted 6 July 1998
Abstract In the radon reference chamber of the PTB, radon and its progenies are measured with different systems for a- and c-spectrometry with the full set of environmental parameters, e.g. temperature, humidity, air pressure and aerosol concentration being controlled. Measurement and control of the environmental parameters is important not only for the quality of the calibration of radon activity concentration but also absolutely necessary for all measurements of activity concentrations of radon progenies and the resulting equilibrium factors. Therefore, the basic design and construction of the chamber, the climate control, the aerosol generation and the air cleaner are properly chosen to provide stable conditions and a wide variation of the parameters. The main advantages of the German radon reference chamber with respect to the variation of environmental parameters is given in this work. ( 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Radon; a-spectrometry; c-spectrometry
1. Introduction Radon or, more precisely, its short-lived progenies are responsible for about 30% of the whole human radioactive exposure. Especially for the respiratory tract and the lungs the dose is predominantly caused by the deposition of radon progenies via aerosols [1,2]. Due to this fact, studies of the activity concentration of radon and its progenies are performed worldwide either at work places (e.g. mines) or at home. It is therefore nessessary to operate calibration facilities, in which this activity
* Corresponding author. Tel.: #49 531 592 8500; fax: #49 531 592 8525.
concentration can be measured under well-defined conditions, e.g. environmental parameters. The environmental parameters temperature, humidity and aerosol size distribution determines the fraction of the activity concentration of radon progenies in air, which is the activity concentration defining the dose. Normally, even in radioactive equilibrium, this activity concentration of progenies in air is not the same as the radon activity concentration. This is due to plate-out of radon progenies to surfaces, therefore the so-called equilibrium factor F normally is smaller than 1. Regulation and control of the equilibrium factor can be achieved via the environmental parameters, especially the aerosol size distribution [3—5]. Since all kinds of calibrations of radon progeny devices
0168-9002/98/$19.00 ( 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 7 8 8 - 8
526
A. Honig et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 525—530
depend on this equilibrium factor, the construction and installation of a climate control unit, an aerosol generator and an air cleaner system is the primary task in setting up a radon reference chamber.
2. The radon reference chamber: Dimensions and construction The German radon reference chamber is set up at the PTB in Braunschweig. It has an inner volume of »"21.035(30) m3 with outer dimensions of (4800]2190]2500) mm3. An air lock of (1270]1210]2440) mm3 has to be traversed to reach the inner part of the chamber. This air lock was installed to minimize the influence exerted by a person entering the chamber during an experimental run. A ground plan of the radon reference chamber is shown in Fig. 1. The walls of the chamber consist of 100 mm polyurethane foam and are clad inside and outside with stainless steel (V4A) 0.6 mm in thickness. Due to this construction, the heat transmission coefficient is smaller than k"0.2 W m~2 K~1. The inner wall is polished and connected to ground. Thus, high temperature stability is achieved and a temperature change can be brought about rapidly, cf. Section 3.
Due to the careful choice of the materials for the radon reference chamber, effects of exhalation or diffusion in the wall material can be neglected [6,7]. As far as possible, all fittings and tubes connected to the radon reference chamber are made of stainless steel. Most of the other materials (e.g. flexible tubes or any pieces made of aluminum) absorb and exhalate radon. Moreover, the use of steel prevents diffusion of radon. In Table 1 several different tubes are compared to an open emanation source. The leakage rate due to an experimental set-up with standardized dimensions and environmental parameters is evaluated. Each tube is 2 m long.
A
¸"
B
*C 0,R/ . *t
(1)
The rate of leakage ¸ is calculated according to the maximum diffusion of radon (activity concentration C ) versus time t. 0,R/ In addition, it is possible to connect active devices through the bypass tube to the chamber. This is preferred for large flows of air or if the system is very large and disturbs the atmosphere inside the radon reference chamber.
Table 1 Rate of leakage in different tubes compared to an open emanation source. The inner diameter as well as the thickness of the wall are given, both in mm Type of tube
Fig. 1. Ground plan of the radon reference chamber. All values are given in millimeters.
