The design of a radon chamber for the
calibration of radon monitors at the Centre for
Applied Radiation Science and Technology,
Mafikeng, South Africa
MM Radebe
orcid.org/0000-0002-0017-4145
Mini-dissertation accepted in partial fulfillment of the
requirements for the degree
Master of Science in Applied
Radiation Science
at the North-West University
Supervisor:
Prof VM Tshivhase
Graduation ceremony: April 2019
Student number: 23051345
i Acknowledgement
I would like to acknowledge and thank people who made the study to be complete. The completion of this study would be impossible without the involvement of the following people:
Professor Victor Tshivhase, my supervisor, for his patience and mentorship throughout the study. His help in identifying the suitable design of radon chamber and suggestion of proper equipment to be used in the study.
Dr Beaulah Ndlovu, for patience and assistance with the iThemba LABS funding.
Mr Thulani Dlamini, for help in understanding analysis of soil samples with the HPGe detector.
Mr Medgar Mojaki and Mr Tobius Mogale, for the assistance with soil samples and calculation of radionuclide activities. As well as their friendliness and availability in time of need. The CARST department staff for assistance in everything regarding the study.
My family, for comforting words and encouragement. I would like to thank God Almighty, for continual strength.
ii Abstract
The radon chamber was designed at CARST. The purpose of the radon chamber design was solely for the calibration of radon monitors in this study. The radon chamber, rectangular in shape was manufactured with a Perspex material of thickness 6 x 10-3 m and of volume of 0.5 m3. Accommodation of radon monitors and radon sources fits the 0.5 m3 radon chamber. A sealable door of the chamber is valuable in allowing the movement of radon monitors in and out of the chamber.
The Tudor - shaft soil samples were used as radon sources. Experimentally; radon concentration, humidity, temperature and pressure were measured with the AlphaGUARDs.
The HPGe detector was used for nuclide activity measurement of the source soil sample. The 226Ra activity in the Tudor-shaft radon source was measured through the
activity of its progenies 214Pb and 214Bi gamma energy line and computed to be
1394.676 ±11.737 Bq. The radon ingrowth was computed through the 226Ra activity to determine secular equilibrium.
The computed radon ingrowth activities were used as a standard for calibrating the experimentally obtained radon activities from radon monitors (AlphaGUARDs). The calibration factors for the study is the change in difference between the radon monitors and the computed radon ingrowth activities determined as 223.97Bq and 339.83Bq. A hyperbolic relationship between the computed radon ingrowth activities and the two radon monitors was observed.
iii List of abbreviations
CARST Centre of Applied Radiation Science and Technology NIST National Institute of Science and Technology
NRSB National Radon Safety Board
KRISS Korea Research Institute of Standards and Science
US United States
HPGe High Purity Germanium Detector PTB Physikalisch Technische Bundesanstalt
ZnS Zinc sulphide
NIRS National Institute of Radiological Science NRPI National Radiation Protection Institute HPA Health Protection Agency
SRM Standard Reference Material CRM Continuous Radon Monitor NPL National Physical Laboratory
E-PERM Electret Passive Environmental Radon Monitor ICRP International Commission on Radiological Protection
FWHM Full Width at Half Maximum
IAEA International Atomic Energy Agency
AARST American Association of Radon Scientists and Technologists
iv List of figures
Figure 1: Decay chain of Uranium-238 (NRC, 2012) ... 2
Figure 2: The emission of radon from radium in underground rocks or soil to water (Hararah, 2007). ... 5
Figure 3: The NIRS inter-comparison exercise (Janik et al., 2009) ... 11
Figure 4: The HPA inter-comparison of radon detectors (Howarth and Miles, 2002) ... 12
Figure 5: Pylon RN-1025 used in most radon chamber as a provider of radon gas (Division, 2018) ... 13
Figure 6: Stainless steel sealed radium source. Inside view of the sealed stainless steel container for radium source (Liang et al., 2018) ... 13
Figure 7: (1) Depiction of the production of radon source in different containers. (1) Radon sent to the chamber from radium source. (2) Radon absorbed by the cold finger (3) Radon in a liquid nitrogen cooled container (4) Radon in a sealed ampoule (Spring et al., 2006). ... 14
Figure 8: Radon chamber (El-Fiki et al., 1993) ... 16
Figure 9: Digital detector (Canada, 2008). ... 18
Figure 10: Activated charcoal (Canada, 2008). ... 19
Figure 11: Charcoal liquid Scintillation (Canada, 2008). ... 19
Figure 12: 1. A double door lock connected to the solenoid valve. 2. Plexiglas windows glued to the radon box by a silicon glue. 3. Gloves for moving contents inside the chamber (Heidary et al., 2011) ... 22
Figure 13: Steel drum radon chamber (Lehnert et al., 2011). ... 24
Figure 14: A 20 litre plastic bag is filled with radon air from the emanation flask (Lucas and Markun, 1988) ... 26
Figure 16: AlphaGUARD and radon comparison in the steel pot chamber (Irlinger, 2015) . 28 Figure 17: Leak tight glass chamber consisting of two E-PERMS, NIST emanation standard for calibration of (Kotrappa and Stieff, 1994). ... 29
Figure 18: Differences in radon concentration in the chamber due to the exposure of charcoal detector and absence of charcoal detectors in the chamber (Al-Azmi, 2009). ... 34
Figure 19: Comparability of the theoretical and experimental approach for calibration of the chamber (Bogacz et al., 2001). ... 35
Figure 20: Constant radon levels in the chamber by the measurement of the AlphaGUARD. (Kotrappa et al., 2004). ... 37
Figure 21: Measurement of radon decay by radon monitors (Kotrappa et al., 2004 ... 38
Figure 22: REF. Intercomparison of labs (Janik et al., 2009). ... 39
v
Figure 24: Different laboratories radon monitors conformity to the ±10% (Gunning et al.,
2014) ... 41
Figure 25: KRISS radon chamber (Lee et al., 2004) ... 42
Figure 26: Egypt radon chamber and the temperature control system (Mansy et al., 2000). . 44 Figure 27: Home-made diffusion chamber (Mansy et al., 2000). ... 46
Figure 28: Radon chamber build in Syria, Atomic Energy Commission (Shweikani and Raja, 2005). ... 46
Figure 29: Tudor Shaft informal settlement ... 49
Figure 30: A diagram showing the sample preparation procedure: (a) unpacking sample into the mortar, (b) crushing of the sample with pestle, (c) sample crushed into powder, (d) samples transferred into Marinelli beakers and (e) measuring of the samples ... 50
Figure 31: HPGe detector used for radionuclide identification and activity concentration analysis: (a) Gama-spectroscopy software for gamma-analysis ready for run-up (b) internal view of the inserted Marinelli beaker in the detector. ... 51
Figure 32: Efficiency calibration curve for the HPGe detector. ... 53
Figure 34: Uranium standard from IAEA (Organization, 2002) ... 55
Figure 35: Energy calibration for HPGe detector using Eu-152 and Ba-133 source. ... 56
Figure 36: The AlphaGUARD for recording radon activity, temperature and pressure in the chamber box and the environment.(Saphymo, 2012). ... 56
Figure 37: Schematic diagram of ionization chamber of the AlphaGUARD (Saphymo, 2012). ... 57
