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DESIGN AND CONSTRUCTION OF A PARALLEL PLATE IONIZATION 
CHAMBER FOR DOSIMETRY IN CONVENTIONAL RADIOGRAPHY 
A THESIS SUBMITTED TO THE 
 
DEPARTMENT OF MEDICAL PHYSICS 
 
SCHOOL OF NUCLEAR AND ALLIED SCIENCES 
UNIVERSITY OF GHANA, LEGON 
BY 
ALI MORROW FATORMAH 
(10703745) 
BSc. Kwame Nkrumah University of Science and Technology, 2014 
IN PARTIAL FULFULMENT OF THE REQUIREMENT FOR THE AWARD OF 
MASTER OF PHILOSOPHY DEGREE 
 IN  
MEDICAL PHYSICS 
 
OCTOBER, 2020 
 
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DECLARATION 
I hereby declare that I have wholly undertaken this project under the supervision of Dr. S. 
N. A Tagoe and Dr. F. Hasford and no other part or whole of this work has been 
submitted for any degree elsewhere in another university, References of other peoples’ 
work have been duly acknowledged.                       
                           ……06/08/2021………..  
      Ali Morrow Fatormah                                                    Date                                                             
               (Student) 
 
     ……19/08/2021………. 
     Dr. Samuel Nii Adu Tagoe                                                     Date                                                             
          (Principal Supervisor)                                       
 
  
               ……19/08/2021………… 
Dr Francis Hasford                                                                         Date                                                             
(Co-Supervisor)   
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ABSTRACT 
The main objective of every radiodiagnostic procudure is to produce an informative 
image with minimum radiation exposure to  patients. To be able to minimize dose to 
patients, and ensure image quality, there must be a regular quality control on the entire X-
ray systems, which involves routine measurement of exposure and exposure rate.  The 
most employed dosimeters for the adjustment and control measurements is the parallel 
plate ionization chamber. It is less intrinsic to energy dependence, hence mostly 
recommended for low dose rate measurement. It is in view of this, that a portable, and 
less expensive detector (parallel plate ionization chamber) has been designed and 
constructed for dosimetry in diagnostic radiography.The chamber comprises of a body 
made of Perspex (1.7 mg/cm²), a bias electrode made of copper plate, a measuring 
electrode made of an aluminium plate, guard rings made of an aluminium plate an 
entrance window made of a paper coated with graphite (shading the paper with HB pencil 
until the paper became electrically conductive) with the uncoated side pasted to a piece of 
unexposed developed radiographic film. The chamber has a sensitive volume of 2.8 cc 
which was vented to the environment. The operational bias voltage of the constructed 
ionization chamber was found to range from 200 V – 400 V. Two different conceptual 
designs  were developed and evaluated. The concept with the highest overall utility value 
was selected and developed. The completed chamber was subjected to several  
performance characteristic and quality control tests: energy dependence, response 
reproducibility and constancy, angular dependence, response linearity and leakage 
characteristics. The chamber was cross calibrated against diagnostic multimeter (Piranha) 
with traceability to a secondary standard dosimetry laboratory (Swedac. Ackredictering, 
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Sweden) and found to have a calibration coefficient (NK) of 1.7 x10
9  mGy/A. Beam 
quality correction factor for chamber could be expressed with a fourth degree polynomial 
equation in terms of HVL (mmAl) using 100 kVp and 20 mAs  (200 mA) as the reference 
exposure parameters.    Response reproducibility and constancy, angular dependence, 
response linearity were all within the International Electrotechnical Commission (IEC) 
61674 stipulated limit. A maximum deviation of 8.6%   was observed at 90O clockwise of 
the gantry angle. This was as a result of cable leakage. A parallel plate ionization 
chamber has successfully been designed and developed and is applicable in a range of 50 
-130 kVp.  
 
 
 
 
 
 
 
 
 
 
 
 
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DEDICATION 
This project, is first and foremost dedicated to the Almighty Allah who has been my 
strength and source of knowledge throughout the entire program. To my late father, Kofi 
Ali Fatormah, though you are gone, your counsel on the ethos of education still lives. 
May the mercy of Allah be with you. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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ACKNOWLEDGEMENTS 
 My profound and sincere gratitude goes to my able supervisors, Dr. Samuel Nii Adu 
Tagoe and Dr. Francis Hasford for their excellent supervisory role and immense technical 
contribution towards the success of this project. 
Special appreciation to Ghana Education Trust Fund (GETFund) for offering me full 
scholarship for this study. 
Special acknowledgement goes to my lovely wife, Hawaawu, my daughter Zaye-nabe, 
and my two sons Baba and Ziad for their understanding during what has been undeniably 
the uttermost fortitude of my physical absence during my study. 
 Again, I would like to thank the staff of the Medical Physics Unit of the National 
Radiotherapy Oncology and Nuclear Medicine Centre, (NRONMC), especially Mr. 
Francis Doughan for his assistance. 
 My heartfelt gratitude goes to Dr. Theresa Dery and Mr. Ernest Eduful, I am indeed very 
grateful to you for your assistance. 
Finally, my greatest thank goes to the Almighty Allah for his mercy, guidance and 
protection in the course of this study. 
 
 
 
 
 
 
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TABLE OF CONTENT 
 
DECLARATION ................................................................................................................. i 
ABSTRACT ........................................................................................................................ ii 
DEDICATION ................................................................................................................... iv 
ACKNOWLEDGEMENTS ................................................................................................ v 
TABLE OF CONTENT ..................................................................................................... vi 
TABLE OF FIGURES ...................................................................................................... xii 
LIST OF ABBREVIATIONS .......................................................................................... xvi 
CHAPTER ONE ................................................................................................................. 1 
INTRODUCTION .............................................................................................................. 1 
1.1 Background ................................................................................................................... 1 
1.2.   Statement of the problem ........................................................................................... 3 
1.3   Aims and Objectives ................................................................................................... 3 
1.4   Scope and delimitation ................................................................................................ 4 
1.5   Thesis organization ..................................................................................................... 5 
CHAPTER TWO ................................................................................................................ 6 
LITERATURE REVIEW ................................................................................................... 6 
2.1   Dosimetry in diagnostic radiology .............................................................................. 6 
2.2   Radiation dosimeters ................................................................................................... 8 
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2.3   Gas filled detectors. .................................................................................................... 9 
2.3.1   Principle of operation of gas filled detectors ........................................................... 9 
2.3.2 Characteristics of different regions across a gas-filled detector. ............................. 11 
2.3.2.1 Recombination Region.......................................................................................... 11 
2.3.2.2   Ionization Chamber Region ................................................................................ 11 
2.3.2.3   Proportional Region ............................................................................................ 11 
2.3.2.4   Limited Proportional Region .............................................................................. 12 
2.3.2.5   Geiger-Mueller (GM) Region ............................................................................. 12 
2.3.2.6   Continuous Discharge Region ............................................................................ 13 
2.4   Ionization chambers .................................................................................................. 13 
2.4.1 Types of Ionization chambers used in diagnostic radiology .................................... 14 
2.4.1.1   Free-Air ionization chamber ............................................................................... 14 
2.4.1.2 Cavity ionization chambers................................................................................... 14 
2.5   Parallel plate ionization chamber. ............................................................................. 15 
2.6   Physics of parallel plate ionization chambers ........................................................... 18 
2.7   Components of a typical parallel plate ionization chamber. ..................................... 21 
2.7.1   Measuring Assembly. ............................................................................................ 21 
2.7.1.1 Cables and Connectors. ......................................................................................... 22 
2.7.2. Chamber Assembly ................................................................................................. 25 
2.7.2.1 The Body ............................................................................................................... 27 
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2.7.2.2 Guard..................................................................................................................... 28 
2.7.2.3   Circuitry of a simple ionization chamber ............................................................ 29 
2.8 Factors Affecting the Measurement of Ionization output ........................................... 30 
2.8.1 Recombination losses............................................................................................... 30 
2.8.2   Leakage Current ..................................................................................................... 34 
2.9   Performance Characteristics of Parallel Plate Ionization Chamber .......................... 35 
2.9.1 Angular dependence ................................................................................................. 35 
2.9.2 Energy dependence .................................................................................................. 36 
2.9.3 Sensitivity ................................................................................................................ 37 
2.9.4 Current Linearity ...................................................................................................... 37 
2.9.5   Reproducibility (Short term stability) .................................................................... 38 
2.10   Ionization chamber calibration ............................................................................... 38 
2.10.1   Cross calibration .................................................................................................. 41 
CHAPTER THREE .......................................................................................................... 42 
MATERIALS ABD METHODS ...................................................................................... 42 
3.1   Introduction ............................................................................................................... 42 
.3.2   Materials used .......................................................................................................... 42 
3.3.1 Acuity Conventional Radiotherapy Simulator ......................................................... 43 
3.3.2 PTW Unidos Electrometer ....................................................................................... 44 
3.3.3   Digital Thermometer .............................................................................................. 45 
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3.3.5 Piranha Diagnostic Multimeter ................................................................................ 47 
3.4   Conceptual Design .................................................................................................... 48 
3.5   Assembling processes ............................................................................................... 50 
3.6   Evaluation of constructed Parallel plate ionization chamber performance .............. 52 
characteristics .................................................................................................................... 52 
3.6.1 Chamber response to Bias voltage ........................................................................... 52 
3.6.2 Angular/Directional dependence ............................................................................. 53 
3.6.3   Pre-irradiation current leakage ............................................................................... 54 
3.6.4 Current Linearity test ............................................................................................... 55 
3.6.6    Energy dependence (Half Value Layer- HVL) ..................................................... 55 
3.6.7 Short term stability test ............................................................................................ 56 
3.6.8 Medium term stability .............................................................................................. 57 
3.6.9 Chamber calibration ................................................................................................. 57 
CHAPTER FOUR ............................................................................................................. 59 
RESULTS AND DISCUSSIONS ..................................................................................... 59 
4.1   Introduction ............................................................................................................... 59 
4.2 Characteristics of the constructed ionization chamber ............................................... 59 
4.3 Preliminary test of the parallel plate ionization chamber ........................................... 60 
4.3.1 Bias voltage of the chamber ..................................................................................... 60 
4.3.1.1   Polarity effect ...................................................................................................... 62 
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4.3.1.2 Ion collection efficiency and recombination ......................................................... 63 
4.3.1.3 Saturation curve .................................................................................................... 64 
4.3.2 Stability check .......................................................................................................... 65 
4.3.2.1 Short term stability ................................................................................................ 65 
4.3.2.2   Medium term stability ......................................................................................... 66 
4.3.3 Pre-irradiation leakage current ................................................................................. 67 
4.3.4 Angular dependency ................................................................................................ 68 
4.3.5 Chamber response linearity ...................................................................................... 69 
4.3.6   Energy dependence ................................................................................................ 70 
4.3.7 Chamber HVL Measurements ................................................................................. 72 
4.3.8:  Beam quality correction factor KQ ......................................................................... 79 
4.3.9   Calibration coefficient of the constructed chamber ............................................... 81 
4.4 Limitations .................................................................................................................. 82 
CHAPTER FIVE .............................................................................................................. 84 
CONCLUSIONS AND RECOMMENDATIONS ........................................................... 84 
5.1   Conclusions ............................................................................................................... 84 
5.2 Recommendations ....................................................................................................... 85 
5.2.1 Radiography centres................................................................................................. 85 
5.2.2 Research community. ............................................................................................... 85 
REFERENCES ................................................................................................................. 86 
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APPENDIX 1 .................................................................................................................... 90 
APPENDIX 2 .................................................................................................................... 96 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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                  TABLE OF FIGURES 
Figure 2.1:  Principle of a gas- filled detectors ................................................................... 9 
Figure 2.2:  Operational regions of s gas filled detector (NET 130,2018) ....................... 10 
Figure 2.3:  Diagram of a well-designed plane -parallel ionization chamber ................... 16 
Figure 2.4:  Schematic diagram of (a) PMMA parallel plate ionization chamber and (b) 
polystyrene parallel plate ionization chamber .................................................................. 17 
Figure 2.5:  Schematic diagram and dimensions of parallel plate ionization chamber .... 18 
Figure 2.6:  PTW Unidos Electrometer ............................................................................ 22 
Figure 2.7: Types of connectors used to connect an ionization chamber to an ionization 
chamber to an electrometer (PTW-Freiburg user manual, 2012) ..................................... 24 
Figure 2.8:  A picture of a complete parallel plate ionization chamber with a connector 
(Rosalina Instruments) ...................................................................................................... 26 
Figure 2.9: Schematic diagram of a parallel plate ionization chamber showing all the in- 
built components with dimensions (Chantler el tal, 2014) ............................................... 27 
Figure 2.10: Schematic diagram of the effects of wall thickness on the chamber response 
(Khan) ............................................................................................................................... 28 
Figure 2.11:  Schematic diagram of a parallel plate ionization chamber showing all the in-
built components with dimension (De Ward-Ion Chambers Instrumentation) ................. 29 
Figure 2.12:  Circuitry diagram of a parallel plate ionization chamber (Podgorsak, 2006)
........................................................................................................................................... 29 
Figure 2.13:  Saturation curve represents a plot of amplitude or charge applied voltage.32 
Figure 3.1: Acuity Radiation Treatment Planning Simulator ........................................... 44 
Figure 3.2: PTW Unidod E lectrometer ............................................................................ 45 
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Figure 3.3:  Digital Thermometer ..................................................................................... 46 
Figure 3.4: Digital Barometer ........................................................................................... 47 
Figure 3.5: Piranha multimeter ......................................................................................... 48 
Figure 3.6:  Conceptual design ......................................................................................... 49 
Figure 3.7:  Material equivalent of the conceptual design ................................................ 49 
Figure 3.8:  Exploded solid CAD model of the parallel plate ionization chamber ........... 50 
Figure 3.9:  Assembled components ................................................................................. 51 
Figure 3.10:  Locally  Assembled paralllel plate ionization chamber .............................. 51 
Figure 3.11:  Set- up for the determination of angular dependency of the constructed 
chamber ............................................................................................................................. 53 
Figure 3.12:  Set -up for the determination of HVL of the constructed ion chamber ....... 55 
Figure 3.13: Set -up for the determination of QC on the Acuity simulation planning 
machine ............................................................................................................................. 57 
Figure 4.1:  Saturation curve of the constructed ionization chamber response ................ 63 
Figure 4.2:  Chamber short term reproducibility response ............................................... 64 
Figure 4.3:  Weekly percentage deviation for medium stability check ............................ 65 
Figure 4.4: Pre- irradiation leakage test. ........................................................................... 66 
Figure 4.5: Normalized chamber response to gantry angle .............................................. 67 
Figure 4.6: A graph of chamber response to voltage ........................................................ 68 
Figure 4.7:  Trend of chamber HVL against detector HVL .............................................. 69 
Figure 4.8:  Chamber HVL response at 60 kVp ............................................................... 71 
Figure 4.9:  Chamber  HVL response at 80kV…………… …………………………….72 
 
