Personnel TLD monitors, their calibration and response

by

Antonia Savva

A dissertation submitted to the Department of Physics,

University of Surrey, in partial fulfilment of the degree of

Master of Science in Radiation Detection and Instrumentation

Department of Physics

Faculty of Engineering and Physical Sciences University of Surrey

September 2010

Antonia Savva 2010

Abstract

Personal dosimeters are worn by workers when they are exposed to radiation

to make sure that a reference limit is not exceeded. Thermoluminescence dosimeters

(TLDs) emit light when they are heated. By measuring this light the dose can be

calculated. The reader and batch calibration are carried out first. CDs are selected

(falling within the range of 0.94-1.06) and by using them, the RCF is automatically

found (0.034127 and 0.027526) for the two different TLD elements in TLD cards.

FDs are also selected (within 0.7-1.3). ECCs are calculated automatically by the

reader and follow a normal distribution with a mean value of 1.0010.001. The

repeatability of the TLDs is measured to be about 1%, lower than the threshold

(2%). The maximum Coefficient of Variation is calculated to be 1.7% where the

limit is 10%. The curve of the energy response has similar shape to the curve given by

the manufacturer. For low energies two different filters are used and the responses are

compared showing that the energy dependence of the TL materials is a function of the

energy spectrum and not just a function of the nominal value of the energy used to

produce this energy spectrum. The angular response is found almost constant (up to

about 70) for both directions and it is reduced to about half its value when the angle

becomes 90. Small difference in the angular response in two directions is due to the

change in the irradiation distance as the TLDs are closer and further away in the x-ray

tube at large angles. The ISO requirement is met for all angles except at 90.

ii

Acknowledgements

I would like to express my deepest gratitude to my supervisor Dr. Stelios

Christophides for giving me this opportunity to carry out my work in Nicosias

General Hospital and for trusting me with the assignment of this very interesting

subject, as well as for his ongoing interest, guidance and support throughout my

projects application.

I am also extremely thankful to Medical Physicists Dimitris Kaolis, Christos

Papaeustathiou and Georgiana Kokona for their essential help during the experimental

part of this work as well as their invaluable assistance in order to overcome crucial

problems that occurred. In addition my deepest appreciation goes to all the Medical

Physicists of the Department for their help and support.

I also cannot express enough gratitude to my family and friends who believed

in me and encouraged me throughout my whole year of studies in Guildford.

Last but not least, my truthful thanks go out to Dr. Paul Sellin for his support

and interest during the length of my course.

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To my family..

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Table of Contents

ABSTRACT.................................................................................................................II

ACKNOWLEDGEMENTS ..................................................................................... III

1. ................................................................................................1 INTRODUCTION

2. ..............................................................................................................3 THEORY

2.1. ............................3 THERMOLUMINESCENCE DOSIMETRY - A GENERAL MODEL

2.2. ..................................................................................................4 TLD READER

2.3. .............................................................6 CHARACTERISTICS OF TL MATERIALS

2.4. ....................................................................................8 TL PROPERTIES OF LIF

2.5. ........................................12 ADVANTAGES AND DISADVANTAGES OF TLD100

2.6. ................................................................................................13 CALIBRATION

2.6.1. ...............................13 Batch Calibration - Element Correction Coefficient

2.6.2. ......................................................................15 Reader Calibration Factor

3. ......................................................................16 MATERIALS AND METHODS

3.1. ...................................................................................................16 MATERIALS

3.2. ............................................................................................17 METHODOLOGY

3.2.1. .........................................17 Selection of Time Temperature Profile (TTP)

3.2.2. .............................................18 Selection of Calibration Dosimeters (CDs)

3.2.3. ...................................18 Calculation of Reader Calibration Factor (RCF)

3.2.4.

...................................................................................19

Calculation of the Element Correction Coefficients (ECCs) and Selection

of Field Dosimeters (FDs)

3.2.5. ..............................................................................19 Irradiation procedure

3.2.6. ............................................................................................20 Repeatability

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3.2.7. ..................................................................................20 Energy dependence

3.2.8. .................................................................................21 Angular dependence

4. ........................................................................22 RESULTS AND DISCUSSION

4.1. ........................................22 SELECTION OF CALIBRATION DOSIMETERS (CDS)

4.2. ............................22 CALCULATION OF READER CALIBRATION FACTOR (RCF)

4.3.

.................................................................23

CALCULATION OF ELEMENT CORRECTION COEFFICIENTS (ECCS) AND

SELECTION OF FIELD DOSIMETERS (FDS)

4.4. ............................................................................................24 REPEATABILITY

4.5. ...................................................................................26 ENERGY DEPENDENCE

4.6. ................................................................................29 ANGULAR DEPENDENCE

5. ..................................................................................................31 CONCLUSION

APPENDIX I EQUIPMENT SPECIFICATIONS...............................................33

HARSHAW BICRON MODEL 6600E AUTOMATIC TLD WORKSTATION ...................33

INSTRUMENT SPECIFICATIONS [8] .............................................................................34

SPECIFICATION OF MAMMOGRAPHY UNIT (PLANMED SOPHIE CLASSIC) [22].......35

SPECIFICATIONS OF MEDCAL EIDOS (RADIOGRAPHY UNIT) [23-24] ........................36

SPECIFICATIONS OF CS SOURCE [25]137 .....................................................................37

DECAY CHAIN OF THE CS SOURCE [26]137 .................................................................37

SPECIFICATIONS OF ION CHAMBER IONEX 2511/3 [27]............................................38

SPECIFICATIONS FOR RADCALL ION CHAMBERS [28].................................................38

APPENDIX II ROW DATA ..................................................................................40

RAW DATA FOR THE SELECTION OF CDS AND FDS AND THEIR CALIBRATION FACTORS

40

DATA FOR THE REPEATABILITY ................................................................................45

vi

vii

DATA FOR THE ENERGY RESPONSE...........................................................................46

DATA FOR THE ANGULAR DEPENDENCE ...................................................................48

ACRONYMS AND ABBREVIATIONS..................................................................50

REFERENCES...........................................................................................................51

1. Introduction

Radiation dosimetry is defined as the measurement, usually, of the absorbed

dose, or other relevant quantities like KERMA, exposure or equivalent dose, which is

produced due to the interaction of the ionizing radiation with a material. That

measurement can be achieved using a dosimeter [1]. A dosimeter with its reader is

called a dosimetry system [2].