Open emanation source Silicon rubber Silicon rubber Norprene Tygon Polyethylene PVC High presure Gas burner PVC Teflon
Tube diameter and thickness (mm]mm)
8]2 8]3 9.6]1.6 10]2 5.3]1.5 8]2 9]4 10]2 8]3 10]1
Leakage rate ¸ (Bq m~3 h~1)
79.2(8) 72.6(12) 73.4(10) 30.6(8) 16.4(6) 11.8(4) 6.36(12) 6.13(12) 4.13(16) 2.18(10) 0.55(8)
A. Honig et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 525—530
Fig. 2. Diagram of the climate control of the radon reference chamber. The sensor (S) gives the value measured for the humidity (u) as an input signal (x) to the three-step control (3). This control adjusts the vaporizer (») and the condenser (C) to the desired value (w). The temperature sensor (0) gives its signal to the two-step controller (2) which works on the condenser and the proportional integral derivative controller (PID) operating the heater (H). At the condenser, maximum (max) is selected according to the incoming signals.
527
Fig. 3. Cooling mode by two-step control: example of desired and actual values for the temperature inside the radon reference chamber.
3. Climate control For the measurement of radon and radon progenies under well-defined environmental conditions, a climate control unit is installed. The air is circulated inside the chamber and can be heated, cooled, dried and moistened. The climate control (see Fig. 2) can be run in two modes: either in the cooling mode from !20°C to 10°C or in the heating mode from 10°C to 40°C. In the case of cooling, the temperature is varied or kept constant by the condenser. The condenser is run by a two-step control without feedback. The dead time of the controlled system is 38(5) s. This mode is independent of the humidity variation. It takes around 2 h to change the temperature from 20°C to!10°C. An example of the temperature variation in the cooling mode from!8°C to!4°C to 0°C is given in Fig. 3. The factor limiting the variation to the actual value is the switching difference X due to the two-step control. In order to S$ achieve a small temperature fluctuation, e.g. by less than 0.5 K, it has to be chosen small enough, in this case X "0.25 K. The unit-step response of the S$ condenser is proportional to the hyperbola function [8].
Fig. 4. Heating mode: temperature inside the radon reference chamber as a function of time. Example of desired and actual values.
In the heating mode, the temperature is adjusted by the condenser and heater. The heater is regulated by a proportional integral derivative (PID) controller [9]. Parallel to this, the condenser works continuously. In this mode, the temperature ranges from 10°C to 40°C. With the PID controller fast temperature changes can be achieved without an offset from the desired value, cf. Fig. 4. With this set-up the fluctuation of temperature is smaller than 0.1 K.
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A. Honig et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 525—530
In the case of heating, the control of the relative humidity is ensured by the vaporizer and the condenser. These systems are controlled by a threestep-controller. A well-defined amount of vapor is mixed to the atmosphere of the radon reference chamber to increase the humidity: the valve of the vaporizer is opened. For a decrease of the humidity, the condenser is used to freeze the water out. This results in a possible variation of the relative humidity from 5% to 95%.
4. Variation of the aerosol concentration The production of aerosols in the radon reference chamber is based on the method of vapor condensation at a well-defined temperature [10], obtaining aerosols of different sizes and concentrations [11]. The material chosen for the aerosols is carnauba wax. It mainly consists of carbon—hydrogen molecules with a preferred chain length of 58. Carnauba wax offers the unique possibility of particle production over a wide range of sizes (starting at about 10 nm aerodynamic diameter up to 1 lm) that are almost spherical, as molecular dynamics simulation [12] and pictures taken by an electron microscope have proved. Carnauba wax aerosols have unique physical and chemical properties as regards shape, size, density, acidic value, etc. From the theoretical and experimental point of view it is, therefore, an ideal choice for a standardized reference aerosol. The spherical shape, in particular, turns out to be an advantage for the comparision of the measured and the calculated adsorption rates of ions on aerosols. Regarding the aerosol radius, both results show a linear dependence of the adsorption rates [16]. Carnauba wax is put into a sample boat [13] elliptical in shape and connected to a condensation volume and heated by an insulated wire cord surrounding it. From the vapor the critical nuclei and the aerosols are formed. Basically, the aerosol generator can be operated in two modes: diffusion and flow mode. For most applications at the radon reference chamber the aerosol generator is operated in the diffusion mode.
