Using Cs-137 Gamma Source. Rasha Osman Khalid

Transcription

1 Calibration of a Radiation Survey Meter Using Cs-137 Gamma Source By Rasha Osman Khalid A Thesis Submitted as Partial Fulfillment of the Requirements of the M.Sc. in Physics Department of Physics Faculty of Science University of Khartoum July 2005

2 ACKNOWLEDGEMENTS I would like to express my gratitude to my supervisors Dr.Farouk Habbani and Eltayeb Hag Musa for their valuable assistance during this work. My thanks also go to all those members of SAEC who helped me during the measurements carrid out at the SSDL. Finally all my gratitude is due to my family for their support.

3 ABSTRACT The survey instrument SmartION was calibrated at the Secondary Standard Dosimetry Laboratory, Sudan Atomic Energy Commission, in terms of Kerma, free in air using Cs-137 gamma radiation. All the calibrations were performed using the reference instrument-spherical ionization chamber LS-01. This reference instrument has been calibrated at the International Atomic Energy Agency, Vienna for x-rays, 137 Cs and 60 Co gamma radiation. The air Kerma calibration factors for the instrument were determined as the ratio of the dose rates obtained with the reference standard chamber LS-01 and the dose rates of the instrument under calibration. The uncertainties for the survey meter SmartION and another survey meter RADOS were obtained and the results compared with the uncertainty for the reference standard chamber. Also, the values of dose rates were obtained for various angles of the incident beam, by changing the angle by which the radiation was incident on the sensitive point of the instrument.

4 الخلاصة تمت معايرة جهاز المسح الا شعاعي SmartION في معمل المعايرة الثانوي التابع لهيي ة الطاقة الذرية السودانية باستخدام أشعة قاما الناتجة من المصدر 137 Cs في الهواء. تمت المعايرة باستخدام الجهاز المرجعي Spherical. Ionization Chamber هذا الجهاز المرجعي تمت معايرته في الوآالة الدولية للطاقة الذرية ڤينا باستخدام أشعة X وأشعة قاما من مصدري. 137 Cs, 60 Co حسبت معاملات المعايرة با خذ النسبة بين معدل الجرعة الاشعاعيه للجهاز المرجعي إلي معدل الجرعة الاشعاعيه للجهاز المراد معايرته.SmartION حسبت قيمة الخطا لجهاز المسح الا شعاعي SmartION وجهاز ا خر للمسح الا شعاعي RADOSوقورنت النتاي ج مع قيمة الخطا للجهاز المرجعي. أخذت مجموعه من القراءات لعدة زوايا للشعاع الساقط لمعرفة قيمة الجرعة الاشعاعيه لجهاز المسح الا شعاعي SmartION بتغيير الزاوية التي يسقط بها الشعاع علي النقطة المرجعية للجهاز.

5 LIST OF FIGURES Fig.2.1 Fig.2.2 Fig.2.3 Fig.3.1 Fig.3.2 Fig.3.3 Fig.3.4 Fig.3.5 Fig.3.6 Fig.4.1 Fig.4.2 Fig.4.3 Fig.4.4 Fig.4.5 Fig.4.6 Fig.4.7 The main processes for interaction of electromagnetic radiation with matter Free ionization chamber The inverse square law Control desks for gamma calibrators with UNIDOS Gamma calibrator OB-85/1 with spherical ionization chamber Gamma calibrator OB-34/1 with SmartION X-ray generator with RADOS and camera Control desk for x-ray generator SmartION and RADOS survey meters Rectangular and triangular distribution Area ambient dose rate Vs Distance for high doses Area ambient dose rate Vs Distance for low doses Dose Rate Vs Angle Uncertainty Vs Energy for spherical ionization chamber Uncertainty Vs Energy for SmartION Uncertainty Vs Energy for RADOS Page No

6 LIST OF TABLES Table 2.1 Table 2.2 Table 3.1 Table 3.2 Table 3.3 Table 3.4 Table 4.1 Table 4.2 Table 4.3 Characteristics of the three radio- nuclides The thicknesses of ionization chamber walls required for establishment of electronic equilibrium The three radioactive sources of OB-85/1 The two radioactive sources of OB-34/1 Gamma rays from Cs-137 and Co-60 The x-ray qualities The calibration coefficients for x- and gamma rays Results of Ka for 137 Cs beam using OB-85/1 calibrator Average values of Ka readings using OB-85/1 calibrator Page No Table 4.4 Table 4.5 Table 4.6 Table 4.7 Table 4.8 Results of Ka for 137 Cs beam using OB-34/1 calibrator Average values of Ka readings using OB-34/1 calibrator Nominal doses obtained by equation (4.15) Nominal doses obtained by equation (4.16) Results of dose rates using OB-85/1 source for SmartION

7 Table 4.9 Table 4.10 Table 4.11 Table 4.12 Table 4.13 Table 4.14 Table 4.15 Table 4.16 Table 4.17 Table 4.18 Table 4.19 Table 4.20 Table 4.21 Table 4.22 Results of dose rates using OB-34/1 source for SmartION Calculation of calibration factor and intrinsic error for SmartION in the high dose range Calculation of calibration factor and intrinsic error for SmarION in the low dose range Measurements of dose rates using OB-85/1 at different angles The average values of dose rate for Table 4.12 Measurement of Ka using different x and gamma-ray energies at distance 2m for spherical ionization chamber Measurement of dose rate using different x and gamma-ray energies at distance 2m for SmartION Measurement of dose rate using different x and gamma-ray energies at distance 2m for RADOS Measurement of uncertainties for spherical ionization chamber Measurement of uncertainties for SmartION Measurement of uncertainties for RADOS Values of uncertainties for spherical ionization chamber Values of uncertainties for SmartION Values of uncertainties for RADOS

