SD9800002

CALIBRATION OF THERMOLUMNESCENT

DOSIMETERS (LiF: Mg : Ti)

AT DIFFERENT X-RA Y ENERGIES

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF

THE REQUIREMENTS FOR THE DEGREE OF THE

MASTER OF SCIENCE IN PHYSICS

BY

AZ1ZA MOB ARK OSMAN

UNIVERSITY OF KHARTOUM

FACULTY OF SCIENCE

DEPARTMENT OF PHYSICS

APRIL 1998

2 9 - 30

We regret thatsome of the pagesin this report may

not be up to theproper legibilitystandards, eventhough the best

possible copy wasused for scanning

n.

CONTENTSPage No:

ABSTRACT I, III ACKNOWLEDGMENTS HI

I CHAPTER ONE 1

INTRODUCTION - - 1

CHAPTER TWO 5

THEORETICAL BACKGROUND 5

2.0 Introduction 52.1 Production of X-rays 52.2 Interaction of X - ray with matter :-- 62.2.1 Photoelectric Effect 72.2.2 Compton Effect 92.2.3 Pair Production 102.3 Radiation Quantities and Units -- - 122.4 Quality and Intensity of X-rays 152.5 Factors affecting quality and intensity of X-rays 162.5.a Tube Voltage - 162.5.b Tube Current - - 162.5.c Tube Filtration - 172.5.d Target material - - 172.6 Radiation Detectors and Dosimeters - 192.6.1 Free air Ionization Chamber — 192.6.2 Thimble lonization Chamber 222.6.3 Calorimetric Dosimetry 232.6.4 Chemical Dosimetry - 242.6.5 Film Dosimeter -- 252.7 Thermoluminescence Dosimetry 252.7.1 luminescent Process — 262.7.1 (i) Fluorescence and Phosphorescence 262.7.2 Thermoluminescence Phenomenon 262.7.3 Theoretical Aspects of Thermoluminescence 302.7.4 The Role of Lattice Defects in (TL) Process 302.7.5 The Glow Curve — 332.7.6 The Characteristics of (TL) Materials 362.7.6. I The (TL) Process in Lithium Fluoride 36

I CONTENTS%:- Page No:IABSTRACT i, ii

ACKNOWLEDGMENTS 111

CHAPTER ONE 1

INTRODUCTION 1

CHAPTER TWO 5

THEORETICAL BACKGROUND 5

2.0 Introduction 52.1 Production of X-rays 52.2 Interaction of X - ray with matter: 62.2.1 Photoelectric Effect 72.2.2 Compton Effect 92.2.3 Pair Production 102.3 Radiation Quantities and Units 122.4 Quality and Intensity of X-rays 152.5 Factors affecting quality and intensity of X-rays 162.5.a Tube Voltage 162.5.b Tube Current 162.5. c Tube Filtration 172.5.d Target material 172.6 Radiation Detectors and Dosimeters 192.6.1 Free air lonization Chamber 192.6.2 Thimble Ionization Chamber 222.6.3 Calorimetric Dosimetry 232.6.4 Chemical Dosimetry - — 242.6.5 Film Dosimeter 252.7 Thermoiuminescence Dosimetry -— 252.7.1 luminescent Process 262.7.1 (i) Fluorescence and Phosphorescence 262.7.2 Thermoiuminescence Phenomenon 262.7.3 Theoretical Aspects of Thermoiuminescence 302.7.4 The Role of Lattice Defects in (TL) Process 302.7.5 The Glow Curve 332.7.6 The Characteristics of (TL) Materials 362.7.6. 1 The (TL) Process in Lithium Fluoride 36

2.7.7 The Role of Magnesium I o n s — 362.8 Relative Photon Energy Response 372.9 Practical Consideration 41 j2.10 Characteristics of TLD 41 j2.11 The Inverse Square law 42 j

CHAPTER THREE - 45

EXPERIMENTAL SET UP - 45

3.0 Introduction 45

3.1 Calibration Factor 453.2 Calibration Set up For X-rays Beams 463.2.1 .i Protection Level X-rays Generator 473.2.1 .ii Filter --- 473.2.1 Mi Shutter (s) 483.3 Irradiation of The TLD - 483.3.1 Calibration Methods 483.3.2 Radiation Qualities 483.4 Annealing procedures 503.5 Thermoluminescence Reader 52

CHAPTER FOUR - — 54

RESULT AND DISCUSSION - - 54

4.0 Introduction 544.1 Experiment Method 544.2 Calibration of Secondary Standard Chamber LSOI 544.3 Determination of the Target to Surface Distance (S)- 554.4 Calibration Factor for The TLD Reader and the Detector — 574.5 The Result of The First part 604.6 The Result of The Second part .... 64

CHAPTER FIVE 77

CONCLUSIONS - — 77

Appendix 7 8

References - ~ 81

ABSTRACT

In this work the distance between the X- ray target (source) and the

reference point on the housing of the newly installed Secondary

Standard Dosimetry Laboratory (SSDL) at Sudan Atomic Energy

Commission in Soba were determined ,using the inverse square law.

Six X-ray qualities were used at different positions.

The results showed that the distance of Ihe source to reference point

is found to be (22±2 cm).

The calibration factors for the (LiF: Mg:Ti) TLD chips with the

Harshow model 2000C reader was determined for x-ray energies for

quality (3) (KV= 80, Filtration (1 mm Al + 1.85 mm CuJ,

11VL- 0.59 mm Cu) and for Quality(4) (KV = 100, Filtration (1 nun

Al + 5.30 mm Cu), HVL =1.15 mm Cu) at 3 meter distance.

The calibration factors for these two qualities is found to be

(0.^?0±0.0002), (0.i0(/S±0.0004) mGray per nano coulomb

respectively .

These values and those obtained earlier at SAEC (1996) lab, by

using Sr-90 inadiator (Beta-energy 2.27 MeV) calibration factor is

found to be (0.1030 mGray per nano coulomb), confirm that within

accuracies needed at radiation protection level, (LiF:Mg:Ti)TLDs

chips can be considered as an energy independent detector in the

studied energy range.

It is suggested that further measurements should be carried for other

energies for determination of calibration factors for the full range of

energies in use.

ACKNOWLEDGMENTS

I would like to send my regards and appreciation to

my supervisor Dr. Omar Ibrahim for the complacent and

achievement of this study as well to express my deep

gratitude to Dr. Osman Dawi for suggesting this project.

Also my thank to Dr. Ibrahim Shaddad for his valuable

assistance.Special thanks for Mr. El-Tayab H.Musafor

his continuos guidance and support, including also the

staff of Radiation Protection for their help to fulfill this

work and in particular Yassir Abdu.

Also thanks are given to the head and the staff of the

Physics Department of U Of K.

Last but not least , 1 can not forget the assistance of

my family in particular my brothers Amir, Abdelrahman

and, my sister Somia lbr their help as they stood by me all

the way through.

HI

i CHAPTER ONE

INTRODUCTION

X-rays constitute part of the electromagnetic spectrum. They have

wave lengths in the range of 0.005 - 1.On M.(Nicholas, 1983). These

short wave lengths are comparable with the interatomic distances and

that is why X-rays are used in imaging techniques.

In the early days of X-ray usage for diagnosis and therapy attempts

were made to measure ionizing radiation by quantifying chemical and

biological effects produced by them (Faiz M. Khan, 1984).rH£ To quantify the effects of ionizing radiation on biological systemsI"'L., one needs information about the amount of energy absorbed per unitU. mass. The quantity absorbed dose has been defined to quantifyi-

I absorption of radiation energy by matter for all types of ionizing. radiations including charged and uncharged particles, and for all

«• energies.

f Dosimetry is the field concerned with the measurement of ionizing

radiation. The main types of measurements are as follows :

1. Measurement of absorbed dose in matter at a point of interest.

2. Measurement of the energy released by indirectly ionizing particles

(photons, neutrons) per unit mass ofa reference material at the point

of interest.

3. Measurement of the number of particles and quanta or their

energies incident at a given point (hank II. Allix, 1986).

Dosimetry is crucial in three main fields: radiation therapy, food

preservation and radiation protection. In each field the typical doses

and the requirements for measurement accuracy are different.

Many different physical methods were used in measurement of X-

radiation namely, free air ionization chamber, calorimetric dosimetiy,

chemical dosimetery, film dosimetiy, thennoluminescence dosimetiy

(TLD) these are discussed briefly in Chapter II and well elaborated by

(Faiz M. Khun, 1984) two of these methods were used in this research.

The thermoluminescence dosimetiy (TLD) and ionization chamber

dosimetry. TLD is now the most widely used method for measurement

of absorbed dose in radiation protection.

The chips used in TLD incorporate small quantities of phosphor,

typically lithium fluoride (combined with small amounts of activators

e.g. magnesium and titanium), a chemical compound which stores

radiation energy and releases it as lower energy photons when the

phosphor is healed. An advantage of the TLD is that when it is

heating anneals the chips it is restored to its original condition and can

be reused.

More precise dose measurements are made with ionization

chambers, which are used as secondary-standard dosimeters. They are

widely used in radiotherapy and for radiation-protection

measurements. Its operation principle is based on the fact that

electrons and positively charged atoms (ions) are produced within a

gas (air) volume under irradiation. So produced charges can be

collected on electrodes and measured by a sensitive electrometer.

Under certain conditions the charge measured is proportional to the

absorbed dose in the air volume.

The bulk of the experimental work for this study was carried out at

the Sudan Atomic Energy Commission in their Secondary Standard

Dosimetiy Laboratoiy (SSDL):- A dosimetiy laboratory designated by

the Competent National Authority to provide calibration services. It is

equipped with secondary standard dosimeters that were calibrated in

primary standard of laboratories. This SSDL is expected to provide

calibration services for field instruments.

The main aim of this study in the first part is to use the inverse

square law to determine the actual distance from the target to a

reference point on the surface of the housing of the machine, to

enable measurement and calculation of dose rates at different

distances form the target.

Also as a secondary goal, the dose rates at different distances were

used to calibrate TLD (chip-reader) system for different X-ray

qualities.

A calibration factor is the quotient of the true value of dose at the

point of interest and the value evaluated from measurement by the

system.

The format of the thesis is as follows:

Chapter two deals mainly with the Physics of X-rays, their production,

and interactions with matter. Further the units and definitions adopted

in X-rays are given. Theoretical aspects of the problem , and the

instrumentation and method used for measurements are also described.

The theoiy behind the Thermoluminescence dosimetiy (TLD),inverse

square law are given.

Chapter three deals with the experimental techniques, annealing

irradiation and readout. In chapter four a full description of the present

work is highlighted. The experimental results obtained are displayed

in both tabular and graphical forms. Then these results are discussed.

The last chapter includes the conclusion of this work and suggestions

for further work. A list of references is included finally.