Figure 4.10: Chamber HVL response at 100 kVp ............................................................ 74 
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Figure 4.11: Chamber HVL response at 120 kVp ............................................................ 75 
Figure 4.12: Chamber HVL response at 130 kVp ............................................................ 76 
Figure 4.13: A graph of normalized KQ values against HVL ........................................... 78 
Figure 4.14: A graph of reference detector response against chamber response .............. 81 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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LIST OF TABLES 
 
Table 3.1: Properties of materials used to construct the chamber .................................... 42 
Table 4.1: Characteristic of the cconstructed paralle plate ionization chamber ............... 59 
Table 4.2: Chamber response with bias voltages .............................................................. 60 
Table 4.3: Polarity correction factors for various bias viltages ........................................ 61 
Table 4.4: Ion collection efficiency for the constructed ionization chamber ................... 62 
Table 4.5:  Weekly corrected response ............................................................................. 65 
Table 4.6: Normalizesd HVL responses ........................................................................... 69 
Table 4.7: Chamber responseto (Al) attenuator at 60kVp ................................................ 70 
Table 4.8: Chamber response to (Al) attenuator at 80kVp………………………………72 
Table 4.9: Chamber response to (Al) attenuator at 100kVp……………………………..73 
Table 4.10: Chamber response to (Al) attenuator at 120kVp…........................................74 
Table 4.11: Chamber response to (Al) attenuator at 130kVp……………………….......76 
Table 4.12: Normalized beam quality correction factor and HVL………………............77 
Table 4.13: Reference detector and chamber  responses .................................................. 80 
 
 
 
 
 
 
 
 
 
 
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LIST OF ABBREVIATIONS 
AAPM                          American Association of Physicist in Medicine 
ADCL                           Accredited Dosimetry Calibration Laboratory 
AFD                              Axis to Film Distance 
CAD                              Computer Aided Design 
CT                                 Computed Tomography 
EMF                              Electromotive Force 
FAD                              Focal   spot to Axis Distance 
 fion                                Ion Collection Efficiency 
GETFund                      Ghana Education Trust Fund 
HVL                              Half Value Layer 
IAEA                             International Atomic Energy Agency 
IEC                                International Electrotechnical Commision 
Kpol                               Polarity Effect 
kQ                                  Beam Quality Correction Factor,  
 Ks                                 Saturated Factor 
KTP                                 Corrected Temperature and Pressure, 
kVp                               Kilo Voltage Peak 
LCD                               Liquid Crystal Display 
Linac                              Linear Accelerator 
Mcorr                                               Corrected Chamber Responses  
mGy                               Milli Gray 
Muncorr                                        Uncorrected Chamber Response 
Nk                                   Calibration Coefficient 
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NRONMC                      National Radiotherapy Oncology and Nuclear Medicine Centre 
pA                                  Pico Ampere 
PMMA                           Polymethylmethacrylate 
SSDL                             Secondary Standard Dosimetry Laboratory 
                                                           
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CHAPTER ONE 
INTRODUCTION 
 1.1 Background 
In medicine, the most common artificial source of public exposure to ionizing radiation is 
diagnostic X-ray and this is due to its extensive use in most diagnostic facilities. In 
Ghana, thousands of diagnostics X-ray procedures are carried out each year. Despite the 
fact that radiation exposure connected with these procedures is unavoidable, there are 
various means to reduce it as much as possible (Halato et al., 2008). To be able to 
minimize dose to patients, and ensure image quality, there must be a regular quality 
control on the entire X-ray systems, which involves routine measurement of air kerma 
and exposure rate. 
Parallel plate ionization chambers are one of most employed dosimeters for the 
adjustment and control measurements in achieving the required accuracy (IEC, 2013). 
The estimation of the skin entrance dose depends largely on the air kerma and the beam 
quality. The right backscatter factor is applied in order to convert the air kerma into skin 
entrance dose. (Alessandro & Caldas, 2008). 
The goal of every radio-diagnostic examination is to provide a clinical informative 
radiographic image while minimizing dose to patients. Periodic quality control is a 
requisite for ensuring the safe use of X-ray equipment facility (Alessandro & Caldas, 
2008). 
The demand of diagnostic radiological services in a developing country like Ghana will 
keep on increasing with complex quality control, quality assurance, patient dose 
management and radiation protection challenges (Inkoom et al., 2011). 
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In considering these, the utmost objective should be arriving at a diagnostic image that 
satisfies clinical requirements whiles ensuring minimum patient dose. 
Again, the critical adjustment of parameters such as tube voltage (kVp), air kerma 
reproducibility and linearity with the tube current product-exposure time (mAs) are 
particularly very important. Also, a better understanding of the relationship between the 
image and these parameters, and the measurement of these characteristics with the right 
tools is necessary. Over time, characteristics of these parameters may change, this calls 
for periodic tests to be performed in order to avoid downtime and ensure that all 
diagnostic X-ray machines are accurately calibrated and function at optimal level and that 
the procedures and images are safe and informative (Hasford, 2018). 
Ionization chambers are used in the determination of the incident air kerma, 
reproducibility, linearity of air kerma and HVL as function of the mAs. Calibration of the 
output of photon beam produced by any X-ray system to achieve optimization of Dose to 
patients is by the use of an ionization chamber ( Parallel plate ionization chamber) 
(Podgorsak, 2005).  Parallel plate ionization chambers, also called surface ionization 
chambers, depending on their sensitive volume and characteristics, have different 
geometries. In order to accept a measurement as valid, it is important for the user to 
consider and well understand the specifications for the design of an ionization chamber. 
(Alessandro and Caldas, 2008). 
This study seeks to design and construct a vented parallel plate type ionization chamber 
and investigate whether the chamber satisfies the routine criteria for calibration of 
diagnostic X-ray machines (conventional radiography) in the clinical setting. 
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1.2.   Statement of the problem 
Delivering clinical informative radiographic images while minimizing dose to patients 
are the primary goals of any radio-diagnostic program. In view of this, the International 
Atomic Energy Agency (IAEA) recommends at least two ionization chambers for a 
diagnostic center. This is a challenge in most diagnostic centers in developing countries 
due to cost implications.  
 Due to the significance of quality control in diagnostic imaging, it is recommended that 
the staff at the facilities should ensure a routine review, for instance quarterly reviews, of 
the control tests, data and images, but this is not the case of most diagnostic centers in 
Ghana, due to cost and the health implications cannot be overemphasized. Preliminary 
checks at some of the major medical imaging facilities in Ghana reveal a lack of the 
availability of parallel plate ionization chamber. As such researchers, who need 
ionization chambers to perform dosimetry needs, always have to rely on Radiation 
Protection Institute (RPI) for their limited dosimeters for their studies. In X-ray systems, 
Parallel plate ionization chambers are the most preferred choice for quality control 
because, they are used for small field dosimetry or low dose rate measurements.  In view 
of this, there is the need to find a less expensive vented parallel plate ionization chamber 
which is environmentally friendly for daily QCs.  
 
1.3   Aims and Objectives 
The main objective of the study is to design and fabricate a vented parallel plate 
ionization chamber from available materials for quality control in X-ray systems 
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 The specific objectives are to: 
 Design and construct a vented parallel plate ionization chamber from available 
materials. 
 Calibration of the constructed ionization chamber for X-ray beam  
 Establishing the traceability of the newly constructed chamber, by cross 
calibrating it with another ionization chamber whose traceability is to a secondary 
standard dosimetry laboratory (SSDL). 
 Determination of calibration coefficient of the chamber to enable reference 
dosimetry measurement with the chamber.  
 
1.4   Scope and delimitation  
The scope of this project is limited to the designing, construction and testing of a portable 
vented parallel plate ionization chamber and validating it against those stipulated in the 
International Electrotechnical Commission (IEC) 61674, 2013, for standard ionization 
chambers. It also include the selection of material, manufacturing processes and cost 
analysis into materials. 
Measurements and procedures to ensure clinical application of the constructed waterproof 
portable parallel plate ionization chamber have been considered. 
 
 
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1.5   Thesis organization 
The project is organized into five chapters. Chapter 1 talks about the Introduction which 
includes the study background, Problem statement, aims and objectives, significance of 
the project and the organization of the thesis report. Chapter 2 discusses literature and 
theories on the various types of ionization chambers. Chapter 3 centres on the materials 
and methods adopted for the study. It highlights the conceptual design of the parallel-
plate ionization chamber, processes of manufacturing, design calculations and parts and 
their functions. Chapter 4 presents the results for the construction and findings for pre-
evaluations measurements of the newly constructed ionization chamber. Chapter 
5pesents, conclusion of this project and suggested recommendations for further work. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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CHAPTER TWO 
LITERATURE REVIEW 
 2.1   Dosimetry in diagnostic radiology 
Dosimetry is an integral component of diagnostic radiology and it requires the use special 
instruments.  The construction and performance of such instruments must correspond to 
the demands of the clinical situation. Also, specialized techniques and knowledge may be 
needed in the use and the interpretation of the result obtained from such instrumentation 
(IAEA, TRS No.457).  
Annals of the American Association of Physicist in Medicine (AAPM) explains that, the 
performances of diagnostic dosimeters, categorized the measurements into two: those 
used to evaluate the risks of radiation damage from X-ray examination and those 
performed as part of quality assurance programs and radiation surveys of workplace and 
areas the general public occupies. Also, the quality assurance measurements can be 
grouped into two: the one used for establishing absolute values of the experimented 
parameters and those used in comparison with the baseline values established for the 
same unit or with parameter values determined for another unit. (AAPM, 1992) 
The most important use of dosimetric quantities in diagnostic radiology is the protection 
of staff and patients from ionizing radiation. Nonetheless, the absorbed dose is the 
amount that best shows the effects of radiation on products or on humans, and thus all the 
amounts associated with safety are centered on it. (IAEA, TRS No. 457, 2007). 
The derivatives of absorbed dose include air kerma, fluence or equivalent dose. The 
absorbed dose is the amount of energy absorbed per unit mass. It is the quantity of most 
interest in diagnostic radiology. From experiments performed on ionization chambers, 
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absorbed dose to air (𝐷𝑎𝑖𝑟) is the quantity used to compare the responses of the different 
detectors and is expressed as 
𝑄
                𝐷𝑎𝑖𝑟 = * W                                                                        (2.1) 
 𝑚
Using the charge (Q) measured by the electrometer in Coulombs and the mass (m) of air 
in kg, and mean energy (W) required to produce an ion pair in air per unit charge (e). 
Currently, the value of W for dry air is 33.97 eV/ion pair or 33.97 J/C, the SI unit of 
absorbed dose is the gray, abbreviated Gy.  
It is important to correct the mass of air for temperature and pressure using the Ideal Gas 
Law, 
𝑃
                             𝜌𝑎𝑖𝑟 =                                                                      (2.2) 
𝑅𝑇
Where 𝜌𝑎𝑖𝑟 is the density of air, 𝑃 is the air pressure, 𝑅 is the ideal gas constant for air, 
and 𝑇 is temperature. Then the mass of air is  
             𝑚𝑎𝑠𝑠𝑎𝑖𝑟 = 𝜌 𝑉,                                                               (2.3)  
Where 𝑉 is the volume of the air cavity.  
Khan (2010), expressed the quantity kerma (K), as the quotient of the summation of the 
primary kinetic energies of all the charged ionizing particles released by uncharged 
particles, 𝑑Etr in a given material of mass 𝑑𝑚.    
𝑑𝐸𝑡𝑟
                            K =                                                         (2.4) 
𝑑𝑚
 
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Khan (2010), defined fluence,Φ as the number of particles or photons, traversing a 
medium. ICRU formal definition for fluence, Φ   is:     
𝑑𝑁
                                            Φ𝐴 =                                                                       (2.5)  
𝑑𝐴
Where 𝑑𝑁 represent the total sum of particles directed on a sphere and 𝑑𝐴 is the cross 
sectional area.   
Evaluating of the health effect of radiation from X-ray diagnostic examination is more of 
uncertainty. But it is important to ensure that such uncertainty in measurement are 
reduced if not eliminated. 
 
2.2   Radiation dosimeters 
Radiation dosimetry is basically the quantitative determination of that energy absorbed in 
a matter (Podgorsak, 2005). 
There is no particular organ identified in man which can detect the presence radiation and 
has not been impossible till date for any scientific initiative to come out with any 
instruments that can amplify human’s response to radiation, in contrast to the case of 
visible light wave (optics). However, there is dependence on instruments to indicate the 
presence of ionizing radiation (Kyere, 2018). 
 Radiation dosimeter helps to detect and quantify ionizing radiation. A radiation 
dosimeter is an instrument that measures the dose of ionizing radiation and consists of a 
measuring assembly: electrometer, and at least one detector assemblies which can either 
be a key component of the measuring assembly or not.  
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In conventional radiography dosimetry, reliable dosimeters such as ionization chambers 
or semi-conductor detectors are used in the measurement of air kerma (K), air kerma 
length product (PKL) air kerma rate (ḱ) and air kerma area product (PKA) in primary beam 
conditions. Measuring of dose is important when it comes to quality control and 
acceptance test (Okuno, 2012). 
 
2.3   Gas filled detectors. 
The most common type of instrument used to indicate the presence of ionizing radiation 
is a gas filled radiation detector. Figure 2.1 is a schematic diagram of s gas filled detector. 
                 
                               Figure 2.1 Principle of a gas- filled detectors 
                    . 
 