Figure 38: The AlphaGUARD records measurements of radon activity, relative humidity, air pressure and temperature in cycles of 60 min, in diffusion mode. (Saphymo, 2012). ... 58
Figure 39: Experiment set up for calibrating radon monitors with soil as a radon source in the chamber box. ... 59
Figure 40: 222Rn ingrowth from the 1394.676Bq radium source. ... 63
Figure 41: 222Rn ingrowth from the 1394.676 Bq radium source... 65
Figure 42: The 222Rn activity from the AG2260 and AG2261 AlphaGUARD monitor. ... 67
Figure 43: The reference 222Rn activities comparison to experimental 222Rn activities over 28 days. ... 68
Figure 44: The variation of temperature, air pressure, relative humidity, and 222Rn parameters in the radon chamber as a function of time... 70
vi List of tables
Table 1: Properties of radon (Mohameed, 2013) ... 3
Table 2: Radium from different types of rocks (Awhida, 2017). ... 4
Table 3: Leakage rate of different tubes compared to an emanation source (Honig et al.,
1998). According to the table, Teflon and PVC tube can be considered for use in chamber compared to silicon rubber, since they diffuse or leak less amount of radon. ... 23
Table 4: E-PERMS response to radon emanated by NIST standard source. IV is the initial
voltage, FV is the final voltage of the E-PERM. EP is the radon concentration measured by the E-PERM. B stands for blank (Kotrappa and Stieff, 1994) ... 32
Table 5: Different flow rates for calibration. NIST DC is the emanated radon activity. AG is
the Aphalguard measured radon concentration. BG is the background and FT is the femto tech detector measured radon concentration (Kotrappa and Stieff, 2007). ... 36
Table 6: Comparability of instruments to the NIST standard. Radon monitors recorded radon
concentration (Kotrappa and Stieff, 2007). ... 36
Table 7: Activity concentration of nuclides in IAEA-RGU-1 1987/01/01 as analysed using
efficiency calibration shown in Figure 32. ... 54
Table 8: The 226Ra activity of the source1 and source2 radon sources ... 61 Table 9: Projected ingrowth of 222Rn from the sources ... 62 Table 10: Experimental 222Rn measurements using AlphaGUARDs: AG2260 and AG2261.64 Table 11: Determination of the radon monitor calibration factor. ... 66
Table 12: The variation of 222Rn activity, temperature, air pressure and relative humidity as a
vii Table of contents
List of abbreviations ... iii
List of figures ... iv
Chapter 1: Introduction and problem statement ... 1
1.1 Introduction ... 1
1.1.1 Background study of radon... 1
1.1.2 Radon in the environment... 3
1.1.4 Radon monitor calibration ... 5
1.2 Problem statement ... 6
1.3 Research aim and objectives ... 6
Chapter 2: Literature review ... 7
2.1 Radioactive decay ... 7
2.1.1 Equilibrium ... 8
2.2 Factors for radon chamber design ... 9
2.2.1 Environmental controls and configurations ... 9
2.2.2 Airflow ... 9
2.2.3 Chamber size ... 11
2.3 Radon source and sample measurement ... 12
2.3.1 Radium as a radon source ... 12
2.3.2 Radon source containers and generation ... 13
2.3.3 Radon standard ... 15
2.4 Components of the radon chamber ... 15
2.4.1 Radon emanation sources ... 15
2.4.2 Temperature unit ... 16
2.4.3 Detectors ... 17
2.4.4 Humidity source ... 20
2.5 Constancy within the radon chamber ... 21
viii
2.5.2 Radon leakage ... 22
2.5.3 Radon concentration ... 26
2.6 Calibration ... 27
2.6.1 Reference devices ... 27
2.6.2 Charcoal detector calibration ... 33
2.6.3 Calibration with steady flow... 35
2.6.4 Calibration by a secondary method using a transfer standard. ... 37
2.6.5 Intercomparison ... 38
2.7 Uses of radon chamber ... 41
2.7.1 The KRISS Radon chamber ... 42
2.7.2 NIS Egypt radon calibration chamber ... 44
2.7.3 Atomic Energy Agency of Syria Radon Chamber ... 46
2.8 Biological effects of radon ... 47
Chapter 3: Methodology ... 49
3.1 Sample collection and preparation ... 49
3.2 Experimental procedure ... 50 3.2.1 Gamma spectrometry ... 51 3.2.2 Radon monitor ... 56 3.2.3 Radon chamber ... 57 3.2.4 Radon source ... 58 3.2.5 Radon sampling ... 58 3.3 Data Analysis ... 59
Chapter 4: Results and discussions ... 61
4.1 226Ra activity in sources ... 61
4.2 222Rn ingrowth ... 62
4.3 Calibration of radon monitors... 65
4.4 Environmental parameters ... 69
ix
5.1 Conclusion ... 71
5.2 Recommendations... 71
References ... 72
1 Chapter 1: Introduction and problem statement
1.1 Introduction
It is of great importance to humankind that the knowledge about the impact of radionuclides is known. Experimental methods for the analysis of radionuclides, which produce useful information about the environment mankind lives in, have been countlessly done. One such radionuclide is radon, reported by Xu et al., (2010) to be an excellent tracer for many geophysical studies including predictions of earthquake and volcanic eruptions, air–sea gas exchange processes and assessment of submarine groundwater discharge. Xu et al., (2010) report that radon is used as tracer through the availability of radon monitors. Prior to tracing radon, it is necessary that radon monitors pass through calibration test at a calibration facility with a tested and well-designed radon chamber.
1.1.1 Background study of radon
Radon is a noble radioactive gas, found in nature. 222Rn is a daughter of 226Ra, which comes from 238U by radioactive decay process as seen in Figure 1. Other radionuclides which produce radon are 232Th and 227Ac which result from 235U decay series. 232Th decay chain produces radon isotope 220Rn while 227Ac decay chain produces radon isotope 219Rn. The half-lives of radon isotopes from thorium and actinium decay chains are too short to be considered in many radon studies, for instance 220Rn has half-life of 56 seconds while 219Rn has a half-life of 54 seconds. Most studies as well as this study focus on 222Rn, with a half-life of 3.8 days, which is longer than other radon isotopes (Jacobi and Andre, 1963).
Radon is a radioactive gas, that decays to radon progeny or radon daughter nuclei. The radon progeny or daughter products are electrically charged particles that attach themselves to dust or smoke particles. Radon itself is an inert element, which means it does not attach or bond to air particles because of its low reactivity. When radon progeny is inhaled, it adheres itself to the lung lining, resulting in DNA damage of the lung cells by alpha radiation from 218Po and 214Po (Khan et al., 1990).
2 Figure 1: Decay chain of Uranium-238 (NRC, 2012)
Uranium undergoes several disintegrations to transform into radon which also undergoes several transformations to reach a stable lead-206 as shown in Figure 1.
Radon has interesting properties that are revealed by a decrease in temperature below the freezing point. At these temperatures phosphorescence is observed. Phosphorescence is the emission of light by the radionuclide during radioactive process.
3 When radon freezes, it changes into a yellow substance and as it gradually freezes, it changes colour to orange-red (Rybalkin, 2012).
At normal temperatures the radon is colourless. Radon can be absorbed and undergo withdrawal process from a substance. Materials with absorbance and transmittance abilities can be used for this process. These materials such as charcoal and silica gel for instance, charcoal absorb radon when exposed to it. An increase in temperature or heat up to 350° results in the removal of radon from the charcoal (Rybalkin, 2012).