CHAPTER TWO

THEORETICAL BACKGROUND

2.0 INTRODUCTION :

In this Chapter we study the production of X-rays, their

interaction with matter and the basic units adopted in radiation

dosimetry. The instrumentation and their merits including the

general theoretical background to thermoluminescence phenomena

are considered. Further, the inverse square law is defined.

2.1 Production of X-rays :

The conventional X-ray tube consists of a glass envelope that has

been evacuated to high vacuum. Inside this tube a filament that

emits electrons, when heated, called the catliode; and a thick rod,

called the anode, are placed. When a high positive potential is

applied to the anode the elections emitted from the cathode and are

then accelerated to the anode.

The spectrum of X-rays produced has two characteristic parts a

continuous energy spectrum called the Bremstrahlung (breaking

radiation) and characteristic X-rays with discrete energy lines.

The first process, Bremstrahlung , is due to the interaction

between a high speed electron and the field of the nucleus, where

part of the electron kinetic energy is lost and propagated in space as

electromagnetic radiation.

Tube current (mA)

High voltage source

Filament Rotating Anode

Glass envelope

Fig. 11.1 : Basic Components of an X-Ray Unit

The second process is due to an incident electron with kinetic

energy E interacting with the inner most elections removing them

from their shell. Elections in the upper levels will fill the vacancy

created by the ejected electrons, thus radiating energy in the form

of electromagnetic radiations, which is called characteristic

radiations.

The emitted photon will have energy hv - E> - E, where [\\ and

E2 are the electron binding energies of the filling and ejected

electron respectively, v is the frequency, h is Planck's constant.

Inner shells transitions, in a high atomic number target, will result

in emission of more energetic characteristic X-ray radiation.

Characteristic radiation have discrete energies unlike the

Bremsstrahlung. (Faiz, 1984).

2.2 Interaction of X-rays with Matter:

When an X-rays beam passes through a medium interaction

between photons and matter can take resulting in energy being

transferred to the medium. This involves the ejection of elections

from the atomic orbits, producing ionization or excitation of the

atoms along their paths.

Electromagnetic radiations (X-rays and y-rays) interact with matter

in three different ways known as the photoelectric effect, compton

effect and pair production process.

2.2.1 Photoelectric effect :

The absorption of X-rays of energy less than 100 KeV mainly

proceeds through photoelectric effect. In this process the entire

energy, hv, of the photon is transferred to an electron in one of the

inner shells (K, L, M, N). Therefore, the electron is ejected from

the atom with kinetic energy Ee almost equal to that of the absorbed

photon

E.= [hv-EJ

where Eg is the Binding energy of the electron. When the resulting

vacancy in the inner shell is filled,( the atom changes to a state of

lower energy). This energy is be released in one of ways; either as

characteristic X-rays or the emission'of an Auger election from

outer shells. For low Z materials the energy of this characteristic

photon is very low and almost locally absorbed. But if the energy of

X-ray photon is less than the K series excitation energy Elo no

photoelectrons will be knocked out of the K shell. The same case is

applicable for the other shells(see Fig. 11.1).

The probability of photoelectric absorption depends on the photon

energy, E, and it can be shown that the photon energy is related to

the attenuation coefficient -cm2/g (Faiz,P

V

i1

I

T

P(2-1)

The data for various martial indicate that photoelectric attenuation

strongly depends on the atomic number Z of the absorbing material

as m

- o c Z 1

P(2-2)

This relationship is the basis of many applications in diagnostic

radiology. By combining the above two equations we have

t Z3

— oc —- (2-3)

The angular distribution of electrons emitted in this process

depends on the photon energy at low energies, the photon is

emitted at 90° relative to the direction of the incident photon . As

the energy increases, the photoelectrons are emitted in a more

forward direction.

Characteristic X-raysAutjer electron

hv (photon)**

A,

" e (Phutoelcctron)

Fig It. 2 Fhutottlccti-ic K.ffeci

2.2.2 Compton Effect:

In this process a photon with energy (iiv) interacts with an

electron of low binding energy (tree electron) in comparison with

the bombarding photon. The electron recoils al an angle 0 and the

photon is scattered at an angle 0 with reduced energy, My applying

the law of conservation of energy and momentum, the following

relationship can he found (I'aiz, 1984)

a(i-cos^)/. _/n, (2-4)I I a(l coa^j

Ihv //>'„ (2-5)

I i u(l cos^j

where

</)COU/-^ l I (X)UUl ^ | |V | ( |,v a m j j; arc | | j C energies of llic

iucidenl photon, the scattered photon , and the emitted election

respectively.and u •• ",, where nii,c2 is the rest mass of the

electron (0.511 MeV).

If the incident photon has much less energy than the rest

energy ol the electron, the Campion scattered photons will have

appioxiuiatcly the same energy as the original pliolon(ei|iialiou.2.5)

Since they arc u dependent, however, if the incident photon has a

very high energy, the photon loses most of its energy lo the

I'ouiplou electron and the scattered photon has much less energy

.for high energy photons with u » I and $ -90

ho - / " V _ ( | . S I J A . / I / | . ( for$-u

ho -h°- -O255A/fi' ( for$-180)a

As (lie pliolon cueigy increases beyond the binding eneigy ol' the

k election, the photoelectric effect decreases rapidly and the

Coiupton effect becomes more dominant.

(\miplon inleniclion is independent of atomic number/, because

it involves essentially tree electrons, but it depends on the number

of electrons per gram. All mallei except hydrogen have

approximately the same number of elections per gram.

It follows that if the energy of the beam is in the region where

C'ouiptou effect is the dominant mode of interaction, approximately

(he same altcuualiou of the beam will occur in any material of

equal density thickness (I'ai/., 198-1). See I'ig. 11.3

"lieu" tlcclruu e |('Oinpluii elctUon)

/

lig.ll. i ; The ( oiiiplou

2.2.3 Pair Production :

Pair production is an interaction between a photon and the

electric field of the nucleus. As a result of the interaction, the photon

10

gives up all its energy in the process by creating a pair consisting of

a negative electron ( e") and positive electron (e').

Since the rest mass energy of the electron is equivalent to 0.51 McV

(he minimum energy required for this process is 1.02 McV. The pair

production process is an example of an event in which energy is

converted into mass, as predicted by 1-Jiistein's equation I'! - me .

Although the nucleus docs not undergo any change, the photon

energy in excess is shared between the particles as kinetic energy.

At most, each particle acquires half the available kinetic energy,

although any other distribution is possible.

Annihilation radiation is the reverse process, in which mass is

converted into energy. The positron created in pair production

loses its energy as it traverses matter by the same type of

interactions as an electron does, by ioni/ation, excitation and

hreinssliahlung at the end of its range the positron combines with

one of the tree elections to give rise to two annihilation photons

each having 0.51 MeV Since momentum is conserved, the two

photons are ejected in opposite direction I'ig (II.'I) . As pair

production results from an interaction with electric field of the

nucleus, the probability of pair production increases rapidly with

atomic number.

The probability for pair production to occur, called the pair

production coefficient, has the form :

K ( m ' ) - NZ"'1(1 •: , / .)

Where:

K. is probability for pair production to occur per unit di»lnnce

traveled.

i i

i'(l:pZ) is a function which changes slightly willi'/. and increases

with H.

hv>1.02Mcy

Riuluu

^ C*|»'OUll(UllJ

I'ig .11.4 : Pair l'io»liu(ion

2.3 Radiation Quantities and Units :

The ICKU (liilcriialional C'oiiiinissiun on Radiological Units] has

provided u clear and unambiguous scl oideiinilions ibr (lie units of

qtianlities used lor radiation dose measurements.

Kxposurc (X): is a measine ol ioni/alioii piutJuccti in air by

photons and is defined as

dm(2-6)

Where dQ h the absolute value id'the total charge of the tons of

one sign produced in air when all the elections, liberated by

12

photons in air, of mass dm arc completely slopped in air. The

special unit of exposure is Roentgen (K)

I R - 2.58X10 ' ( ' . k g '

A b s o r b e d d o s e : is the mot>( important (|tiuutily wiiicli is a

statement of amount of energy absorbed per unit mass of an

irradiated material and is defined a s :

dl<: . (2-7)ilm

Where di! is the mean energy imparted by ionizing radiation to

material of mass dm. Absorbed dose is therefore a point function

and is continuous and differenliable. We may refer to its gradient

mill its rale. Absorbed dose may be specified in any medium for any

type of ionizing radiation. This includes charged and uncharged

particles, all material and energies.

The Gray is (he unit for absorbed dose and is given by:

I (iy •- IJ kg1

The absorbed dose rale is defined by :

I) - d l ) (2-8)dl

The mill for absorbed dose rate is JkgV

I (iy s ' -- I Jkg's '

13

thus the relation between gray and Kadis: I (iy -100 Rad(the

Rud being the old egs unit of absorbed dose).

The sulnmit, cent Gray (cGy), has often been used in the transition

period as it is equivalent to Kad

Dose equivalent: since the biological effects of the radiation

depend not only on dose hut also on the type of radiation,the

dosimeliic quantity relevant to radiation protection is dose

equivalent (II) it is defined as:

II-D.Q.N (2-9)

Where:

I) is absorbed dose.

Q is a quality factor lor the radiation (which lakes into account the

relative biological equivalence of the radiation ).

N is a product of all other modifying factors.

The Si unit for both dose and dose equivalent is Joue per Kilogram

hut the special name for the SI unitof dose equivalent is 'Sic vert

(Sv), defined as:

ISv - I J/.Kg.

If the doic is expressed in units oliad ,lhe special unit for dose

equivalent is called the tern .

II (iem)-l)(rad)Q.N

Since Q and N are factors and have no units

I rein-1U'2 J/Kg

14

M); APPROXIMATE QUALITY MCTOUS FOH UADIATJON:

• • I 8 ' 'fPJTTj 1 X-ray, y-rays, cleeiron,

I Neutrons

Heavy particle

QuaMyfttfor *' <

i

3-10 depending on energy

J-20

3.

2,4 Quality and Intensity of X-Rays:

The quality of an X-ray beam :

The i|ualily describes the penetrating power of an X-ray beam. If

the radiation is homogeneous (monochromatic or monoenerge(ic),

(lie quality is completely described by its wave length. The beam

from an X-ray lube, however, is invariably heterogeneous. In order

to describe completely the quality of such a beam it is necessary to

give (he spectrum of (he radiation. The quality is usually specified

by the following,

a- The generating voltage

b- The beam filtration

c- The half value layer or the effective photon energy is ikl'meil as

the thickness of any given absorber required lo attenuation which

reduces the intensity of the incident beam lo half.

The intensity of a given beam is defined as the quantity of

radiation energy flowing per unil lime through a unil area of a plane

perpendicular lo the direction of propnuulion. It in mctisured m

joules/me(eiJ/second ( Wall/m2).