2.3.1   Principle of operation of gas filled detectors 
The principle behind the operation of the gas-filled detector that when ionizing radiation 
is allowed to pass through air or gas, the molecules of the air or gas get ionized. The 
positive ions will be attracted to the negative side of the detector (the cathode) when a 
high voltage is imposed between two areas of the gas filled vacuum, and the free 
electrons will migrate to the positive side (the anode). The anode and cathode, forming a 
very small current in the wires going to the detector, collect these charges. The current is 
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measured and shown as a signal by putting a very sensitive current measuring device 
between the wires of the cathode and the anode. The more radiation reaches the chamber, 
the more current the instrument shows. 
 According to Ahmed (2007), the design of a gas fill detector is of three essential 
components: an anode, a cathode, and a gas enclosure. Most detectors use the wall of the 
container that holds the gas and a wire inside the container as cathode and anode 
respectively. 
The design of gas-filled detectors and applied voltage constitute their classification as 
either ionization chambers, proportional counter, or Geiger- Muller counters. Figure 2.2 
shows the various regions of a gas filled detector. 
 
  
Figure 2.2: Operational regions of s gas filled detector (NET 130, 2018) 
 
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2.3.2 Characteristics of different regions across a gas-filled detector. 
2.3.2.1 Recombination Region  
In the recombination region, the voltage applied is low, and hence low electric field 
strength exist within the chamber. This leads to ion recombination since only small 
charges are collected from the gas. The recombination of positive and negative ions 
within the air cavity reduces the amount of charge collected. At this region, not all the 
signals produced are collected as charges due to the recombination effect (Mayles et-al., 
2007). 
 
2.3.2.2   Ionization Chamber Region   
Increasing the electromotive force (EMF) accelerate more electrons from the 
recombination zone. The region of full ionization (i.e. the ionization chamber region) is 
then reached. This is also known as saturation region.  At the saturation region, the 
applied voltage is high enough to prevent recombination and low enough to prevent 
secondary ionizations. The signal becomes constant over a wide voltage range.  All 
primary ion pairs are collected on electrodes. The operating voltages of an ionization 
chamber can be found within a range of 500–1000 V (Knoll, 2010). 
 
2.3.2.3   Proportional Region   
Further increase of the Electromotive force (EMF) beyond ionization chamber region,  
forces the electrons produced in the gas to be accelerated to such an extent that they 
produce additional ionization of the gas. This is called the proportional region and there 
is increases in the charge collected on electrodes.  It is called proportional region because 
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there is direct proportionality between number of ions pairs collected and number of ions 
pairs initially produced in the detector by radiation if the voltage remain constant. This 
means that for a given detector and particular voltage its gas amplification factor is 
always a constant (Mayles et-al., 2007).  
 
2.3.2.4   Limited Proportional Region  
At the proportional region, increase in the Electromotive force (EMF) results in the 
expansion of the ionization zone and significant gas amplification is obtained.  Further 
increased in voltage, results in space charge effect starts to reduce the effective electric 
field and affects the gain. The limited proportionality region is then entered. This is not a 
very useful range for radiation detection. The collected charge becomes independent of 
the number of primary ionizations. The secondary ionization leads to photo ionization 
and the constant of proportionality is no longer exact (Knoll, 2010). 
 
2.3.2.5   Geiger-Mueller (GM) Region   
As the Electromotive force (EMF) is increased, beyond limited proportional zone, a point 
of massively multiple, successive ionizations of most of the gas within the detector is 
reached. No more gas amplification is possible once all the gas is involved. This implies 
that any further increases in voltage has little or no effect on the size of the pulse of 
current.  The detector is said to be operating in Geiger–Müller region and hence called a 
Geiger–Müller counter. In the Geiger –Muller region, the size of the current pulse does 
not depend on any small changes in voltage, (Knoll, 2010). 
 
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2.3.2.6   Continuous Discharge Region  
 At the continuous discharge region, electric field intensity becomes so strong that no 
initial radiation event is needed to completely ionize the gas by increasing voltage 
beyond the Geiger-Müller zone. Secondary ionization is propagated by the electric field 
itself, and complete avalanching takes place. There is no feasible practical radiation 
detection (Knoll, 2010). 
 
2.4   Ionization chambers 
The International Electrotechnical Commission (IEC) defines ionizing radiation detector 
as a chamber enclosed with air, in which an electric field insufficient to produce gas 
multiplication is provided for the collection at the electrodes of charges associated with 
the ions and the electrons produced in the measuring volume of the detector by ionizing 
radiation (IEC 61674, 2013). 
Both electrodes are separated with a high-quality insulator to reduce the leakage current 
when a polarizing voltage is applied to the chamber. They are in different sizes and 
shapes, depending on the specifications.  The operation of the chamber changes as the 
voltage increases.  Generally, they operate in the near saturation or saturation region 
(Okuno, 2012).  
For any high-quality technical measurements, ionization radiation detector is often 
employed. It the most preferred choice for over a very long period. (Ross, 2009.) 
It is recommended that the ionization chambers used in diagnostic radiology be vented 
(the air inside the vacuum interacts with the environment), making the air mass 
dependent on temperature, pressure and humidity conditions (Okuno, 2012). 
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2.4.1 Types of Ionization chambers used in diagnostic radiology 
Ionization chambers used in diagnostic radiology can be grouped into two types namely 
free-air ionization chambers, and cavity ionization chambers: 
 
2.4.1.1   Free-Air ionization chamber 
This type of ionization chamber is called free –air chamber because principle, the walls 
of the chamber do not perform any role in its response. They are used to measure direct 
exposure now air kerma in photon beams. 
The measurement involves the collection of all ions which are produced by the radiation 
beam as a result of direct transfer of energy from photon to primary electron in a defined 
volume of air. They are practically limited to photon energy below 0.3 MeV. It is 
practically impossible to build such a chamber for higher energies due to the increase in 
secondary electrons (Mayles et al., 2007). 
 
2.4.1.2 Cavity ionization chambers  
Cavity ionization chambers consist of an envelope surrounding a gas volume (air in this 
case) between electrodes (-polarizing and central electrode) with an applied voltage. An 
electric field is created when a voltage is applied to the two electrodes inside the cavity. 
This electric field collects the charges resulting from the ionization of the air by electron 
entering the cavity. The amount of incoming electrons is proportional to the radiation 
received.  
Basically, cavity chamber are grouped into two:  cylindrical (thimble) chamber and 
parallel plate (plane parallel). Both chambers are designed to behave as Bragg Grey 
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cavities in megavoltage photon and electron beam and their designs features can best be 
understood in the light of this theory (Mayles et al., 2007). 
Cherry et al. (2012), advises that the gas cavity should be sufficiently small to prevent the 
disturbance of the particle fluence could result in same number and energy of electrons 
traversing through the volume. 
Measurements of air kerma in air for energies in the ranges of 0.6 to 1.5 MeV is done by 
cavity ionization chambers Ions are collected in a defined volume, inside a cavity, 
enclosed by graphite body which is thick enough to provide full build-up of secondary 
electron. Cylindrical chambers are uniformly sensitive around their central geometrical 
axis. The chambers used for measurement in the X-ray beam have effective volume of 3 
cm3 to 6 cm3 
 
2.5   Parallel plate ionization chamber. 
Parallel plate ionization chamber (PPIC) is also referred to as plane parallel chamber 
because both electrodes are parallel to each other and to the entrance window (Podgorsak, 
2005). Both electrodes serve different purpose, one serving as an entry window and 
polarizing electrode and the other as the back wall and collecting electrode. It has a guard 
electrode for reduction of leakage. 
In conventional diagnostic radiation dosimetry, parallel plate ionization chambers have 
long history. There have been a number of important designs, for the absolute 
determination of exposure (now air kerma), and extrapolation chambers in which one can 
gradually reduce the distance between the electrodes by mechanical means (Nahum and 
Thwaites, 2011).  
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In 1914, William Duane, the first American Biophysicist developed the first plane 
parallel chamber to overcome the wall effect and measured Villard unit which he called 
intensity. He defined “dose” as “intensity multiplied by time in seconds” (Almond, 
2009). 
Subsequent chambers are developed in relation to the original parallel plate ionization 
chambers, but differ in terms of composition of wall material, central electrode, and/ or 
chamber sensitive volume and guard thickness. Figure 2.3 presents schematic diagram of 
a plane-parallel ionization chamber (IAEA TRS 398). 
 
                                   
Figure 2.3 Diagram of a well-designed plane -parallel ionization chamber. Indicated in 
the diagram are the height a of the air cavity, the diameter d of the entrance window (1) 
the diameter m of the collecting electrode (2) and the width g of the guard ring (3). 
 
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Figure 2.4 is a schematic diagram of PMMA parallel-plate ionization chamber and 
polystyrene parallel-plate ionization chamber (Souza et al) 
 
 
Figure 2.4: Schematic diagram of (a) PMMA parallel plate ionization chamber and (b) 
polystyrene parallel plate ionization chamber 
 
Today, parallel plate ionization chamber remains the most common type of ionization 
chamber for diagnostic radiological measurement of air-kerma (Mayles et al., 2007). 
For dosimetry of electron beams with energies below 10 MeV, and higher photon beams, 
plane parallel is the mostly preferred choice.  Its major limitation is the angular 
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dependence response. It should always be placed perpendicular to the radiation beam 
(Dance, 2014). Schematic diagram and dimensions of standard plane-parallel chambers is 
presented in Figure 2.5.  
 
      
Figure 2.5 Schematic diagram and dimensions of parallel plate ionization chamber w: is 
the outer/ polarizing electrode, b: inner/ measuring electrode, g: guard ring, a: the height 
of air cavity, ∅ : the diameter of the collecting electrode. g: the width of the guard ring 
(Solimanian et al, 2005). 
 
2.6   Physics of parallel plate ionization chambers 
Ahmed, (2015), “explains that as radiation interact with gas, it ionizes the gas molecules, 
if its energy is higher than the ionization potential of the gas. By the application of an 
external electric field, the ion pairs created can thus be made to move in opposite 
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direction. The result is an electric pulse can be measured by an associated measuring 
device 
Andreo et al, (2005), explains that, when ionizing radiation, such as x-ray is allowed to 
pass through the entrance window (polarized electrode) it ejects a high energy electron 
into the sensitive volume. The electron ionizes the air in the sensitive volume, yielding 
cation and low energy electrons. The electronegative oxygen attracts the low energy 
electron, producing negative ions. The positive electrons are attracted by polarizing 
electrode while negative ions goes to the measuring electrode creating a charge Q in the 
known sensitive volume of air and the total charge measured. If voltage applied is 
relatively low, the electric field E is too weak to efficiently separate the negative and 
positive charges. Primary and secondary ions produced within the gas are separated by 
Coulombic effects. 
Dose in air is calculated using Bragg Gray cavity theory (Andreo et al., 2005) in equation 
2.6:  
 
Q Wair 
                               Dair                                                                         (2.6)                                                                                                           
mair  e 
                   
 where: Dair  is dose in air  Q  is ionization charge mair is mass of air in the sensitive 
volume 
Wair 
     is the mean energy required to produce an ion pair in per unit charge. 
 e 
  (33.97J/C ±0.06 for dry air, ICRU 1984) 
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Dose in a medium is deduced from Bragg-Gray and Spencer-Attix cavity theory (Andreo 
et al., 2005), which links dose in air, dose in a medium and dose in the wall according to 
the following correlation  
 
     en en Q W    gas
   
D    en med  Dwall  Dgas Swall ,gas   Swall ,gas                   (2.7)  
 p
 
 p
 m e  
med ,wall  med ,wall   
p med ,wall
 Where Swall ,gas  is the ratio of restricted mass collision stopping power for cavity wall and 
gas. 
Q Wgas 
In practice:      D                                                (2.8) med   Swall ,gas Pfl Pdis Pwall Pcelm  e 
where: Pfl   electron flounce  perturbation correction factor. 
Pdisl   the correction factor for displacement of effective measurement. 
Pwall   wall correction factor. 
            Pcel      the correction factor for central electrode. 
 
Podgorsak (2005), explains that, the Bragg-Gray cavity theory is applicable under the 
following conditions:   
i.  The cavity must be small when compared with the range of charge particles 
incident on it, so that it presence does not perturb the fluence of charge particles 
in the medium. 
ii.  The absorbed dose in the cavity is deposited solely by charge particles crossing it. 
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According to Seco (2014), the quantity of charge induced, 𝑑𝑄, by either an ion or a single 
electron moving a distance   within an electric field, E in an ion chamber can be 
calculated by:   
1
                       𝑉 20 𝑑𝑄 = 𝑞𝐸𝑑𝑙                                                                           (2.9)               
2
where V0 is voltage difference across the electrodes and 𝑞 is the electron or ion charge. 
   