8 CONTENTS Acknowledgement Abstract Abstract (Arabic) List of Figures List of Tables.i.ii.iii. iv. v CHAPTER ONE INTRODUCTION 1 Purpose of Calibration 2 Objectives 2 CHAPTER TWO THEORETICAL BACKGROUND Ionization Ionizing Radiation Types of Ionizing Radiation Radioactive Gamma Sources Interaction of Electromagnetic Radiation with Matter Ionization Chamber The Ionization Process in Gases Design and Operation of DC Ion Chambers..10

9 2.5 Bragg-Gray Cavity Theory The Inverse Square law Radiation Quantities and Units Particles Number and Particles Flux Radiant Energy and Energy Flux Particle Fluence and Particle Fluence Rate Energy Fluence and Energy Fluence Rate Energy Imparted Absorbed Dose and Absorbed Dose Rate Kerma and Kerma Rate Exposure and Exposure Rate Ambient Dose Equivalent..20 CHAPTER THREE EXPERIMENTAL SET UP AND MEASUREMENTS Gamma Calibrators Gamma Calibrator OB-85/ Gamma Calibrator OB-34/ Spherical Ionization Chamber Electrometer The X-ray Set Up Survey Meters SmartION RADOS Auxiliary Calibration Equipments..31

10 3.7 Experimental Measurements Calibration Measurement Evaluation of Uncertainties 33 CHAPTER FOUR RESULTS AND DISCUSSION Calibration Factor Relative Intrinsic Error Activity Measurement Uncertainties...37 CHAPTER FIVE CONCLUSION References 80.82

11 CHAPTER ONE INTRODUCTION In 1896 Henri Becquerel found that uranium salts emitted nuclear radiation which was traced to natural radioactivity by the element uranium. This radiation can be in the form of particles (alpha and beta particles) or electromagnetic radiation (gamma rays) or both. Nuclear radiations have several applications in our life, so we need instruments to detect these radiations. But these detectors must be calibrated before their first use and then should be recalibrated periodically. The calibration is defined as the quantitative determination, under a controlled set of standard conditions, of the indication given by a radiation measuring instrument as a function of the value of the quantity the instrument is intended to measure. The calibration is normally carried out by National or Secondary Standard Laboratories. The lack of calibration facilities in some countries led the International Atomic Energy Agency (IAEA) and the World Health Organization (WHO) to consider supporting the establishment of Secondary Standard Dosimetry Laboratories (SSDLs) in various developing countries, including Sudan. The main task of the SSDLs is to transfer the calibration from the Primary Standard Dosimetry Laboratories (PSDLs) to the user. It is assumed that calibrations either in terms of air kerma or exposure have been carried out for the SSDLs at the PSDLs. The beam qualities and the irradiation geometries at the PSDLs and at the SSDLs need to be as similar as possible in order not to introduce errors in this transfer.

12 Another important task for the SSDLs is to advise and help the users to calculate the necessary correction factors for their instruments. Although these laboratories were primarily concerned with the calibration of radiotherapy instruments, they are increasingly becoming more concerned with calibrating radiation protection instruments. To perform the calibration a reference instrument, ionization chamber is usually required, which has itself been calibrated against a reference instrument of higher quality. The reference instrument must be calibrated for the range of energies and air kerma that are intended to be used. The reference radiation used to calibrate the reference instrument should, if possible, be the same as that used for the calibration of radiation protection monitoring instruments. Purpose of Calibration: The aim of the calibration is to insure that an instrument is working properly and to determine, under a controlled set of standard conditions, the indication of an instrument as a function of the measured value. Also to adjust the instrument calibration so that the overall measurement accuracy of the instrument is optimized. Objectives: The objectives of the present work were: 1/ To calculate the intrinsic error and the calibration factor of a survey meter. 2/ To determine the uncertainties of two different survey meters and a reference instrument. 3/ To compare the results of the uncertainties obtained by the two survey meters with that obtained by the reference instrument.

13 Chapter two deals with the basic concepts of the ionization chambers and their operation. It discusses the types, sources and interactions of electromagnetic radiation with matter, and the basic concepts of radiation dosimetry. Chapter three describes the equipments which were used in the present work, and the method of calibration of the survey meter. Chapter four presents the results obtained for the reference instrument and the two survey meters. It also discusses the results obtained for the calibration factor, intrinsic error and the uncertainty of the instruments used.