15

| i . 2.5 Factors Affecting Quality and Intensity of X-Ray:

The quality and intensity of X-Rays are delennined by the

following lour factors: a- Tube voltage (kV) b- tube current (inA)

| c- tube filtration and d- the target material in the tube.

p 2.5,a Tuba Voltage:

If The value of llie applied voltage affects both the quality and

intensity of the X-mys produced Fig. (11.5).

T .. The applied voltage influences the spectrum of X-ray in the

r following niauner: as the iipplied voltage is increased, the spectrum

- extends to higher photon energies resulting in increases in the half-

vulue layer and the effective photon energy of the radiation. The

f maxiniuin photon energy depends on (he peak value of the applied

voltage.

.• The intensities for all |>hoton energies present increase us the

applied voltage is increased; (his results in the (olai inlcnsily which

is given by the area under the . curve, being approximately

proportional to the applied voltage squared.

2.5.b Tube Current:

The value ol lube cuiicul (niA) only affects the intensity of the

beam but not its ijiialily. As the lube current rises the intensity

increases. It represents the number of electrons passing from the

filament to the anode. The (olal beam inlcnsily is proportional to

the average value of the tube current. FhjOl.6).

2.5.C Tuba Filtration:

Filters arc materials which arc inserted into the X-ray bca/n lo

improve Ilie i|iiality of the beam ami lo reduce the beam's intensity.

A filler is usually a thick sheet of metal such as aluminum, copper,

tiu or lead, which absorb most of the low energy photons and

ininsinils most of the high energy photons. l'ig.(ll.7).

2.5.d Target Material:

The atomic number of the target material affects the intensity of the

coiilinuos spectrum of the X-rays produced. Also, changing the

utomic number will change the photon energy of the characteristic

radiation and therefore affect the quality of the beam by changing Ihe

line spectrum produced. The atomic number does not alter the

quality of the continuos spectrum it only affects Ihe intensity. Fig.

(11.8).

TAUl.b. (2-2); I UK. VAUIOIIS VM IOK.S WHICH INI'I.DKNCK QIJAI.I'I Y AND INTENSITY

Kxiiongia^tucior

Raising tub voltage

Raising lube current

Increasing tube

tillration

Increasing atomic

number of target

material

Quality '-. '

Increased

No effect

Increased

change line

spectra

only

. /'4. 'I'd/// ''':

increased

Increased

Reduced

increased

17

NiL ^

so /5 too(k|V|

spectrum of changing tho

tube voltttgu ( KV ),

. ^ / 3 0 0 I I IA | fulJ«

I'llUlUI)

Jig. [I'/ Effect on X-ray

tipttctrum of changing the tube

current ( mA).

W'tlulu(

V V i l l i l i l l u f . • • • • - .

\ ,

ct on X- um of adding Win filtration

"I'jll JIOIIIII.

I .i» JI.MIII,;

ilwlnt)i<

vi tjy

' : j :

\

..•urn o f

2.6 Radiation Detectors and Dosimeters :

For (lie measurement of adsorbed dose and dose rale the basic type

of radiation detectors and their relevance to different situation arc

discussed in this section.

2.6.1 Free air ionization chamber:

loui/.ation technique was one of the pioneering methods employed

for the detection and measurement of ionizing radiation.

The free air chamber is used for measurement of exposure,

generally it is used only for the calibration of secondary instruments

designed for Held use. The free air ioni/aliou chamber installations

are thus confined principally to some of the national standards

labontloiies. ll is employed more often than any other calibration

system because of its sensitivity and ease of quantitative

measurement compared to other methods of dosimetry.

A free an chamber is represented schematically in Fig. (II.9). An

X-ray beam originating from (he focal spot is defined by the

diaphragm I) and passes centrally between a pair of parallel plates. A

high voltage is applied between the plate to collect ions produced in

the air between the plates. The ioni/alion is measured for a length L

defined by the limiting hues of force to edges of the collection plate

C. The lines of force are made straight and perpendicular to collector

by aground ring (i.

The electrons produced by the photon beam in the specified volume

(shaded in Fig. (11.9) must spend all their enemy '*y ioni/nlion of the

nit' between the plates.

The exposure \ , ill I IK* center ol specified \olnnie (pninl I') is

H (? 10)

,\(,) is ilk1 diilif-V i i i lkvlnl ill Coulonil), pis density (K|.'/iir ) oliiir

A,, is llu1 «.russ - si\lioiiiil an'ii (in )til tin' luiiiii ;ii point I'

I (in) is Icn^lli ol collecting volnnie

Somc convclion needed lo niipio\e ;iceniiiey ol niLiisiiiemcul wild

liee-iiii loni/iiiion cliiiinhii:

i. ('i)irection loi an aitemiation

ii.Collection lor iccomlunation ol ions

iii.Collection loi ellecls ol leinpeiitlure, pressine mid Iminidily on

l i ol'iiii

loi ioni/alion piodneed by sealleied pliolons

Diaphragm

Guard Win*

I-cad- Kincd Box

1-lcLlioiiiclci

,9); A scluiiiiiUi' riiugmm of Kite air ioiii/alion tliiiinhii

lilctliixlc

21

2,6.2 Thimble loiiization Chamber:

The ihiiiihle iom/atiuii chamber is one ol' (he Held iiistruiricnls

which is calibrated by lice air ioni/ation chamber.

Air Shell

( t )

Air Cavil)

Thimble W.ill

Solid Air Shell

Air C'uvily

liiMilalor

An uivi

(auralblecirotlc

FigNo(H.IO)ScheilUllie (llllgltm> illllbdalill^ (he lUtllllC of llie ThimMc loai/ation Cliumbcr A, Air Shdl vvilliAir ('mil) C, The ilnmlilc tliiiinl)cr

22 •

It is illustrated in rig. 11.10 (u,l>) and cosist of a coaxial cable with a

ground shield connected lo the ioni/.ation chamber. The cable is

connected to the centra! electrode and the grounded shield is

connected to the guard. The cap of the thimble chamber is made of a

material which has approximately the same atomic number as air

(e.g. graphite, plastic), some of the energetic electrons produced in

(he cap by the radiation are able to penetrate into the air surrounding

(lie central aluminum wire electrode and are attracted towards it

because of the positive charge upon it applied by external electrical

supply. The inside surface of (he cap is coaled with conducting

material and may be 'earthed' thus a potential difference exists

between the cup and the central electrode enabling (he electrons to

experience a force due lo electric field.

By suitable choice of materials and size of the caps the thimble

chamber behaves as if it were "air equivalent" such a device is

calibrated over several photon energy ranges against radiation

"standard" ami the correction factor is used lo convert the indicated

reading of current or total charge to a (rue absoibed dose.

2.6.3 Calorimetric Dosimetry i

Culorimelry is a basic method of determining energy absoibed in a

medium which appears as heal energy as distinct from other fractions

(hut may appear in other forms e.g. form chemical change (Faiz.M

Khan 1984)

This results in u small increase iit the temperature of absorbing

medium which, if measured can be related lo the energy absorbed

per unit mass or absorbed dose.

21

The small volume of medium is thermally isolated from the

..;. remainder, the absorbed dose (I)) is defined as ->?•-*

1 ) - ^ , ^ (2-11)dm dm

Where dF|, is energy appearing as heat in absorber of mass dm and

(H's is energy involved in other modes of energy transfer (l.aughlin

el. ul., l%7;(imm, I«J76).

2,6.4 Chemical dosimetery:

Ferrous Sulfate (Fricke dosimeter):

The energy absorbed from ionizing radiation may produce a

chemical change It this change can be quantified, it can be used as a

measure of absorbed dose (Faiz.JWM).

The ferrous sulfate or i'ricke dosimeter is considered lo be the most

developed system. It consists of I m mol/liter ferrous sulphate, lm

mol/lilcr NuCI and 0<l mol/litci sulphuric acid Na( "I is used in the

solution lo counteract (he effects of organic impurities. When the

solution is irradiated, the ferrous ions, l;e'' are oxidi/ed lo ferric ions

Fe ' which can be determined by spectrophotometry as Fe ' shows

measurable absorption peaks in the ultraviolet light range (at

wavelengths of 224 and 30-1 nm (ICKU Report, 1069).

The number of molecules oxidized per 100 eV of energy absorbed

is known as (lie (i-valuc. Thus, measuring yield ol ferric ions enables

calculation of the energy absorbed, when (he G-value is known. A

drawback is that only doses greater than 30 (Jy can be measured with

reasonable accuracy.

2.6.5 Film dosimeter:

This is radiographie film consisting of transparent film base coated

with an emulsion containing very small crystals ol silver bromide

when the Him is exposed to ioni/ing radiation a chemical change

tukes place within the exposed crystals to form what is referred to as

a latent image, when the Him is developed the affected crystals are

reduced to small grains of silver. The unaffected granules arc

removed by using fixing solution, leaving a clear film in their place

while the reduced metallic silvei causes darkening of the film, thus

(he degree of blackening of an urea on the film gives the amount of

silver deposited and consequently, a measure ol (he energy

absorbed.

Dcnsilomelers are used to determine the degree of blackening by

determining the optical density 01) which is defined as

01) -log1-1 (2-12)

Where I,, is the intensity of light measured without the film and 1, is

(hut through the film. This method is useful for checking radiation

mul obtaining quick qualitative patterns ol radiation distribution.

2.7 Thermoluminescenco Dosimotry:

Thermolumiuescence Dosimetry is one oflhe available solid slate

system for dosimetry of ioni/ing radiation.

25

2.7.1 Luminescent process:

Luminescence describes (he process of emission of optical radiation

from a material from causes other than heating it to incandescence.

Luminescent materials can ahsoih energy, store fraction of it, which

may be converted into optical radiation which is then emitted.

(Sclmlman,l%7).

2.7.1 (i) Fluorescence and phosphorescence :

(i) fluorescence: luminescence which persisted only as long as

the excitation continued. Fig. (II.) I-it).

(ii) Phosphorescence, the luminescence observable alter removal

of the exciting source, Fig. (II. 11-b)

The decay lime of fluorescence is. essentially independent of

temperature, being determined by the probability of the transition

from an excited energy level \\ to the ground stale \\, but in the

phosphorescence the decay lime depends on the temperature.