2.7   Components of a typical parallel plate ionization chamber. 
The components of a typical parallel plate ionization chamber can be grouped into two 
major forms; the measuring assembly and the chamber assembly 
 
2.7.1   Measuring Assembly   
The measuring assembly for the parallel-plate ionization chamber includes an 
electrometer and a power supply for polarizing voltage of the ionization chamber. The 
electrometer should preferably be provided with a digital display and should be capable 
of four-digit resolution; it should allow 0.1% resolution on the reading. The variation in 
the response should not exceed ± 0.5% over 1 year period (i.e. long-term stability) (IAEA 
TRS-381). The electrometer (Figure 2.6) and ionization chamber can be calibrated 
separately. In centers provided with many electrometers or chambers, this is particularly 
useful. The electrometer is an integral part of the dosimeter in some situations, however, 
and the ionization chamber and the electrometer are calibrated as a single unit only. Due 
to the determination of polarity effect and ion collection efficiency,  the electrometer  
polarising voltage polarities must be reversible,. 
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Figure 2.6 PTW Unidos Electrometer 
 
2.7.1.1 Cables and Connectors.  
Cable and connectors play a major role in dosimetry. The chamber and the electrometer 
are coupled together by high-quality low-noise triaxial cable and a connector. To ensure 
good contact and rigidity, the connectors are mostly made male and female. Some 
connectors may have protective covers which veil the real shape of the connector.  
The cable has three conductive wires which are separated by insulators. The following 
are the desired cable characteristics:  
i. Low capacitance. 
ii. Pliability  
iii. Short equilibration time ( less than one minute)  when the high voltage is  
changed 
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iv.  Low radiation-induced signal 
v.  Low microphonic noise  
vi.  Low leakage (less than 10-14 A). 
Basic problems uncounted by users with the connectors of the chamber cable and the 
electrometer includes, mismatches of connectors resulting to improper contacts, dirt, 
breakages in connection of the cable leading to the connector contacts, moisture, 
misaligned pins, strain of the cable where it meets the connector, and slightly different 
sizes of the same nominal connector. It is important that the two devices are from the 
same manufacturer and consider male and female connectors so that they easily match 
(Mayles, 2007). 
Connectors are of various types and sizes as shown in Figure 2.7.  
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Figure 2.7 Types of connectors used to connect an ionization chamber  to an electrometer 
(PTW-Freiburg user manual, 2012) 
 
 
 
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2.7.2. Chamber Assembly  
The design of a plane parallel ionization chambers as shown in Figure 2.8 and Figure 2.9 
are such that the entrance window faces the radiation source. According to (IAEA TRS-
381). they are usually characterized by the following constructional details  
 The air volume is a disc-shaped right circular cylinder, one flat face of which 
constitutes the entrance window. The central electrode is conducting circular disc 
inserted at the centre of the sensitive volume which forms the other flat face of the 
cylinder opposite to the entrance window and operates with positive voltage and 
collect negative charge. It is separated from chamber wall by an insulator of high 
quality to minimize leakage current once the voltage is applied across the 
electrodes.  
 The sensitive volume is that fraction of the total air volume through which the 
lines of electrical force between the inner and outer electrodes pas. 
  The inner and outer electrodes are mounted in a supporting block of material (the 
chamber body) to which the connecting cable is attached. The cable usually exits 
the body in a direction parallel to the entrance window.  
 The sensitive volume is typically between 0.05 cm3 and 0.5 cm3.  
 The polarizing potential is applied to the outer electrode and the signal charge is 
collected from the inner electrode; 
  There is usually a third electrode surface between the other two which is not 
connected electrically to either of them, but which is designed to be held at the 
same potential as the inner electrode. If the chamber assembly is fully guarded 
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this third electrode will be present in the air volume as a ring around the inner 
electrode.  
Plane-parallel chambers for photon radiation have the following typical dimensions:   
 The entrance window thickness is 1 mm or less 
  The distance between the inner and outer electrodes is 2 mm or less 
  The diameter of the inner (collecting) electrode is 20 mm or less. 
 
 
Figure 2.8: A picture of a complete parallel plate ionization chamber with a connector 
(Rosalina Instruments) 
 
 
 
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Figure 2.9: Schematic diagram of a parallel plate ionization chamber showing all the in- 
built components with dimensions (Chantler et al., 2014) 
 
2.7.2.1 The Body 
The body of parallel plate chambers are mostly disc shaped with diameters of Ranging 
from 30mm to 65mm and 10m thick. 
It must be noted that, the bigger the chamber, the more sensitive it becomes 
The body or shell is usually a block of non-conducting material (usually Perspex, acrylic 
or polystyrene). 
Adequate wall thickness is necessary to give enough mechanical protection and also to 
achieve electronic equilibrium. However increased wall thickness result in attenuation of 
some photon flux, thus reducing chamber response. Contrary, decreased wall thickness 
than that required for equilibrium or maximum ionization, produces too few electrons in 
the wall, causing low chamber response as depicted in Figure 2.4 (Khan, 2010). 
 
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Figure 2.10 Schematic diagram of the effects of wall thickness on the chamber response 
(Khan) 
 
2.7.2.2 Guard  
An ionization chamber does not need a guard electrode to operate correctly, but having a 
guard provides wonderful benefits and performance enhancements (Hooten, 2000). 
The guard electrode absorbs the leakage current and allows it to directly flow to the 
substrate, bypassing the collecting electrode. It guarantees field continuity in the sensitive 
volume of chamber, with resulting advantage in the collection of charges. (Hartmann, 
2012).  
Some parallel-plate ionization chambers require significant fluence perturbation 
correction because they are provided with an inadequate guard width. Ionization 
chambers designed for high-accuracy dosimetry measurements use guard materials that 
are nearly tissue equivalent or air equivalent in atomic composition such as plastics or 
graphite. Use of metals lead to enhanced photoelectric effect and pair production due to 
high atomic number. .  Figure 2.10 presents schematic diagram of a parallel-plate 
ionization chamber showing major components 
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Figure 2.11 Schematic diagram of a parallel plate ionization chamber showing all the in-
built components with dimension (De Ward-Ion Chambers Instrumentation) 
 
2.7.2.3   Circuitry of a simple ionization chamber 
The circuitry diagram of the parallel plate ionization chamber is shown in Figure 2.11. 
 
 
 
Figure 2. 12 Circuitry diagram of a parallel plate ionization chamber (Podgorsak, 2006) 
 
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The polarizing electrode is connected to the power supply directly. The measuring 
electrode is wired to the ground to measure the current or charge generated in the 
sensitive volume of the chamber via the low impedance electrometer. The Guard 
electrode is directly grounded and serves two purposes: it determines the sensitive 
volume of the chamber and prevents chamber leakage currents from being measured. 
 
2.8 Factors Affecting the Measurement of Ionization output 
Beam energy (ke) temperature and preasure (ktp) ion recombination (pion) and polarity 
effect (kpol) are the major factor to be corrected when determining exposure now air 
kerma with an ionization chamber. It is expressed as: 
              Exposure (X) =MNXK                                                                    2.11 
 
               K= ke.ktp.kpol.pion                                                                                                                     2.12 
where X is exposure measured in C/kg, M is meter reading in C, NX is the calibration 
factor of an ion chamber for standard beam energy from a national or accredited 
dosimetry calibration laboratory. K is a composite correction factor, Ke is the energy 
correction factor, Ktp is the correction for the air density due to the temperature and 
pressure, Kpol is correction for the polarity, and Pion is the ion recombination correction 
factor. 
 
2.8.1 Recombination losses 
Within the ionization chamber, ion recombination losses is one of the factors that affects 
the current or charge measured. If the chamber is well designed and constructed, the ion 
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recombination effects becomes very small and are often ignored and not corrected for in 
X-ray systems.  Such small losses are not accounted for by the user of the chamber since 
the calibration coefficient are also left uncorrected. (IAEA-TECDOC-1585, 2008). 
If the Q1 and Q2 are the charges collected at the chamber potential of V and V/2 V, 
respectively, the   ion recombination Pion is calculated as: 
4 𝑄1
 P  = ⟮ − ⟯ -1                             ion                                                    2.13 
3 𝑄2
Practically the charge Q¹ and Q are not the same. 
Q¹ = Charge at low voltage, where recombination occurs. 
Q = Charge at high voltage, where the chamber is saturated. 
The charge Q1 measured less than charge Q generated within the chamber due to 
recombination of ion pairs within the gas molecules (Atix, 1986).  
According to Ross (2009), in the absence of an electric field, ion recombination will 
always occur. To reduce this effect, the voltage applied to the ionization chamber, must 
be higher enough to cause ion separation quickly to ensure that all the ions created are 
collected. The chamber is said to be saturated when all ions created successfully 
accounted for and ion recombination is totally absent. 
The decrease or increase in charge is best explained by electrical design of the chamber 
and physics of ion transport in the chamber sensitive volume. Poor ionization geometry 
results in persistent recombination. For example, internal corners of chambers should be 
rounded not sharp. 
The voltage range at which an ion chamber should be operated to achieve saturation is 
shown as saturation region in Figure 2.12. 
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 Initially the curve rises linearly with increase in voltage as shown as recombination 
region (where ion saturation has not been achieved and complete ion collection does not 
take place) until a saturation is reached and finally at higher voltages it breaks down.  
 
                              
Figure 2.13: Saturation curve represents a plot of amplitude or charge against applied 
voltage. 
 
Khan (2010), defines polarity effects as change in magnitude of the ionic charge collected 
as the polarity of the collecting voltage is reversed. Changing the polarizing potential 
polarities results in change in the ionization chamber current measured. This polarity 
effect is generally neglected especially in photon beams, but very significant in electron 
beams or plane parallel ionization chambers. 
The correction for polarity effect, 𝐾𝑝𝑜𝑙 is calculated using equation 2.14 the following 
relation:  
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|𝑀+|+|𝑀−|
                       𝑘𝑃𝑂𝐿 =                                             (2.14) 2𝑀
                                                                                
Where, 𝑀+ is the reading with positive polarity; 𝑀−is the reading with negative polarity; 
and 𝑀 is the polarity chosen from the saturation test. The polarity is made reference to 
the polarity of the bias voltage supplied to an ionization chamber. Polarity effect may be 
caused by extracameral current. A good example of extracameral current is current 
collected outside the sensitive volume of the chamber. It is caused by Compton current 
and irradiation of the cable connecting the chamber with the electrometer.  
IEC (2013), recommends a polarity effect, K𝑝𝑜𝑙 limit of within 1.0%.  
Furthermore, Khan (2010) recommends that for any radiation beam quality the variation 
of the chamber response between the positive and negative polarizing potential should be 
less than 0.5%.  
Meanwhile, it is necessary to estimate the magnitude of the charge deficiency  
Q – Q¹ or ion collection efficiency: 
𝑄1
                          𝑃𝑖𝑜𝑛 =                                                              (2.15) 𝑄
 And make a correction to obtain a charge Q produced in the ion chamber. Where Q¹ is 
the measured charge by the electrometer and Q is the charge produced in the gas of the 
ionization chamber. 
The actual measured charge, Q for the measurement of exposure, is the absolute value of 
mean of the two charges: 
𝑄+ + 𝑄−
         Q =⃒ ⃒                                                                                         (2.16) 
2
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The actual measured charge, Q for the measurement of exposure, is the absolute value of 
mean of the two charges: 
(𝐾𝑝𝑜𝑙−1Error% = )x100 for Q--                                                                       (2.17) 
𝐾𝑝𝑜𝑙+1
1− 𝐾𝑝𝑜𝑙
Error % =( ) x100 for Q+                                                                     (2.18) 
1+𝐾𝑝𝑜𝑙
 
2.8.2   Leakage Current  
Leakage current refers to any signal change recorded by the measuring assembly that is 
not generated by radiation (IEC, 2013). 
Any current produced by the complete dosimetry system in the absence of radiation is 
classified as leaked current. No matter how well an ionization chamber system is 
designed and constructed, there is always a small amount of non-radiation, inherently 
exhibited due to surface and volume leakage currents which flows between the central 
and the polarizing electrodes of the chamber. Since, current measured by the electrometer 
is of the   order of pico or nano amperes, any current that leaks across the insulator can 
greatly affects the signal output (Ross, 2009). 
To prevent or reduce the effects of current leakage, guard rings or electrode is used. Use 
of insulators and guard electrode minimizes intrinsic current leakage. In order to ensure 
that no leakage of charges are induced across the inner insulator of the guard electrode, 
the central electrode and the guard electrode are at the same potential (TechNote4812-00, 
2015). Mechanical stress like twisting and bending can induce cable leakage currents and 
must be avoided. 
According to (IEC 61674, 2013) standard, the leakage current shall not exceed 5% of the 
minimum effective air kerma rate for the range in use. The indicated value shall not 
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change by more than 1% per minute, when a dosimeter is left in measurement mode after 
being exposed to the maximum effective air kerma value.  
A clean insulating surface is very important and must always be maintained when 
constructing any ionization chamber since moisture and contaminants can reduce the 
resistivity of the chamber. Even the smallest microns of dust can cause large amounts of 
leakage. 
 
2.9   Performance Characteristics of Parallel Plate Ionization Chamber 
The IEC 61674, (2013) specifies the performance and some related constructional 
requirements of diagnostic dosimeters intended for the measurement of air kerma, air 
kerma length product or air kerma rate, in photon radiation fields used in radiography, 
with generating potentials not greater than 150 kV. 
The complete dosimeters systems  to be  used for adjustment and control measurements 
must be of satisfactory quality and must therefore fulfil the special requirements or 
performance characteristics that international standard specifies. Appropriate 
performance characteristics should be applied to correct for any deviation from the 
reference conditions. Some of the corrections for influence quantities that must be 
accounted for in a dosimetry system include: angular dependence, energy dependence, 
sensitivity, linearity, and reproducibility.  
  
2.9.1 Angular dependence  
When Radiation is incident from a different angle, the detector response may vary from 
another different angle. This variation in response to different angle of the incidence of 
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the radiation source the source detector distance of 100 cm describes the angular 
dependence of the dosimeter (Hartmann, 2012). 
The directional or angular dependence of any ionization chamber largely depends on its 
construction and aesthetics and the energy of the incident radiation. The angular 
dependence of a parallel ionization chambers might be very significant at large angle of 
incidence. 
IEC 61674, (2013), recommends that at an incident angle of ±5o, from normal 0o, a 
response variation of ±3% upper limit is expected. 
 
 2.9.2 Energy dependence  
Radiation energy determines the response of every dosimeter. In diagnostic dosimetry, 
the beam quality (X ray spectrum) greatly affects dosimeter readings. The quality of the 
beam is specified by the beam half value layer (HVL). Determination of HVL in 
diagnostic radiology, is a way of characterizing the hardness of the x-ray beam.  Half 
value layer (HVL), Is the thickness of material needed to reduce intensity of an x-ray 
beam to one-half of its initial value, when introduced into the path of a given beam of 
radiation. (I /I o = 0.5).  
                        HVL = 0.693/ µ                                                                               (2.19) 
HVL, usually measured in millimeters (mm) of aluminum (Al), is use to determine the 
effective energy. All calibrations are done at a specified beam quality, therefore it is 
important to correct for beam quality if the user beam quality is not the same as the 
calibration beam quality. The difference in response to different radiation qualities is 
account for by the beam quality correction factor KQ.  
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For a radiation quality Q, the correction factor KQ is the ratio of the calibration factors for 
quality Q to the reference radiation quality.   
IEC 61674, (2013), standard recommends a ± 5% upper limit of variation of energy 
response in the 50 - 150 kV range while IAEA TRS 398, 2000 proposes the limit of ± 2.6 
of the same ranges. 
 
2.9.3 Sensitivity 
 Signal output largely depends on air kerma. The least air kerma needed to produce a 
signal output describe the seensitivity of a detector. A good sensitivity dosimeter, will 
produce a higher charge (or current) than a less sensitive dosimeter for the same air 
kerma (rate) 
Large ionization chambers are mostly used in fluoroscopy (low air kerma rate 
measurements) because chambers with larger active volumes are more sensitive 
compared those with smaller active volumes. In radiography, where better spatial 
resolution is required, smaller chambers are used at higher air kerma rates. 
 