External dosimetry is a measure of absorbed doses, produced from radiation

sources, which are outside of the body of the exposed worker. For this kind of doses a

personal dosimeter is used, which is usually called badge, and has to be worn by the

worker every time that he/she is exposed to the radiation, in order to make sure that a

reference limit is not exceeded. If a worker works with sources which are not sealed,

then the radioactive material gets into his/her body and absorbed by tissues or organs

in the body. Therefore, internal doses should also be measured, using specific

monitors, with the aim of calculating the total effective dose to the worker from

internal and external exposures [3].

Absolute dosimeters are used in order to measure directly the dose without the

need of a calibration in a known radiation field (e.g. calorimeters) [1]. On the other

hand, secondary or relative dosimeters provide indirect measurement of dose but have

to be calibrated using a primary (absolute) dosimeter at reference conditions. An

example of secondary dosimeters is the TLDs (thermoluminescence dosimeters)

which are normally calibrated using an ion chamber dosimeter [4].

TLDs have been developed significantly over the years and a lot of materials

were studied to see if they are suitable for applications for different areas in dosimetry

[4]. TL materials store energy inside their structure when they are irradiated, as

electrons and holes are trapped in trapping centers due to defects. When that material

is heated, electrons and holes recombine, at luminescence centers, and thus light is

emitted. The light is measured using a PMT (photomultiplier tube) inside the reader

device [5]. The photons which are emitted are in the visible region and they comprise

the TL signal. Preferably, one photon is emitted from each trap center. Therefore, the

1

measured signal is an index of the number of electron/hole pairs and it is proportional

to the absorbed dose [6].

TLDs are mainly used for personal monitoring of workers who are exposed to

radiation that is higher than 3/10 of the dose equivalent limits. The individual

monitoring of those workers is essential in order to make sure that the limit of the

equivalent dose does not exceed the maximum permissible dose [4]. They are used to

determine the dose of an individual at a specific depth of his/her body, most of the

times at 0.07 and 10 mm. At 0.07 mm the effective dose of the workers skin

(Hp(0.07)) and at 10 mm the dose of the organs inside the body (Hp(10)) are

measured (see figure 1.1). Additionally, at depth 3 mm the dose in the eyes of a

worker is calculated [3]. TLDs can also be used for environmental, neutron and

clinical monitoring. Moreover in vivo techniques, in radiotherapy as well as in

diagnostic radiation measurements. Furthermore, for dating of materials and for

analysis of the meteorites [4].

Figure 1.1: External exposure Estimation of dose at depths equal to 0.07 and 10 mm [3].

In this project, the TLDs used for individual monitoring are fabricated from

LiF and doped with Mg and Ti, will be examined. Firstly, their calibration will take

place and then their repeatability, their energy and angular response will be studied.

2

2. Theory

2.1. Thermoluminescence Dosimetry - A general model

Luminescence is a process in which, a material that is irradiated, absorbs

energy which is then emitted as a photon in the visible region of the electromagnetic

spectrum. Thermoluminescence is a form of luminescence in which heat is given to

the material which results in light emission [4].

In a crystal, electrons (e-) are found in the valence band (see figure 2.1.1a).

When the material is irradiated, e- move from the valence to the conduction band

where they move freely. Therefore, a hole (h) remains in the valence band (absence of

electron) which can also move inside the crystal. Due to impurities and doping of the

crystal, e- and h traps are created in the band gap between the valence and the

conduction band. Thus e- and h are trapped at defects (figure 2.1.1b). If these traps are

deep, the electrons and holes will not have enough energy to escape. By heating the

crystal their energy is increased, they leave the traps and recombine at luminescence

centers. As a result light is then emitted (figure 2.1.1c) [4-5].

(a) (b) (c)

Figure 2.1.1: The mechanism of TL dosimetry [5].

A TLD can be considered as an integrating detector in which the number of e-

and h, which are trapped, is the number of the e-/h pairs which are produced during

the exposure. Preferably, every trapped e-/h emits one photon. Consequently, the

number of emitted photons is equal to the number of charge pairs, which are also

proportional to the dose which is absorbed by the crystal [6].

3

The probability of a charge carrier to escape per unit time (p) is given by the

Randall-Wilkins theory using the equation:

/1 E kTp e

(1) where,

= the mean half-life of a charge carrier in a trap = the frequency factor E= the energy of the trap (eV)

k= the Boltzmans constant = 8.62 *10-5 eV/0K

T= the Temperature (0K)

By increasing the temperature, the escape rate is increased and the mean half-life of e-

/h is reduced. This rate, as it is increased, reaches a maximum at a specific

temperature and then is rapidly reduced. But as the intensity of the emitted light is

proportional to this rate, it could be realized, that there would be a creation of a peak

in the graph of intensity versus temperature, called glow peak, and the graph called

glow curve. Both are explained in detail in subsection 2.4 [1].

2.2. TLD Reader

A schematic diagram of a TLD reader is shown in figure 2.2.1. The dosimeter

is placed on a tray (support made of metal) inside the chamber. There it is heated by a

heating coil, which is in good contact with the dosimeter and the tray. A thermocouple

is also used to measure the temperature of the heating cycle in the chamber. Nitrogen

gas is used to reduce the signal produced from impurities in the air [5].

Due to the thermoluminescence effect, light is emitted and as it passes through

optical filters, it enters the PMT through the light guide and then it is measured. As

the output of the PMT is proportional to the number of photons which are generated,

it becomes also proportional to the absorbed dose when the output is integrated.