Fig. 5. Normalized aerosol size distributions of the diffusion (N (d)) and of the flow mode (N (d), 1 l/min) at ¹"110°C. Both D F the diameter of the maximum and the mean diameter of the distributions are given.
This mode is easy to characterize by the temperature at the outlet of the sample boat. It provides very stable aerosol size distributions, long-term stability of all environmental parameters due to low thermal disturbance in the chamber and nearly no surface contamination. Finally, all changes are made slowly enough. It is therefore the preferable mode for long-time continuous operation. In contrast to the diffusion mode with only one operational parameter, the flow mode of the aerosol generator is characterized by two parameters: temperature at the outlet of the sample boat and air throughput. The flow and the temperature control (which are independent of each other) offer the possibility of fast changes in the aerosol concentration and its size distribution. The aerosol size may be controlled by a well-defined air flow through the sample boat. The aerosol size distribution of the so-called “aerosol jet” [14] is therefore shifted to smaller diameters, the distribution has a smaller full-width at half-maximum (fwhm), and the integral concentration is increased. For comparison, Fig. 5 shows normalized aerosol size distributions of the diffusion and of the flow mode for the same temperature of 110°C. The integral aerosol concentration in the radon reference chamber can be varied from 108 to 1013 m~3 using the aerosol generator. Typically, the diffusion mode is used for 108—1010 m~3, whereas the flow mode operates from 109 to
A. Honig et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 525—530
529
Table 2 Environmental parameters of the radon reference chamber of the PTB. The value or the range of the parameter is given
Fig. 6. Isometric plot of the aerosol concentration inside the radon reference chamber as a function of size and time. The aerosol size distributions N(d) are given as a function of time while the air cleaner is operated.
Environmental parameter
Value or range of parameter
Inner volume Temperatue Relative humidity Integral aerosol concentration Mean diameter of aerosol size distribution
21.035(30) m3 !20—40°C 5—95% 106—1013 m~3 30—300 nm
(Model 3071), and their concentration is measured with a Condensation Particle Counter (Model 3022) [18]. The system is capable of measuring the aerosol concentration as a function of the aerodynamic diameter (from 10 nm to 1 lm). Full aerosol size spectrometry can be performed in about 1 min. Thus, dynamic processes can be studied or the stability of the aerosol distribution in the reference chamber surveyed.
5. Conclusions 1013 m~3. The mean diameter of the aerosol size distribution varies from 90 to 300 nm in the diffusion mode and from 30 to 120 nm in the flow mode. The endpoint of the size distribution, that is the maximum diameter, ranges from 90 to 800 nm. For the reduction of the aerosol concentration an air cleaner system [15] is installed inside the radon reference chamber. With this system the integral aerosol concentration can be reduced to a clean-room atmosphere of 106—107 m~3 particles in less than 2 h, cf. Fig. 6. The combination of the highly efficient aerosol generator and the air cleaner results in a variation of the integral aerosol concentration over seven orders of magnitude. This is the basis for the variation of the equilibrium factor [5]. For the measurement of aerosols (size and concentration) a Differential Mobility Particle Sizer (DMPS) or a Scanning Mobility Particle Sizer (SMPS) and a condensation nucleus counter [17] are used. The DMPS/SMPS measures the size distribution of submicrometer aerosols using an electrical mobility detection technique. The particles are classified with an Electrostatic Classifier