2.9.4 Current Linearity 
The linear proportionality of an ionization chamber output to the dosimetric quantities 
describes the linearity of the chamber. At certain range of air kerma, almost all ionization 
chambers exhibits current linearity, but above a certain range, non-linearity sets in, this 
depends on the type of chamber and it design characteristics. 
It is important that all manufacturers state the dosimeter linear response range to air 
kerma (rate).  For air kerma equation 2.21 rate must be satisfied IEC 61674, (2013) 
𝑅𝑚𝑎𝑥−𝑅𝑚𝑖𝑛
                               ≤ 0.02                                            (2.20) 
𝑅𝑚𝑎𝑥+𝑅𝑚𝑖𝑛
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where Rmax is the maximum response over the rated range of air kerma rate and  
 Rmin is the minimum response 
2.9.5   Reproducibility (Short term stability)  
IEC 61674, (2013), states that the limits of variation of response when the detector 
assembly is irradiated in a reproducible field shall not be greater than ±2.0 % per year. 
Due to the fragility of ionization chambers, any mechanical damage that may occur with 
little or no visible sign may result in slight changes in response of the reference chamber 
with time. The change is easily noticed with soft radiations unlike medium or high 
radiations. It is therefore recommended that stability check is done periodically to ensure 
that the chamber is consistent in its functions. Basically stability check of a reference 
chamber is achieved by either measurement in gamma beam or using stability source 
check like strontium-90 (90Sr).  Short-term stability is evaluated by the repeatability test 
while long-term stability uses reproducibility test (IAEA-TECDOC-1585, 2008). 
The AAPM Diagnostic X‐Ray Imaging Task Group No. 6 recommends that the value 
obtained in each test must not differ from the reference value by more than 0.5% for short 
term stability. This must be determined from ten consecutive measurements made within 
1hour 
  
2.10   Ionization chamber calibration  
An Accredited Dosimetry Calibration Laboratory (ADCL) is responsible for the 
calibration of ionization chamber in X ray and gamma beams. 
To achieve high level of accuracy, precision and quality productivity, of ionization 
chambers, both manufacturers and users are required to calibrate the instrument. This 
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requires air kerma calibration factor, 𝑁𝐾 or an absorbed dose at the radiation quality Q.  If 
the chamber has a 𝑁𝐾 factor, the air kerma, 𝐾𝑎𝑖𝑟 at the reference depth, 𝑍𝑟𝑒𝑓, is given by:  
                                           𝐾𝑎𝑖𝑟 = 𝑀𝑄𝑁𝐾              (2.21)  
Where 𝑀𝑄 is the dosimeter reading corrected for influence quantities.   If the chamber is 
not sealed, there is need to correct 𝑀𝑄 for temperature and pressure as the density of air 
in the cavity is influenced by environmental temperature, pressure and humidity, should 
be applied to convert the cavity air mass to the reference conditions. The air density 
correction factor, 𝐾𝑇𝑃 to account for variations in temperature and pressure is given as;   
                                         𝐾 (273.15+𝑇)𝑃
𝑇𝑃= 𝑜                                                                                     (2.22)
 
(273.15+𝑇𝑜)𝑃
  
                         
Where kTP is the correction factor, P and T are the ambient pressure and temperature 
during the air kerma measurement  
  P0= 101.32 kPa (1 atm) T0= 293.2 K or 20°C are the values of the calibration reference 
conditions.                                                   
If the calibration coefficient was done at relative humidity 50%, there is no need to 
correct for humidity since it fall within 20% to 80% relative humidity.   
The corrected ionization chamber response MQ is given as: 
       M = 𝑀 𝐾 𝐾 𝐾Q 𝑟𝑎𝑤 𝑇𝑃 𝑑𝑖𝑠𝑡 𝑠𝑡𝑎𝑏𝐾𝑜𝑡ℎ𝑒𝑟𝑠                               (2.23) 
Where 
𝑀𝑟𝑎𝑤 is the mean value of the readings taken after the instrument is settled, 
𝐾𝑇𝑃 is a factor to correct for departure of air density from reference conditions, 
 𝐾𝑑𝑖𝑠𝑡 is a factor to correct for the deviation of chamber position from the 
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reference position, 
𝐾𝑠𝑡𝑎𝑏 is a factor to correct for the stability of the SSDL reference standard, 
𝐾𝑜𝑡ℎ𝑒𝑟𝑠 is a factor including all the corrections whose uncertainties are too 
small to consider individually in the uncertainty budget, because they are 
estimated to be much less than 0.1%. Nevertheless, SSDLs are advised to 
review and assess these factors independently to ensure that their overall 
contribution is indeed negligible (less than0.1%). 
𝐾𝑜𝑡ℎ𝑒𝑟𝑠 is given by: 
          𝐾𝑜𝑡ℎ=𝐾𝑒𝑙𝑒𝑐𝐾𝑙𝑖𝑛𝐾𝑠𝐾𝑙𝑒𝑎𝑘𝐾ℎ𝐾𝑝𝑜𝑙𝐾𝑟𝑜𝑡𝐾𝑓𝑠𝐾ℎ𝑜𝑚              (2.24) 
Where: 
𝐾𝑒𝑙𝑒𝑐 is the calibration coefficient of the measuring assembly, in case the 
chamber and measuring assembly are calibrated separately, 
𝐾𝑙𝑖𝑛 is a factor to correct for non-linearity of the measuring assembly sensitivity, 
𝐾𝑠 is a factor to correct for the lack of saturation due to recombination 
𝐾𝑙𝑒𝑎𝑘 is a factor to correct for leakage current (possibly converted from an 
additive correction). 
𝐾ℎ is a factor to correct for any departure of humidity from the reference 
condition: 50% relative humidity (RH). 
𝐾𝑝𝑜𝑙 is a factor to correct for any change in the reading due to changing the 
polarizing voltage from its value at calibration. 
𝐾𝑟𝑜𝑡 is a factor to correct for any misalignment (rotation, tilt) of the chamber in use, 
𝐾𝑓𝑠 is a factor to correct for departure of the field size from the reference condition, and 
𝐾ℎ𝑜𝑚 is a factor to correct for radial non-homogeneity of the beam 
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IEC 61674 explains that sealed chambers, in which the air volume does not change, are 
not suitable for diagnostic radiology dosimetry; their necessary wall thickness may cause 
unacceptable energy dependence, while the long term stability of the chambers is not 
guaranteed. 
  
2.10.1   Cross calibration  
Cross calibration is a general term used to describe the direct comparison of reference 
instrument that has been calibrated at a secondary standard dosimetry laboratory (SSDL) 
and a field or newly constructed instrument in a suitable quality beam (IAEA-TRS 457, 
2007).  
The two methods used in cross calibration include tip-to-tip and substitution method. For 
the tip to tip method, both the reference ion chamber and the one to be calibrated are 
irradiated together and readings are recorded for each of them (IAEA-TECDOC-1585, 
2008). To ensure that, scattered radiation for each of the detectors are the same, the two 
detectors are of a similar design. (IAEA-TRS 469, 2009).  
For the substitution method, reference output rates of the beam with the reference 
detector are determined through a set of readings. The detector to be calibrated is put 
under the same conditions and similar sets of reading are taken and recorded.  This 
method is adopted when the chambers to be compared are very different in size or shape. 
With the substitution method, the beam non uniformity accuracy is less compared to the 
tip-to-tip calibration method (IAEA-TECDOC-1585, 2008). 
                                                      
 
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CHAPTER THREE 
MATERIALS AND METHODS 
3.1   Introduction 
In this chapter, the conceptual design, materials and experimental methods are presented. 
The construction processes includes designing, machining, drilling, bonding and 
assembling of the components. The pre-evaluation tests to ensure that the ionization 
chamber satisfies the IEC  61674 (2013) standard has also been presented. 
 
.3.2   Materials used 
Table 3.1 presents a brief description of the major materials that were used for 
constructing the ionization chamber. 
Table 1.1 Properties of materials used to construct the chamber 
S/N Material Properties/Characteristics 
1 Aluminium Aluminium is a good conductor of heart and electricity 
with a melting point of 660 OC. It is light in weight, 
malleable, ductile and non-corrosive.  
2. Copper Copper has a melting point of 1080 OC. It a good 
conductor of electricity and heat. Its malleable, ductile 
and non-corrosive 
3 Polymethylmethacrylate PMMA Is one of the durable materials used for the 
(PMMA)  construction of ionization chamber. It is transparent 
and easy to machine. It is used as an electrical  
insulator  
 
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3.3   Equipment needed to evaluate the performance characteristics. 
i. Acuity Conventional Radiotherapy Simulator  
ii. PTW Unidos electrometer 
iii. Digital thermometer 
iv.  Digital barometer 
v. Piranha Diagnostic Multimeter 
 
3.3.1 Acuity Conventional Radiotherapy Simulator 
Acuity Radiation Treatment Planning Simulator (MPN: 316900714) is a KV X-ray 
machine and detector, that is attached to a machine that emulates the movements of 
a radiotherapy treatment machine as linear accelerator (LINAC). Therefore it makes it 
possible to produce X-ray images from the patient body under positioning conditions 
simulating a LINAC and make it possible to control all parameters on the treatment plan 
such as the field size, beam directions and collimator setting. The gantry and collimator 
(to shape the desired field size and direction) can rotate about angle of ±190° and ±185°, 
respectively, and a frame Laser positioning or alignment system (LAP Apollo). 
 It has exposure parameters of fluoroscopy and radiography modes, up to 150 kVp and 
300 mAs (radiography) and 4 mA (fluoroscopy) with 2.5 mmAl filter and 30 kW high. It 
was manufactured by Varian Medical Systems in the year 2008 and the X- ray tube was 
changed in 2019.  This Radiation Treatment Planning Simulator shown in 3.1 was used 
during the performance characteristic test of the newly constructed chamber.  
 
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Gantry 
Collimator 
Gantry Angle 
Hand Control 
Couch 
 
                  Figure 3.1: Acuity Conventional Radiotherapy Simulator 
 
3.3.2 PTW Unidos Electrometer 
The PTW Freiburg Unidos Electrometer (SN: 1053, Version: 2.40i- Germany) is used to 
supplies bias voltage to the ionization chamber. It can operate in radiological mode for 
measuring small current. It has a large liquid crystal display (LCD). It has a bias voltage 
of ± 0 – 400 V with 50 V increment. The polarity of the bias voltage can be selected by 
the use of a switch at the back of the electrometer. The electrometer was calibrated 
together with its accompanying ionization chamber. 
When taking dosimetric measurements, the chamber was connected to the electrometer to 
obtain the chamber response to radiation. During measurement in this study, the 
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electrometer was operated in current mode, hence the measurement parameters are in 
pico amperes (pA). Figure 3.2 shows the PTW Freiburg Unidos Electrometer used in this 
study. 
 
                                                   
              
                      Figure 3.2 PTW Unidod Electrometer 
                 
3.3.3   Digital Thermometer 
The Dostmann Electronic Digital thermometer (SN P700, Germany) with 
dimensions 200 mm x 93 mm x 44 mm was used during charge 
measurements to correct the effects of temperature differences between the 
standard reference temperature of (20.0 ºC) and the temperature recorded 
at the facility under different environmental conditions. It has a detachable 
measuring probe that is fixed into position before it can be used. Figure 3.3 
shows the thermometer that was used in this study. It measures-temperature 
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in oC – Celsius and oF - Fahrenheit with allowable operating temperature 
of 0 °C to +40 °C and  highest accuracy degree of ±0.03 °C 
 
                         Figure 3.3 Digital Thermometer 
 
 3.3.4 Digital Barometer 
A hand held Digital Barometer (Model: XA1000, Germany) for barometric air pressure 
800 - 1100 mbar with dimensions 170 mm x 62 mm x 34 mm  was used during current 
measurements to correct the effects of  pressure differences that may exist between 
standard reference pressure (101.325 hPa) and pressure recorded at the study centre. It  is 
simple operated by touchscreen and has ±0.5 mbar degree of accuracy. It is used for 
accurate measurement of the barometric air pressure up to 4000 m latitude above sea 
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level.  Figure 3.4 shows the barometer that was used. It has external sensor for climate 
measurement application for temperature, humidity and air flow. 
 
      
                     Figure 3.4 Digital Barometer 
 
3.3.5 Piranha Diagnostic Multimeter  
 Piranha Diagnostic Multimeter (SN: CB2-15020088, RTI-Sweden) is used for quick 
quality control (QC) test in radiography, fluoroscopy, computed tomography (CT), 
dental, and mammography. The unique features of the Piranha Diagnostic Multimeter 
makes it possible to check the position (position check) of the detector before measuring. 
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This verifies that the detector area is fully irradiated. It is a solid-state detector and hence 
no need to compensate for temperature and pressure. Its compatibility with the Ocean 
diagnostic software and provides quick and repeatable Quality Control (QC) activities. It 
measures and presents all parameters instantly and simultaneously.  Figure 3.5 shows the 
Piranha multi that was used to validate the newly constructed chamber.  
 
                             Figure 3.5: Piranha multimeter 
 
3.4   Conceptual Design 
The conceptual design of the parallel plate ionisation chamber was developed and 
evaluated. This concept of a portable parallel plate ionization chamber design comprises 
of an entrance window which is mechanically secured on top of the main body by means 
of a screw. The bias electrode is positioned in a way to reduce current leakage. The 
triaxial cable support is firmly secured into the body by means of glue. It has cork at the 
end to hold the cable firmly into position. Due to proximity of the measuring and the 
polarized electrode, recombination was anticipated to be greatly reduced. With this 
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design, effective functioning of the ionization chamber was expected. Figure 3.6 and 3.7 
shows sketches of the design concept. 
 
            
                     Figure 3.6 Conceptual design 
                                                                 
             
Figure 3.7 Material equivalent of the conceptual design 
 
 
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Figure 3.8 shows the exploded solid computer aided design (CAD) model of the parallel 
plate ionization chamber in stepwise arrangement of the components. 
                           