More properties of radon such as the boiling point, the melting point, vapour pressure and density are presented in Table 1 with its corresponding values (Mohameed, 2013). Table 1: Properties of radon (Mohameed, 2013)
Property Value
Volume at 1Bq of Rn-222 at NTP 1600 m3
Boiling point -61.8 °C
Melting point -71.0 °C
Vapour pressure at -144°C 0.13Pa
Density at NTP 9.96 kg/m3
1.1.2 Radon in the environment
The origins of radon in soil, starts at the decay of uranium to radium, to beget radon. The concentration of radon in soil differs due to the concentration of its parent radionuclide. Radon rocks containing a higher concentration of uranium are dark shale, volcanic rocks and phosphates (Hararah, 2007) and its mineral oxides possess higher uranium concentration (Awhida, 2017).
4 Table 2: Radium from different types of rocks (Awhida, 2017).
Rock
226Ra
(Bq kg-1) Soil
226Ra
(Bq kg-1)
Granite, normal 25 - 80 Till, with normal Ra content 15 - 65 Granite, uranium
rich 100 - 500 Till, with fragments of granite
130 - 125
Sandstone 1 - 60
Till, with fragments of U-rich granite
125 - 360
Limestone 5 - 40 Gravel 30 - 75
Shale 10 - 125 Sand 5 - 35
Black shale 10 -2000 Silt 10 - 50
Alum shale
125 -
4300 Clay 10 - 100
Uranium ore 12000
2.5-105 Soils with fragments of alum shale
175 - 2500
In Table 2, different types of rocks and soil are listed with corresponding radium concentrations. Materials indicate potential soil or rock that will emit a high radon to the atmosphere. Uranium and phosphate tailing can contribute up to 20% of radon in the atmosphere (Awhida, 2017).
The groundwater contains radon due to the movement of water through radon producing rocks consisting of varying levels of uranium and radium content underground. 222Rn is in abundant concentration in underground water as it is trapped and not free as in the surface of the earth in rivers and lakes (Hararah, 2007). The methods of extraction of groundwater for the survival of mankind including boreholes and wells disperse the abundantly resting radon to the surface. This consequentially poses a risk of a silent disease that mostly gets evident in several years due to intake of radon in great quantities unacceptable to the level of dosage the public should be exposed to (Hararah, 2007). Figure 2 shows the movement of radon as it is dispersed to water and rocks.
5 Figure 2: The emission of radon from radium in underground rocks or soil to water (Hararah, 2007).
1.1.4 Radon monitor calibration
A radon chamber is a housing or container constructed to perform radon detector calibration for research purposes. López-Coto et al., (2007) state that calibration is done according to different aspects such as radon concentration, range control, volume, material, radon source, reference radon detector, humidity, temperature control and aerosol concentration. Radon chambers small in size, are used for testing limited numbers of radon monitors, while those big in size are used to test numerous radon monitors simultaneously (Azimi-Garakani, 1992). Chambers differ in design and in sizes. There is a flow-through, walk-in and accumulation type of a radon chamber (Kotrappa and Stieff, 2012). The walk-in type allows the placing of materials to be used and monitored by personnel through their entry with protective clothing. The flow-through type is used for a performance test of radon monitors by certified radon professionals. Accumulation type of a radon chambers are small in size and cost less to build than the flow-through type.
The purpose of a radon chamber is to perform calibration by exposing radon detectors to a steady flow of radon concentration in a chamber under parameters such as relative
6 humidity and temperature (Tin, 2014). It is the alignment of the tested monitor to known reference values.
1.2 Problem statement
A large amount of uranium tailings are produced from mining processes in South Africa (Hnizdo et al., 1997) that have a potential of providing high level of radon gas. People in close proximity to tailings need to monitor the levels of radon, to ensure that they are exposed to acceptable levels of radon by utilising radon monitors (Hnizdo et al., 1997). For the workers to measure the radon dose accurately over time, radon monitors must be calibrated. There are detectors in certain facilities around South Africa (RGM, 2008 ), but there are no known calibration facilities within the country. Therefore, there is a need to design radon chambers, to meet the needs of the country in calibrating radon monitors.
1.3 Research aim and objectives Aim
The aim of this study is to design a radon chamber for the calibration of radon monitors at the Centre for Applied Radiation Science and Technology
Objectives
The objectives of the study are to:
measure the radon activity, relative humidity, air pressure and temperature,
calibrate the radon monitor,
measure the radon ingrowth, and
7 Chapter 2: Literature review
2.1 Radioactive decay
Radioactive decay is the emission of energy from an unstable atom by emitting radiation. Different kinds of unstable atoms emit different types of ionizing radiation including the release of beta, alpha and gamma particles from an unstable atom. This study is based on the unstable atoms or materials that emit alpha particles. Alpha particles are positively charged mono-energetic helium nuclei with the energy particle emissions ranging from 5 to 9 MeV while gamma rays are electromagnetic radiation particles with a high penetration power (Santawamaitre, 2012).
The number of disintegrations per second in radioactive decay process as unstable nuclide decays to a stable nuclide is called the activity. Activity of a spontaneously decaying radionuclide is expressed by equation (1) and is measured in Becquerel (Bq).
𝐴 = −𝑑𝑁
𝑑𝑡 = ƛ𝑁 (1)
Where 𝐴 is the activity of the radionuclide which is equal to the number of the radionuclide represented by 𝑑𝑁 disintegrating in a given time 𝑑𝑡. The fraction , −𝑑𝑁
𝑑𝑡
is proportional is proportional to the number N of radionuclide present at time 𝑡. ƛ for the decay constant and the negative sign show that the radionuclide decrease with the increase of time.
The number of radionuclide present at time t in equation 1 is obtained by solving equation 1 and is expressed as equation (2)
𝑁(𝑡) = 𝑁0𝑒−ƛ𝑡 (2)
Where 𝑁0 is the number of radionuclide at time t.
The half-life of a radionuclide in radioactive decay process is time it taken for one-half of the atoms of a radionuclide to decay and is given by equation (3)
𝑡1 2
= ln 2
8 2.1.1 Equilibrium
Equilibrium in radionuclides studies is the state of quantitative equality between the parent nuclide and daughter nuclide. It is a process for equilibrium to be reached as the parent nuclide decays and the daughter nuclide grows to reach the activity of the parent nuclide. There are two types of equilibrium that radionuclides undergo named secular and transient equilibrium. Chakravarty and Dash (2014) differentiate between secular and transient equilibrium. In secular equilibrium Chakravarty and Dash (2014) state that the parent nuclide half-life is bigger than the daughters’ half-life while, in transient equilibrium the “activity of the ratio of the half-lives of the parent and daughter is less than 10”(Chakravarty and Dash, 2014).
In Figure 15, an example of secular equilibrium between parent and daughter nuclide is shown. The activity of radon is denoted by a purple line while the activity of the parent nuclide, radium is denoted by a blue line. Santawamaitre (2012) in his work shows that secular equilibrium can be reached within 30 days. A red dotted line in Figure 15 shows the point where the parent and daughter nuclides are equal, indicating that secular equilibrium is reached.
Figure 3: The condition of secular equilibrium is obtained in less than 30 days (Santawamaitre, 2012).
9 2.2 Factors for radon chamber design
The factors taken into account during radon chamber design are environmental controls, airflow, chamber size, radon source and monitoring devices.
2.2.1 Environmental controls and configurations
The consistency of radon concentration is a point of emphasis and the control of parameters such as temperature and humidity must be perfectly executed to ensure good performance of radon monitors undergoing testing. Values of temperature and humidity must be kept at a constant range (NRSB, 2012). The, temperature must be around 18°C to 27°C and humidity must be around 20% to 75%. According to AARST (2015) the temperature should be between 20°C and 22°C during the device test.