The radon reference chamber of the PTB was designed and constructed to provide variable but stable environmental parameters for the calibration of radon and radon progeny devices. This is achieved by the installation of a climate control unit, a highly efficient aerosol generator and an air cleaner system. The stated environmental parameters are defined, controlled and measured within an uncertainty of 0.5—5.0%. This provides ideal conditions for the calibration of both, passive and active radon devices. Moreover, it provides the possibility of varying the equilibrium factor F as well as the attached and unattached fraction of radon progenies, which is a fundamental prerequisite for the calibration of radon progeny devices; up to now, equilibrium factors from 1.00(8)]10~2 to 1.00(3) have been obtained. Since 1991, i.e. since the PTB’s radon reference chamber is in operation, the environmental parameters temperature, humidity and aerosol size distribution have been carefully observed. The results are summarized in Table 2. It is interesting to note that the studies of absorption and transport of
530
A. Honig et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 525—530
radioactive ions on carnauba wax aerosols have led to a more fundamental understanding of the equilibrium factor as well as to the design of a highly efficient aerosol jet. This jet is already used as an instrument in fundamental nuclear physics for the collection and transport of neutron-rich radioactive ions (fission products) at the LOHENGRIN mass separator at of ILL, Grenoble, France [14,19]. Acknowledgements We would like to express our thanks to all those involved, for the support of this work in context 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; especially the successful cooperation under contract no. 6108 between PTB and BfS (St. Sch. 4008/3-6). References [1] W. Jacobi, Lungenkrebs nach Bestrahlung: Das RadonProblem, Naturwissenschaften 73 (1986) 661. [2] Thieme-Stratton, Seminars in Respiratory Medicine, Thieme-Stratton, New York, 1980. [3] J. Porstendo¨rfer, G. Ro¨big, A. Ahmed, J. Aerosol Sci. 10 (1979) 21. [4] J. Porstendo¨rfer, T.T. Mercer, J. Aerosol Sci. 9 (1978) 469. [5] A. Paul, A. Honig, S. Ro¨ttger, U. Keyser, Erzeugung und Einsatz von Referenzaerosolen an der RadonnormalKammer der PTB und anderen Radon-Referenzkammern. PTB-Laboratory Report PTB-Q.2-1997, PhysikalischTechnische Bundesanstalt, September 1997.
[6] K.H. Folkerts, G. Keller, H. Muth, Rad. Prot. Dos. 9 (1) (1984) 27. [7] M. Durcik, F. Havlik, J. Radioanal. Nucl. Chem. 209 (1996) 307. [8] Arbeitskreis der Dozenten Regelungstechnik, Regelungstechnik in der Versorgungstechnik, C.F. Mu¨ller GmBH, Karlsruhe, 1988. [9] M.K. Juchheim GmbH & Co, Moltkestr 13-31, D-6400 Fulda, Jumo DICON SC Universeller Kompaktregler fu¨r Industrie- und Proze{regelung, 1989. [10] Kommission Reinhaltung der Luft VDI, Herstellungsverfahren fu¨r Pru¨faerosole, VDI-Richtlinien VDI 3491, Verein Deutscher Ingenieure, July, 1980. [11] A. Paul, U. Keyser, Nucl. Instr. and Meth. 368 (1996) 819. [12] F. Gunkel, Anwendung der Moleku¨ldynamiksimulation auf Carnauba-Wachs und Phospholide, Ph.D. Thesis, Technische Universita¨t Carolo-Wilhelmina zu Braunschweig, 1997. [13] K.W. Tu, J. Aerosol Sci. 13 (1981) 363. [14] A. Paul, U. Keyser, A new aerosol generator for the collection and transport of radioactive isotopes, Nuclear fission and fission-product spectroscopy, 94FA05T, 161—168, 1994. [15] R.C. Klein, Health Phys. 68 (1995) 116. [16] A. Paul, Untersuchungen zur Adsorption und zum Transport von radioaktiven Ionen auf Carnauba-Wachs-Aerosolen, Ph.D. Thesis, Technische Universita¨t Carolo-Wilhelmina zu Braunschweig, 1995. [17] E.O. Knutson, K.T. Whitby, Aerosol classification by electrical mobility: apparatus theory and application, J. Aerosol Sci. 6 (1975) 443 foll. [18] TSI Incorporated, St. Paul, MN 55164, USA. Model 3934 SMPS, Scanning Mobility Particle Size, March 1993. [19] A. Paul, S. Ro¨ttger, U. Keyser, Research with Fission Fragments, Q values of the low and high spin isomer of b 146La, S. 252—257, ISBN 981-02-3140-7, World Scientific, London, 1997.