                         
Figure 3.8 Exploded solid CAD model of the parallel plate ionization chamber 
          
3.5   Assembling processes 
The bias electrode was first fixed onto the bottom cover. The measuring electrode was 
fixed on top of the bottom cover. These three components were securely fixed into the 
body. The top cover, polarizing electrode and the guard ring are all aligned into position 
to take the screws. The crews are further tightened to ensure firmness. The various 
components were assembled by using adhesive and screws. 
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                           Figure 3.9 Assembled components 
                                                                                        
                            
 
 
 
           
 
     
  
 
 
  
           Figure 3.10: Assembled parallel plate ionization chamber 
 
 
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3.6   Evaluation of constructed Parallel plate ionization chamber performance  
characteristics 
To satisfy international standards as stipulated in BS EN 61674 2013, the following 
dosimetric measurements were carried out to evaluate the performance characteristics of 
the constructed parallel plate ionization chamber. 
i. Bias voltage response. 
ii. Angular /Directional dependence 
iii. Pre- irradiation current leakage  
iv.  Current Linearity 
v. Half value layer (HVL) 
vi. Short term stability (Consistency) 
vii. Repeatability 
viii. Chamber calibration. 
These performance characteristics test were carried out at National Radiotherapy 
Oncology and Nuclear Medicine Centre at the Korle – Bu Teaching Hospital (KBTH) 
Accra, Ghana. 
 
3.6.1 Chamber response to Bias voltage 
The bias voltage was determined by selecting various voltages from the electrometer and 
irradiating the chamber, while corresponding readings were recorded. An open field of 10 
× 10 cm2,   100 cm SSD technique was adopted. The chamber’s voltage ranged between -
400 V to +400 V with 50 V incremental intervals. The ion collection efficiency, polarity 
effect and saturation factor were all determined by the bias voltage response.  
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  Effective polarity effect = (1- mean Kpol) ×100%                                       (3.1) 
Where Kpol =   polarity factor 
The bias voltage results are presented in Table 4.2. 
 
3.6.2 Angular/Directional dependence 
The angular dependence check was carried out using the Acuity simulation machine. An 
open field of 10 × 10 cm2, 100 cm SSD technique was adopted. With increments of 10o 
the gantry was moved from 0o to 90o anticlockwise and clockwise, while the various 
responses were recorded for each angle. Figure 3.11 shows the angular dependency set 
up. Results for angular dependence is presented in Appendix 2, Table2.B. 
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Constructed 
Chamber 
 
Figure 3.11: Set- up for the determination of angular dependency of the constructed 
chamber. 
 
3.6.3   Pre-irradiation current leakage 
Prior to pre-irradiation current leakage test, the electrometer was first switched on for a 
period of five minutes as readings were monitored and recorded without the constructed 
chamber connected. The constructed chamber was later connected to the electrometer and 
then switched on while leakage current was monitored and recorded at integral time of 
100 sec. In this case, the chamber was not exposed to any radiation. Results for pre-
irradiation current leakage is presented in Appendix 2 Table 2B. 
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3.6.4 Current Linearity test 
In the current linearity test, an open field of 10 × 10 cm2 at 100 cm SSD technique was 
used. The electrometer was set to a bias voltage of +300 V.  The various current 
responses were monitored and recorded for each kVp starting from 50 kVp to 130 kVp. 
The chamber responses were plotted against the kVp. Results for current leakage test is 
presented in Appendix 2, Table 2E. 
 
3.6.6    Energy dependence (Half Value Layer- HVL) 
A narrow field size of 4 × 4 cm2,   100 cm SSD technique was used for assessing the 
energy dependence. The beam was collimated to match the sensitive area of the detector 
to minimize scattering. The electrometer was set to a bias voltage of +300 V. Figure 3.12 
shows the set up for energy dependence. Ten (10) pieces of alumunium plate measuring 
50 mm x 50 mm x 1.47 mm were used during the measurements. The set up shows the 
angular dependency set up 
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Constructed 
Thermometer 
Chamber 
Sensor 
Digital 
Thermometer 
                        
Figure 3.12 Set -up for the determination of HVL of the constructed ion chamber 
 
First, measurement was taken without the aluminium plate and recorded. In an order of 
one, two, three, four etc, the plates were placed on top of the chamber until half of the 
measured value (output) obtained without plate is obtained. A graph of chamber response 
(pA) was plotted against thickness of the aluminium plates in (mmAl). The graph is 
presented in Figure 4.7, 4.8, 4.9, 4.10 and 4.11 
 
3.6.7 Short term stability test 
An open field of 10 × 10 cm2,   100 cm SSD, 80 kVp, 200 ms and 20 mAs technique was 
used. The electrometer was set to a bias voltage of +300 V. The responses were 
monitored and recorded, as the same kVp was repeated ten times. A graph of chamber 
responses was plotted against kVp (Figure 4.2) 
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3.6.8 Medium term stability 
Repeatability test was performed at an open field of 10 ×10 cm2, 100 cm SSD, 80 kVp, 
200 ms and 20 mAs technique. The electrometer was set to a bias voltage of +300 V. The 
various current responses were monitored and recorded as the same kVp was repeated in 
a week interval for three weeks. For all measurements, the laiser positioning alignment 
was used  to achieve  the same set up.  The chamber responses was plotted against the 
kVp. Results for the repeatability test is presented in Figure 4.3. 
 
3.6.9 Chamber calibration 
A Piranha Multimeter with Serial number CN2-15020088 was used to cross calibrate the 
newly constructed chamber. Substitution method was used for the cross calibration. 
Measurements were taken in an open field size of 10 × 10 cm2 at 100 SSD irradiating 
technique.  The Piranha multimeter was irradiated from 50 kVp to 130 kVp and their 
corresponding exposure and exposure rates were recorded. In the same set up, the 
developed chamber was irradiated from 50 kVp to 130 kVp while the electrometer 
reading were recorded. The readings were corrected for the influences of pressure, 
temperature, polarity and ion recombination. Exposure in mGy recorded from the piranha 
was plotted against the charges in pA recorded from the electrometer to determine the 
calibration factor using the equation 2.11.     The figure below shows the quality control 
set up 
 
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Figure 3.13: Set -up for the determination of QC on the Acuity simulation planning 
machine. 
 
In the same set up, the developed chamber was irradiated from 50KVp - 130KVp while 
the electrometer reading were recorded. The readings were corrected for the influences of 
pressure, temperature, polarity and ion recombination. Exposure in mGy recorded from 
the Piranha was plotted against the charges in pA recorded from the electrometer to 
determine the calibration factor using the equation 2.11. 
  
 
 
 
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CHAPTER FOUR 
RESULTS AND DISCUSSIONS 
4.1   Introduction 
The methodology technique use for the construction of the parallel plate ionization 
chamber is validated for dosimetry in X-ray systems. It discusses and evaluates the 
results of the construction and the findings of the performance characteristics of the 
developed chamber. 
 
4.2 Characteristics of the constructed ionization chamber 
The chamber was designed based on the specifications of cavity ionization chambers.  
It  comprises of a body made of Perspex (1.7 mg/cm²), a bias electrode made of copper 
plate, a measuring electrode made of an aluminum plate, guard rings made of an  
aluminum plate, an entrance window made of a paper coated with a pencil ( graphite), a 
sensitive volume of air 2.8 cm³, triaxle cable, and a connector. The polarization voltage 
ranges from 200V – 400 V.  
The specifications compare favourably with the standards except that the measuring 
electrode diameter is a little above the recommended 20 mm due to systematic error on 
the machine used. Table 4.1 shows the chamber characteristics and what has been 
recommended by Andreo et al (2005). 
 
 
 
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Table 4.1 Characteristic of the constructed parallel plate ionization chamber 
C0MPONENTS RECOMMENDED USED 
MEASUREMENT MEASUREMENT 
Body/Shell ∅(mm) 30 -75mm 49mm 
Entrance window tw (mm) ≤ 1mm 0.3 mm 
Collecting electrode ∅(mm) ≤ 20mm 21mm 
Sensitive Volume (cm3) 0.05 – 3.5cm3 2.8cm3 
Electrodes separation ≤ 2 1.5 
Guard electrode width   7mm 5mm 
Cable Triaxial Triaxial 
 
 
4.3 Preliminary test of the parallel plate ionization chamber 
4.3.1 Bias voltage of the chamber 
Table 4.2 shows the response of the newly constructed chamber response with bias 
voltage. The responses were corrected for temperature and pressure, (KTP), and polarity, 
(Kpol), using equations: (2.21), and (2.14), respectively. Equation 2.23 was used to 
calculate the corrected chamber responses (Mcorr). 
  
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Table 4.2 Chamber response with bias voltages 
(V) M(uncorr) KTP Kpol Mcorr  
50 91.724 1.018 0.804 75.165 
100 105.414 1.018 0.891 96.741 
150 108.607 1.020 0.937 98.814 
200 110.545 1.020 0.971 109.552 
 250 110.921 1.018 0.984 111.214 
300 112.634 1.019 1.002 115.082 
350 112.351 1.018 0.995 114.232 
400 113.924 1.018 0.997 115.741 
-50 89.447 1.018 0.800 72.910 
-100 98.546 1.018 0.887 86.192 
-150 102.523 1.018 0.933 97.524 
-200 108.295 1.019 0.968 106.821 
-250 110.712 1.019 0.979 110.631 
-300 113. 171 1.019 0.997 114.910 
-350 111.302 1.019 0.988 112.145 
-400 110.713 1.019 0.992 108.627 
 The corrected chamber responses (Mcorr) were used to determine polarity effect (Kpol), ion 
collection efficiency (fion), ion recombination and saturation curve. 
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4.3.1.1   Polarity effect 
The polarity effects listed in Table 4.3 was determined based on the polarity correction 
factors. Equation 2.14 was used to calculate the polarity effect of the constructed 
chamber. The mean value of the polarity correction factor was determined, then 
subtracted from unity, which is the ideal polarity correction factor. The result was then 
expressed as a percentage using the equation 3.1. 
 
 The mean value of the polarity correction factor   was 0.97275 which resulted in a 
polarity effect of 2.7 ±0.03%. (M+ Positive bias reading and M- negative bias reading). 
Table 4.3: Polarity correction factors for various bias voltages 
(kVp) M+ M- Kpol 
50 91.745 89.452 0.979 
100 105.452 95.332 0.901 
 
150 108.641 102.542 0.948 
 110.563 108.281 0.971 
200 
 250 110.908 110.712 0.991 
300 112.625 113. 145 1.004 
350 112.374 111.365 0.995 
400 113.932 110.706 0.993 
 
The polarity effect of the chamber was found to be above tolerance limit of 1% as 
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recommended by IEC 61674, (2013). The effect of this is that, all the ion created are 
collected. This can be mitigated by re-designing of the electrodes 
 
4.3.1.2 Ion collection efficiency and recombination 
The constructed chamber ion collection efficiency (fion) response with bias voltage (V) 
are shown in Table 4.4.  Equation 2.15 was used to estimate ion collection efficiency. The 
inverse of the efficiency is the saturation value (Ksat). 
 
Table 4.4: Ion collection efficiency for the constructed ionization chamber 
 V M+ fion Ksat 
    
50 91.70 
0.81 1.21 
100 105.40 
0.93 1.07 
150 108.60 
0.96 1.04 
200 110.50 
0.98 1.02 
 250 110.90 
0.98 1.09 
300 112.60 
1.00 1.00 
350 112.30 
0.99 1.01 
400 113.90 
1.01 1.00 
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IAEA-TECDOC-1585 explains that for conventional diagnostic and mammographic 
ionization chambers, the air kerma rate and air kerma per pulse values at which the ion 
collection efficiency of the ionization chamber falls to 95 % when the normal polarizing 
voltage is applied. Andreo et al, 2005, explains that Ionization chamber operated at near- 
saturation region where the ion collection efficiency is greater than 0.97.From the table, 
recombination was prominent from 50 –150 V, hence saturation voltage was 200 V. 
 
4.3.1.3 Saturation curve 
Graphs of corrected chamber responses against bias voltages were plotted in order to 
establish the saturation voltages of the chamber, for positive and negative bias voltages as 
seen in Figure 4.1. The curves were a good representation of a typical saturation curve 
shown in Figure 2.12.   
Polarizing voltage
150
100
50
0
-500 -400 -300 -200 -100 0 100 200 300 400 500
-50
-100
-150
Polarizing Voltage (V)
 
         Figure 4.1: Saturation curve of the constructed ionization chamber responses 
 
 
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To determine the positive bias voltage equation 2.16 was used. At 350 V, the saturation 
factor was found to be high.   The graphs were found to saturate at +200 V for positive 
bias voltage and -250 V for the negative bias voltage. At this linear range, change in 
voltage in the operational region does not have much effect on the chamber output  This 
means that the newly constructed ionization chamber would have 99% ion collection 
efficiency if the maximum electrometer bias voltage was +350 V. The effects of ion 
recombination and charge multiplication would essentially be negligible as a result no 
saturation correction factor would be needed if the electrometer was operating at +350 V. 
 
4.3.2 Stability check 
4.3.2.1 Short term stability 
Figure 4.2 shows a short term stability graph of normalized chamber responses against 
time in (seconds). The purpose is this measurement was to check for reproducibility of 10 
times integral repeated response. Appendix 2: Table 2B: shows the raw data for the 
chamber reproducibility.   
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Relative chamber response
2  
1.8
1.6  
1.4
1.2  
1
0.8
0.6  
0.4
0.2  
0
0 20 40 60 80 100 120  
Time(sec) 
 
Figure 4.2: Chamber short term reproducibility response 
IEC 61674. (2013), recommends ±0.5% tolerance. With standard deviation of 0.4%, the 
short term stability check was within the limit stated.  
 
4.3.2.2   Medium term stability 
Table 4.5 shows the weekly consistency result. To establish consistency, weekly 
corrected response was plotted against percentage deviation test for medium term 
stability,  
 
Table 4.5: Weekly corrected response 
Date Mcorr % Deviation 
25/09/2020 117.9 
 0.0 
2/10/2020 112.9 
2.5 
9/10/2020 113.9 
2 
  
 
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Series1 Linear (Series1)
119
118
117
116
y = -2x + 117.9
115
R² = 1
114
113
112
0 0.5 1 1.5 2 2.5 3
% Deviation
                     
 
       Figure 4.3 Weekly percentage deviation for medium stability (consistency) check 
 
A graph of weekly output measured charges against percentage deviation is show in 
figure 4.3.  The results for medium stability showed that the mean weekly deviation 
percentage for measured charges was 1.3% which is 0.3% above the IEC 61674 (2013), 
±1.0% as recommended. This as a result of the small sample number used due to time 
constraints. 
 