Some chambers are dynamic, while others are static in terms of airflow. If the chamber is static, then the pathway of the transference of radon concentration to the reference monitor which is, secondary must be implemented and demonstrated to work accordingly (AARST, 2015). Dynamic chambers are chambers that have no constant airflow, therefore the radon concentration is altered. The chamber must be designed in such a way that radon concentration is monitored and controlled by monitoring devices hourly (AARST, 2015).
Based on a design desired by a manufacturer, whether a large chamber or small chamber, all the required instruments to be placed inside the chamber must be able to be accommodated within the chamber space (NRSB, 2012). The radon source must be of known concentration and the activity of the source must be chosen based on the design and operation of the chamber. With the proper design, a source with an activity concentration in the range of 37 kBq to 370 kBq should yield a range of radon chamber activity concentration between 74 Bq/m3 and 1850 Bq/m3 (NRSB, 2012).
2.2.2 Airflow
The airflow is the movement of air in the chamber to maintain a certain control range of parameters such as temperature and pressure of the designed chamber. The airflow in the radon chamber is in two ways namely, static and dynamic (Sciocchetti et al., 1994). The airflow is important in transferring the radon to the internal chambers volume. Sources that provide the air flow are air pumps and fans (Lee et al., 2004).
10 Vargas et al., (2004) used the air pump system for the airflow of radon to the internal volume of the chamber.
Dynamic radon chambers
Dynamic chambers are chambers that have no constant but different airflow. The radon concentration is controllable by the changes of airflow to reach a desired concentration. If the radon concentration within the chamber is more than the desired concentration needed to calibrate the radon monitor or exposing samples, it can be decreased by the calculated withdrawal of air from the chamber (Vargas et al., 2004).
The dynamic chamber can operate in two conditions called dynamic open circuits and dynamic recirculation condition. In the dynamic recirculation condition, the air that is circulated in the part of the chamber is recirculated so that it is reintroduced to the chamber for maintenance of environmental conditions (Sciocchetti et al., 1994). In dynamic open circuit condition, depending on the design of the chamber, the radon in-existent in the chamber is introduced to the chamber by a medium of air transferred to the chamber.
Static radon chambers
Anthony et al., (1995) maintain that static chambers are those non-steady state chambers that do not have circulation of air to keep radon concentration, pressure and humidity at stable conditions. Static chambers include an airtight chamber, that does not circulate air and does not have fans that spread the air inside the chamber for steadiness of environmental parameters Khalid et al., (2014). The Health Protection Agency (HPA) radon chamber is a static type radon chamber because of the continuous discharge of radon in the chamber (Howarth and Miles, 2002).
The manufacturing costs of dynamic chambers, are higher than static chambers. The advantage of a dynamic chamber is the ability to maintain environmental conditions within the chamber, the elimination of significant concentration build-up of the target and the traceability of radon (Vargas et al., 2004). The disadvantage of the dynamic chamber is that the airflow might change the pressure (Sciocchetti et al., 1994).
11 2.2.3 Chamber size
The larger the chamber size, the bigger the space for accommodation of detectors or devices at the same time. A bigger space inside the chamber gives the chamber personnel the ability to expose numerous detectors to steady conditions for calibration and also the evaluation of the detectors performance under the set calibration conditions (López-Coto et al., 2007).
The National Institute of Radiological Science (NIRS) and National Radiation Protection Institute (NRPI) radon chambers are the biggest chambers. The NIRS radon chamber has a volume of 25 m3. By this volume, it accommodates a lot of detectors such as the alpha track detector as seen in Figure 3. The NRPI big radon chamber has the inner volume of 45 m3. This volume is occupied by the humidifier, dehumidifier,
power supply for fans, air cleaner, carnauba wax, aerosol generator, pump and the AlphaGUARD (Howarth and Miles, 2002).
The advantage of a large chamber is the ability to simulate living room conditions; the aerosol concentration and ventilation. Large chambers are quantitatively good for inter-comparison of radon detectors. The HPA radon chamber in Figure 4, with a volume of 48 m3 accommodated thirty eight sets of detectors from different laboratories for inter-comparison of detectors (Howarth and Miles, 2002).
12 Figure 4: The HPA inter-comparison of radon detectors (Howarth and Miles, 2002). In small chambers only few devices can be inter-compared due to the size limit. Although radon concentration can be maintained in small chambers, house room conditions like ventilation, temperature and aerosol concentration cannot be simulated in a small chamber.
2.3 Radon source and sample measurement
Radon source is a provider of radon to the chamber in different concentration depending on the volume that can be emanated from the radon source.
2.3.1 Radium as a radon source
Radium exists in two phases of matter. It exists as a solid and can exist as a liquid when it is transitioned from a liquid. If radium is trapped in soil and rocks, it can be dissolved by hydrochloric acid into a solution that can be absorbed into some materials used as a radon source such as a manganese fibre. Radium source for radon consists of dissolvable radium chloride salt mixed in hydrochloric acid solution and small amount of a water soluble salt named barium chloride (De Felice, 2007). A good example of a radon source is that of radium absorbed onto a material after manganese fibre was soaked with radium for use as a source of radon emanation (Xu et al., 2010).
The most used radon source in radon chambers is the Pylon RN-1025, in Figure 5, which has a certain radium content and radon content. López-Coto et al., (2007) radon chamber used Pylon RN-1025 with radium of activity 23.7 kBq and emanation factor of 100% of radon gas. Venoso et al., (2009) designed the radon chamber with height and volume of 15 cm and 50 cm diameter respectively. The source of radon in the
13 chamber is RN-1025 with 100 kBq of radium, whereby the radium is enclosed in an aluminium container which has a dry radium powder. The radon extracted from the radium source is stored in a glass bulb where it is released to the chambers volume.
Figure 5: Pylon RN-1025 used in most radon chamber as a provider of radon gas (Division, 2018).
2.3.2 Radon source containers and generation
Figure 6(1) shows a radon source made of radium salt inside the stainless steel and the stainless-steel container made in such a way that leakage is prevented by the filter as seen in Figure 6(2).
Figure 6: (1)Stainless steel sealed radium source. (2) Inside view of the sealed stainless steel container for radium source (Liang et al., 2018).
14 The Swiss national metrological institute uses an ampoule for the containment of radon. A dry radium powder source is used for producing radon gas which transforms from a gas phase to a solid phase as shown in Figure 7. Radon permeates the chamber and changes to a liquid phase and is trapped by the cold finger. In this process the activity of radon is measured by the number of alpha counts. Then, the radon is emptied from the chamber by heating the cold finger. The radon is transferred to a liquid nitrogen cooled container, where it is solidified inside the ampoule and sealed. Once sealed in an ampoule, it can now be used for calibration of radon detectors (Spring et al., 2006).
Figure 7: (1) Depiction of the production of radon source in different containers. (1) Radon sent to the chamber from radium source. (2) Radon absorbed by the cold finger (3) Radon in a liquid nitrogen cooled container (4) Radon in a sealed ampoule (Spring et al., 2006).
Dersch and Schötzig (1998) generated radon from liquid and solid radium source. In their experiment, liquid radium standard solution was placed in a sealed glass. The radon of 56 kBq was produced using the liquid radium solution which is transferred to a glass bulb. Radon was generated from solid radium solution, using small tubes for containment of solid radium solution. According to their experiment, solid radium sources produce more activity of radon than liquid sources. The solid radium sources
15 produced radon of activity of 2 MBq and 500 kBq of the total activity which was sent to the glass bulb for transference into the chamber for calibration.