2.7.2 Thermoluminescence Phenomenon:

In ihcriuohuuiucsccucc experiment the system is irradiated al a

temperature al which the phosphorescent intensity is low (long decay

lime) and later healed through a temperature range where the

phosphorescence is bright (very short decay lime), until a

lempcialiirc is leached al which all the cenlus have been thermally

exciied out of iheir inelaslable levels and the luminescence

completely disappears.

luminescent materials are commonly referred to as "phosphors",

whether or not iltcy exhibit plioipliorcicencc llii'icleut

lliamolmwficscciil phosphors liavc high concentration of election or

hole Imps, provided by structural defects and impurities (or

activators) in %(IU2)(Sclmlinan,l%7)Allix>l07<1)

(i) The irradiation process:

The irradiation produces free electrons and holes, the electrons ate

(hen lice to travel through the solid in the conduction hand for short

limes. They may he ultimately cither:

(a) happed ul defects i.e. the melaslahle energy slate I1!,,, or,

(h) tall hack into the valence hand and rceoinhine either

radialively (lluoresccnee) or non-radioactivcly with holes,

(c) he captured al luminescent centers already activated hy holes

as a result of the iiradiation, and de-aclivale the center with llic

emission of light. This last process "radiolumincscence" is the hasis

of scintillation counting and can he used for dose-rale measurements

Similarly .holes can move freely through the valence hand before

\vn\£ iva^K«i AI vkkws , c\\ Kcoinhmmo. nu\uvu\c\\ ov \unv-

Kuhoicnwh \\ul\ clccuons. vn u\\Mn\uum£ v;u\u\u\c\\ al c\cci\oi\

activated luminescent centers, (see l'ig. (11.12-a)

(ii) Heating process:

The effect of (he subsequent heating process is illustrated in

l ig ( l l 12-h) the electron trapped al the melaslablc cneigy stale

,aie given suH'icient thermal energy to escape from the traps into the

conduction hand again, where they are free to travel The electrons

trapped at the metaslable energy state, are given sufficient thermal

and have three possible fates as before They may either he retrapped

al defects, or fall hack into the valence hand and recomhine

tadiatively or mm- tadialively with holes (activated luminescence

ccnici) The light emilled hy the last process is ihcimolumincscence

Similaily, holes can be thermally liberated iiom either traps, and

ungrate via the valance band ,and reeoinbine radialivcly or non -

radialivdy vvilli electrons, or recoinbinc radioaclivdy at an electron

uctivated hiiiiiiiesceni center, also producing llicimoliimiiiescciice.

Emissionm

Emission

(6) lrradlatio»A

Ion , —>

ion

p - - - •

Emission

(c) Uudioliuninebcenco

(b) Heating

L Xl

Emission

_ . J

w ' ^ ^ f e S . S)"tt"-i——Bt delect.

Eloctron-ttctivaUd

g |e -o- Hole trapHole-activated

R centreII

i 2.7.3 Theoretical Aspects of Thermoluminscence:

I• Introduction:

; The mechanism of (II .) is complex, although general theoretical

ff models can he postulated, difficulties arise when specific dosiinclric

materials are considered.

j | The models have been developed on the husis of experimental

evidence. One oUhe majoi pioblems wuh ihcsc models is \\uu many

of the experimental observations on which they have been based can

be observed only al very high levels of absorbed dose (Al-Mckincly,

I UK I) it is difficult to assess how far can one relate this effect to

prediction ol'effects at the normally low levels of absorbed dose.

A general theoretical mechanism lor I I , may be developed by

ictening to the simplest ol all multi-atomic crystalline structures, the

alkali halides. They consist of two inter penetrating cubic lattices of

alkali and halogen ions as illustrated in Fig.(II. 13).

the structure shown represents that which would exist in the ideal

case of a peilccl crystal. All real crystals contain lattice defects of

various kinds and these play an important role in the I I . process.

2.7.4 The Role of Lattice Defects in (TL) Process:

The presence of defects in a material is important for (I'L) process.

Consider the role of intrinsic defects in the electron trapping process,

as illustrated in rig. (11.14 a,b).

A negative ion vacancy l ig. (14.1)) is essentially a region of excess

positive charge and as such may be regarded as a potential trap for a

f: free electron, an electron captured creates; an l; center similarly, a

%• region of excess negative charge will be a potential trap for free

| positive charge (holes) the analogue (anli-eenleij of an I1'- center

| would he a hole trapped at a positive ion vacancy (a v - center) but

|, there is doubt as lo whether this particular center generally exists

I (Pick, 1982).f

The energy band structure lor an ideal crystal may he represented

by an energy band as shown in I'ig. (II. Ma).

The valence baud is representative of all electrons held in bound

slates, and the conduction baud is representative of all electrons in

band states which are free lo migrate through the crystal lattice. In

the case of an ideal electrically insulating crystal under discussion

the conduction band will be empty and all electrons will reside in the

valence band, the conduction band and the valence band are widely

separated in energy by the so-called forbidden gap. Without the

influence of external forces, ll is highly improbable fur an electron lo

be able to traverse the forbidden gap from valence to conduction

baiuUAI-Mckinley, I % I).

However in the case of real disordered crystal containing defects of

simple or complex nature, other allowed energy levels exist in

forbidden gap as in I'ig. (II. 14.b).

In (lie description of the general model which follows wo shall

suppose (hat the energy level labeled I! represents an electron trap

and that level II represents a hole trap. I, is luminescence center

where electrons and holes may recombine with photon emission.

1*1 Ibl

• 0 * 0 * 0 « 0 * 0 • 0 « 0 « 0 « 0 * 0O t O t O « U « 0 0 0 « 0 * 0 * 0 « 0 »

O » O » U » U » U » O • O • O • llil|)U O •

• o t o * o t o » o • o • o • o • o k o

jjO t O I O l O l 0 1 0 # 0 » 0 # 0 » 0 #• 0 « 0 * 0 * 0 « 0 • 0 • 0 • 0 * | U o

0 « 0 * 0 » 0 * 0 * 0 * 0 * 0 * 0 * 0 *1 0 « l ) t O l l l l O » 0 # 0 » 0 » 0 » 0

Fig. (11.13)

Ionic siiticdtrc of (a) an ideal perfect alkali lialidc ciyslal

(I)) a real imperial crystal containing dciccls of various types.

Also iluiwn is a divalent Magnesium ion-alkali metal ion vacancy

dipolc

^ halogen ion Q alkali metal ion

do d>)

Li>!ldui;[ion hinid (Oiuliicjion band

II

valence bund Valence hand

l'ig.(M.M)

Mncryy band iliagram of (a) an "ideal" electrically insulated crystal

(h) 'iciil' crystal coutuining defects giving rise to various

centers.

32

2.7.5 The Glow Curve :

Consider a material containing defects which give rise to a single

electron trap, the energy depth of the ground slate is K below llie

bottom of (he conduction band. A trap may also have several excited

slates If at some lime I a single electron trap contains n-eleclrons,

the energy distribution ol electrons within Ihe trap will be described

by lite Hull/man distribution and hence Ihe probability of release of a

single electron is given by

(Ml.

Whcie K is Holl/uian constant,

S is frequency factor associated with the particular lattice defect

and I is temperature of Ihe material; Ihe rale of release of electrons

liom (he (rap is

)

Where u is the number of electrons.

Assuming thai no elections released from Ihe traps are rclrappcd

bill that ail under (Tl ) transitions, Ihe intensity of Ihe (Tl,) glow I

depends on ihe rate of the photon emission and therefore on lite rate

of release of electrons from traps and the rale of airival of

luminescence centers

dl KT (2-15)

Where C is a constant related to Inininescence efficiency, if the

material is healed at uniform rate R -dt

diiThen , - I f|. *[ • I ~K ,, by substituting in equation-n (2-14)

lit v il | y V lu / (It

we get

)"SCX|( Kl) (2-16)

by integrating one gets

'•(,:j-i: (2-17)

Where n0 is the number of electrons present in Ihe trap at lime Io and

leiuperalure in

1'iually substiliiting - lor n in e(|ui|lion.(2-l5)

no('ex|) Se'^'dT Se (2-18)

This is the expression lor Ihe glow intensity I from electrons trapped

at a single (rapping level H.

The plot of I against T is termed. The glow curve and is shown in fig

(11.15).

o*a:

I

üll)U

tWV)

ai.

Utti i

UJ(11

(KUC

110

0(1IU

10'

11'

IU'

Hi'mJ

«u

"'• Iliuufuiitjl yluw i. m vu > luf pMu»(iliuik wuli »mylu U«p U«plh f milliut|uuncy laciuf > HiailiLk ul jl l'J4'J.'iu|innluii willi Illy puiiuikkiun ul Ilia Oétiniluu

Hio>», U A I U I Ü I

P»uk

1

/

]

I

ib

hull-lil»

lüiimt1 Uo,OllHJdlll l

61! ( tunMl »u-i'inui>iJ'«Ut

imnpviuiii'« rc i

y u i m - • vuiow tuivu» lur Lit- M j Ii I U U IUU) innualuU luf 1 It ü 4ÜU'Clüliuvvudüy. A. Luoluiu l l ü ' C min ') IU I IUI I IK I I JMIIJIUIH |IIIII|<UIIIUI<I; U. lullJIII IUJI ill UÜ (.', lulluwuU by iKailKliun H I * <|J|JIU«I'H4I« valu« ul Um hull-

lilu ul oji.li )juak it «Itu khuwn (Mjbun m il IU/b)

2.7.6 The Characteristics of TL materials :

2.7.6,1 the TL Process In Lithium Fluoride:Many crystalline materials act as (TL) phosphors. Example, lithium

fluoride (l.ii), lithium borate (I^H.iOy) and calcium fluoride (Oil^).

lithium fluoride doped with magnesium and Titanium (l.iF:Mg:Ti),

is most widely am) intensively studied and was fust investigated by

Daniels et al (1953).

lithium fluoride is an alkali halide with a density of 2.64 g cm'1 and

has a photon effective atomic number of Zu j l- 8.2,compared with

7 1 lot soft tissue, this makes this material veiy suitable for clinical

dosimelry, and for most applications it can be considered to be

approximately tissue - equivalent.

The thcriuoluntinesccnce process of l.iF:Mg:Ti is, complex and

critically dependent on a number of factors including : the amount

and the type of impurities present, the chemical form and method of

miioduuiou into lattice and the thermal, optical and mechanical

treatment of (he phosphor during its manufacture and use.

2.7.7 The Role of Magnesium tons:

lil.Mg:Ti is given a pre-irradialion anneal at 400 V for one hour

and cooled miickly to normal ambient temperature, the resulting

glow curve after irradiation contains al least six glow peaks between

normal ambient temperature and 300V as in Fig. (11.16).

Hy convention these are named peak I (60"C), 2 (I2O°C), 3 (170V)

•I (190V), 5 (21(A ) and 6 (285V), peak 5 is the one normally used

for practical itosimctry. It is possible however, to reduce effectively

the number of electron traps with which the low temperature peaks

are associated by thermally annealing the material for 1-2 h at 400°C

or 16-24 It at 80"C prioi to irradiation. This procedure results in the

iinich more satisfactory glow curve shown in Fig (11.16) (Al\

Mckinlay, \n\).