14 CAPTER TWO THEORETICAL BACKGROUND 2.1 Ionization:- Ion formation plays an important role in almost all methods used for the detection and measurement of nuclear radiations. The average energy required to form an ion pair is considerably greater than the ionization potential of the absorber. Ionization occurs in all substances in gas, liquid or solid form. Ion collection is easily achieved in gases and these form the basis for most radiation measuring devices Ionizing Radiation: Ionizing radiations are defined as those that can ionize matter directly or through the action of some secondary radiation. The process of ionization occurs when enough energy is transferred to a material to eject electrons from the atoms or molecules. The energy deposited by these radiations is capable of disrupting the energy bonds which hold together and characterize an atom or molecule. The ionization is not the only process, there is a second important phenomenon, excitation, in which an atom or molecule is raised to a higher energy state by the action of radiation, but is not given enough energy to permit the escape of an electron. Usually the primary radiations dealt with have many times the energy necessary to ionize or excite atoms or molecules of materials. In traversing matter, the energy of these radiations is degraded, and at the same time secondary radiation of lower energy is generated. The energy deposited by the radiation which has insufficient energy to excite or ionize the material is

15 small enough so that it needs not be specifically considered. Energetic charged particles produce ionization and excitation through direct Coulomb force interaction with the electrons of the matter being traversed. X and γ ray photons interact by processes in which energetic secondary charged particles are generated, and these in turn produce more of the excitation and ionization in the material. Neutrons of all kinetic energies down to thermal energies may give rise to energetic secondary charged particles by various processes [5, 6, 7] Types of Ionizing Radiation: Ionizing radiation may be categorized into four general types as follows: - Electrons. - Heavy charged particles. - Electromagnetic radiation. - Neutrons. Electrons include beta particles emitted in nuclear decay, as well as energetic electrons produced by any other process. Heavy charged particles denote a category that encompasses all energetic ions with mass of one atomic mass unit or greater, such as protons, deuterons, alpha particles, ions, fission products, or the products of many nuclear reactions. The above two types are called charged radiation, whereas, the other two types are known as uncharged radiation. The electromagnetic radiation includes x rays emitted in the rearrangement of electron shells of atoms, and gamma rays which originate from transitions within the nucleus itself. Neutons generated in various nuclear processes comprise the final major category, which is often further divided into slow neutrons and fast neutrons [6].

16 2.2 Radioactive Gamma Sources: Radionuclides such as Radium-226, Caesium-137 and Cobalt-60 have often been used as sources of γ-rays for calibration of instruments. These γ- rays are emitted from the radionuclides as they undergo radioactive disintegration. The advantages of using the gamma rays emitted by Cs-137 is that it requires less shielding, and has a long half life of about 30 years. It emits monoenergetic γ-rays of energy MeV. The main advantage of using the gamma rays emitted by Co-60 is its high specific activity which allows fabrication of small sources required for some special applicators. It has a half life of about 5.26 year. It has higher average photon energy than Cs-137 and Ra-226 gamma rays. Radium is much more expensive and has greater self absorption of its radiation than either Caesium or Cobalt. These characteristics for the three radionuclides are compared in Table2.1[4]. Table 2.1: Characteristics of the three radio nuclides Radionuclide Half life / years γ-ray energy/ MeV Radium Caesium Cobalt , 1.332

17 2.3 Interactions of Electromagnetic Radiation with Matter: When X or γ- ray beam passes through a medium, interaction between photon and matter can take place with the result that energy is transferred to the medium. The initial step in the energy transfer involves the ejection of electrons from the atoms of the absorbing medium. These electrons transfer their energy by producing ionization and excitation of other atoms along their path. The main three processes by which the electro-magnetic radiation interacts with matter are [6, 7]: - The photo electric effect. - Compton scattering. - Pair production. (See Fig.2.1). In the photo-electric effect the incident photon of x or γ-ray gives all its energy to an electron in the atom of the material, which is ejected out of the atom, leading to ionization. In the Compton effect the photon of x or γ-ray gives part of its energy to an electron in the atom of the material, which recoils, and the photon itself is scattered with reduced energy. In pair production the photon of x or γ-ray interacts in the field of the nucleus of the atom of the material leading to the creation of an electronpositron pair, which is then ejected out of the atom, and the photon disappears.

18 2.4 Ionization Chambers: Several of the oldest and most widely used types of radiation detectors are based on the effects produced when a charged particle passes through a gas. The primary modes of interaction involve ionization and excitation of the gas molecules along the particle track. The majority of the gas- filled detectors are based on sensing the direct ionization created by the passage of the radiation through the gas. Ion chambers in principle are the simplest of all gas-filled detectors. Their normal operation is based on collection of all the charges created by direct ionization within the gas through the application of an electric field The Ionization Process in Gases: An atom is ionized when one or more of the orbital electrons are removed, leaving a positively charged ion and one or more free electrons. At a minimum, the ionizing particle must transfer an amount of energy equal to the ionization energy of the gas molecule to permit the ionization process to occur. The average energy lost by the incident particle per ion pair formed is usually greater than the ionization energy. In the presence of an electric field, the drift of the positive and negative charges represented by the ions and electrons constitutes an electric current. The rate of formation of ion pairs will be balanced by the rate at which the ion pairs are lost, either through recombination or by diffusion or migration from the volume. Measurement of ionization current is the basic principle of the DC ion chamber. In such an ion chamber the electric field can be created by the application of an external voltage. At equilibrium, the current flowing in the external circuit will be equal to the ionization current collected at the electrodes and this current can be measured by a sensitive ammeter.