 
4.3.3 Pre-irradiation leakage current 
The raw data for chamber response before irradiation is presented in Appendix 2, Table 
2J.  Figure 4.4 shows a graphical presentation of pre-irradiation leakage current. On the 
curve are displayed correlation equation, y and regression (R2) for the dotted line of best 
fit to chamber responses for pre-irradiation leakage current.  
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8
7
y = 0.0003x2 - 0.0703x + 7.1364
6 R² = 0.965
5
4
3
2 Series1
1 Poly. (Series1)
0
0 20 40 60 80 100 120 140
Time (S) 
 
 Figure 4.4: Pre- irradiation leakage test. 
          
The results shows that initial stray charges are collected and with time their magnitude 
reduces. 
The ionization chamber required a minimum of 2 minutes to stabilize. The maximum 
leakage current was 3.35pA. 
 
4.3.4 Angular dependency 
A graph of normalized chamber responses to 00 gantry angle response 
against the gantry angles is given in Figure 4.5.  
 
 
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1.2
1
0.8
0.6
0.4
0.2
0
0 50 100 150 200 250 300 350 400
Gantry Angle (degree) 
             
 
                        Figure 4.5: Normalized chamber response to gantry angle 
                    
The IEC 61674. (2013), standard recommends ±3% variation of response at incident 
angles of ±5o from 0o.  The chamber 2.99% recorded was within the recommended range.   
 
A maximum deviation of 8.6%   was observed at 90O clockwise of the gantry angle. This 
was as a result of position of the chamber cable support and the cable its self being 
irradiated. Furthermore clockwise measurements had a mean deviation of %while the 
anticlockwise was It is therefore clear that the  clockwise  gantry movement  
deviated 1.1% more than the anticlockwise gantry movement due to the  position of the 
cable support and the cable its self being irradiated. 
 
4.3.5 Chamber response linearity 
 
Appendix 2, Table 2E shows corrected chamber response to energy.   A graphical 
representation of the chamber response to energy is shown in figure 4.6. A line of best fit 
to the chamber responses to dose represented by dotted line was drawn on each graph. A 
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correlation equations, y and regressions (R2) for the lines of best fit to chamber responses 
are show 
160
140 y = 0.2276x + 51.669
R² = 0.9924
120
100
80
60
40
0 100 200 300 400 500
Tube Appiied voltage (KVp)
               
               Figure 4.6:  A graph of chamber response to voltages 
 
The chamber responses to voltages were linear with a coefficient of correlation and R2 
equivalent to 0.9924. 
 
4.3.6   Energy dependence 
 The constructed ionization chamber energy dependence was evaluated by plotting a 
graph of normalized HVL values of both detector and chamber against voltage as shown 
in Figure 4.7. The normalized HVL of the detector and chamber responses to various 
voltages are shown in the Table 4.6 below  
 
 
 
 
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Table 4.6: Normalized HVL responses. 
 
kVp Normalized Detector Normalized Chamber 
Values Values 
1.0 
60 1.0 
1.3 
80 2.5 
1.4 
100 6.0 
1.8 
120 7.9 
1.9 
130 8.5 
    
Normalized Detector Values Normalized Chamber Values
Deviation error
16
8
4
2
1
45 55 65 75 85 95 105 115 125 135
KVp
            
 
Figure 4.7: Trend of chamber HVL against detector HVL 
From the graph, deviation error decrease as kVp increases and becomes 
stable from 100 kVp upwards. This means that at low kVp deviation error 
is high and diminishes at 100 kVp. It can be seen from the graph that, 
above 100 kVp the chamber does not depend on energy. 
 
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4.3.7 Chamber HVL Measurements  
The result of HVL measurement for 60 kVp using aluminum   attenuator 
of size 50 mm x 50 mm x 1.47 mm as shown in Table 4.7. 
 
Table 4.7:  Chamber response to Aluminium (Al) attenuator at 60 kVp 
 
kVp Thickness Chamber 
response(pA) 
(mmAl) 
60 0.00 38.91 
60 1.47 37.24 
60 2.94 31.53 
60 4.41 19.55 
60 5.88 9.74 
 
 
 
 
 
 
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Chamber response Expon. (Chamber response)
60
50
40
y = 48.361e-0.233x
30 R² = 0.8607
20
10
0
0 1 2 3 4 5 6 7
Thickness(mmAl)
               
                        Figure 4.8: Chamber HVL responses at 60 kVp 
From Table 4.7 and Figure 4.8, it can be concluded that at 60 kVp, the intensity of the 
beam decreases as the thickness increases till all the beams generated are attenuated.  It 
can be concluded that an alumimium of 3.91 mm thick was used to attenuate the beam to 
half. .Therefore the HVL of the beam at 60 kVp is 3.91 mmAl. 
 
 
 
 
 
 
 
 
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Table 4.8:  Chamber response to Aluminium (Al) attenuator at 80 kVp 
 
kVp Thickness Chamber 
(mmAl) response (pA) 
80 0.00 113.64 
80 1.47 91.06 
80 2.94 77.42 
80 4.41 74.15 
80 5.88 49.39 
80 7.35 45.74 
 
Chamber response(pA) Expon. (Chamber response(pA))
120
100
y = 113.22e-0.125x
80 R² = 0.9563
60
40
20
0
0 1 2 3 4 5 6 7 8
Thickness(mmAl)
      
             Figure 4.9:  Chamber HVL response at 80 kVp 
From Table 4.8 and Figure 4.9, it can be concluded that at 80 kVp, the intensity of the 
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beam decreases as the thickness increases till a point where all the beam are attenuated.  
It can be concluded that an aluminium of 5.52 mm thick was used to attenuate the beam 
to half. .Therefore the HVL of a beam at 80kVp is 5.52mmAl. 
 
Table 4.9:  Chamber response to Aluminium (Al) attenuator at 100kVp 
kVp Thickness(mmAl) Chamber 
response(pA) 
80 0.00 196.17 
80 1.47 167.65 
80 2.94 153.49 
80 4.41 130.24 
80 5.88 117.51 
80 7.35 105.97 
80 8.82 91.75 
 
 
 
 
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Chamber response(pA) Expon. (Chamber response(pA))
250
200 y = 193.23e-0.084x
R² = 0.9959
150
100
50
0
0 2 4 6 8 10
Thickness (mmAl)
 
Figure 4.10:  Chamber HVL response at 100 kVp  
From Table 4.9 and Figure 4.10, it can be concluded that at 80kVp, the intensity of the 
beam decreases as the thickness increases till a point where all the beams generated are 
attenuated.  It can be concluded that an aluminium of 8.50mm thick was used to attenuate 
the beam to half. .Therefore the HVL of a beam at 100 kVp is 8.50 mmAl. 
 
 
 
 
 
 
 
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Table 4.10:  Chamber response to Aluminium (Al) attenuator at 120 kVp 
KVp Thickness(mmAl) Chamber 
response(pA) 
120 0.00 310.36 
120 1.47 283.43 
120 2.94 274.38 
120 4.41 234.44 
120 5.88 218.72 
120 7.35 197.46 
120 8.82 150.16 
120 10.29 135.53 
 
 
Chamber response(pA) Expon. (Chamber response(pA))
350
300
y = 329.98e-0.081x
250 R² = 0.9567
200
150
100
50
0
0 2 4 6 8 10 12
Thickness (mmAl)
 
Figure 4.11:  Chamber HVL response at 120 kVp. 
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From Table 4.10 and Figure 4.11, it can be concluded that at 120 kVp, the intensity of the 
beam decreases as the thickness increases till a point where all the beam will be 
attenuated.  It can be concluded that an aluminium of 9.32 mm thick was used to 
attenuate the beam to half. .Therefore the HVL of a beam at 120 kVp is 9.32 mmAl. 
Table 4.11:  Chamber response to Aluminium (Al) attenuator at 130 kVp 
kVp Thickness(mmAl) Chamber 
response(pA) 
130 0.00 335.92 
130 1.47 306.35 
130 2.94 297.73 
130 4.41 264.54 
130 5.88 243.56 
130 7.35 224.52 
130 8.82 207.57 
130 10.29 166.48 
130 11.76 160.44 
 
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400
350
y = 347.44e-0.064x
300 R² = 0.975
250
200
150
100
50
0
0 2 4 6 8 10 12 14
Thickness(mmAl)
         
Figure 4.12:  Chamber HVL response at 130 kVp. 
From Table 4.11 and Figure 4.12, it can be concluded that at 120 kVp, the intensity of the 
beam decreases as the thickness increases till a point where all the beam will be 
attenuated.  It can be concluded that an aluminium of 10.45 mm thick was used to 
attenuate the beam to half. .Therefore the HVL of the beam at 130 kVp is 10.45 mmAl. 
 
4.3.8:  Beam quality correction factor kQ 
 The Beam quality correction factor kQ was determined by using equation 2.11. 
Table 4.12 shows normalized chamber beam quality correction factor kQ at 100 kVp and 
HVL. Figure 4.13 presents a graph of normalized kQ values and HVL. 
 
 
 
 
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Table 4.12: Normalized chamber beam quality correction factor KQ and HVL 
KVp kQ(mGy/pA
2) Normalized kQ  Values HVL(mmAl) 
60 1.046 0.914 3.914 
80 0.892 1.072 5.526 
100 0.956 1.000 8.501 
120 0.977 0.976 9.323 
130 1.028 0.930 10.459 
 
 
Normalized KQ  Values Poly. (Normalized KQ  Values)
1.1
1.08
1.06
1.04
1.02 y = -0.0009x4 + 0.028x3 - 0.3405x2 + 1.7931x - 2.3652
1 R² = 1
0.98
0.96
0.94
0.92
0.9
0 2 4 6 8 10 12
HVL
 
Figure 4.13: A graph of normalized kQ values against HVL. 
From the graph, the beam quality correction factor of the constructed chamber can 
expressed with a fourth degree polynomial equation in terms of HVL (in mmAl) using 
100 KVp and 20 mAs  (200 mA) as the reference exposure parameters. 
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4.3.9   Calibration coefficient of the constructed chamber 
Table 4.13 shows the corrected parallel plate ionization chamber and the reference 
detector responses after cross calibration by substitution. 
 
 Table 4.13: Reference detector and chamber responses 
 
Tube Voltage Reference Chamber response 
(KVp) Detector Mcorr  ( pA)              
Exposure (mGy) 
50 0.0336 18.0421 
60 0.0692 38.9324 
70 0.1146 78.5021 
80 0.1723 113.6154 
90 0.2424 153.5324 
100 0.3188 196.1258 
110 0.4057 250.9145 
120 0.5153 310.3712 
130 0.6219 355.9452 
 
 
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Series1 Linear (Series1)
0.7
0.6
y = 0.0017x
R² = 0.9946
0.5
0.4
0.3
0.2
0.1
0
0 50 100 150 200 250 300 350 400
Chamber responses (pA) 
       
 
Figure 4.14: A graph of reference detector against chamber responses 
 
 
 The corrected responses for the reference detector and constructed chamber were used to 
determine the calibration coefficient of the local chamber using Equation 2.22. The results 
showed that the calibration coefficient Nk of the constructed chamber was 1.7x109 mGy/A 
with uncertainty of ±0.01%. With a regression R2 of 0.9946, the chamber response is 
linear to the detector response 
 
4.4 Limitations  
 Due to time constraints, and unavailability of a lower kVp X-ray source, like 
mammography machine, sensitivity test could not be done. Sensitivity test 
was to be done to find out the minimum energy required by the chamber to 
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give a signal. 
 A solid state ionization chamber (Piranha multimeter) which was calibrated 
by Swedac Ackredictering was used for the cross calibration   due to the non-
existence of parallel plate ionization chamber in the country.   
 
 
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CHAPTER FIVE 
CONCLUSIONS AND RECOMMENDATIONS 
5.1   Conclusions 
The ultimate goal of this work was to design and fabricate a portable, and less expensive 
parallel plate ionization chamber from available materials to minimize cost, a device that 
will assist medical physicist, radiographers, and radiation protection officers for 
dosimetry in conventional radiography in Ghana.  
 Indeed a vented parallel plate ionization chamber has been designed, fabricated, and 
calibrated for non calibrated beams and for scattered radiation. The results of the test 
highlighted some other important concepts of dosimetry, such as, angular/directional 
dependence, linearity, ion recombination, and leakage current. 
The ion chamber was shown to be consistent, and capable of demonstrating some of the 
fundamental principles involved in ionizing radiation measurement.    
The performance characteristics tests on bias voltage response, angular dependence, dose 
linearity check, HVL, pre-irradiation, current leakage, and energy dependence were all 
within tolerance limit of IEC 61674, 2013.  
The calibration coefficient for the chamber was determined to be 1.7x109 Gy/A with 
uncertainty of 0.01%.The chamber is applicable in X-ray beam quality ranging of 50 – 
130 kVp. The chamber wall was made of Perspex (PMMA) which had given the chamber 
a unique property of tissue equivalence and also relatively less mechanical fragility.  
Furthermore, the chamber has a maximum pre-leakage current of 3.35pA current which 
stabilizes after two minutes.  
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5.2 Recommendations  
5.2.1 Radiography centres 
The local vented parallel plate ionization chamber ionization chamber provides an 
alternative for dosimetric measurement of conventional X-ray machines and megavoltage 
x-ray beams.  Facilities that are financially constrained to use commercial ionization 
chambers has a choice to make now.  
 