Environmental control of the German radon reference chamber A. Honig, A. Paul, S. Ro¨ttger, U. Keyser* Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany Received 6 April 1998; accepted 6 July 1998
Abstract In the radon reference chamber of the PTB, radon and its progenies are measured with different systems for a- and c-spectrometry with the full set of environmental parameters, e.g. temperature, humidity, air pressure and aerosol concentration being controlled. Measurement and control of the environmental parameters is important not only for the quality of the calibration of radon activity concentration but also absolutely necessary for all measurements of activity concentrations of radon progenies and the resulting equilibrium factors. Therefore, the basic design and construction of the chamber, the climate control, the aerosol generation and the air cleaner are properly chosen to provide stable conditions and a wide variation of the parameters. The main advantages of the German radon reference chamber with respect to the variation of environmental parameters is given in this work. ( 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Radon; a-spectrometry; c-spectrometry
1. Introduction Radon or, more precisely, its short-lived progenies are responsible for about 30% of the whole human radioactive exposure. Especially for the respiratory tract and the lungs the dose is predominantly caused by the deposition of radon progenies via aerosols [1,2]. Due to this fact, studies of the activity concentration of radon and its progenies are performed worldwide either at work places (e.g. mines) or at home. It is therefore nessessary to operate calibration facilities, in which this activity
* Corresponding author. Tel.: #49 531 592 8500; fax: #49 531 592 8525.
concentration can be measured under well-defined conditions, e.g. environmental parameters. The environmental parameters temperature, humidity and aerosol size distribution determines the fraction of the activity concentration of radon progenies in air, which is the activity concentration defining the dose. Normally, even in radioactive equilibrium, this activity concentration of progenies in air is not the same as the radon activity concentration. This is due to plate-out of radon progenies to surfaces, therefore the so-called equilibrium factor F normally is smaller than 1. Regulation and control of the equilibrium factor can be achieved via the environmental parameters, especially the aerosol size distribution [3—5]. Since all kinds of calibrations of radon progeny devices
0168-9002/98/$19.00 ( 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 7 8 8 - 8
526
A. Honig et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 525—530
depend on this equilibrium factor, the construction and installation of a climate control unit, an aerosol generator and an air cleaner system is the primary task in setting up a radon reference chamber.
2. The radon reference chamber: Dimensions and construction The German radon reference chamber is set up at the PTB in Braunschweig. It has an inner volume of »"21.035(30) m3 with outer dimensions of (4800]2190]2500) mm3. An air lock of (1270]1210]2440) mm3 has to be traversed to reach the inner part of the chamber. This air lock was installed to minimize the influence exerted by a person entering the chamber during an experimental run. A ground plan of the radon reference chamber is shown in Fig. 1. The walls of the chamber consist of 100 mm polyurethane foam and are clad inside and outside with stainless steel (V4A) 0.6 mm in thickness. Due to this construction, the heat transmission coefficient is smaller than k"0.2 W m~2 K~1. The inner wall is polished and connected to ground. Thus, high temperature stability is achieved and a temperature change can be brought about rapidly, cf. Section 3.
Due to the careful choice of the materials for the radon reference chamber, effects of exhalation or diffusion in the wall material can be neglected [6,7]. As far as possible, all fittings and tubes connected to the radon reference chamber are made of stainless steel. Most of the other materials (e.g. flexible tubes or any pieces made of aluminum) absorb and exhalate radon. Moreover, the use of steel prevents diffusion of radon. In Table 1 several different tubes are compared to an open emanation source. The leakage rate due to an experimental set-up with standardized dimensions and environmental parameters is evaluated. Each tube is 2 m long.
A
¸"
B
*C 0,R/ . *t
(1)
The rate of leakage ¸ is calculated according to the maximum diffusion of radon (activity concentration C ) versus time t. 0,R/ In addition, it is possible to connect active devices through the bypass tube to the chamber. This is preferred for large flows of air or if the system is very large and disturbs the atmosphere inside the radon reference chamber.
Table 1 Rate of leakage in different tubes compared to an open emanation source. The inner diameter as well as the thickness of the wall are given, both in mm Type of tube
Fig. 1. Ground plan of the radon reference chamber. All values are given in millimeters.