19 Increasing the voltage further can not increase the current because all charges are already collected and their rate of formation is constant. This is the region of ion saturation in which ion chambers are conventionally operated. Under these conditions, the current measured in the external circuit is a true indication [6] Design and operation of DC Ion Chambers: The design of an ionization chamber varies widely, depending on the particular type and intensity of radiation to be measured. Parallel plate or planar geometry leads to a uniform electric field between the plates. Also common is a cylindrical geometry in which the outer shell of the cylinder is operated at ground potential and a central conducting rod carries the applied voltage (see Fig.2.2). Short range radiations may be admitted to the chamber through a thin film or window. In general, these chambers operate at atmospheric pressure, and the window is not subjected to a pressure differential. The problem is to obtain a maximum energy absorption in the gas with a minimum of energy loss to the walls. Ionization in the gas can be increased by using dense gases. In the design of an ionization chamber for measuring gamma- radiation, when a photon is absorbed by the walls of the ionization chamber or by the gas within it, high speed electrons are produced. These electrons travel through the gas, producing further ions until their kinetic energy is spent. Gamma ray chambers may have walls of high atomic number material to increase the photon absorption and hence obtain greater ion currents. The walls of the chamber should be made of a material having an atomic number close to 7 or 8, the values for nitrogen and oxygen. Carbon with an atomic number of 6 is reasonably close, and chamber walls are often

20 made of bakelite or similar plastic. The inner surfaces of the chambers are coated with graphite to make them a conductor for the collection of the ions. The wall thickness will be a comprise, since the optimum thickness depends upon the energy of the radiation. Because of the relatively high density of the walls, most of the gas ions are produced by electrons originating in the walls. Low energy photons will produce only low energy electrons, and many of these will lose a large fraction of their energy before emerging into the gas. Very high energy photons will produce some high energy electrons, and these will be only partly absorbed by the gas. At the optimum wall thickness there will be electronic equilibrium between the wall and the gas with a minimum of attenuation of the incident radiation. Table 2.2 shows the thicknesses of the ionization chamber walls required for the establishment of electronic equilibrium for different photon energies [6].

21 Fig.2.2 Free Air Ionization Chamber

22 Table 2.2: The thicknesses of ionization chamber walls required for establishment of electronic equilibrium Photon energy (MeV) Thickness (gcm -2 )

23 2.5 Bragg Gray Cavity Theory: Gray considered a medium whish is uniformly irradiated by photons and of sufficient dimensions that electronic equilibrium is established at a place within it at which a small gas filled cavity is introduced. He showed that the introduction of the cavity did not modify the number, energy or directions of the electrons crossing the cavity surface, provided that the scattering of electrons in the cavity could be neglected. He felt this was justifiable if the cavity was sufficiently small, i.e. that the electrons lost only a small fraction of their energy in crossing the cavity. If both the solid medium and the gas have the same number and energy of electrons passing through them, the fraction of the electron energy lost per unit mass in the two materials will be the same as the ratio of the mass stopping power of the solid medium and the gas for the electrons concerned. Gray then made the assumption that the ratio of the energy absorbed per unit mass of the gas is: where: J.W g / e J The charge per unit mass resulting from the ionization produced by the electrons. W g The average energy required to produce an ion pair in the gas. e Electronic charge. The absorbed dose, D m, in the medium is then given by: where: S m. g D Wg = S m. g. J (2.1) e m. A weighted mean ratio of the mass stopping power of the medium to that of the gas for the electrons crossing the cavity. This is the Bragg Gray relation [3, 4].

24 2.6 The Inverse Square Law: Electromagnetic radiation travels in straight lines. Consequently, if one takes a source of small physical size, the rays diverge in all directions from the point source in straight lines. Because the rays are spreading out, the intensity of the radiation decreases with increasing distance from the source. The relationship between the intensity and the distance from the source is an inverse square law as shown in Fig.2.3 [1]. The inverse square law is represented by the equation: K int ensity = (2.2) ( dis tan ce) 2 where K is a constant, which also may be expressed as I I d1 d 2 = = (2.3) 2 1 d1 d 2 2 where I 1 intensity at distance d 1 I 2 intensity at distance d 2 For X and gamma radiations, the inverse square law is usually stated in terms of dose rate, and defined as: 2 D 1 d = 2 2 (2.4) D2 d1 where: D dose rate and d distance.

25 Fig.2.3: Diagram Demonstrating the Inverse Square Law

26 2.7 Radiation Quantities and Units: Particles Number and Particles Flux [3]: The particles number, N, is the number of particles that are emitted, transferred inside a medium or received in another medium. The particles flux, N & is the quotient of dn by dt, where dn is the increment of particles number in the time interval dt: dn Ν& = (2.5) dt The unit of flux is s Radiant Energy and Energy Flux: The radiation energy, R, is the energy of the particles that are emitted, transferred inside a medium or received in another medium. The radiant energy is expressed in J. For particles of energy E (excluding rest energy), the radiant energy R is equal to the product NE, where N is particle number. And, the energy flux, R & is the quotient of dr by dt, where dr is the increment in radiant energy in the time interval dt : dr R & = (2.6) dt The energy flux is expressed in J.s -1 or W Particle Fluence and Particle Fluence Rate: The particle fluence, Φ &, is the quotient of dn by da, where dn is the number of particles incident on a sphere of cross sectional area da, thus dn Φ = (2.7) da Its unit is m -2.