The magnesiuni ions are presumed to form election traps in

combination with certain defect centres in the lattice . The influence

of titanium in the trapping process is unclear and its role is thought

to be primurialy in the formation of luminescence recombination

centers.

the divalent magnesium ion (Mg2') is introduced into a lattice

consisting of an array of inouovalent lilhium(l.i')and fluorine(F")

ions, (he substitution ol'l.i' ion by a ( Mg ') ion results in an excess

positive charge at the lattice site .Coulombic attraction results in

the formation of nearest -neighbour pairs(dipoles) consisting of a

substituted (Mg2') ion in combination with a l.i'ion vacancy, as

illustrated in fig(ll.l.l.h).

2-8 Relative photon energy response:

The total II. emitted by an irradiated phosphor is proportional to

the total radiation energy absorbed by it .The mass energy

absorption cofficcnt of any TL phosphor calculated from the

formula:

(V) - I r w,

where i™. , is the mass energy absorption coefficient of the

ith elemental constituent of the phosphor, and W, is the fraction of

(hut clement in the phottphoi. The mttiti energy Hbioiplion covifittiint

of any element a function of photon energy, and is dependent on the

37

main photon absorption and other interaction processes, photoelectric

effect, Compion scatter, pair production and (of relatively minor

importance) Rayleigh (elastic)scatter. Tables of theoretical mass

energy absorption coefficients and photon interaction cross sections

have been compiled by several authors (e.g. Storm and Israel l%7).

All coefficients and cross sections are dependent to varying degrees

on the atomic number /. of the target atoms, and on the photon

energy I As a I I . phosphor comprises many atoms of Ihe basic

lattice, plus relatively few dopant alums, the simultaneous absorption

and scattering processes are complex. The total photon interaction

cross section per atom may be written as:

where o^.o^and a,,,, are Ihe individual interaction cross sections for

photoelectric effect Complon scatter, and pair production,

respectively. The approximate dependence of these interaction cross

sections as functions of atomic number Z and photon energy (I is

shown in table (2.3).

table (2.J).

Approximate dependence of photon interaction cross section on the atomicnumber (Z) of the absorber.

Processphotoelectric effect

complon scatterpair production

Approximate dependencevaries us iA for Low energy photonsvaries as Zs for high-energy photonsvaries as /vanes as z*(-1.02 Me V)

i-'or elements of low atomic number, and for pholon energies up to

approximately 15 KeV, the photoelectric effect is dominant, but

U*;iz ail-.t * , Is i'j ',lc'. ( /::,/.;;, >,*;.?/ , ; jAuySKU' '.X V?M fM

approximately 20-I01 KeV. l\n elements of high atomic number,

winch 11. dopant materials often are, the photoelectric process is

dominant up to several hundred keV. The photon energy response of

a II phosphor may be expressed in different ways und a commonly

used method is lo compare the response of the phosphor normalised

at a particular photon energy .often Co''" gamma energy (1.25

MeV), with that of air or (issue.

£ Q

— Cttl:a Ma

mid Him

1 . I i . 4 » » i l l » - I *-

) IO(J 3UU IUU0 2UU0

liMucilvu l:iiu(uy (KuV)

II |"/V Wil'ONit LUWt ful l Itf (llfJ-IU)J.Ciif,: MI IANI ) AI'HOIOGftAI'HICfllM

2.9 Practical Consideration:

As slated earlier .alhcrmoluinuncsccneeul dosimeter must be

calibrated before it can be used lor measuring unknown dose .since

(be response of the 111) material is affected by their previous

radiation history and thermal history the material must be suitably

annealing procedure lor I il; is III of healing at '100 '(' an then 24h at

80 V Slow heating namely 2-lh at K0 V .removes peaks I and 2 of

the glow curve,by decreasing the" Happing efficiency "peaksland 2

may also be eliminated by post irradiation annealing fbi 10 mm at

IOUV. The

need for eliminating peaks 1 and 2 arises from the fact that the

magnitude of these peaks decrease relatively fast with lime after

inadiation. Uy removing these peaks by annealing the glow curve

becomes more stable and therefore predictable (I'ai/.. M. Khun 1984)

The II I) response is defined as Tl. output per unit absorbed dose in

the phosphor.

The energy response curve ofl.il' ( I I I ) - 100) for photon energies

below mega voltage range is shown in I1 ig (11-17) (I'ai/.. M. Khan

I •>«•»).

2.10 Characteristics of TLD:

I lew limitations on the si/e of dosimeters, thus suitable for mosl

applications m radiological protection and radiotherapy..

1 Wide dose range, and a linear response over the greater part.

.V Kesponse independent of dose-rale over all ranges of practical

interest.

•I Near tissue eiunvuicnl kyvlcm uehtcvable.

41

1. Reasonable accuracy attainable.

2. Resistant to variations in temperature, pressure, and lunnidily, and

therefore suitable for use in most terrestrial climates.

V t oug-lcrm retention of stored dose.

4 Mecltauically tugged dosimeters.

Y Rapid on - site evaluation, if required.

(> Reusable, therefore low cost per read out.

7 Coded dosimeters available, suitable lor automatic readout and

record - keeping.

2.11 The Inverse square law:

lleclromaguclic radiation travels in straight lines; this is known as

rectilinear propagation. ('onsc<|ucully, if one lakes a source of small

physical si/.c (i.e. a point source), the rays diverge in all directions

from the point source in straight lines, because (he rays are spreading

out, the intensity of (he radiation decreases with increasing distance

from the source. The relationship between the intensity and the

distance from the source is an inverse square law.

Reduction in intensity vacuum (in free space) is due only to the

geometrical divergence and not lo any absorption or scattering of the

rays Tig. (II. IH) illustrates the divergence of rays from a point

source (), Ol\ OQ, OR and OS represent the rays which pass through

the corners a, b, c and d of unit urea at I meter from 0 . At 2 meters

from 0 , (he same rays pass through the corners of the area

represented by e, f, g and li. liy die geometry of similar triangles,

side of is ct" equal to twice side ab, and side f'g is equal to twice side

be, therefore area efgh is four times the area a b c d,

•42

As (licie is no lobs of energy by absoiplion or seaHering, all ihe

eucigy passing through urea abed also passes, through areaefgh.

Therefore, ihe intensity at 2 meters is one quarter of the intensity at 1

meters.

Definition:

The inverse square law states that Ihe intensity of Ihe radiation from

a point source varies inversely as Ihe square of the distance from the

source, provided thai there is no absorption or scullering by the

medium.

This law is represented by the equation :

intensity - , (2-19)(distance j

K is a constant, also may be expressed as

Where:

11 - intensity at distance d|.

I..- intensity at distance i\>

for \ and gamma radiation, the inverse square law is usually slated

in leims of dose rate and uol intensity ,and defined as:

In practice the inverse square law applies, to X-rays traveling through

air if they are generated at voltages above about 50 KV1\ At lower

voltages absorption and scattering by the air are uol negligible; they

cause the exposure rate lo decrease with distance more rapidly (ban

would be expected from Ihe inverse square law.

43

IHi ill 18) I)IA(<I<AM DliMONSIHATINU Till: INVliHSl: SQIJAKIi

CHAPTER THREEEXPERIMENTAL SET UP

3.0 INTRODUCTION:

In this chapter the measurement of the absorbed dose, calculation

of ilie parameters, the calibration equipment and the different

components of the system arc discussed.

A complete cycle of the use of Tl . dosimeters consists of

annealing, irradiation and read out.

The purpose of (his chapter is to identify sources of errors and

asses magnitude of errors and (heir possible effects on precision

and rcprodueibilily of the results.

3.1 Calibration Factor:

lor a radiation dosimeter, the determination of the calibration

factor, always involves the use of at least one calibrated dosemelry

system in the same field of radiation with the system being

calibrated under same geometric and environmental conditions.

The calibration factor is the quotient of the true value of the

quantity being measured, (in this case, the value measured with a

secondary standard dosimeter corrected using the calibration factor

provided by the calibrating lab. as well as correction for prevailing

temperature, pressure and humidity and irradiation conditions),

divided by the vulue indicated by the system being calibrated

concctcd also for prevailing environmental conditions.

3.2 Calibration Set up for X-ray Beams :

A schematic diagram of a suitable lay oul of llic apparatus lor

calibrating dosimeters with X-raclialion is shown in

I I)

^hicldiny

Xioylulii:

Dcttni AXIB

Slitiiiir

(HI)

. I lie calibration bd-uj) consists of au X-ray generator with a

protective housing mound the X-ray Itihe, Sliiillei(S); lillei (I'),

Kelerence ioni/alion clmiul)ei(K) and ioni/ation chamber of (he

insinnneni to lie calil)iateil(l). The diiicrenl components of the

calibration set -up are mounted on a bench with suitable holders

and trolley lor precise adjustment. These components, including

holder and trolley should he rigidly mounted, produce the minimum

radiation scatter and be totally out side the useful beam.

3.2.1 (I) Protection levol X-ray generator:

The X-ray generator should be of constant potential in which any

superimposed alternating vollage does not exceed l% ol" the mean

vollage at llie tube current employed lor the calibration. The

applied lube voltage selector should enable adjustment ol the lube

voltage with a precision of ±|%.

lor medium energy X-ray qualities the maximum vollage is 150

KV willi 3 in A lube current, and for the low energy the vollage is

•ID KV with tube current 10 mA The X-ray qualities used in (his

woik are shown in lahle(.l-l)

In the SSDI. the X-ray lube is mounted in a protective (shielding)

housing thai peimils no appreciable radiation lo emerge in any

direction oilier than that of (he useful beam.

Surveying for radiation leakage from the housing showed

significant X-ray leakage only on the right side of the X-ray

housing resulting from the opening lo incorporate the cable from

the High Vollage (I IV) generator.

3.2.1 (II) Filter:

I'or calibration purposes, the X-ray beam requires additional

filtration. This should be chosen ,so that the radiation qualities

used in the calibration are similar lo those used in practice. Filters

are usually made from metals of highest purity and should be

mounted as close as possible to the shutter, with the highest atomic

47

number tillers nearest to the X-ray lube window. A suitable set of

(he (liters are mounted on a wheel to facilitate changing.

3.2.1(111) Shutter (S): Is the part of the x-ray tube housing,

attenuates the radiation to a safe level for personnel. This provides

improved x- ray beam stability by making il unnecessary to switch

on and off the high voltage to the x- ray tube for each irradiation.

3.3 Irradiation of the TLD

3.3.1 Calibration methods:

The calibration was made using TIT) ehips(Lil':lvlg:Ti), the chips

lii illy were annealed to remove residual effects, and then placed in

suitable holders and a Irolley for precise adjustment of the distance.

The T i l ) was irradiated for a suitable duration of time with all the

possible X-ray qualities at each position (distance from the X-ray

source).

3.3.2 Radiation Qualities:

Calibration has been performed with X-ray qualities according to

ISO 40.17 standard (1979).