2.3.3 Radon standard
De Felice (2007) defines radon standard as the total activity or the radon concentration of radon produced. The radon standard is stored in a glass bubbler or suitable containers like ampoules and compressed gas cylinders (Picolo, 1996).
NIST is the manufacturer and distributes standard reference materials. SRMs are primary radon standards used for calibration of radon detectors. They are packaged in a polyethylene capsule with radium solution of different activities per polyethylene capsule. The radium in the polyethylene capsule decays to radon that diffuses through the polyethylene walls. The diffused radon is transferred to the chamber or a vessel. This makes it a primary standard when it is transferred to be used to calibrate radon detectors (Volkovitsky, 2006).
2.4 Components of the radon chamber
The components of the radon chamber are materials used for production and measurement of the radon and environmental parameters in the radon chamber. They include detectors, temperature units and radon emanation sources.
2.4.1 Radon emanation sources
Radon is only obtained from radium and measured with various instruments. Shweikani et al., (2005) calibrated solid state nuclear detectors using 1.5 kg of uranium ore (pitchblende) as a source of radon. To measure the accumulation of radon in this chamber from pitchblende, a silicon surface barrier detector was used to get the counts of the decay of radon as well as daughters 218Po, 214Po for a period of time. As chambers are designed differently, radon is emanated differently, in some designs radon emanates from outside the main chamber and is injected into the main chamber. One such chamber is an exposure chamber made by the Radioisotope Unit at the university of Hong Kong (Leung et al., 1992). This exposure chamber has the conditioning chamber and the main chamber. Radon gas, water vapour and aerosol are mixed in the conditioning chamber, after being mixed they are injected into the main chamber where exposure takes place. While in the NIRS walk-in radon chamber, radon gas emanates from ceramic sources of 226Ra located outside of the chamber (Janik et al., 2009).
16 In some chambers, especially of small size, a dry radium, 226 Ra is used without being mixed with other solutions. At the University of Kebangsaan in Malaysia, an airtight chamber was constructed to determine the emanation and activity of radon (Khalid et al., 2014). The radon source used was a mineral called xenotime in quantities of 0.15 kg, 0.25 kg and 0.50 kg. Xenotime has a high concentration of the radioactive radium which decays to radon. In these airtight chambers, radon was generated by exposing the xenotime to air in the chamber that is the same as air outside the chamber. The radon in the chamber was measured by a continuous radon monitor (Khalid et al., 2014). 2.4.2 Temperature unit
The temperature of the chamber must be controlled to meet guidelines set by calibration institutions, such as NRSB. Components used to control temperature are the freezer, heater, condenser and air conditioner. The air conditioner in the NIRS walk-in chambers maintains the temperature and humidity. The air conditioner cools/dehumidifies the chamber to acceptable levels. It does this by a fan which distributes air throughout the chamber. Leung et al., (1992) designed exposure chamber made of an insulator that makes it to work with its own temperature, different from the room temperature. The exposure chamber has a freezer and a heater for temperature control. The chamber has a heating coil which converts the supplied electricity to heat and warming the chamber gas inside to a temperature desired for chamber operation. The gas is passed through the freezer to the chamber for cooling the exposure chamber. Leung et al., (1992) reports that the exposure chamber temperature ranges from 0°C to 60°C.
17 El-Fiki et al., (1993) constructed a radon chamber seen in Figure 8, with an internal volume of 540L and the chambers temperature is controllable in the ranges of 20°C to 90°C through thermal radiation, released by hot objects as a result of charged atomic particles. The hotter the object the greater the temperature. The ultra-thermostat controls the temperature within the radon chamber by pumping water at the desired temperature through copper tubes which radiate the heat necessary to maintain temperature. The temperature sensor is placed at different sides of the chamber for homogeneity test of the chamber.
2.4.3 Detectors
There are two types of radon detectors made for radon detection; passive detectors are detectors that require no electrical power while active detectors require electrical power to function. Passive and active detectors in the chamber are used for measuring radon concentration, monitoring temperature and humidity, for instance, the AlphaGUARD which is an active detector (Janik et al., 2009).
Alpha track detector
The alpha track is a radon detector which has a film in its container and the film gets damaged as radon enters the detector. The damage is caused by alpha particles causing tracks on the film (Tommasino, 1990). The advantages of using alpha tracks are affordability, easily usable and require no electrical power supply. They are not suitable for short time period measurements and do not provide accurate measurement when the concentration of radon is low.
Electret ion chamber
Electret ion chamber consists of ion chamber, paper filter, plunger and electret. The plunger opens and closes the device. When open, the radon gas enters the chamber through a filter. As radon gas inside the chamber decays due to its short-half life, ions and electrons are produced, and while electrons are attracted by electrets, ionization occurs (Organization, 2002). As ionization occurs the electret loses its charges. The voltage on the disk is decreased called the electret charge. The difference in voltage on the disk is determined to get the radon concentration. Electret ion chambers are reusable and results can be given immediately although they are sensitive to gamma radiation
18 and altitudes (Tin, 2014). This detector requires training as it may give errors due to determining humidity and temperature in different environments.
Digital detector
Digital detectors are plugged into the wall and monitor radon continuously and they are based on ion chamber, as shown in Figure 9. Digital detectors give an average radon concentration from two days and onwards (Canada, 2008).
Figure 9: Digital detector (Canada, 2008). Activated Charcoal Absorption
Activated charcoal detector is made of an airtight container, activated charcoal and a filter as seen in Figure 10. Sampling is done by exposing parts of the detector to the air. Activated charcoal inside the detector absorbs the radon in air. After 2 to 7 days the detector is wrapped and sent to the lab for analysis by use of a liquid detector in the lab (Canada, 2008). This detector is affected by environmental conditions and is cheap (Organization, 2002).
19 Figure 10: Activated charcoal (Canada, 2008).
Charcoal liquid Scintillation
Charcoal liquid scintillator seen in Figure 11 is similar to activated charcoal. Radon and radon progeny diffuse into charcoal and get trapped. This is done for a period of 2 to7 days. After these periods, the trapped radon and its progeny are put into plastic vials and collected for radon concentration in the laboratory. At the laboratory, the charcoal is treated with scintillation fluid, followed by scintillation counter for radon analysis (Canada, 2008).
20 Continuous radon monitoring (CRM)
Continuous radon monitoring device is an active monitor which records real time continuous measurements of radon. The use of CRM normally ranges from 48 hours and beyond. Radon is pumped or diffuses into the chamber and results are readily available within short periods (Organization, 2002).
Continuous working level monitor
Continuous working level monitor does sampling by continuous pumping of air into the filter (Organization, 2002). The trapped alpha particles on the filter are counted to measure radon concentration. Models of this device vary in functionality. Some measure environmental conditions such as temperature, barometric pressure and humidity. They are expensive and require frequent calibration (Canada, 2008).
2.4.4 Humidity source
To make the air humid in the chamber, water is used. Water is evaporated and its vapour is spread in the chamber by the air. The humidity system has a heater while a fabric holds the evaporated water which is passed through so that the water vapour is spread in the chamber (Lee et al., 2004).