Instead of integration, pulse counting can take place. That means that the output is

converted into pulses which are counted. The reader device is connected to a PC and

the measured results are either stored in the hard disk of the PC or printed out [5].

4

Figure 2.2.1: A TLD reader [5].

The PMT consists of a photocathode which converts the incident light into

current. That current is amplified inside the PMT which gives an output that can

easily be measured [6]. Most photocathodes have a peak sensitivity of about 400nm

wavelength. So it is very important to choose a suitable TL material (phosphor) which

generates light in the blue region of the electromagnetic spectrum. The selection of a

suitable material will be discussed in section 2.3. A good reader should have a large

transmission of light and be able to measure different TL materials [5]. PMTs with

low response are mostly used for the detection of low levels of light from TL

materials [7].

There are more than one ways to heat a TL material (dosimeter). In figure

2.2.1 the tray and dosimeters are in contact with a heating coil (element). The increase

in the temperature can also be produced by an electric current. These methods are

called ohmic heating and are the most commonly used methods [4].

Another way to rise the temperature is a non-contact method. This method

could include a hot air heating method (hot nitrogen gas), radiofrequency (RF) heating

or optical heating method. In the RF heating the heat is produced from the current of

the RF induction heating spool. In the optical method the increase in temperature is

due to a heating lamp. By using the non-contact methods the reproducibility of the

5

heat is easier and there is no contamination produced between reader and dosimeter.

Nevertheless, it is simpler to control the temperature using a contact method [4].

More details for the Harshaw (model 6600E) reader used for this project and

its specifications are shown in Appendix I.

2.3. Characteristics of TL materials

Although there are more than 2000 TL materials available, only 8 are used as

they are more appropriate for measuring radiation dose. Four of them have low atomic

number (Z) and are characterized as tissue equivalent materials, as they have a

respond similar to that of human tissue. These are lithium fluoride (LiF), lithium

borate (Li2B4O7), beryllium oxide (BeO) and magnesium borate (MgB4O7). They are

used for medical application as well as for personnel monitoring for industrial

applications. The other four materials over-respond due to their higher Z. Thus, they

have higher sensitivity and are characterized as non-tissue equivalent materials. These

materials are calcium sulphate (CaSO4), calcium fluoride (CaF2), aluminum oxide

(Al2O3) and magnesium orthosilicate (Mg2SiO4) and are used for environmental

monitoring [7].

Different commercial TLDs holders (badges) are shown in figure 2.3.1. The

personal information of the worker is outside of the dosimeter. Their shape makes

sure that they are correctly placed inside their holder. It is very important that a

worker wears correctly the badge as well as keeping it clear and dry [3].

TL materials are not ideal for measuring dose. Many factors have to be taken

into account in order to find the most suitable material. The availability is very

important as well as the stability of its produced signal. A low fading rate is important

(lower than 5% per month) as well as simple glow curves with a plain anneal heating

cycle. Although the sensitivity of a tissue equivalent material is not very high, it can

be increased by adding impurities called activators. As there are more impurities in

the material, more traps are included and thus more light is emitted during

thermoluminescence process. Therefore the efficiency of the material is increased.

6

Figure 2.3.1: Commercially available TLDs holders [3].

Except for high sensitivity and efficiency, a low variation in the signal of the

background is needed with the aim of measuring low dose thresholds with high

accuracy (doses lower than 100 Gy). Moreover, a flat energy response over a large range of energies is required [7]. An ideal dosimeter should have a linear response

over a large range of doses and its response should not be affected from the dose rate.

Their response variation due to the different angles of the incident radiation to the

dosimeter should be well known. Finally, the dosimeter must have small dimensions

in order to be able to measure point doses with high spatial resolution [2].

The most widely used material is the LiF with added magnesium and titanium.

TLD100 is LiF:Mg,Ti which consists of 92.5% of 7Li and 7.5% of 6Li. This is the TL

material used in this project. Also the TLD600, with more 6Li in it, and TLD700, with

only 7Li, are available. Their sensitivity to -rays is the same but it is different to neutrons, as 6Li have high thermal neutron absorption coefficient. The properties of

the TLD100 are discussed in section 2.4.

MgB4O7 has similar behavior as LiF with higher sensitivity (5-10 times higher

than LiF). Its disadvantage is that an additional anneal irradiation is needed to reduce

the fading as it is very sensitive to light. Li2B4O7 has less sensitivity compared with

LiF (1/10th of LiF) and is hydroscopic. BeO is a more tissue equivalent material that

LiF with almost the same sensitivity, but toxic and very sensitive to light [7]. Table

2.3.1 summarizes the properties of the different TL materials.

7

Table 2.3.1: TL properties of different materials [7].

Material Sensitivity per

unit mass

Dose Threshold

(Gy) Fading factor

(5 loss at 200C)

Energy

response ratio

LiF- TLD100 1 50 ~5y-1 1.3

BeO 0.2-1

dose is the 5th peak. The dosimetry peak should have large enough temperature in

order not to be affected by the room temperature but also not to high in order not to be

affected by the black body emission of the TLD disc. The half-life of each peak is also

shown on figure 2.4.1.

Figure 2.4.1: Glow curve of TLD100 (A) after pre-heating procedure (B) The half-lives of each

peak can also be seen. [4]

The problem is that at low temperatures the fading is high. Thus electrons

have enough energy to leave the traps and de-excite without the need of heat. That

affects the sensitivity of the dosimeter. It is possible to transfer the TL sensitivity of

low temperatures to the dosimetry peak by pre-heating just before the read-out. Thus

the background signal is removed and therefore, the dosimetry peak is much more

distinct (figure 2.4.1-curve B).