TAI1I.K (J-l)'niK X-RAY QIJAMTIKS*

Qimiiiy

Qi

Q2

Q3

04

05

Highvoltage and

Current

(KV/wA)40/10

60/6

80/10

100/22

120/20

150/3

v . ' - I ' m * • • - • » . . ii - y ' • • ^ • • ^ T ' ^ f / T

Additionalfiltration to (row)

3AHO.3OCu

IAiiO,59Cu

IAI»1.85Cu

JAJ 15.30 Cu

IAX3.00tfun.0Sn

IAH0.OOcut2.5Sn

IWfVrti*'Layer (HVl)

in (mm)

2.7AI

0.24Cu

0.59Cu

I.I5CU

\.1MM

2.40Cu

48

1111 WO S1Wi'II

| ; ) ) IM

cniCOWVIHVfOHntlVIIWI

"I 01

now

inn

s

3.4 Annealing Procedures:

l;oi cucli II. material used in dosemetric up|)licalioiis, it is

extremely important lo know the procedure lor restoring its basic

conditions alter irradiation. This procedure is called annealing and

has two aims:

Hie lirsl is lo empty Ihe Haps of phosphor completely alter the

irradiation.

The second is to stabilize the election traps in order to obtain,

within narrow limits, the same glow curve even alter repeated

irradiation and thermal treatments.

All phosphors display some change in their thermoluminescence

characteristics depending on Ihe thermal treatment which they

receive. To ensure complete read out of storage signal and repealed

use of phosphor without significant change in its

ihennoluminesceiice sensitivity, thermal annealing is almost always

required. Before making radiation measurements all dosimeters

should be identically annealed, us far us it is practically possible lo

standardize their sensitivities and background.

lor some phosphors Ihe annealing may he simple, but for others it

may be complex. Such as for l.il?:Mg:Ti, pre-irradiation annealing

is especially important in order to remove all the residual TL signal,

to establish Ihe Tl. sensitivity and to eliminate unstable low

temperature glow peaks. A comprehensive study of ihe annealing

characteristics of TM) 100 by Zimmerman cl til (1965) confirmed

the optimum anneal for i hi at 400"C, followed by 16-24 hrs at

HOT. The effects of temperature variations within the range 80-400

Y are shown in fig. (III.2), it has uiso been observed that repealed I

5(J

In 400"C anneals procedure decrease TL sensitivity of Lil; by up

lo 18% alter 100 cycles (Wald et al 1977). The effect of the 8OUC

anneal is particularly important with regard to the elimination of the

low temperature glow peaks.

I'or a dosimeter which receives a veiy high absorbed dose, a high

temperature anneal must be used; while it is not necessary for

relatively low observed dose.

Although individual dosimeters may be annealed in the reader,

when a long term anneal is required many dosimeters can be

annealed, in an external annealing oven. Some of these ovens can

easily achieve uniformity of temperature through out the entire

volume of the oven, and hence a reduction of the temperature

gradients in the dosimeters. The Oven should be kept clean and

preferably be used only for one type of phosphor to prevent cross

contamination and inter mixing.

Table (3-2) shows the annealing procedures for several TL

materials used in practice.

TAHI I (J-2); Till. ANNKAUNG IMiOCKIUJUES

s Uintucia)'^':1

l.il'01.1)100)

( a r , Dy(TI. 1)200)

l.iF(I'll. 700)

Ann^ltng procedures> i < <

Hir.Ml4OO"Ci24

his. at 80 V or 2 his

at I0OT

1 hr. at 400"C

240-250 V in the

reader

lUitiin. at 100 u C

10 min. at 100° C

10 min. at IOO°C

51

3.5. Thermoluminescence Reader:

The llarshow model 2(100 series o! llieniioliiiiiiiiesccncc reader,

used in ihis study, has a wide range of applications. It has many

applications including:

I.. Kadiulion protection.

2 Huvironmciiliil dosimeliy.

3 Diagnostic and the therapeutic radiation dosiinelry.

•I Kescarch application to Geology and Archeology.

The Lutsic function of the model 2000 series olTL readef.is to

heal the iherinolumincscenee material using a reproducible and

controlled temperature cycle and lo delect the light (Tl.) emitted by

the material. The emitted light is measured hy a photomultiplicr

tube (I'MT) which con veils the light inlo an electrical current. The

current is liien amplified and . measured by a recorder or counter,

see tig (III. i)

Mui;ni:uTUIili

f~ 01'III Al, HI.TJ'.K

i.Kiiii

Ill

HliATINfi UJI'

:ATI:K I'OWHH SUI'1'l.Y

S( I I K M A T K D I A G R A M II , I )K I A D I U

CHAPTER FOURRESULTS AND DISCUSSION

4.0 Introduction :

In this (liii|>lcr a hi id' description of the present work is

outlined, the cxpciimculal IC.MIIIs obtained arc displayed in holh

tabular and graphical forms.

4.1 Experimental Method:

Different positions were used in the experiment of the

calibration in (lie SSDL lust, the calibration was made using the

loni/iiliou chamber which was placed in a suitable holder on the

liolley lor precise adjustment of the distance.

The chamber was irradiated for suitable duration of lime

with all possible X-ray (jualilies (six i|ualities were used). At

each energy setting the distance between the reference point

and the detector (chamber) was varied and the dose measured.

The values of dose from distances I to 7 meters from the

reference point were taken

4.2 Calibration of Secondary Standard ChamberLSOI:

The system consists of a spherical chamber lype I,SOI with the

following physical properties and parameters.

l.SOI-Scr No. 912.

Outside diameter I ) - MO mm

( liambei volume (nominal): V - 1000 cm J

54

Chamber Material: I'olyacelal mixture (1)I:I,KI.NR)

Manufacturer : Austrian Research Center Seibers dorl

Hie Kelereiue Conditions under which liie system wasoriginally ealihraled by (lie I'lit; "Cciinun PrimarySfaudimli/atioii I.ah11

Reference point: center of sphere 0 marks beam direction.

1. ('dumberhigh voliage : I I V - 1500V

2. Atmospheric conditions :-

Air lemperalnic :- I,, - 20"(>

Air pressure :- l\, - 101.3 Kpa (760 mm llg)

Kcl. humidity :- K I . - 40 -60%

lal)lc(-t-l) iiililiiiitidii l'actors(('|) provided by I'I'll

Quqlily•••%1 *:<i:f.

<h

CliHiiiber Culibratioi) factor mi

2.762x10(

2.762x10"1

2.753x10'

2.77Sxl0'

2.775x10'

2.779x10'

4.3 Determination of The Target to SurfaceDistance(S):

('alibi ation of the SSDl - for absorbed dose in (iy/inin. was

carried out using (he D( I 8500 standard electrometer.

55

The output of different x-ray tjualittcs were calculated. The

calculation procedure is shown below :

Dose Mi-.('.(1,.l1"ii'(tiy) M-l)

(>0 A(>( )A Dose , , . . , , ».

Dose Kate- (in(jy/h) (4-2)l

where:

Mr is the average of meter reading in volt.

( ' is capacity in f .

C, is Chamber calibration factor in (iy/Couloinb . (see lab. (4-1)

I n . is collection factor for temperature T and pressure p.

2 / M S i T MH.125

"' 2y.HS * I'

I is collection lime in seconds .

The measurement ol (he meter reading (MR) for different

t|ualilies were tabulated (Appendix(AI-A7)).

Alter installing a new X-rays unit it is important to know the

actual position of the X-kay target and in the tube housing of

the machine

lor the determination ol the position oflhe target of the X-ray

unit from the reference point on (he housing (source (lis(anee),wc

adopt the following procedure using the inverse square law,

where we measure the distanced,, and the dose rale I ) . Which

are then related by the inverse square law relation

Hic' ileleiiuiiialioii ol die source liiblancc coi responding lo

inuisc M|ii>tic l<m is ululai (o distance d, und die rclciencc

lenyih s

1 aiycl

\ K\\ I ulic

i)

Oelccl

<III\C(^C M|ii.tic l a u ) ( I I )

i . A(4-2)

A0 ' , i l i o i n I ti> 7

l«l > > ) '(•1-3)

w l k l C i l | i i , > i )

A'

I ,1 >

v'D A' A'

. I

, i IMIUICC II) MllliUC llbliilltC) (t ^)

\ i i l , wcic |)l»Ulcil aiui llic litlcil bliaighl

l l i I,-, l , l , \ ( l ) VNCIC l l i l l l 10 (icICIIIIIIIC lIlC blHIU'C 10the unlace iliilaikc ( i ) accoulmg lo lliü cijuulum

Souivc thitliiiicc uilciccpi/slope (l-(>)

llic incastiicii dihc und coiK:>|)nmliii^ tlosc idle wcic

u^bkud , die Kbiilb ohlaiiicd loi du* six qualme:» were

l.llml.lUj (l.llik -. I In (i )

4.4 Calibration factor for the TLD Reader and theDetector:

lo i (lie calilii.ilKnii of die nieabiiieiiieiil system ( i f 111)

Keattei s>ileni), A gioup ol 11 |) ( l i l M g l i ) chipb

A nuiiiliei ill' Ji j |» (100 chips) ivcie annealed and. divided into

two yioups I ,uli gioup " ' 50 chip* wcie Iwlhei subdivided

into live Miligioup* each coiilmning ten chip*

I .uli <>iit)^iuii|) was luadialcd in the SSDI wlicic they

|)l,iccd on die liollcy ill u dibdnice o lM incteii. j w» (|uali(ici

U L I C chosen, niiincly (,), .nul (,), I he iiiadiulion vvab earned out

l(»i <.), wall (lie lime iniciiids I, 2, .1, -t and S nun ,

euiiei|h»nding to do^e o l 8 5 400, 170 KK(I, 2^> 130, 311 76(1 and

All _'0

l u i the second tjualil) l | , the i.nne lixcil dbtance wab n^eil but

\wlli the tune inteivuU 2,1 ,(>,8 and 10 nun , conespondiiuj to

|(tl (<()(). JdlJltO, )()l «()0, •!()() 100 uiui 508 (JO ) i ( iy

I lie I I I ) Jiijib \seie lead using llai^hovv Model 2000C

I lieiiiuilunnne^enl deiedoi and the leadings in nuno coulombs

\seie l d

I lie awiage leadingb lor 111) clii|b o( the two gjuiips u(

tlilleieul |)obiiii)ii \ u i e taken, the lesull obtained vveie l.ibnlaled

in tab led 10,11)

Calculations:

I o ealeulale llie calibialiou lai loi loi llie I I I ) leudei, the

yi.ij)hi aie plotted a* I I I ) leading in uano coulombs

the given dt»e in i i i i c io t i ia^ ioi each gioup, which in gi

( \ , h ) we lound eijiialiou ol the hue I he data aie filled to

btiai^hl line

y a \ > b

uilli slope u ami inleiccpl I)

I he line plotted using the measurements o! liihle (10) are

slimsu m g u t p M A ) It has u slope of UO(W7iO(K)O2, intercept

ol 0 0%UtO 167..