Aerosol generator
As radon decays to radon progenies in the chamber, absorption takes place. Radon is non-sticky and is measured by alpha detectors. The concentration of radon progenies is measured by the particle sizes which they are absorbed into. To achieve this, particles are generated within the chamber, for the attachment and measurement of radon progeny. Aerosol particles from a material, are generated in the chamber by vapour condensation at a suitable temperature for vapour condensation (Honig et al., 1998). Before radon progenies attach themselves to aerosol particles, they are in an equilibrium state with radon. The radon activity and radon progeny activity are in transient equilibrium (Moore and Kearfott, 2005).
The production of aerosol particle inside the chamber is by a material called carnauba wax (Paul and Keyser, 1996). With carnauba wax, aerosol particles of different sizes resulting in different concentrations are determined and obtained by the use of computer equipped with aerosol size spectrometry software. Aerosol particles are measured from the range of 10 nm to 1µm aerodynamic diameter. Paul and Keyser (1996) state that the
21 reason why carnauba wax is chosen is because of the uniqueness of its physical and chemical properties such as shape, size, density and acidic value. Furthermore, carnauba wax is an ideal material choice for standardized reference aerosol from an experimental and theoretical point of view (Honig et al., 1998).
For generating aerosol particles, carnauba wax is placed in a sample boat, which is elliptical in shape. The sample boat is then connected to a condensation volume and heated by an insulated wire surrounding it (Honig et al., 1998). A vapour is then formed with critical nuclei and aerosol.
An example of a chamber with aerosol generator, determining radon progeny concentration is the exposure chamber built at Radioisotope unit in the University of Hong Kong (Leung et al., 1992). For generating aerosol, a high output atomizer (Model 3076, TSI Inc, USA) was used together with a diffusion dryer, which is able to maintain a constant aerosol size and concentration. The atomizer has a pump which adjusts the concentration of aerosol to a desired level, using the connected computer. Then, for counting aerosol particle concentration, a condensation nuclei counter is used (Model 3760, TSI).
2.5 Constancy within the radon chamber
The constancy within the chamber is made possible by an airtight chamber that prevents radon from leaking and keeps other environmental factors such as temperature and air pressure constant.
2.5.1 Homogeneity
Homogeneity is the constant radon concentration in the chamber. This is achieved by maintaining minimal leakages of radon for a stable radon concentration everywhere in the chamber. Heidary et al., (2011), Shweikani and Raja (2005) used the same method to test for radon homogeneity in the chamber. They placed CR-39 detectors at different positions within the chamber. They exposed them to a constant radon concentration for a certain period of time. Then, CR-39 detectors were etched in 6.25 N NaOH at 70°C for 6 hours. To find the radon concentration similarity amongst the CR-39 plastic detectors, counting of tracks by optical microscope to find the track density which is proportional to radon concentration was done. This resulted in same track counts in all CR-39 detectors, implying that there’s a homogenous radon distribution in the chamber.
22 2.5.2 Radon leakage
Radon leakage is the escape of radon from the inside of the chamber. This can be caused by poor design of the radon chamber and factors such as uncontrolled airflow, personnel entry especially in large chambers to change or insert samples.
Figure 12: 1. A double door lock connected to the solenoid valve. 2. Plexiglas windows glued to the radon box by a silicon glue. 3. Gloves for moving contents inside the chamber (Heidary et al., 2011).
Solutions to prevent radon leakage from many radon chamber designers have always being based on tightening the chamber and its joints with tightening material such as O-rings and silicon. Some chambers are sealed with silicon to be airtight, so radon escape possibility is minimal. Hosoda et al., (2009) used the NIRS radon chamber connected to the stainless-steel tank and radon measuring system. To prevent radon leak, he tightly covered all the joint connections. The materials used for tightness of joint connection used by Hosoda et al., (2009) are glycerine and silicon rubber. Heidary et al., (2011) also constructed radon chamber using tightening material called O-rings and a silicon glue. The silicon glue was used to stick the Plexiglas which makes the contents inside the chamber viewable to the inspector, to the radon chamber structure. The double door lock also plays a part in prevention of radon leakage, it is attached to a solenoid valve seen in Figure 12. A solenoid valve in this chamber carries the radon to and fro. It functions as a connection of the chamber and the vacuum pump in conveying radon. When radon is lost in the chamber, it goes to the vacuum pump. The
23 vacuum pump, pumps back the radon to the chamber, to keep radon levels constant, so that there’s no radon loss in the chamber.
Radon loss
Other materials used for the radon chamber can cause a loss of radon concentration. Therefore, if radon concentration is to be kept at a particular rate or constant level, the material chosen for the build-up of the radon chamber must never tamper with radon inside the chamber.
One of the most used materials is stainless steel. The stainless steel does not absorb a large amount of radon compared to other materials when used as a tube or a covering for walls of the chamber. Honig et al.,(1998) sheds light about various material absorbance and exhalation of radon by experimentation. Different materials used as tubes were connected to the radon reference chamber which had a radon emanation source. The results of the experiment showed how various materials absorbed and exhaled radon. So these materials add to the radon loss in the chamber. The materials used as tubes include silicon rubber, norprene, Teflon, tygon and PVC. The radon leakage as seen in Table 3, is determined by calculating the radon concentration with respect to time.
Table 3: Leakage rate of different tubes compared to an emanation source (Honig et al., 1998). According to the table, Teflon and PVC tube can be considered for use in chamber compared to silicon rubber, since they diffuse or leak less amount of radon.
Type of tube
Tube diameter and
thickness Leakage rate L (mm x mm) (Bq m-3 h-1)
Open emanation source 8 x 2 79.2 (8)
Silicon rubber 8 x 3 72.6 (12) Silicon rubber 9.6 1.6 73.4 (10) Norprene 10 x 2 30.6 (8) Tygon 5.3 x 1.5 16.4 (6) Polyethylene 8 x 2 11.8 (4) PVC 9 x 4 6.36 (12) High pressure 10 x 2 6.13 (12) Gas burner 8 x 3 4.13 (16) PVC 10 x 1 2.18 (10) Teflon 0.55 (8)
24 The steel drum radon chamber in Figure 13 was built with the aim of preventing the loss of radon (Lehnert et al., 2011). The steel drum radon chamber lid was sealed with rubber at one end while the other end was shut. The chamber consisted of wood with the radon source on top of the wood and radon monitors below the wood. When the accumulated radon activity in the chambers was measured by radon monitors, it was found that some radon was lost (Lehnert et al., 2009). To stop the radon loss, a duct- tape was used to provide extra sealing to the steel radon drum chamber. The radon monitor used for the chamber is the charcoal canister, which absorbs radon from air. So, the amount of radon absorbed by the charcoal canisters must be known in order to know the amount of radon that is diffused outside the chamber. Therefore, it will be easy to know whether the tape is a good sealer for the chamber or not.
Figure 13: Steel drum radon chamber (Lehnert et al., 2011). Radon leakage determination
Prediction or mathematical modelling of equations of radon activity and radon leakage must be developed for evaluating radon activity to meet and verify the expected output. Moore and Kearfott (2005) state that in an airtight chamber, the activity of radon is calculated by:
25 where, the radon activity is 𝐴(𝑡)𝑅𝑛 and 𝐴𝑅𝑎 is the radium source activity and 𝜆𝑅𝑛 is the
decay constant of radon.