After the TLDs are read-out, they are annealed in order to ensure the signal

has been completely removed and the TLD is again ready for use. For the TLD100

the annealing is not as simple, as it is first heated at 4000C for an hour and then at

800C for 16 to 24 hours. If the used annealing temperature is more than 4000C the

sensitivity of the material is reduced [4].

The area under the glow curve, after the appropriate calibration, corresponds

to the absorbed dose which is measured using the TLD reader. If the rate of the

9

temperature is constant the glow curve is the TL intensity against the time. A good

reproducibility of the heating cycles is very important for accurate measurements [2].

A typical read-out cycle, which includes the pre-heating, heating, annealing and

cooling periods, is shown in figure 2.4.2.

Figure 2.4.2: A typical read-out cycle [1].

At higher temperatures (300-4000C) a spurious TL signal is produced called

triboluminescence. This signal is produced due to the combination of effects of the

absorbed gases and the dirt and humidity of the TL material. It can be reduced using

an oxygen-free gas, like nitrogen or argon, around the TL material during the read-out

cycle. This problem should be taken into account especially for low dose rate

measurements [1].

The dose response curve of the TLD100 is shown in figure 2.4.3. The TL

intensity is linear for low doses (3 to 10 Gy). For dose equal to zero the TL signal is

not zero, but it is equal to a background signal which determines the threshold of the

absorbed dose, that can be measured by a dosimeter. For higher doses, the response is

10

supralinear. The signal is increased reaching a maximum called saturation and then

decreases quickly [4].

Figure 2.4.3: TL signal against absorbed dose [4].

The saturation is related to fill traps or to the beginning of radiation damage

[9]. 20% less than the saturation dose is, in practice, the maximum limit. Over this

dose more calibration factors are needed and therefore the error of the measurement is

increased [4]. Supralinearity and saturation can lead to problems of under and over

estimation respectively. They can both be affected prior to radiation exposures.

Therefore, re-use of these dosimeters have different dose response. In order to avoid

annealing, heating procedures are required [9].

Only a small part of the incident ionizing radiation is absorbed as dose and

measured in a TLD material when is heated. Thus, the ratio of the TL light which is

emitted per unit mass over the absorbed dose is called intrinsic efficiency of the TLD.

The intrinsic efficiency of the TLD100 is found to be equal to 0.039% with the rest of

the dose, approximately 99.6%, is converted to thermal radiation. That is the main

reason why the reproducibility conditions are very important for the read-out of a

TLD as only a small part of the total energy deposited is actually measured to

determine the whole dose [1].

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2.5. Advantages and Disadvantages of TLD100

The most important advantages and disadvantages of the TLD100 are shown

in Table 2.5.1 below [1-2].

Table 2.5.1: Advantages and Disadvantages of TLD100 .

ADVANTAGES DISADVANTAGES

Large availability of TLDs and readers from

many manufactures

The signal is read only once- The signal is erased

during the read-out cycle

They are available in many forms It is easy to lose the reading

They are tissue equivalent Loss of TL signal due to fading

They have small size and therefore they can be

used for point dose measurements

TLDs have different sensitivities- Calibration is

essential for accurate measurements

They have large range of dose TLDs are sensitive to light

Their response is not depended on the dose rate The storage in a TLD is not stable Annealing

heating cycle is needed

By using annealing procedures they can be

reused many times before they are completely

damaged from radiation

Spurious TL signals are produced due to scraping of

a TLD or its contamination by dirt or humidity

Due to their reusability their cost is decreased

The sensitivity is decreased or increased after a

large dose received by a TLD- an additional anneal

procedure is then needed

The read-out is quick and it does not require any

wet chemicals It is not recommended for beam calibration

Automatic readers which are connected to PCs

are available

1-2% reproducibility can be achieved using

calibration

In a single exposure many TLDs can be exposed

Due to 6Li they are sensitive to neutrons

TLD600 and TLD700 have different

sensitivities to neutrons

12

2.6. Calibration

2.6.1. Batch Calibration - Element Correction Coefficient

Even though dosimeters are irradiated to the same uniform dose at the same

geometrical conditions, their sensitivity (efficiency) is different. The TL efficiency

can be expressed as the TL light which is emitted per unit of absorbed dose. The

variance in the sensitivity of a typical batch of TL dosimeters is unavoidable but can

be reduced from 10-15% to 1-2% when dosimeters are calibrated. Thus, calibration is

critical.

The Element Correction Coefficient (ECC) is a correction factor which relates

the TL efficiency of a specific dosimeter to the average TL efficiency (TLE) of the

Calibration dosimeters and is given by:

jj

TLEECCTLE

(2)

Where

ECCj= the ECC of a dosimeter j

= the mean TLE of the Calibration dosimeters

And TLEj= the TLE of the dosimeter j from the Field dosimeters

In order to calculate the average TLE a small subset of all the dosimeters is used,

called Calibration dosimeters (CD). The average value of all the CDs is compared

with the efficiency of each one of all the dosimeters, called Field dosimeters (FD), to

calculate the ECC for each one individually.

But the TLE is proportional to the TL response (TLR) of the dosimeter

TLR K TLE (3) where

K= proportionality constant

and TLR= the quantity which is measured and produced when ionizing radiation is

incident on the dosimeter

Therefore, by reducing the variance of the efficiency (or sensitivity), the variation of

the response is also reduced. This correction factor is then multiplied with the

13

response of each dosimeter and its efficiency becomes identical to the mean of all the

dosimeters. As a result, all the TLDs have similar efficiencies (see figure 2.6.1.1).

Figure 2.6.1.1: ECC factors.

The actual quantity measured by the Reader is the charge which is produced

during the TL process. Consequently, the ECC can be expressed by:

jj

QECCQ

(4)

Where

= the average measured charge of the Calibration dosimeters

and Qj = the charge measured from dosimeter j

By reducing the variance in efficiency the variation of the measured charge is also

decreased.

If new dosimeters are added, the ECCs are re-evaluated with the aim of having

similar efficiency with the existing ones. In order to achieve this, the sensitivity of the

Calibration dosimeters must remain constant [8].