27 And, the particle fluence rate, Φ&, is the quotient of d Φ by dt, where d Φ is the increment of the particle fluence in the time interval dt: dφ Φ& = (2.8) dt The particle fluence rate is expressed in m -2.s Energy Fluence, and Energy Fluence Rate: The energy fluence, Ψ, is the quotient of dr by da, where dr is the radiation energy incident on a sphere of cross sectional area da: dr Ψ = (2.9) da Its unit is J m -2 And, the energy fluence rate, Ψ &, is the quotient of d Ψ by dt, where d Ψ is the increment of the energy fluence in the time interval dt: dψ Ψ & = (2.10) dt And expressed in J.m -2.s -1 or W.m Energy Imparted [2]: The energy imparted, M, by ionizing radiation to matter in a volume is given by: M = R in R out + Σ Q (2.11) where: R in is the radiant energy incident on the volume, i. e, the sum of the energies (excluding rest energies) of all charged and uncharged ionizing particles entering the volume. R out is the radiant energy emerging from the volume, i. e, the sum of the energies (excluding rest energies) of all charged and uncharged ionizing particles leaving the volume.

28 Σ Q is the sum of the rest mass energies of nuclei and elementary particles in any interactions occurring in the volume (decreasing : positive sign, increasing: negative sign ). Unit J Asorbed Dose and Absorbed Dose Rate: Absorbed dose, D, is defined by: de D = (2.12) dm where de is the mean energy imparted by ionizing radiation to matter of mass dm. Unit J.kg-1. The special name for the unit of absorbed dose is gray [Gy]. And absorbed dose rate,, is the time derivative of absorbed dose: dd D & = (2.13) dt where dd is the increment of absorbed dose in the time interval dt. Unit: J.kg-1.s-1. The special name for the unit of absorbed dose rate is gray per second [Gy.s-1] Kerma and Kerma Rate: The kerma, K, is the quotient of de tr by dm, where de tr is the sum of the initial kinetic energies of all the charged ionizing particles liberated by uncharged ionizing particles in a material of mass dm: detr K = (2.14) dm Its unit is J.kg -1 The special name for the unit of kerma is gray [Gy].

29 And kerma rate, K &, is the quotient of dk by dt, where dk is the increment of kerma in the time interval dt, thus: dk K & = (2.15) dt Its unit is J.kg -1 s-1. The special name is gray per second [Gy.s-1] Exposure and Exposure Rate [3]: The exposure symbolized by X, is the quotient of dq by dm, where dq is the absolute value of the total charge of the ions of one sign produced in air when all the electrons and positrons liberated or created by photons in air of mass dm are completely stopped in air, thus: dq X = (2.16) dm Unit: C.kg -1 The exposure was formerly expressed in Roentgen: 1 R = 2.58 x 10-4 C.kg -1 Exposure rate, X &, is the quotient of dx by dt, where dx is the increment of exposure in the time interval dt: dx X & = (2.17) dt Its unit is C.kg -1.s Ambient Dose Equivalent, H*(d) [2]: The ambient dose equivalent, H * (d), at a point in a radiation field, is the dose equivalent that would be produced by the corresponding expanded and aligned field at depth d (expressed in millimetres) in the ICRU sphere, on the radius opposing the direction of the aligned field. Its unit is J.kg -1. The special name for the unit of ambient dose equivalent is Sievert [Sv]. (ICRU= International Commission for Radiological Units).

30 For strongly penetrating radiation, a depth of 10 mm is currently recommended. The ambient dose equivalent for this depth is then denoted by H * (10). For weakly penetrating radiation, a depth of 0.07mm for the skin is employed, and the ambient dose equivalent is denoted by H * (0.07).

31 CHAPTER THREE EXPERIMENTAL SET UP ANDMEASUREMENTS General: A calibrating laboratory for photon radiation should at least consist of two separated irradiation rooms, one for gamma rays and another for x- rays and a control room. If possible, the rooms should be large to obtain a sufficient useful range of distances from the source. Sufficient shielding of the irradiation rooms is required for the protection of people. The irradiation rooms should have suitable track and carriage systems to move the reference instruments and the instruments to be calibrated along the beam axis. An aligning instrument, preferably using a laser beam, should be used. The concrete walls should have special holes for cables with a cross section allowing for additional installations. Air conditioning of the laboratory space is necessary to decrease room temperature. Control of humidity fluctuations is also necessary [2, 12]. 3.1Gamma Calibrators: These calibrators were developed and designed at the Dosimetry Laboratory of the International Atomic Energy Agency. The calibrators are enclosed in cylindrical, steel-enveloped shielding containers. The shielding containers enclose the calibrators (the radioactive sources). The radioactive source in each calibrator is controlled via a control desk, where a timer controls the irradiation period. The time needed by the radioactive source to reach the irradiation position is about 2 seconds. All control and operating elements are laid out on the control desks (Fig.3.1).

32 FIG.3.1: CONTROL DESKS FOR GAMMA CALIBRATORS WITH UNIDOS

33 The control desk is provided with power pack, automation device, buffered emergency operation control and timer. The timer has a 6 digit display indicating the irradiation period entered in hours, minutes and seconds Gamma Calibrator OB-85/1: - Type: OB-85/1 (Fig.3.2). - Manufacturer: Buchler GmbH. - It consists of three radioactive sources, of one activity each, as given in Table 3.1 [8]: Table 3.1: The three radioactive sources of OB-85/1 Nuclide Activity Half life time (G Bq) (year) Ra Co C s Gamma Calibrator OB-34/1: - Type: OB-34/1 (Fig.3.3). - Manufacturer: Buchler GmbH. - It consists of two radioactive sources, of three activities each, as given in Table 3.2 [11].