Open emanation source Silicon rubber Silicon rubber Norprene Tygon Polyethylene PVC High presure Gas burner PVC Teflon
Tube diameter and thickness (mm]mm)
8]2 8]3 9.6]1.6 10]2 5.3]1.5 8]2 9]4 10]2 8]3 10]1
Leakage rate ¸ (Bq m~3 h~1)
79.2(8) 72.6(12) 73.4(10) 30.6(8) 16.4(6) 11.8(4) 6.36(12) 6.13(12) 4.13(16) 2.18(10) 0.55(8)
A. Honig et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 525—530
Fig. 2. Diagram of the climate control of the radon reference chamber. The sensor (S) gives the value measured for the humidity (u) as an input signal (x) to the three-step control (3). This control adjusts the vaporizer (») and the condenser (C) to the desired value (w). The temperature sensor (0) gives its signal to the two-step controller (2) which works on the condenser and the proportional integral derivative controller (PID) operating the heater (H). At the condenser, maximum (max) is selected according to the incoming signals.
527
Fig. 3. Cooling mode by two-step control: example of desired and actual values for the temperature inside the radon reference chamber.
3. Climate control For the measurement of radon and radon progenies under well-defined environmental conditions, a climate control unit is installed. The air is circulated inside the chamber and can be heated, cooled, dried and moistened. The climate control (see Fig. 2) can be run in two modes: either in the cooling mode from !20°C to 10°C or in the heating mode from 10°C to 40°C. In the case of cooling, the temperature is varied or kept constant by the condenser. The condenser is run by a two-step control without feedback. The dead time of the controlled system is 38(5) s. This mode is independent of the humidity variation. It takes around 2 h to change the temperature from 20°C to!10°C. An example of the temperature variation in the cooling mode from!8°C to!4°C to 0°C is given in Fig. 3. The factor limiting the variation to the actual value is the switching difference X due to the two-step control. In order to S$ achieve a small temperature fluctuation, e.g. by less than 0.5 K, it has to be chosen small enough, in this case X "0.25 K. The unit-step response of the S$ condenser is proportional to the hyperbola function [8].
Fig. 4. Heating mode: temperature inside the radon reference chamber as a function of time. Example of desired and actual values.
In the heating mode, the temperature is adjusted by the condenser and heater. The heater is regulated by a proportional integral derivative (PID) controller [9]. Parallel to this, the condenser works continuously. In this mode, the temperature ranges from 10°C to 40°C. With the PID controller fast temperature changes can be achieved without an offset from the desired value, cf. Fig. 4. With this set-up the fluctuation of temperature is smaller than 0.1 K.
528
A. Honig et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 525—530
In the case of heating, the control of the relative humidity is ensured by the vaporizer and the condenser. These systems are controlled by a threestep-controller. A well-defined amount of vapor is mixed to the atmosphere of the radon reference chamber to increase the humidity: the valve of the vaporizer is opened. For a decrease of the humidity, the condenser is used to freeze the water out. This results in a possible variation of the relative humidity from 5% to 95%.
4. Variation of the aerosol concentration The production of aerosols in the radon reference chamber is based on the method of vapor condensation at a well-defined temperature [10], obtaining aerosols of different sizes and concentrations [11]. The material chosen for the aerosols is carnauba wax. It mainly consists of carbon—hydrogen molecules with a preferred chain length of 58. Carnauba wax offers the unique possibility of particle production over a wide range of sizes (starting at about 10 nm aerodynamic diameter up to 1 lm) that are almost spherical, as molecular dynamics simulation [12] and pictures taken by an electron microscope have proved. Carnauba wax aerosols have unique physical and chemical properties as regards shape, size, density, acidic value, etc. From the theoretical and experimental point of view it is, therefore, an ideal choice for a standardized reference aerosol. The spherical shape, in particular, turns out to be an advantage for the comparision of the measured and the calculated adsorption rates of ions on aerosols. Regarding the aerosol radius, both results show a linear dependence of the adsorption rates [16]. Carnauba wax is put into a sample boat [13] elliptical in shape and connected to a condensation volume and heated by an insulated wire cord surrounding it. From the vapor the critical nuclei and the aerosols are formed. Basically, the aerosol generator can be operated in two modes: diffusion and flow mode. For most applications at the radon reference chamber the aerosol generator is operated in the diffusion mode.