14

2.6.2. Reader Calibration Factor

If a constant geometry, constant operational conditions and similar response of

the TLDs are achieved, the only part of the whole system that is not stable for long

time is the Reader. Thus a Reader Calibration factor (RCF) should be applied which is

given by:

QRCFL

(5) where

= the mean charge measured of a set of Calibration dosimeters

and L= a radiation quantity expressed in generic units (gU). 1 gU is the radiation

delivered in 1 second at a specific geometry by a specific source at a constant distance

from the source.

The set of Calibration dosimeters used for the calculation of are

automatically selected by the software. These are the dosimeters which have the

lowest difference from the average value of the charge measured for the generation of

the ECCs. These dosimeters are equal to 1-2% of the total number of dosimeters used

for calibration.

It is very important to find a relation between a gU and Gray (for absorbed

dose) or Sievert (for equivalent dose). The dose measured from a dosimeter j is given

by:

jjq ECC

D jRCF K (6)

where

qj= the charge measured by the Reader from a Field Dosimeter j

ECCj and RCF are as defined above

and K is expressed by:

LKD

(7) where

L and D are as defined above.

K is measured in gU/Gy

15

From equation (6) the quantity RCF*K is measured in nC/gU and shows a

relation between the dose and the internal units of the Reader. The time between

irradiation and read-out or irradiation and preparation is not important but it should

remain constant. Using the RCF an accurate conversion from charge to dosimetric

units is achieved [8].

3. Materials and Methods

3.1. Materials

The materials and equipment used are listed below:

- Harshaw Bicron TLD Reader (Model 6600E)

More details and specification of the reader are presented in Appendix 1.

- TL dosimeters (TLD100)

In total 124 TLD100 cards were used. TLD cards are tacked on pieces of

polystyrene (felizol), using pins, during the irradiations. More details and

characteristics of the TLD100 can be found in theory section 2.3-2.4.

- 137Cs source

The source is used for the Readers and TLDs calibration and for the study of

the energy response of these dosimeters at high energies (662 keV). For the

specifications of the source see Appendix I.

- Mammography unit (Planmed Sophie Classic)

For the exposure of the TLDs at low energies and the examination of the TL

response at these energies (28 33 keV) a mammography unit is used. For the

specification of this unit see Appendix I.

- Radiography unit (Mecall EIDOS)

For the irradiation of the TLDs at energies between 47 and 120 keV a

diagnostic radiography unit is used. For the specification of this unit see

Appendix I.

To ensure that the dose remains constant, different ion chambers are used for the

measurement of dose at the different irradiation units.

16

- IONEX 2500/3 ion chamber (for the 137Cs source)

- Radcal Model 10X9-6O (for the radiography unit)

- Radcal Model 10X9-6M (for the mammograpgy unit).

Specifications of these ion chambers are given in Appendix I.

3.2. Methodology

The TLD readers calibration as well as the TLDs and batch calibration are

carried out following the steps given in the TLD readers manual [8]. The

methodology sequence is described below.

3.2.1. Selection of Time Temperature Profile (TTP)

The TTP corresponding to the TLD100 material, shown in Table 3.2.1.1 is set

on the Reader [8].

Table 3.2.1.1: Time Temperature Profile for TLD100.

PREHEAT

Temp (C)

Time (sec)

50

0

ACQUISITION

Max Temp (C)

Time (sec)

Rate (C/sec)

300

13.33

25

ANNEAL

Temp (C)

Time (sec)

0

0

17

3.2.2. Selection of Calibration Dosimeters (CDs)

For the selection of the CDs, the TLDs are annealed in the reader so that any

residual exposure is removed and are then stored in a subdued UV environment at

temperature less than 30 C.

The TLDs are exposed in a Secondary Standard Dosimetry Laboratory at a

dose of 500 mR (= 4380 Gy) using a 137Cs source (figure 3.2.2.1). The TLDs are then stored for 30 minutes at a maximum temperature of 30 C, to allow for the

shallow TL peak (peak 1 in figure 2.4.1) to fade out. The time between irradiation and

read-out must remain constant to have for all dosimeters the same fading.

The TLDs are subsequently placed in the reader and read.

The Reader automatically designates the TLD cards as CDs those that fall

within a specified range around the normalized mean value of their response. Usually

this range is narrower than 0.9 to 1.1 (10 %).

Figure 3.2.2.1: Irradiation of TLDs.

3.2.3. Calculation of Reader Calibration Factor (RCF)

For the creation of the Reader Calibration Factor the selected CDs are

annealed, exposed and read repeating the cycle described in 3.2.1. The RCF is

calculated automatically by the reader.

18

3.2.4. Calculation of the Element Correction Coefficients (ECCs) and Selection of Field Dosimeters (FDs)

The ECCs are generated by repeating for the third time the annealing,

exposure and reading cycle for the rest of the dosimeters (not the CDs). ECCs are

calculated automatically by the Reader (one for each TLD element). The mean value

and the standard deviation for each ECC are calculated.

The Reader automatically designates the TLD cards as FDs falling within a

specified range around the normalized mean value of their response. A typical batch

of FDs has a variation less than one relative standard deviation.

3.2.5. Irradiation procedure

The TLDs are positioned 2 meters from the irradiation source with the aid of a

laser beam. The TLD cards are pinned on a piece of polystyrene support with

dimensions 20 x 20 cm2 (20 TLDs for each irradiation - see figure 3.2.5.1). The

Figure 3.1.5.1: 20 cards were placed on a block of polystyrene (The maximum distance of a TLD

element from the centre of the irradiation beam is 12.5 cm).

maximum distance of a TLD element from the centre of the irradiation beam is 12.5

cm. The distance between this TLD and the irradiation source is 2.0039 meters (figure

3.2.5.2) which is well within the positioning error for such a calibration set-up. It was

19

assumed that all TLDs are exposed to the same dose independently of their position at

the edge or in the middle of the polystyrene piece.