So the calcululion is

(MKW7

cV.'o. //

I he line plotted using the ineasiiieineiil o l lah le (I I) is shownin (imph ( l i ) It has u slope of 0O09I i()(KKM, inlciccpt of0.1778•») IMW.

So the ciililiialion lacloi is caletilated below

I nC

the culiluatioii t k t o i is (oumi to lie 0 [O'fiiu ( iy per uC

aiul the line plolled IIMII^ the uieasuieinent of table (10 ) , (11 ) aie

slunsu in (iiaph (( ) it has slope of (MMWV So the ciilihiatiou

lacloi is tiiiiiut lo he (Mtt$i»i(iy per u( this lesull sliovs that the

calihialion lacloi is the same loi i|iiali!\ (1) and (|uahty ( I ) ,

licike the I il is eneigy nulepcndenl delecloi in this cncigy

lunge

4.5 the Results of the first part:

l u l > l c ( l ) : < , ) u u l i i ) ( I ) :

K V t o

lit A 10

t t inat to i t n u n A l i 0 10 n u n ( u, I I V I . 2 7 A l n u n

Di&uuiccmclcr

I

•>

1

•I

•>

(i

7

Collectionlime in sec

5

20

20

50

too

Ml

SO

Average of themeter reading

(volt)

d87 | (> i 0 0<>7<»

K 1816 .• 0 0176

3 835 )0011

5 5974 i 0 0114

7 |9(i(u 0 0121

2.424(i!OOO84

1 7760 i 0 0048

DoscmGy". ! • . . ' , , , ( ; i

v . • / •

' • A : . [

0 01910

002298

001077

0 00113

0 0202

000<>8

0 004988

Dose ratewOy/h

13.897

4.136

1 9390

1.320

0.7277

04904

0 3592

in A (i

(2)

n u n A l . • 0 5 9 n u n ( i t , I I V I . - 0 2 1 C u n u n

Distance in»npler;\

\ >

I

3

4

S

0

7

Collection

)

8

15

30

50

50

Si)

Average of llict

7 8OOOi 0 0012

9 4080 i 0.012

8 2()9V ! 0 00%

9 8910 10 0158

10 420 i 0.0132

7 2582 ! (I 0038

5 1058 t 0 0026

0.02191

0.02097

0 02.122

0.2779

0.02927

002038

0 0518

.Dose

39437

11 393

5.5745

3.335

2.1074

1468

1 09 J 3

I able ( i ) :Q.u!il) ( i )

KV 80

m A Id

I dilution I imii AI ' I 85 mm CM, IIVI.-O 59('u mm

• , , -yv.v

1

>

3

4

i

(>

7

• mw1^

1

8

15

.mso

Ml

Ml

w a i t , • , - > * • - • • . n " «

^i'(voU)' "7 Kill) i (10124

H MM •! 0 0144

7 0244 l0 0084

y 11%i OOI54

9.645810 0122

<> 82161 OOlbH

5 10201 0 0052

3pw:

0 0IV«

0 1)2427

0 021.147

002553

OÜ27I y

o.oiyoyy

O.ÜI4284

.nil::tnQy/li15 786

10 92.12

5.1433

.1.0639

20078

1375

1 0285

luhlc (4) MJuulil) (4):

K\ loo

I I IA 21

I illi.ilion I nun AI > 5 10 nun cu. 11VI - 1.15 Cu nun

SiMM III, Collection

. " . - • » . ; •

4

10

15

30

50

50

50

y.5582l()O()74

7 1668 i 0.0024

5 . o y y 2 1 ( » o i l

6.0752 t 0.0126

6 4074 i 0 008

4 54021 0 0046

3 478Oi 0 004

0.02697

000226

0.014.19

OOI7I4

0(11808

OOI28I

0 009815

24.2774

7.28

34532

2057

I 30196

0923

0.707

(•I

5) :<M>lii) (S):

UK

iiiA

I i l i u i i o n I nun A l i 5 ( ' u i 4 I nnn Sn, I I V I - I 74 C u m i n

Pisltqiçcinnic(cr '

1

>

1

•1

s

()

7

Co)lwüou|Ûiiic tco >:

]

Ю

h

io

Ml

Avçwjwafth(?|meter reading*

(voh)

8 40X8 t 0 00 Id

К 1 И2 i 0 0074

5 У410 ( 0 0052

7 0W4 ! 0 0072

7 4122 ( 0 0242

5 2У И» ! 0 0054

4 < п « ( и OOOlO

0 0217.1

0 02 »55

0 01678

0 02001

0 02ОУ18

0(1 Nl>

OOII45

tei i й •

28.477

847V

4 025

2404

1 50<)

(11080

0 825

laMc ((>) :Quiility (6):

K\ ISO

III \ i

I i l i i.i imn I mm Al • о (К) пни ( К < 2 S пни Su, II VI 2.-I0 Cumin

м \a h. Average Qfr

1

10

15

10

5(1

50

50

8 48(>8 i 0 0016

К M)II ! 0(H)7

(>.U24iOOO65

0 02198

0O2IIK

0.01727

28.783

8.707

4.146

5 8680! 0 0108

7 6766 i 0.0202

5 5 9 0 2 ( 0 1482

0 01658

0.02169

0 01579

1.990

1.562

Ш75

4 II46 10 0054 0.01162 ÜM11

lav cue ul' I lie mjuurc runt of (lie dusc rule lor tucli quulily :

(iiiiance/in

1

>

>

I

0

7

0 2<>K

0 19 J

0 718

ti V-Hi

1 172

•

•

0 I V)

0 290

0 12-1

0 i 1«

0 (>89

0 825

0 957

Q3

0 Io7

0 303

0 -112

0 171

0 708

0 851

0 98<>

0 203

0 371

0 538

o <>97

0 87(>

1 0-11

1 189

•;

/ * W j:

7 . • '•••

u 1H7

0 313

o m

0<>|5

0 815

«

I 101

• • • & , • • • • ' • « • • •

0 186

0339

0 491

•

0 800

OV37

1093

t.iluo . IK li.4 uiilmLiI In lliv ^l.ijili ilu- In liul ,i,.<.til.ilv III lilt. IHL.iiiiUllli.lll lli.il !u\C Uxil

I u\>\c (K):iltc m u l l ul I lie sum i e Jiiluiuc from flit.-

graph

1

2

i

I

S

0

0

0

0

0

1)

Slope u

225oi 0.0009

1327 10 0009

1 in-> i 0 0007

| 6 " > 6 I 0 0014

IS 50 1OOOI6

|M)l> i 0 0044

0 0412)

0 02591

0 02% i

0 0399 i

0 037.3 1

0 0373 i

fcpt^*!

0 0029

00040

0 0011

0 0061

00068

0 0010

Source dis

18

19

21

24

24

24

22

26

55

90

10

18

50

03

V>) I lii» (ul>lc »lum the Miiumuruc lor source distance uf uli quufitie&.

Quality

O.i(Jo

•lui e

source distance

2124242422

9010185001

4-6 Result of the second part:

Also flic icstillb ul the bccoiui pail ol llic woik included a[;MIIJ) til I I D i wiucli u a c i b c d lot the dclcnninalion oi liic

ul (lie II I) icadci iibiii^ I I I M J ^ li

I lie Aui . i j 'c ic.nling ol llic I I.I) in nano couloinl) coi ioponding

to £i\cn doic i aic lalnilalal in llic lal)lci (10,11 )

lu i i l f (KI) :

d u m p one, "ill J u p i (I il M g l i )

<.>u.ilH>(l) K\ ' Hi) MA 10

d i l u t i o n I nun At > I 8.5 nun Cu, IIVI- 0.5V('u nun

Oblancc > ntclci

Group Number

liioup one

(noup luo

lnoup line*.

(noup tout

(iiuup tivc

liradiation Timein jnii).

1>

• )

i

, Given Dose ^MGy: m :

85410

I7OK8O

25<>31O

It 1 <>70

•127 200

Average RcAdui#in nano coulomb,94710.152

I 720 1 0.176

2 59110 150

.1.422 J 0.17V

4 570 tO 141

l.»iKilit) (4) KV 100 MA 22

I I I I I I I Al • •> »i)niiii ( i i, I IV I . l . |5 ( i i nun

3 inclci

Group No

(iioii|> one

(noup hsu

( ili)U|) lillCC

iiu)ii|) loin

tilOU|l il\C

IiTAdiatioii :' liine/inin ''

1

«

to

(Jivt'iidosi;'.';.• • • • ' • • • ' • ' n d y - H " 1

101 600

2(l.V2O(l

104 800

406 400

SOHLHIO

1 .160 JOO82

2 171 10 181

3.24810 298

3.%7 10.213

5.|.Wi 0.316

Discussion

I lie newly installed SSDI. iit SAI r is authorized lo pi ovule

calihiation icaiccs This uoik being (lie liisl one in this lal). aimed

in Us liibl pail ul tibiiig (lie uueise squaie law to delenniiie (lie

actual distance between (he X-iay largcl(souice)aiui die icieiciice

point on die lube lunging ,(lie second pan localibiate (he T i l )

(l)clcctoi- Keadei System) llarshow mudel 2000C for X -lay

enemies ul qualify (luee ( 80 Rv, liltiuliou IminAh l.U^iuiu t'u,

IIVI -0 59 nun (11 )am) ijimlily four (100 Kv, lilliiiliuJi

(htiiiiAli V.KhiiiiiCd, IIVI. ~ I I') iiiiiiCu )

Hie oulptil wlteie die X-iay expobiueh weie ineabiiied ul dilieienl

tlbl.incci tiling equation (4-1 ),\vcic lalmluled us shown Tliese

outputs ucic used to calculate (lie souicc disluuces liom the X-iay

laigel to a uleieucc point on (lie bin lute ol* flic lube housing lor

each tjualily using e(|iiulion (•)-(>) and (he t;iaphs( I to 6 ) .