Fig. 5. Normalized aerosol size distributions of the diffusion (N (d)) and of the flow mode (N (d), 1 l/min) at ¹"110°C. Both D F the diameter of the maximum and the mean diameter of the distributions are given.
This mode is easy to characterize by the temperature at the outlet of the sample boat. It provides very stable aerosol size distributions, long-term stability of all environmental parameters due to low thermal disturbance in the chamber and nearly no surface contamination. Finally, all changes are made slowly enough. It is therefore the preferable mode for long-time continuous operation. In contrast to the diffusion mode with only one operational parameter, the flow mode of the aerosol generator is characterized by two parameters: temperature at the outlet of the sample boat and air throughput. The flow and the temperature control (which are independent of each other) offer the possibility of fast changes in the aerosol concentration and its size distribution. The aerosol size may be controlled by a well-defined air flow through the sample boat. The aerosol size distribution of the so-called “aerosol jet” [14] is therefore shifted to smaller diameters, the distribution has a smaller full-width at half-maximum (fwhm), and the integral concentration is increased. For comparison, Fig. 5 shows normalized aerosol size distributions of the diffusion and of the flow mode for the same temperature of 110°C. The integral aerosol concentration in the radon reference chamber can be varied from 108 to 1013 m~3 using the aerosol generator. Typically, the diffusion mode is used for 108—1010 m~3, whereas the flow mode operates from 109 to
A. Honig et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 525—530
529
Table 2 Environmental parameters of the radon reference chamber of the PTB. The value or the range of the parameter is given
Fig. 6. Isometric plot of the aerosol concentration inside the radon reference chamber as a function of size and time. The aerosol size distributions N(d) are given as a function of time while the air cleaner is operated.
Environmental parameter
Value or range of parameter
Inner volume Temperatue Relative humidity Integral aerosol concentration Mean diameter of aerosol size distribution
21.035(30) m3 !20—40°C 5—95% 106—1013 m~3 30—300 nm
(Model 3071), and their concentration is measured with a Condensation Particle Counter (Model 3022) [18]. The system is capable of measuring the aerosol concentration as a function of the aerodynamic diameter (from 10 nm to 1 lm). Full aerosol size spectrometry can be performed in about 1 min. Thus, dynamic processes can be studied or the stability of the aerosol distribution in the reference chamber surveyed.
5. Conclusions 1013 m~3. The mean diameter of the aerosol size distribution varies from 90 to 300 nm in the diffusion mode and from 30 to 120 nm in the flow mode. The endpoint of the size distribution, that is the maximum diameter, ranges from 90 to 800 nm. For the reduction of the aerosol concentration an air cleaner system [15] is installed inside the radon reference chamber. With this system the integral aerosol concentration can be reduced to a clean-room atmosphere of 106—107 m~3 particles in less than 2 h, cf. Fig. 6. The combination of the highly efficient aerosol generator and the air cleaner results in a variation of the integral aerosol concentration over seven orders of magnitude. This is the basis for the variation of the equilibrium factor [5]. For the measurement of aerosols (size and concentration) a Differential Mobility Particle Sizer (DMPS) or a Scanning Mobility Particle Sizer (SMPS) and a condensation nucleus counter [17] are used. The DMPS/SMPS measures the size distribution of submicrometer aerosols using an electrical mobility detection technique. The particles are classified with an Electrostatic Classifier
The radon reference chamber of the PTB was designed and constructed to provide variable but stable environmental parameters for the calibration of radon and radon progeny devices. This is achieved by the installation of a climate control unit, a highly efficient aerosol generator and an air cleaner system. The stated environmental parameters are defined, controlled and measured within an uncertainty of 0.5—5.0%. This provides ideal conditions for the calibration of both, passive and active radon devices. Moreover, it provides the possibility of varying the equilibrium factor F as well as the attached and unattached fraction of radon progenies, which is a fundamental prerequisite for the calibration of radon progeny devices; up to now, equilibrium factors from 1.00(8)]10~2 to 1.00(3) have been obtained. Since 1991, i.e. since the PTB’s radon reference chamber is in operation, the environmental parameters temperature, humidity and aerosol size distribution have been carefully observed. The results are summarized in Table 2. It is interesting to note that the studies of absorption and transport of
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A. Honig et al. /Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 525—530
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