Figure 3.2.5.2: Diagram showing the positioning error for the TLD cards irradiation with the

137Cs source.

3.2.6. Repeatability

The repeatability of the TLDs is examined be exposing each time 20 FDs

using the 137Cs source. The FDs are stored for 30 minutes for fading and then are read.

This procedure is repeated 10 times. The mean value of each exposure, the

standard deviation and the standard error of the mean are calculated. The TL response

of the TLDs (relative to the first exposure) is represented in a graphical form as a

function of the number of exposures.

3.2.7. Energy dependence

By keeping the dose constant (4.38 mGy or 4.38 mSv equivalent dose) the

energy dependence is investigated within the energy range for which the TLDs will be

used routinely.

Due to physical limitation the irradiation distance for the mammography unit

is 0.6 m and for the radiography unit 1.0 m.

The TLD irradiations are repeated 4 times for 20 dosimeters per irradiation

using the 137Cs source and 20 times for 4 dosimeters (see figure 3.2.7.1) using the

mammography unit (positioning error=0.6017 m) and radiography unit (positioning

20

error=1.0010 m), so as to minimize the irradiation distance error from the centre to the

edge in the case of the mammography and the radiography units set-up.

Figure 3.2.7.1: 4 cards were placed on a block of polystyrene (The maximum distance is 4.5 cm).

The mean responses and their uncertainties are calculated. The energy

response of the TLDs is represented in graphical form as the relative response (TL

response/ response of the 137Cs) as a function of irradiation beam energy. For low

energy photons (mammography unit) the TL response is measured and compared at

two different energy qualities (Rh and Mo anode filtration).

3.2.8. Angular dependence

The angular dependence of the TLD cards is investigated at the same dose and

energy. The TLDs are positioned as for the energy dependence but at an angle to the

direction of the irradiation beam (as an example see figure 3.2.8.1). 5 groups of 4

TLDs, were irradiated by keeping the positions of the TLDs exactly the same for all

the measurements. The measurement was repeated 4 times for each group for each

angle.

The mean responses and their errors are calculated. The angular relative

response (TL response at an angle/TL response at 0) against the angle is given in

graphical form.

21

Figure 3.2.8.1: Irradiation at an angle equal to 10.

4. Results and Discussion

4.1. Selection of Calibration Dosimeters (CDs)

The reader automatically selects the TLD cards that are within the set selection

limits. Initially the selection limit was set to 0.9-1.1. As all TLDs felt within this limit,

the limit was reduced and the CDs were selected as those within the range of 0.94-

1.06 ( 6%).

The readings for the 124 TLD cards are given in Appendix II. 42 TLDs are

within the selection limits and are designated automatically by the reader as the CDs.

The TLDs that are indicated in Appendix II as bad will be used to select the

FDs that will be used for the repeatability, energy and angular response

measurements.

4.2. Calculation of Reader Calibration Factor (RCF)

The RCF was calculated automatically from the reader and was found equal to

0.034127 and 0.027526 (no units). Two different RCFs were calculated as each card

22

consists of 2 TL elements. Thus the first RCF is for the TLD in position ii and the

second one for position iii. According to [10] the RCF of this reader changes by 1.3%

over 5.5 years proving that its system is reliable and stable. Using the RCF, a

measurement of the charge is automatically converted into dose by the reader (see

equation 6).

4.3. Calculation of Element Correction Coefficients (ECCs) and Selection of Field Dosimeters (FDs)

For selecting the FDs to be used for the energy and angular dependence of the

TL response a range of 0.7-1.3 ( 30%) of the normalized mean value of the response

was chosen. For the TLD element falling in this range their ECCs were calculated (a

different one for each TLD element). Those falling outside this range were rejected.

These ECCs are used by the Reader to multiply automatically each charge

measurement in order to have a similar sensitivity (efficiency).

According to [10], ECCs follow a Gaussian distribution with mean value equal

to 1. The values of the ECCs of the selected FDs are shown in graph 4.3.1. As ECCs

are correction factors given by the Reader, it was assumed that they do not have an

error.

0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25-5

0

5

10

15

20

25

30

35

Num

ber o

f dos

imet

ers

ECC values Figure 4.3.1: The normal distribution of ECCs.

23

A Gaussian curve is fitted to the ECC distribution (red curve in figure 4.3.1).

This is obtained by using equation

222

0

cx xwy y A e

given by the Origin Pro 8 program, where y0=0.620.14, A=30.821.30, xc=1.0010.01 and

w=0.060.01. The measured mean value of the normal distribution (xc) is equal to

1.0010.01 (no units) and is similar to the theoretical mean value. The standard

deviation of that distribution is equal to 0.060.01 (w).

The 57 FDs that are within this range of standard deviation are chosen to be

used for the repeatability, energy and angular dependence response measurements.

4.4. Repeatability

The repeatability of the FDs was checked by irradiating them with the same

dose and energy. The energy was 662keV, as a 137Cs source was used, and the dose

was equal to 4380 Gy. As badges were not used, there was no filter to separate the elements for skin or deep dose. The mean value of all the FD responses for each

exposure was calculated with its uncertainty (standard error of the mean). As the

Reader converts the charge measurement into dose using correction factors is was

assumed that the error due to the Reader is very small compared to the error of the

mean value and only this uncertainty is taken into account. As a more accurate value,

the uncertainty of the measured dose assumes to be one standard deviation at 95%

coincidence level instead of the standard error of the mean.

According to [11] the repeatability of the TLD 100 should be within 2%. The

measured dose per cycle, relative to the dose measured in the first cycle, as a function

of the number of exposures [12], is illustrated in figure 4.4.1, showing that the

reproducibility is within the threshold (2%) of all the exposures. The actual

reproducibility observed in this work is about 1 %. The error bars shown in figure

4.4.1 are calculated by adding the errors in quadrature i.e.:

2

21

21 1

j jSD D SDRD D

24

where

SD1 and SDj are the standard deviation of the 1st and j exposure accordingly (j=1-10)

and 1D and jD are the mean measured dose (response) of the first and j exposure

respectively.