34 FIG.3.2: GAMMA CALIBRATOR OB-85/1 WITH SPHERICAL IONIZATION CHAMBER

35 FIG.3.3: GAMMA CALIBRATOR OB-34/1 WITH SmartION

36 Table 3.2: The two radioactive sources of OB-34/1 Nuclide Activity (MBq) 7.4 Cs Co Half Life time (year)

37 3.2 Spherical Ionization Chamber: This is the reference instrument (shown as black sphere in Fig.3.2): -Model/ type: LS 01 -Serial Number: 912 -Manufacturer: PTW Freiburg -Chamber size: 1000 cc 3.3 Electrometer: This device is used to record the readings of the reference instrument: -Model/ type: UNIDOS -Serial Number: Manufacturer: PTW Freiburg (shown at top right in Fig.3.1). 3.4 The x-ray set up: The x-ray set up consists of an x ray generator with a protective housing around the x ray tube, a shutter and filters (Fig.3.4). Filters are usually made from metals of highest purity and should be mounted as close as possible to the shutter, with the highest atomic number filters nearest to the x ray tube window. A set of suitable filters is mounted on a wheel to facilitate changing. The shutter attenuates the radiation to a safe level for personnel. The shutter is opened and closed, with no need for switching on and off of the high voltage to the x ray tube for each irradiation. The x ray generator is operated via a control desk (PANTAK High Resolution Control). A filter changer allows choice of the x-ray quality (as shown at top right of Fig.3.5) [1].

38 FIG.3.4: X-RAY GENERATOR WITH RADOS AND CAMERA

39 FIG.3.5: CONTROL DESK FOR X-RAY GENERATOR

40 3.5 Survey Meters: SmartION: This is the instrument used for calibration and uncertainty evaluation, as shown to the right of Fig.3.6: - Type: SmartION - Serial Number: E RADOS: This instrument was used for uncertainty evaluation, as shown to the left of Fig.3.6: - Type: RDS Serial Number: Fig.3.6 shows SmartION and RADOS survey meters. 3.6 Auxiliary calibration equipments: 1. Mercury thermometer. 2. Device for measuring calibration distances. 3. Barometer 4. Closed circuit television consisting of controlled camera and monitor. 5. Aligning instrument using laser beam. 3.7 Experimental Measurements: Calibration Measurements: First, the spherical ionization chamber (the reference instrument) was placed in a suitable holder on the trolley for precise adjustment of the distance between the source and its sensitive point. The chamber was irradiated with Cs gamma rays and the dose was measured. The values of dose from a distance 1 to 5 meters were taken using gamma calibrator OB 85/1, and from a distance 30 to 80 centimetres were taken using gamma calibrator OB-34/1.

41 FIG.3.6: SmartION AND RADOS SURVEY METERS

42 Then, two graphs were plotted for area ambient dose (ambient dose equivalent) versus distance and nominal doses were chosen. Then the SmartION (instrument for calibration) was placed in a suitable holder on the trolley and readings of dose rate for different distances were obtained in the two ranges (high dose range using gamma calibrator OB-85/1 and low dose range using gamma calibrator OB-34/1). Also, measurements of dose rate for SmartION at a distance of two meters were obtained using different angles Evaluation of Uncertainties: For the evaluation of uncertainties measurements of dose at two meters for the spherical ionization chamber were obtained, using different energies (gamma beams from Cs -137 and Co - 60 as shown in Table 3.3 and six x ray qualities as shown in Table 3.4). Furthermore, the measurements of dose rate for two survey meters (SmartION, RADOS) at distance two meters were obtained using the same energies. Table 3.3: Gamma rays from Cs-137 and Co-60 [2] Gamma-ray Mean Energy Conversion source (kev) coefficient Cs Co

43 Table 3.4: The x-ray qualities [2] X-ray High voltage & current quality (kv/ma) Mean Energy (kev) Conversion coefficient Q1 40/ Q2 60/ Q3 80/ Q4 100/ Q5 120/ Q6 150/

44 CHAPTER FOUR RESULTS AND DISCUSSION The reference instrument was calibrated in terms of air kerma. The radiation protection monitoring instruments were calibrated in terms of dose equivalent quantities. Area dosimeters were calibrated in terms of ambient dose equivalent, H * (10), without any phantom, i.e. free in air. The reference instrument used is the spherical ionization chamber which has been calibrated with its electrometer at the IAEA Dosimetry Laboratory, Vienna, for Cs - 137, Co -60 gamma radiation and various x ray qualities. The calibration coefficients, N k, in terms of air kerma are listed in Table 4.1 [9, 10]. The calibration coefficients are established at the conditions: T = 20.0 ο C P = k Pa Relative Humidity = 50.0% 4.1 Calibration Factor [2]: The calibration factor, Cf, is defined as the conventional true value of the quantity the instrument is intended to measure, H, divided by the indication, M, given by the instrument, i. e. Cf = H/M (4.1) The calibration factor is normally only quoted for one reference radiation. Also it is dimensionless because the indicated value and the measured one have the same units. A perfectly accurate instrument should have a calibration factor of one.