Moore and Kearfott (2005) further state that when the chamber is not airtight, the radon escapes the chamber. The activity of radon and radon leakage are calculated as:
𝐴(𝑡)𝑅𝑛= 𝐴𝑅𝑎
𝜆𝑅𝑛+𝜆𝑙𝑒𝑎𝑘[1 − 𝑒
−(𝜆𝑅𝑛+𝜆𝑙𝑒𝑎𝑘)𝑡 ] (5)
where λleak is the leak constant. In an airtight chamber, if the radon emanation source is
removed, the radon concentration inside the chamber decreases. The decrease in radon concentration is calculated by:
𝐴(𝑡)𝑅𝑛 = 𝐴(𝑡0)𝑅𝑛𝑒−(𝜆𝑅𝑛)𝑡, (6)
where 𝐴(𝑡)𝑅𝑛 is the radon activity at time t, 𝐴(𝑡0)𝑅𝑛 is the initial radon concentration.
A radon leaking chamber’s radon activity can be calculated by equation (7), when the radon emanation source is removed:
𝐴(𝑡) = 𝐴(𝑡0)𝑅𝑛𝑒−(𝜆𝑅𝑛+𝜆𝑙𝑒𝑎𝑘)𝑡. (7)
Radon leak test
Khalid et al., (2014) built small airtight chamber with silicon used to join parts of the chamber to be airtight and tested the airtight chamber with tissue for leakage. The airtight chamber was filled with blue-dyed water inside, and covered with tissue outside to test for leakage. Results showed that the chamber was airtight because the tissue was not wet at all.
Lucas and Markun (1988) designed plastic radon chamber for calibration of radon monitors which was filled with a specific amount of radon from radium source. To measure the radon leak from the plastic, it was placed in a tank and enclosed. The amount of radon that escaped from the plastic was trapped in the tank. By simply determining the difference between the radon in the plastic bag and the radon in the tank, the radon leakage amount was known. Therefore, the strength or weakness of the airtightness of the plastic bag was known. In Figure14, the radon air is pumped in the plastic bag from the emanation flask, as the bag is filled some of the radon is absorbed by the plastic.
26 Figure 14: A 20 litre plastic bag is filled with radon air from the emanation flask (Lucas and Markun, 1988).
The test for leakage resulted in negligible loss of less than 0.02 percent per day radon leakage into the tank. This clearly gives an indication that the plastic bag can be used for calibration of radon monitors. By inspection, the design of the plastic bag chamber is affordable and cheaper than the complex radon chamber design.
2.5.3 Radon concentration
The radon concentration in the chamber is determined by the activity concentration of the radium. The airflow, radon leakages are controlled to reach a steady-state concentration of radon. The airflow has to be constant and no radon has to escape the chamber (Fisenne and Cavallo, 1999). The devices such as ZnS scintillation counter (lucas cell counter), nuclear solid state, germanium detector and alpha guard are frequently used for measurement. The lucas cell counter has a filter where radon progenies stick and radon passes, the filter is removed and radon progenies are counted. The alpha guard neglects radon progenies and counts/measures radon only. Some chambers like the environmentally controlled radon chamber built by Wharton Jr (1991), can control radon by injecting radon in the chamber, when radon concentration drops. The radon concentration drop is detected by the computer software. This computer system is also responsible for the injection of 222Rn into the chamber.
In a chamber where continuous radon monitors are exposed to radon for a long period of time, the radon concentration varies as a function of time. The radon concentration
27 is calculated and obtained at different radon exposure times. Mansy et al., (2000) states that the radon concentration can be calculated at any given time by:
𝑅𝑆 =𝑓𝐴𝑅𝑎 𝑒−𝜆𝑅𝑎𝑇𝐷(1−𝑒−𝜆𝑅𝑛𝑇𝐴)
𝑉 (8)
where 𝑓 is 222Rn emanation fraction from the source, 𝐴
𝑅𝑎 is the for 226Rn source
activity, 𝜆𝑅𝑎 is the 226Ra decay constant, 𝜆𝑅𝑛 is the 222Rn decay constant, 𝑇𝐷 is the time interval from the certified 226Ra activity reference time tr to the start time t = 0 of the accumulation period (𝑇𝐷 = 𝑡0 + 𝑡𝑟), 𝑇𝐴 is the time interval for the total duration of accumulation and 𝑉 is the air volume inside the chamber that is corrected for volume
stp
V at standard pressure and temperature (1013.25 mbar; 273.15 K):
𝑉𝑠𝑡𝑝 =𝑉
𝑝
273.151031.25
𝑇 (9)where p is the pressure in mbar and temperature (T) is in Kelvin for the air volume inside the accumulation chamber.
2.6 Calibration
Calibration of detectors is a requirement that must be performed periodically to avoid incorrect measurements of radon in the evaluated field of choice or environment. It is very important to keep a steady flow of radon concentration during calibration, for quality calibration results (Kim et al., 2013). This process of calibration is used to get quality results from the calibration chamber. Detectors and samples exposed, include charcoal canisters, alpha track detectors and radon progeny integrating samplers. When analysed, some give results quickly while others take time to get results. The working level monitor and radon monitors provide quick results, and are useful in establishing calibration factors in the calibration chamber. During calibration measurements they are made in different exposure times to get conversion factors used to obtain radon concentration results.
2.6.1 Reference devices
Reference devices are useful for the calibration of other radon monitors. They serve as a standard for comparison of the radon concentration when calibrating other radon
28 detectors. Irlinger (2015) experiment is an indicator of how reference devices can be compared for the use in other chambers as calibrated devices.
One way of doing comparison to get the difference or calibration factors is by exposing them in an enclosed chamber to a radon concentration. The difference can then be used to estimate the real value of the radon concentration for exact radon measurement according to a calibrated device.
Irlinger (2015) used a steel pot chamber with pitchblende as the radioactive source for radon. The AlphaGUARD and Rad7 were the reference devices being compared in the steel pot. The idea behind the comparison is that since reference devices are exposed to a radon in an airtight chamber, a partially correct suggestion is that they must record similar measurements of radon concentration. When this is done, the quantitative similarity can be used as one radon device in another chamber as a reference device. The similarity or constant difference obtained becomes a clue for the traceability of one radon device to multiple radon devices undergoing calibration tests. The similarity is seen in the results of the steel pot chamber comparison of two devices as seen in Figure 16.
Figure 15: AlphaGUARD and radon comparison in the steel pot chamber (Irlinger, 2015).
29 The difference between the AlphaGUARD and Rad7 lies in the radioactive nuclide detection. The AlphaGUARD detects radon from pitchblende while the Rad7 detects both the radon and the thoron. The detection capabilities of both the radon devices are expressed quantitatively in Figure 15.
Irlinger (2015) found that the AlphaGUARD and Rad7 average radon concentration mean varies by a small percentage (1.4%). A conclusion is reached that there is no need for the determination of calibration factors, which helps in estimating the real value of the Rad7 compared to the calibrated AlphaGUARD. This means that both the AlphaGUARD and the Rad7 need calibration factors, and if they are used to measure environmental radon they will both give the same values. One can argue that for accuracy sake, the calibration factors are needed for precise measurement no matter how negligible the uncertainties of the radon devices may be.
Kotrappa and Stieff (1994) designed a calibration chamber for the calibration of E-PERM AND CRMs. NIST sources were used for the radon emanation. Theoretical and experimental values were compared and found to be in a certain range. Meaning that they can be traceable to the certified radon source from NIST. NIST is an organization that produces standard reference material of radon and other radioactive nuclides. E-PERM predicted theoretical results is 5% (Kotrappa and Stieff, 1994).