0 2 4 6 8 100.97

0.98

0.99

1.00

1.01

1.02

1.03 Measured dose relative to the measured dose in the first exposure Upper limit (+2%) Lower limit (-2%)

Rep

rodu

cibi

lity

Number of exposure

Figure 4.4.1: The reproducibility against the number of exposures.

According to [13] the ISO requirement is that the Coefficient of Variation

(CV) should not exceed 10%. The CV of each exposure is shown in Table 4.4.1 and

as indicated the CV is less than 10% for all exposures and is calculated by [14]

(%) 100%SDCVMean

Table 4.4.1: CV for 10 different exposures.

Number of

exposures 1 2 3 4 5 6 7 8 9 10

Coefficient of

Variation (%)1.4 1.5 1.0 1.3 1.3 1.7 1.5 1.3 1.2 1.7

25

4.5. Energy dependence

FDs were irradiated to 4.38 mSv using photons of energies 28 and 33 keV

(mammography unit), 47, 60, 80, 100 and 120 keV (radiography unit) and 662 keV

(137Cs source). As the TL signal is proportional to the dose absorbed by the TLD, the

energy response was defined as the TL response relative to the 137Cs response, at the

same irradiation conditions. Whereas the calibration was carried out using the 137Cs

source, the accuracy of the measured doses in diagnostic x-ray energies is not affected

[15].

For the measurements of the energy dependence no phantom was used.

According to [16] the relative response of TLDs is similar whether the irradiation is

on phantom or in air.

The energy response relative to the 137Cs source response is portrayed in figure

4.5.1. The mean response, standard deviation and standard error in the mean for all

the measured energies were calculated. The maximum error for the TL response at

energy j was found using:

2 2max maxj jSD SD

where

SDj is the standard deviation at 95% coincidence level for an energy j

and SDmax is the maximum standard deviation at 95% coincidence level found from

the repeatability in part 4.4.

The uncertainty of the relative response, indicated by the size of the error bars in

figure 4.5.1, was calculated by adding the errors in quadrature i.e.:

2

2max max

. 2j j Cs

E RCs Cs

DD D

where

max j and max cs are the maximum error of the jth energy response and 137Cs response respectively

and CsD and jD are the mean measured signal of the 137Cs and j energies.

26

Figure 4.5.1: Energy response (relative to 137Cs response) versus the energy.

Figure 4.5.1 demonstrates the influence of the x-ray beam energy quality by

the points on the graph obtained with the mammography unit by generating x-rays

with the same nominal energy but with different beam filtration, thus producing a

different energy spectrum. In such cases the energy response of the TL material is that

of a specific energy spectrum characteristic of the x-ray beam anode and beam

filtration material used. Molybdenum (Mo) and Rhodium (Rh) filtrations give

different TL energy response due to the different spectra produced by the x-ray tube.

These spectra are illustrated in figure 4.5.2 and 4.5.3 using the XCompW program

[17].

Figure 4.5.2: Mo and Rh filtration at 28 keV.

27

For the radiography unit the same filter (1.7 mm Aluminum-Al) was used. The

respective energy spectra for the radiographic unit are shown in figure 4.5.4. It can be

concluded, therefore, that the energy dependence of the TL materials as used in

Diagnostic Radiology is a function of the energy spectrum (energy quality) and not a

function of the nominal value of the energy used to produce the energy spectrum.

Figure 4.1.3: Mo and Rh filtration at 33 keV.

Figure 4.5.4: Different spectra of incident beams used for Radiography unit.

28

Compared with the energy response curve given from Harshaw TLD-100 [18],

the shape found is similar, although in [18] only one filter is used for low energies.

According to [13] the ISO requirement is that the response should be within

the range of 0.5-1.5 assuming monoenergetic beams. Even so, in this work, for the

energy range of 47-662 keV the TL response is within this range. Using the Mo filter

in mammography unit this requirement is satisfied. With the Rh filter the response at

33 keV is about 1.6 and thus the ISO requirement is not met at that energy.

4.6. Angular dependence

The angular dependence of the TLDs was studied at an incident energy equal

to 120 keV with the radiographic unit where the energy response is constant (figure

4.5.1). The piece of polystyrene was rotated around the central axis of the incident

beam at 10, 30, 50, 70 and 90 degrees in both directions. The clockwise rotation

assumed to be the positive values of the angles and the anticlockwise the negative

ones.

The mean value, standard deviation and standard error in the mean were

calculated for each angle for all the groups of the irradiated TLDs. The TL response

normalized to 0 (TL response at an angle/TL response at 0) as a function of the

angle is shown in figure 4.6.1 using polar coordinates.

Figure 4.6.1: TL response normalized to 0 against the angle.

29

The uncertainty of the angle is estimated to be 1. The maximum error of the

response at each angle is the same as explained in part 4.5 for energy dependence. The

uncertainty for the response normalized to 0 was calculated adding the errors at the

same way as explained in part 4.4 and 4.5.

Figure 4.6.1 shows that the TL response is roughly constant for both positive

and negative angles for up to about 70. The TL response is reduced to about half its

value when the angles become 90 in both directions. This means that when a worker

stands at 90 with respect to the incident beam, the TLDs measure half of the dose

compared to the dose which would be measured at 0.

It is worth pointing out that the small difference in the angular response

noticed in figure 4.6.1 for the angular range of -30 to -65 compared with those of 30

to 65 is due to the decrease in the irradiation distance as the TLDs are closer in the x-

ray tube (see figure 4.6.2). As the dose is proportional to the inverse square of the

distance (1/d2), for smaller distance the measured dose is relatively higher [19]. For

longer distances the measured dose is relatively lower as illustrated in figure 4.6.2.