45 Table 4.1: The calibration coefficients, N k Radiation quality Chamber + electrometer N k (µgray/nc) 137 Cs γ rays 25.1 ± Co γ rays 24.5 ± kv x rays 25.0 ± kv x rays 23.9 ± kv x rays 24.1 ± kv x rays 24.2 ± kv x rays 24.3 ± kv x rays 24.5 ± 0.2

46 4.2 Relative Intrinsic Error: The relative intrinsic error, I, is defined as the quotient of the error of indication, H M, of a quantity, by the conventional true value of the measured value, H, i.e. 4.3 Activity: H M I = (4.2) H The activity, A, is the quotient of the expectation value of the number of spontaneous nuclear transitions from that energy state ds by the time interval dt: ds A = (4.3) dt Its unit is the Becquerel, defined as: 1Bq = 1dps (4.4) (dps =disintegration per second). 4.4 Measurement Uncertainties: The uncertainty associated with a measurement is a parameter that characterizes the dispersion of the values that could reasonably be attributed to the measured one. This parameter is normally an estimated standard deviation. An uncertainty has no known sign and usually assumed to be symmetrical. There are two statistical methods, type A and type B, used for estimating uncertainty. In this work type B was used. Type B uncertainties can be described by a rectangular probability density, i.e. that they have equal probability any where within the given maximum limits M and + M (see Fig.4.1).

47 With this assumption type B uncertainty, U B, is given by M U B = (4.5) 3 Alternatively, if the assumed distribution is triangular, we are led to the relation: U B = M (4.6) 6 In this work the triangular distribution was used. ½M 1/M -M 0 +M (a) 0 -M +M (b) Fig.4.1: The rectangular (a) and triangular (b) distributions [2]. In determining the overall uncertainty the correction for some parameters like pressure, temperature etc are considered. The calibration factor, Cf, is then given by [2]: Cf = ( M M Η (10) K 0 ) K pr t K T K d (4.7) where: H * (10) =ambient dose equivalent at depth 10mm for strongly penetrating radiation K t corrects for the decay of the source K t = exp {-t ln2/t ½ } (4.8)

48 K pr corrects for the deviation of the actual air pressure P from the reference pressure P o = 1013 mbar K pr = P o /P (4.9) K T corrects for the deviation of the actual air temperature T from the reference temperature T o = K. K T = T/T o (4.10) K d corrects for the possible deviation of the actual distance of the reference source to the measuring instrument from the nominal calibration distance. K d = 1.00 M the mean value of the indication of the measuring instrument at the time of calibration. M o the mean value of the indication of the measuring instrument when the reference source is removed (background). The relative combined uncertainty, U, is then given by (where U stands for uncertainty) [2]: U uh H = (10) (10) ( M M ) u( K ) u( K pr ) u( K ) u( K ) u + M M K t t + K pr + K T T + K d d 2 (4.11) where: uh H (10) (10) = 0.2 as given in the calibration certificate of the primary laboratory, and ( K ) u K pr pr d = p 6 K pr P (4.12) where: d p the deviation from the mean pressure, P

49 ( K ) u K T T d = T 6 K T T (4.13) where: d T the deviation from the mean temperature, T 2 2 U M M ) = u ( M ) + u ( ) (4.14) ( 0 M 0

50 Results of five readings for Ka at Cs-137 beam of OB-85/1 source, using the spherical ionization chamber at various distances (1-5m) are shown in Table 4.2. The average values of the readings of Table 4.2 are shown in Table 4.3. Results of five readings for Ka at Cs-137 beam of OB-34/1 source, using the spherical ionization chamber at various distances (30-80cm) are shown in Table 4.4. The average values of the readings of Table 4.4 are shown in Table 4.5. From Table 4.3 a graph has been plotted for area ambient dose rate versus distance, with the curve shown in Fig.4.1 obtained from the fit of the results. The curve obtained in Fig.4.1 was found to follow the following equation: y= x (4.15) Equation (4.15) was then used to obtain the nominal doses shown in Table 4.6. From Table 4.5 a graph has been plotted for area ambient dose rate versus distance, with the curve shown in Fig.4.2 obtained from the fit of the results. The curve obtained in Fig.4.2 was found to follow the following equation: y= x (4.16) Equation (4.16) was then used to obtain the nominal doses shown in Table 4.7.

51 Table 4.8 shows several readings of SmartION that were taken for each nominal dose value selected from Table 4.6, using Cs-137 of OB-85/1 source. Table 4.9 shows several readings of SmartION that were taken for each nominal dose value selected from Table 4.7, using Cs-137 of OB-34/1 source. Calibration factors were then obtained for Cs-137 of OB-85/1 calibrator using Table 4.8, as shown in Table The calibration factors for Cs-137 of OB-34/1 calibrator using Table 4.9, are shown in Table 4.11.

52 Measurements were conducted for the dose rates obtained from Cs-137 of OB-85/1 source at different angles for the positioning of the survey monitor, at the same distance from the source. The aim of this exercise was to find the optimum position for the monitor. The results of the measurements are given in Table The average values of the dose rates are given in Table A plot for the average dose rate versus angle is shown in Fig.4.3. It can be seen from the figure that the best position for the placement of the monitor with regard to the Cs-137 gamma source is at the angle 0 (360 ), when the monitor is directed at this angle towards the source. The dip shown in the figure for the case when the monitor is turned 180 from the source (at the same distance).