In the accumulation chamber, by the use of the NIST radon source, theoretical radon concentration over time can be calculated. The calculated theoretical radon concentration must be comparable to the experimental radon concentration obtained by radon monitors.
Figure 16: Leak tight glass chamber consisting of two E-PERMS, NIST emanation standard for calibration of (Kotrappa and Stieff, 1994).
30 Kotrappa and Stieff (1994) designed a radon chamber with a leak-tight lid for calibration of E-PERM. As seen in Figure 17 the glass chamber consists of the NIST radon source standard, two E-PERM and temperature strip. The radon emanated by the NIST radon source is used in the chamber for calibration of two E-PERM in an accumulation glass chamber. Its activity over an accumulated time is compared with the activity of radon measured by the E-PERM. Therefore, calibration factors can be determined and used for traceability of E-PERM to NIST radon emanation source standard. An equation for determining the radon concentration in the glass chamber at any set time is necessary. The equation (10) that Kotrappa and Stieff (1994) used is similar to the one used by Mansy et al., (2000).
A A Rn Ra Rn V T fA A (1exp( )) (10)
The emanation rate of NIST radon source is represented by f. ARN stands for radon
concentration and ARA stands for the radium of the NIST emanation standard source.
VA stands for glass chamber volume, the λ is the decay constant for radon. TA is the
time taken to measure radon concentration at a particular time. Table 4 shows how SST E-PERM can be traceable to NIST standard. This is done by dividing the measured radon activity of E-PERM by the radon activity emanated by the NIST standard radon source. The results of the division give the factor that can be used for tracing SST E-PERM to NIST standard source.
32 Table 4: E-PERMS response to radon emanated by NIST standard source. IV is the initial voltage, FV is the final voltage of the E-PERM. EP is the radon concentration measured by the E-PERM. B stands for blank (Kotrappa and Stieff, 1994).
Source NS NS EI IV FV2 EP EP --B (EP -- B) / Av.
(Ra Bq) (Bq.m-3) (Bq.m-3) NS ratio CP5 4.2602 232 SK6439 573 523 250 217 0.934 CP5 4.2602 232 SK4701 573 525 239 206 0.886 0.910 CP6 4.5460 248 SK4697 447 390 301 268 1.079 CP6 4.5460 248 SK1126 462 409 276 243 0.981 1.030 CP7 4.7121 257 SI3944 595 539 281 248 0.966 CP7 4.7121 257 SK3612 598 540 292 259 1.008 0.987 CP27 4.7532 259 SK1301 557 499 296 263 1.014 CP27 4.7532 259 SK4734 549 487 319 286 1.103 1.059 CP28 4.6167 252 SJ6073 350 288 340 307 1.219 CP28 4.6167 252 SK4733 368 315 285 252 1.000 1.110 CP30 3.6889 201 SK3574 356 307 262 229 1.139 CP30 3.6889 201 SK3786 358 311 250 217 1.080 1.109 CP31 4.7304 258 SK1255 439 381 307 274 1.063 CP31 4.7304 258 SK4722 443 388 290 257 0.994 1.028 CP32 4.9893 272 SK6435 367 313 291 258 0.947 CP32 4.9893 272 SK6412 375 319 302 269 0.988 0.968 CP33 4.7321 258 SK6446 274 230 239 206 0.799 CP33 4.7321 258 SK1118 262 211 283 250 0.968 0.884 CP35 5.0071 273 SK1217 162 110 299 266 0.975 CP35 5.0071 273 SK4696 176 130 260 227 0.832 0.903 CP36 4.7704 260 SK3601 354 296 316 283 1.087 CP36 4.7704 260 SK3533 339 284 300 267 1.024 1.055 CP37 5.0289 274 SK6405 266 215 283 250 0.910 CP37 5.0289 274 SK4715 254 200 302 269 0.981 0.945 CP38 5.6069 316 SJ3779 573 503 361 328 1.038 CP38 5.6069 316 SK3728 575 505 631 328 1.037 1.037 CP40 4.9055 268 SK6434 745 688 274 241 0.901 CP40 4.9055 268 SK6415 744 677 327 294 1.100 1.001 CP41 5.1324 280 SK4692 746 690 269 236 0.842 CP41 5.1324 280 SK6422 748 689 285 252 0.898 0.870 CP42 5.2977 289 SK1253 749 690 284 251 0.870 CP42 5.2977 289 SK6448 735 675 291 258 0.893 0.881 CP44 5.3052 289 SI3986 230 175 311 278 0.960 CP44 5.3052 289 SK3768 229 171 329 296 1.024 0.992 blank 0.0000 0 SI1457 664 653 33 GRD A 0.986 blank 0.0000 0 SI1419 655 644 33 % STD 9.299
33 As seen in Table 4 two SST E-PERMs are used in one glass chamber under the heading, source. According to Table 4 E-PERM can be traced to the NIST standards. For instance, if CP5 E-PERM can be traced to NIST by dividing its measured radon activity with 0.998, which is the factor for accurate tracing to NIST standard.
2.6.2 Charcoal detector calibration
The charcoal detector takes up radon from the air. It absorbs it causing a decrease of radon concentration in the air or chamber. The charcoal detector consists of activated carbon that absorbs radon (Gervino et al., 2004). One may ask if a lot of charcoal detectors can be placed in homes or mining areas of high concentration to reduce the radon which leads to cancer.
An example of how radon concentration can be reduced by the charcoal detectors in the built chamber is graphically displayed in Figure 18. Al-Azmi (2009) distinguished the radon concentration variability by the radon concentration measured at the availability and non-availability of charcoal detector in the chamber for calibration of detectors. It is seen in Figure 18, where curve 1 and 3 show the radon concentration when there’s no charcoal detector. Curves 2 and 4 show when the charcoal detector is added to the chamber.
34 Figure 17: Differences in radon concentration in the chamber due to the exposure of charcoal detector and absence of charcoal detectors in the chamber (Al-Azmi, 2009). Both Al-Azmi (2009) and Bogacz et al., (2001) showed models of calibrating charcoal detectors in small chambers. Al-Azmi (2009) built chamber has a volume of 101 L , while Bogacz et al., (2001) used a 211dm3 chamber. For calibration of charcoal
detectors, the experimental approach has to be tested against the theoretical approach. So the calibration factor can be used for traceability, to a certified radon source or standard. Bogacz et al., (2001) observed a good agreement between theoretical activity concentrations values and those measured experimentally using the AlphaGUARD. This comparability is seen by the illustration of the linear fit graphs in Figure 19. From Figure 19, a constant difference is seen of the radon concentration between the theoretical equation implementation and the experimental measurement by the AlphaGUARD when charcoal detectors are exposed.
35 Figure 18: Comparability of the theoretical and experimental approach for calibration of the chamber (Bogacz et al., 2001).
2.6.3 Calibration with steady flow
Calibration with a steady flow method takes less time than the accumulation method. Different flow rates of air during the calibration process are the reason for longer periods taken by the steady flow method. The different flow rates of air to the chamber play a role in affecting the chambers radon concentration. The higher the flow rate, the smaller the radon concentration detected by the radon monitor or emanated by the NIST radon source (Kotrappa and Stieff, 2007).
The calibration process starts by placing different or several radon monitors into the chamber for exposure to a steady flow of radon air generated from the NIST standard source. After exposure having used a certain flow rate, measurements are recorded of the radon concentration. In Table 5, we see the different flow rates used to calibrate AlphaGUARD and femto-tech monitor. Their comparability to NIST radon standard source is of a good ratio.