5.2.2 Research community 
 
 Test for sensitivity was not carried out since the Acuity simulator planning 
machine used has an energy range from 40 kVp to 150 kVp. There is the need to 
test for sensitivity to know the minimum kVp required to produce a signal. 
 A similar test can be done by comparing the constructed chamber to another 
detector which calibration is based on ionization chamber. 
 Although the chamber satisfies IEC 61674, 2013 recommendations, it is 
recommended that the calibration coefficient must be confirmed by a secondary 
standard  dosimetry laboratory (SSDL) 
 
 
 
 
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REFERENCES 
Ahmed, S. N. (2015). Physics and Engineering of Radiation Detection. Ontario, Canada: 
Academic Press Inc. 
Alessandro, M. C., & Caldas, L. V. E., (2008), Plane-parallel  ionization  for  X-radiation 
of conventional radiography and mammography, Radiol Bras vol.41 no.1 São 
Paulo 
Almond, P. (2009). A brief history of dosimetry,calibration protocol, and the need for 
accuracy. AAPM Summer School.  
Andreo, P. E. D. R. O., Seuntjens, J. P., & Podgorsak, E. B. (2005). Calibration of photon 
and electron beams Radiation Oncology Physics. Vienna: IAEA. 
Andreo, P., Almond, P. R., Mattsson, O., Nahum A. E., And Roos, M. (1995). The Use 
Of Plane-Parallel Ionization Chambers In High-Energy Electron And Photon 
Beams, An International Code Of Practice For Dosimetry. IAEA Code of Practice 
for plane-parallel ionization chambers. Vienna: IAEA TRS-381. 
Atix, F. H. (1986). Introduction to Radiological Physics and Radiation Dosimetry. 
Wisconsin: WILEY-VCH VerlagGmbH & Co.KGaA. 
Bailey, D.L., Humm J.L., Todd-Pokropek, A., and Aswegen-van, A. (2014). Nuclear 
Medicine Physics: A Handbook for Teachers and Students. Vienna, Austria: 
IAEA. 
Cherry, S. R., Sorenson, J. A., and Phelps, M. E. . (2012). Physics in Nuclear Medicine 
4th . Elsevier Health Sciences. 
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De Souza C. N., Caldas L. V. E., Sibata, C. H., Ho, A. K., And Shin, K. H. (1995). Two  
New Parallel-Plate Ionization Chambers For Electron Beam Dosimetry. Pinheiros, 
Sao Paulo, S.P., Brazil 05499, 66. 
DeWerd, L.A., Davis, S., Bartol, L., and Grenzow F. . (2006). Ionization Chamber 
Ionization Chamber Instrumentation. Wisconsin: University of Wisconsin & 
ADCL University of Wisconsin & ADCL Madison. 
Goitein, M. (2007). Radiation Oncology: A Physicist’s-Eye View. Windisch, 
Switzerland: Springer. 
Halato, M. A., Suliman, I.I., Kafi, S. T., Ahmed, A. M., Sid Ahamed, F. A., Ibrahim, Z.,               
and Suliman, M. F., (2008), Dosimetry for Patients undergoing  Radiographic 
Examinations in Sudan . IX Radiation Physics & Protection Conference, Nasr 
City - Cairo, Egypt , 157-162. 
Hartmann, G. H. (2012). Radiation Dosimeters. In J. Izewska, and G. Rajan. Review of 
Radiation Oncology Physics: A Handbook for Teachers and Students. 
(Heidelberg: IAEA. 
Hooten, B. (2000). Ionization Chamber Construction. Standard Imaging, Inc. 
IAEA, Technical Reports Series No. 398. (2000). Absorbed Dose Determination In 
External Beam Radiotherapy. Vienna - Austria: International Atomic Energy 
AgencY. 
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IAEA, Technical Reports Series No.457. (2007). Dosimetry In Diagnostic Radiology: An 
International Code Of Practice. Vienna, Austria: International Atomic Energy 
Agency. 
IAEA-Tecdoc-1585. ( 2008). Measurement Uncertainty: A Practical Guide For 
Secondary Standards Dosimetry Laboratories. Measurement Uncertainty A 
Practical Guide For Secondary Standards Dosimetry Laboratories IAEA, Vienna, 
2008 Austria: IAEA,. 
IEC 61674 (2013).  Medical electrical equipment — Dosimeters with ionization 
chambers and/or semiconductor detectors as used in X-ray diagnostic imaging. 
BSI Standards Publication. UK: BSI Standards Limited. 
Inkoom S., Schandorf C., Emi-Reynolds G. and Fletcher J. J., ( July 2011 ), Quality 
Assurance and Quality Control of Equipment in Diagnostic Radiology Practice - 
The Ghanaian Experience, Radiation Protection Institute, Ghana Atomic Energy 
Commission, Accra,  https://www.researchgate.net/publication/221913318 
Khan, F. M. (2010). Physics of Radiation Therapy, The, 4th Edition. Lippincott Williams 
& Wilkins. 
Knoll, G. (2010). Radiation Detection and Measurement. 4th ed. New York, Wiley; . 
Kyere, A. (2018). Lecture notes: Radiation Detection Principles and and Instruments. 
Radiological Protection. University of Ghana - Legon, Ghana: NET 130:. 
Mayles, P., Nahum, A., & Rosenwald, J. C. (Eds.). (2007). Hand book of radiotherapy 
physics, theory and practice. Boca Raton,USA: CRC Press. 
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Okuno, E. (2012). Instrumentation for Dosimetry. In J. C. Hourdakis and R. Nowotny, 
Review of Diagnostic Radiology Physics: A Handbook for Teachers and 
Students. S. Paulo, Brazil,: IAEA. 
Podgorsak, E. ( 2005). Radiation Oncology Physics: A Handbook for Teachers and 
Students. Vienna,Austria: International Atomic Energy Agency . 
Podgorsak., E. (2006). Set of 189 slides on Calibration of Photon and Electron Beams. In 
E. a. Pdgorsak, Radiation Oncology Physics: A Handbook for Teachers and 
Students (p. 9.2.1 Slide 1). Montreal, McGill University: IAEA publication. 
PTW Unidos Electrometer. (n.d.). ptwdosimetry.com. Retrieved from 
https://image.app.goo.gl/yQuZtTVCqvbC3wRe9 
Ross, J. (2009). Design Of Ionization Chambers For Use In Teaching X-Ray Dosimetry . 
Oklahoma: East Central University. 
Seco, J., Clasie, B., & Partridge, M. (2014). Review on the characteristics of radiation 
detectors for dosimetry and imaging. 59(20), R303. Physics in medicine and 
biology, 59(20) R303. 
Solimanian, A., Ensaf, M. R., and Ghafoori,M. (2005). Design, Construction And 
Calibration Of Plane-Parallel Ionization Chambers At The SSDL Of Iran. Karaj - 
Iran: Atomic Energy Organization of Iran (AEOI). 
Standard Imaging, Inc. (2015). Ionization Chamber And Leakage Measurements: Best 
Practice Guide. Standard Imaging, In.  
 
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                                 APPENDIX 1. 
 1A: Ionization Chamber Assembly Design Drawings 
 
 
 
 
 
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APPENDIX 2 
 
 Preliminary measurements of the parallel plate ionization chamber 
 
Table 2A: Positive bias voltage electrometer readings for the chamber. (80KVP, 200mA 
and 20mAs) 
 
 Electrometer P (hPa) T (OC) KTP 
Voltage reading (pA) 
(+V) 
50 91.7 1010.2 24.6 1.0187 
100 105.4 1010.3 24.6 1.0186 
150 114.6 1010.2 25.1 1.0204 
 
 
200 115.5 1010.3 25.1 1.024 
250 117.4 1010.3 24.7 1.0189 
300 117.8 1010.1 24.7 1.0191 
350 118.3 1010.1 24.6 1.0187 
400 128.9 1010.1 24.6 1.0187 
 
 
 
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Voltage     Electrometer P (hPa) T (OC) KTP 
(-V) reading (pA) 
50 -86.4 1010.1 24.6 1.0188 
100 -98.3 1010.1 24.6 1.0188 
 
150 -102.5 1010.1 24.6 1.0188 
200 -114.2 1010.3 25.0 1.0199 
250 -110.7 1010.3 24.0 1.0199 
300 -113. 1 1010.2 24.7 1.0190 
350 -107.3 1010.2 24.7 1.0190 
400 -103.7 1010.2 24.7 1.0190 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Table 2B:  Repeatability/Consistency electrometer readings for the chamber using a bias 
voltage of +300V and a setup of 80KVP, 200mA and 20mAs. 
  (KVp) Time  (S) Electrometer Normalizes 
reading (pA) 
Values 
80 10 112.6 1.0000 
80 20 113.1 0.9955 
80 30 112.4 1.0017 
80 40 111.4 0.9757 
80 50 115.4 0.9757 
80 60 114.6 0.9825 
80 70 113.6 0.9911 
80 80 110.4 1.0199 
80 90 113.5 0.9920 
80 100 114.2 0.9859 
Mean   113.12 Std Dev. 0.004227 
  
 
 
 
 
 
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Table 2C: Corrected reproducibility electrometer readings for the chamber using a bias 
voltage of +300V in medium term stability 
 
Date Muncorr. P (hPa) T (OC) KTP Kpol Mcorr. 
(pA) (pA) 
25/09/2020 117.6 1010.5 26.1 1.023 0.9808 117.9 
2/10/2020 112.6 1010.5 22.7 1.0119 0.9911 112.9 
9/10/2020 114.3 1010.7 22.0 1.0093 0.9878 113.9 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Table 2D:  Temperature and pressure corrected electrometer readings for the chamber at 
different gantry angles normalized to the 0o gantry angle response  
 
Muncorr (pA) KTP Mcorr Normalized  
Angle  (o)  to 0o 
0 117.8 1.023 120.05 1.00000 
10 113.9 1.023 116.51 0.96689  
 
20 99.7 1.023 101.99 0.84634 
30 98.3 1.023 100.56 0.834532 
40 95.6 1.023 97.79 0.811544 
50 93.7 1.019 95.48 0.79494 
60 86.7 1.019 88.34 0.73311 
70 84.7 1.020 86.39 0.71690 
80 75.5 1.020 77.01 0.43903 
90 55.5 1.019 56.55 0.46929 
270 56.3 1.020 57.42 0.47652 
280 77.4 1.020 78.94 0.66384 
290 83.7 1.020 85.37 0.70844 
300 87.5 1.020 89.25 0.74060 
310 90.6 1.019 92.32 0.76609 
320 91.7 1.019 93.44 0.77539 
330 92.0 1.019 93.74 0.77593 
340 93.4 1.019 95.17 0.78976 
350 104.3 1.019 106.28 0.88193 
  
 
 
 
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Table 2E:  Temperature and pressure corrected electrometer readings for the chamber 
response to different energy levels to determine current linearity. 
 
(KVP) M1 M2 M3 Mean P (hPa) T (OC) KTP Mcorr. 
Muncorr (pA) 
50 17.4 16.8 19.3 17.8 1010.5 22.7 1.0119 18.0 
 
60 29.3 45.5 41.0 38.6 1010.6 22.1 1.0097 38.9 
  
70 74.0 79.5 80.4 77.9 1010.7 21.9 1.0089 78.5 
`80 121.4 117.7 98.8 112.6 1010.7 22.0 1.0093 113.6 
90 168.8 149.5 138.4 152.2 1010.8 21.9 1.0088 153.5 
100 195.4 197.4 190.6 194.4 1010.7 21.9 1.0089 196.1 
110 258.9 245.3 242.3 248.8 1010.8 21.9 1.0088 250.9 
120 321.4 322.5 278.5 307.4 1010.7 22.1 1.0096 310.3 
130 336.4 356.4 365.5 352.7 1010.7 22.0 1.0093 355.9 
140 368.2 369.4 396.5 378.0 1010.7 22.1 1.0096 381.6 
150 384.2 465.3 454.6 434.7 1010.7 21.9 1.0089 438.5 
 
 
 
 
     
 
 
 
 
 
 
 
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Table 2F:  Chamber response to different energy levels to determine HVL. 
 
 
Voltage Mcorr P1 P2 P3 P4 P5 P6 P7 P8 
(KVP) 
60 38.9 37.2 31.5 19.5 9.7 --    
80 113.6 91.0 77.4 74.1 49.3 45.7 --   
100 196.1 189.3 167.6 153.4 130.0 117.5 105.9 91.7 -- 
120 310.3 283.4 274.3 234.4 218.3 197.4 150.1 135.5 -- 
130 355.9 306.3 297.7 264.5 243.5 124.5 207.5 166.4 160.4 
 
 
NOTE: P1 means one piece of aluminium, plate, P2 means two pieces and in  that order. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Table 2G: Piranha multi (Reference detector) KV reproducibility response. 
R/N  KVp Ref.Detector Response(mGy) 
1 80 79.34 
2 80 79.19 
3 80 79.40 
4 80 79.47 
5 80 79.51 
6 80 79.31 
8 80 79.39 
9 80 79.65 
9 80 79.37 
10 80 79.47 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Table 2H:   Piranha multi (Reference detector) response to different energy levels to 
determine the calibration coefficient.( KV Accuracy) 
KVP HVL (mmAl) Total Filtr. Exposure (mGy) 
(mmAl) 
50 3.40 11.0 0.0336 
60 4.09 9.6 0.0692 
70 4.79 9.8 0.1146 
80 5.42 9.7 0.1723 
90 6.03 9.9 0.2424 
100 6.59 10.0 0.3188 
110 7.05 10.0 0.4057 
120 7.52 10.0 0.5153 
130 7.95 10.0 0.6219 
 
 
 
 
 
 
 
 
 
 
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Table 2I: Temperature and pressure corrected electrometer readings for the chamber 
response to different energy levels for cross calibration. 
Voltage M1 M2 M3 Mean P (hPa) T (OC) KTP Mcorr. 
(KVP) Muncorr (pA) 
50 17.4 16.8 19.3 17.8 1010.5 22.7 1.0119 18.0 
60 29.3 45.5 41.0 38.6 1010.6 22.1 1.0097 38.9 
  
70 74.0 79.5 80.4 77.9 1010.7 21.9 1.0089 78.5 
`80 121.4 117.7 98.8 112.6 1010.7 22.0 1.0093 113.6 
90 168.8 149.5 138.4 152.2 1010.8 21.9 1.0088 153.5 
100 195.4 197.4 190.6 194.4 1010.7 21.9 1.0089 196.1 
110 258.9 245.3 242.3 248.8 1010.8 21.9 1.0088 250.9 
120 321.4 322.5 278.5 307.4 1010.7 22.1 1.0096 310.3 
130 336.4 356.4 365.5 352.7 1010.7 22.0 1.0093 355.9 
140 368.2 369.4 396.5 378.0 1010.7 22.1 1.0096 381.6 
150 384.2 465.3 454.6 434.7 1010.7 21.9 1.0089 438.5 
   
 
 
 
 
 
 
 
 
 
 
 
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Table 2J:   Chamber  Pre irradiation current leakage response 
(80 KVp,200 mA,  20 mAs abd 100 SSD) 
Time (s) Chamber response (pA) 
10 6.85 
20 5.60 
30 5.00 
40 4.70 
50 4.45 
60 4.30 
70 4.05 
80 3.45 
90 3.40 
100 3.35 
110 3.25 
120 3.30 
Mean 4.308333 
 
 
 
 
 
 
 
 
 
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107