53 As regards the evaluation of the uncertainties for the two survey meters. Table 4.14 shows the results of the measurements of Ka for the spherical ionization chamber (SIC) using Cs-137, Co-60 of OB-85/1 gamma calibrator and various x-ray qualities at a distance of 2 m. Table 4.15 shows the values of dose rates for SmartION (SI) at a distance of 2 m using the same sources used for the reference instrument. Also the values of the background are shown in the table. Table 4.16 shows the results of measurements for the second survey meter (RADOS) using the same sources and at the same distance of 2 m, with the values of the background shown. The low dose rate obtained at the x-ray quality 40 is due to its low energy and the low dose rate obtained for the gamma-rays of Co- 60 is probably due to the fact that the source with a half-life of about five years, has not been changed since 1985, leading to a much lower count-rate as compared to the Cs-137 source, with a half-life of about 30 years. At the beginning the two sources were received with the same activity (in 1985). However, at the time of the measurements Co-60 decayed much more than Cs-137, with much lower count-rate. No other explanation could be found. The results obtained in the three tables were then used to calculate the uncertainties for the three instruments. Table 4.17.a, 4.18.a and 4.19.a show the calculations of the compound uncertainties using equation (4.11) and OB-85/1 gamma calibrator, for spherical ionization chamber, SmartION and RADOS monitors, respectively.

54 Tables 4.17.b, 4.18.b and 4.19.b show the determination of uncertainties at various x-ray qualities for the three instruments. The final results of the compound uncertainties for each energy for the reference instrument, SmartION and RADOS are given in Tables 4.20, 4.21 and 4.22, respectively. Graphs plotted for uncertainty versus energy are shown in Fig.4.4, Fig.4.5 and Fig.4.6 for the three instruments.

55 Table 4.2 : Results of five readings for Ka at Cs-137 beam of OB-85/1 source at each distance Chamber used :1000cc spherical ionization chamber Type: LS-01 S/N :912 Calibration Factor: 25.1mGy/µc Distance Meter Reading m µgy/min mgy/h msv/h Table 4.2 (continued): Distance Meter Reading

56 m µgy/min mgy/h msv/h Table 4.3: Average values of Ka readings using OB-85/1 calibrator Distance Meter Reading m µgy/min mgy/h msv/h

57 Fig 4.1 :Area Ambient Dose Rate Vs Distance Area Ambient Dose Rate/ msv/h y = x Distance/ m Table 4.4 : Results of five readings for Ka at Cs-137 beam of OB-34/1 source at each distance Chamber used :1000cc spherical ionization chamber Type: LS-01 S/N :912 Calibration Factor: 25.1mGy/µc Distance Meter Reading cm µgy/min µgy/h µsv/h

58 Table 4.4 (continued): Distance Meter Reading cm µgy/min µgy/h µsv/h Table 4.5 : Average values of Ka readings using OB-34/1 calibrator

59 Distance Meter Reading cm µgy/min µgy/h µsv/h Fig 4.2 :Area Ambient Dose Rate Vs Distance Area Ambient Dose Rate/ Sv/h y = x Distance/ cm

60

61 Table 4.6 : Nominal doses obtaind by the equation (4.15) Dose Rate H*(10) Distance msv/h m Table 4.7 : Nominal doses obtained by the equation (4.16) Dose Rate H*(10) Distance µsv/h cm

62 Table 4.8 : Results of dose rates at Cs-137 beam of OB-85/1 source for different nominal doses Survey meter under calibration : Type:SmartION S/N :E Nominal Dose Distance Temperature Pressure Reading msv/h m ºc mb msv/h

63 Table 4.9 : Results of dose rates at Cs-137 beam of OB-34/1 source for different nominal doses Survey meter under calibration : Type:SmartION S/N :E Nominal Dose Distance Temperature Pressure Reading µsv/h cm ºc mb µsv/h

64 Table 4.10 : Calibration of Radiation Protection Level Dosimeters Survey meter under calibration: Type:Smart Ion S/N:E Type :SmartION S/N : E

65 Radiation Source:Cs-137 of OB-85/1 Measurement performed on Ambient Temperature:Tmin=29ºC, Tmax=30ºC Ambient Pressure:Pmin=968.0 mb, Pmax=968.3 mb SDD H*(10) Mi <Mb> <Mi> <Mi>- <Mb> I% Cf m msv/h msv/h msv/h msv/h

66 Cf Average 1.08 [Results after subtraction of background] Table 4.11 : Calibration of Radiation Protection Level Dosimeters Survey meter under calibration: Type:Smart Ion S/N:E Type : SmartION S/N :E Radiation Source:Cs-137 of OB34/1 Measurement performed on Ambient Temperature:Tmіn=30ºC,Tmax=31ºC Ambient Pressure:Pmin=967.8 mb,pmax=968.5 mb <Mi>- <Mb> I% Cf SDD H*(10) Mb Mi <Mb> <Mi> m µsv/h µsv/h µsv/h µsv/h µsv/h µsv/h

67 Cf Average 1.00 [Results after subtraction of background] Table 4.12 : Measurements of dose rates for Cs-137 beam of OB-85/1 source at different angles Survey meter under calibration : Type :SmartION S/N :E Angle Reading Angle Reading msv/h msv/h