It can be seen lumi the lesulls thut the dose ia(e iuereases with

decici^e ol the dulance, an observation vthieli is expected loi Us

cuiibblcucy with die iu\eise square law Ihe appaienl inciease in

\alue tor quality (.*>) ul seven nteleis, may be attributed lo a

combination of statistical vuiiulion (measured value being so low)

ami possible back scatter (VOID (lie I'ar wall which is very close lo

(he dclecloi

On die oilier hand .equation (-1-5) was used lo plot die graphs ol die

IIIU-IM: ol die ^quaie tool ol'lhe dose tale versus distance for all

qualilies using lable(7)

I tie source distance tut all qualilies weie calculated tioiii the graph

using relationship (4-6) and tubulated in (able (8 ) and the average

\<iluc loi ihc souice distance was found lo be (22 l 2cm )

Ill lite second pail olihih \\oik a group o f l I.I) diips(I.il":Mg:Ti),

wcic used I'oi lite dcici mutation ot tlic ealil»ation tactoi ot tlic l.il'

111) chips with llaislum iiuule) 2U00 C icudei

(lie valihuliou lucloi loi die i|iialily ,(.M ( KV -KO.Iiltnilioii

(liniiiAl 11 tCSnini ( 11) , l l \ 1 0 V) nun ('u)and i|iiality O l

(KHi Kv, lilliaUoudiiiiuAl'.V.KliiiinCul, 11VI. 115 nun ( u )

vvcic cakulalv.tl .ULOHIIIIJ; lo table (10) and giii|)li (A) vvilli slojic ol

((IUIW7IUIKIUJ), wheie a that liuiu table ( I I ) with die slope

Ol 10 0091 • OOOIM)

I lie obiciv.ilion blioui that the culiluulion tacluib lor the two

i|tialilic:> arc aliito^i idcntieal ami were found to be

(01"JtliO 0002).(O tO|'Ji0 000-1) nt(u.iy pci iiano coulontl),

In and eailiei uoik in SAI.C (in Nov-19%) I il was talibialed by

iiiin^ '"Si luaduitvii (lictu -cucig^ 2 27MeV) it vva:> obtained (hat

the caliliialmn I.KIOI b Imnul to be 0 HIM) m (iy pci nC .

I he^c liudnuj Lonlinu dial vvilluii ucciiiiicic^ needed al radiation

level, H I ) * chij)s(l i l M g l i ) ean be consideied ui an

iudc|>emieiil dclcclor it) die Mudied eneigy utnge.

It h bii^geilcd thai liullier ineabUicuicnts should be canied for

olhei Liicigitb Idi dctciinitiation ot the ealibialion laelois lor the lull

ol eiieigiCb in use

l l a s e lal)k(4-2) »him the risult* ul the lulibiutiini futtoin for

(lie 11 l)(l.il :Mg: I i) chips in dillertnl energies :

looSi l)0( Hel.t - cueiyy(2 27MeV))

cAlibraiicm liwtpr in in fyy Bf uCf^^

*» U' O 1 0.0(102

mo>iai ooooi0.10U)

(.7

nor I 0

no

GRAPH (1): QUALITY (1)

14

y - 0 0412 + 0 2256 *x

06rr(A

£ 04

02

00

al (JalaKitted data

i

5

Distance (in)

GRAPH (2): QUALITY (2)

I8Q>

10

08

y = 00259 + 0 1327 *x

I 06

30

04

02• Experimental data

Fitted data

00

0 t

1 1 1 1 1

6 7 8

Distance (in)

oo<u

o -O i;

feeo

0 6

2 0 4

GRAPH (3): QUALITY(3)

2

12

y - 00206+ 0 1305'x

o

r

02

00 1

2

i

3

i

4

Distance

. 5

(m)

Experimental-- Fitted data

i

B ,

data

i '

7

7o

oCO

1)6

GRAPH (4): QUALITY (4)

0)to

a)no

bo

I 4

I 2

10

y - 0 0'JUU + 0 Ibbb " x

c

oenui 02

00

• Expuruneiital datahtteiJdalii

3 4 5

Distance (in)

71

no0>r u a

GRAPH (5): QUALITY (5)

I 2

0)ra

= 0 03/3 ^0 1530* x

o

u>I

a>•>

r

06

0 4

00

I'lllud

• r

7

u (ill)

72

GRAPH(6): QUALITY(6)

0)(U

U)IAOOU)

r

5^CO

nto

at(A

a>•>

r

12

I 0

04

0 2

00

y - 0 0373 i 0.1509 *x

3 4 - 5

DlbldllCb (in)

hilluil (luld

i

6

Graph (A): Calibration Curve for Q3 (80 Kv)

45

40

3.5

3 0

'3 2 0

15

1 0

Oh

00

y = 0.0060 + 0.0087 ' x

Moubinod avuiiiuu (JalaHllocl data

50 100 150 200 250 300 350 400 450

Dose (|i Gy)

Graph (B): Calibration Curve forQ4 (100 Kv)

or

QI

60

i>.5

50

45

40

:ib

10

2 b

JO

I b

10

05

0.0

y - 0 3778+ 0 0091 *x

tooi

200

I

300

i

400

uvomyo (lulii

500 600

Dose

75-

Graph (C): Calibration Curve for Q3 and Q4

60

bb

50

4b

40

O 3 6

r

O

I- 2 b

I 5

10

Ob

00

y-02171 » 000U5*x

Avora(jod data for Q3• Avoruyod data lor Q4

- Filler! (Jala

i i i i i i i i i i . . ( . . .

bO 100 IbO 200 250 300 3bO 400 450 500 550 600

Dose (jiGy)

CHAPTER FIVE

CONCLUSIONSThis work being (lie first in die Secondary Standard Dosimclry

laboratory (SSDI ) Aimed in Us first purl al using the inverse

square law to determine the actual distance from x- ray target to a

reference point on (he surface the X-ray luhe housing The dose rale

for different X-ray qualities were measured a! distances ( I -7 meter)

from reference point. The average value of the source to a surface

was obtained. The average distance obtained from the six qualities is

found to be (2212 cm).

I he second part ol this work dealt with the determination of the

calibiation factor of the Tl.D delecloi- reader system (Ilarshow

model 2000 (') lor x-ray energies quality (3)(KV HO, filtration

(hum Al i .110 mm Cu), 11VI.- 0.5'Jmm Cu) and quality (4)

(KV 100, i ihiat iou(lnini AH 1.85 mm Cu), I IVI - i.15 i i i iuCu) .

A group of I I Ds clnps(l.il ;:Mg :Ti) were irradiated aflej they had

been divided into S groups each of 10 Chips. My the X-ray machine

for different doses ol the quality three and quality four at llircc meter

distance with aid of graph ( A) and ( l i ) we obtained that the

calibration factors were found Id be (O.JftJOj0.0002), and

(0^fV/iJiO.OOOl) m (iy /nC respectively.

These values and those obtained earlier al SAliC for using ' Si

inadiiilor beta energies (2.27 MeV) confirm that wilhui accuracies

needed at radiation protection level, Tl.I)s chips (l . i l ' .Mg :Ti) can

be considered as an energy independent detector in the sluded energy

range.

It is suggested that further measurements should be carried for oilier

energies for determination of the calibration factors for llie full range of

energies in use.

7?

AppendixCalibration of secondary standard chuinticr LOSI: the system consists of

spherical chamber type I .OSI .serial No. 912.

The calibration of the SSPI. for absoilied dose in Gray was carried

out using the IX'I 8500 standard electrometer . The out put of

different \- ray i|iialilies at distance (I-7m) were calculated using:

Dose » Mr • ( ' • ( ' , • • l'i,.((iy).

Mr: is the average of Meter reading in volt.

(' is capacity in I .

(', is the chamber calibration factor in (Jy / c (See lab (4-1).

In1 is correction factor for temperature (T) and pressure (I1)

l ' ' i i - - .27 i lS i | • IOJJ25293.15 I1

The measurement were 23(.'° and 760 nun llg.

The measurement of Meter reading (MU) for different distance (l-7m) for

different qualities were tabulated (A I-A7).

Table (AI)Mder leading (Ml

6.926

6 9.12

6.926

d.785

6.789

aveiaye

SO

6 8710

00771

1) f(ir each (|iiuli(y t

HOPS7.794

7.801

7.798

7.804

7.803

7.8000

0 0041

7.072

7.083

7.086

7.082

7.182

7.10100.0446

it distance 1 meter

9.543

9.564

9.555

9.567

9.562

9.55820<MW

Mm8.413

8.409

8.410

8.410

8.411

8.40880 0045

mom.8.496

8.487

8.483

8.485

8483

8.48680 0054

/K

Table (A2)

8

8

8

8

8.

202Ib7165179195

average

SD80

.1816

0l<)5

9.408

9.399

9.438

9.393

9.405

9.4080

0.0174

8 660

8.643

8.685

8.688

8.671

8.6614

0.0180

77.7.7.7.7.1

166168171172157668

0.0060

8.356

8.336

8.331

8.347

8.356

8.3452

0.0114

80

Mill8.583

8.567

8.577

8.558

8.558

.5694

.1090

Table (A3)Meter readiuy (MR) for ea

3844

3.8-17

3.849

3.850

3.785

average

SO3.3350.028

i;i0^'*8.293

8.261

8.264

8.260

8 267

8.269

0 0217

cli (juttl'ily u

7.628

7.627

7.628

7.604

7.635

7.6244

0.118

distance

"04 '.5.115

5.098

5.065

5.108

5.107

5.0992

0.0185

3 meter

055.947

5.952

5.941

5.940

5.935

5.9430

0066

* QS '6.122

6.101

6.109

6.111

6.119

6.1124

0.0084

Table (A4)Meter t for each quality at distance 4 meter

5.574

5.614

5.599

5.602

5.5l)2

SI)5.5974

0.0146

9.880

9.920

9.909

9.883

9.881

98940

0.0186

mom*9.120

9.090

9.111

9.134

9.143

9.1196

0.02061

6.072

6.089

6.087

6.049

6.079

6.0750

0.0161

7.100

7.087

7.094

7.106

7.110

7.0994

0.0092

HI5.857

5.870

5.873

5.888

5.852

5.8680

0.0142

ri

Table (A5)Meier rcnriiui' (MM) lor null uu;ilily ut clislitiuc 5 meter

017,108

7.208

7 , 1 %

7216

7.195

SI)7. I960

0,0182

0210 411

10.416

10.410

10.414

10.401

10.420

0.0168

039.669

9.717

9.701

9.698

9.692

9.696

0.0176

6.405

6.428

6.401

6.191

6.412

6.4074

0.0118

7,4717.191

7.414

7.195

7.188

7.4122

0.0141

7.691

7.614

7.674

7.646

7.686

7.67660.0184

Table (A6)

Oi2 417

2 421

2 111

2 419

2111

SI)

2425

00100

027.264

7.256

7.251

7.259

7.261

7.2582

00050

ioa--6.820

6.810

6815

6.821

6.822

6.822

0.0089

Q44.541

4.512

4.547

4 544

4.517

4,5402

0.0059

5.299

5.298

5.286

5.284

5.291

5.2910

0 0056

5.961

5.506

5.494

5.996

5.494

5.5902

0.2071

Table (A7)Meter reading (MK) for each quality at distance 7 meter

0! 07.781 5.402 5.095 1.476 4.051 4.115

.771 5.408 5.102 1487 4066 4,125

5.410 5.1 .1.476 4.056 4.117

.779 5.404 5.096 1.479 4.055 4.107

.777 5.4058 5.106 1.472 4.06.1 4.109

average 1.7760 5.4058 5.1020 1.4780 4.0586 4.1146

SI) 0.0064 0,0026 0.0067 0.0056 0.0056 0.0071

7/i

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82