Figure 4.6.2: The difference in distance in which the incident beam travels for 45 .

As reported by [20] the angular response of LiF:Mg,Cu,Na,Si (instead of

LiF:Mg,Ti) is reduced rapidly over 50-60. The main difference is that in [20] badges

were used in order to measure the deep dose. The badges have filters inside them

which affect the TL response of the TLDs. As reported by [21] where no filter was

used and the angular response of TLD 700 was measured, the results are almost stable

and reduced when TLDs are rotated to 90 (vertical).

30

According to [13] the ISO requirement states that the response over all angles

should not be more than 15%. At 90 this requirement is not met as the response is

reduced to about half of its initial value.

The raw data obtained for all the measurements made in this project are given

in Appendix II in Table form.

5. Conclusion

Personal dosimeters (Thermoluminescence dosimeters-TLDs) are worn by

workers every time they are exposed to radiation that is higher than 3/10 of the dose

equivalent limits, to make sure that a reference limit is not exceeded. When TLDs are

heated, they emit light and by measuring it with a PMT inside a reader, the dose is

measured in mGy or in mSv for the whole body.

In this project, the TLD-100, fabricated from LiF and doped with Mg and Ti,

is examined. The reader and batch calibration are carried out first using a 137Cs

source. Then the repeatability of the TLDs is checked by repeating the same

procedure 10 times using a 137Cs source. Their energy and angular response are also

studied at different energies and angles respectively, but at a constant dose of the

incident beam, with the aid of a 137Cs source, a radiography and a mammography unit.

CDs are first selected as they fall within the range of 0.94-1.06. Using CDs the

RCF is calculated automatically by the reader and is found equal to 0.034127 and

0.027526. FDs are then selected falling within the range of 0.7-1.3 ( 30%) and are

used for the rest of the measurements. ECCs are also calculated automatically by the

reader, one for each TLD element, which follow a Gaussian distribution with a

theoretical mean value equal to 1. The measured Gaussian distribution is shown in

figure 4.3.1 and has a mean value of 1.0010.01.

After the calibration the charge measurements are converted into dose with a

similar sensitivity for all the TLDs. Their repeatability is found about 1% which is

31

within the limit (2%) for all the exposures. The maximum CV is measured 1.7%,

much lower than the maximum allowed one (10%).

The energy response is then measured and the curve which is found has

similar shape with the curve of Harshaw TLD-100 (manufacturer). At low energies

two different filters are used (Mo and Rh) producing two different spectra. The

measured responses of the TLDs at these energies with the different filters are not the

same, showing that the energy dependence of the TL materials, as used in Diagnostic

Radiology, is a function of the energy spectrum (energy quality) and not a function of

the nominal value of the energy used to produce the energy spectrum.

The angular dependence is studied using photons of maximum energy equal to

120 keV. The angular response was almost constant for all the angles up to about 70

for both directions and it is reduced to about half its value when the angle becomes

90. Small difference in the angular response (figure 4.6.1) for the angular range of -

30 to -65 compared with those of 30 to 65 is due to the change in the irradiation

distance as the TLDs are closer and further away from the x-ray tube. The ISO

requirement is met at all angles except at 90 for both directions.

Different results, especially for the angular response, were expected to appear

if badges with filters were used, as different spectra would be produced in order to

measure the deep and skin dose. In future work, badges can be used to check if the

response would be reduced for lower angles and not only for 90. Also,

monoenergetic beams could be used as the ISO requirements are referred to them.

Spectra produced by the x-ray tube are continuous energy distribution with the

maximum energy the one used to plot the graphs. Different filters absorb different low

energy photons and thus different spectra are produced, giving dissimilar response. In

such situation a better energy value could be the effective spectrum energy and not the

maximum energy value as used in this project.

Finally, in future work, the TLD elements could be removed from the TLD

cards. As the TLD cards are made of aluminum, the incident beam is absorbed by it

and the response of the elements is affected, especially for higher angles where the

beam hits first on the metal and is partially absorbed before it reaches the TLD.

32

Appendix I Equipment Specifications

Harshaw Bicron Model 6600E Automatic TLD Workstation

The Harshaw Automatic TLD Workstation (Model 6600E) is the reader used

for this project and is used for whole body and extremity TLD measurements. This

projects main interest is for the whole body dose measurements. A non contact

heating system is used which utilizes a stream of hot nitrogen gas. Glow curves and

data calculated, using algorithm software, are shown on an electroluminescence panel

of the Reader. The Reader can be connected to a computer where all measured data

can also be displayed and stored. The PC is connected with a printer for printing the

results [8].

Up to 200 TLD cards can be placed in the reader. Each card consists of four

chips as shown in figure 1. In this project only the two chips for each card are used,

which are made of TLD100. The holders (where the cards are placed in) are made of

two different filters thicknesses, 0.07mm (for skin dose measurements) and 0.1mm

(for deep dose measurements). One corner of the TLD card is notched to make sure it

is placed correctly in the Reader (see figure 1). The holder protects the cards from the

environment and keeps the filtration media which attenuate different types of

radiation in order to ensure selective entrapments of the TLD100. Apart from the

chips, each card has an ID number in barcode and numeric layout [8].

Figure 1: Dosimeter card [8]

33

The Reader holds two cartridges, where the first one is empty and the second

with the TLD cards to be read. When a card is read, is moved to the empty cartridge.

If its barcode can not be read, is moved to a Rejected Card Drawer in order to be

removed at the end of the whole read-out cycle.

Element Correction Coefficients are generated during the calibration and

saved in the reader together with the Reader Calibration Factors. Both are used during

the read-out cycle and the reader provides directly a dose measurement in Sv instead of charge. A different RCF can be saved for up to ten sets of Time Temperature

Profile (TTP). In a TTP the operator can change the time and temperatures for the

heating cycle (pre-heat, heat and anneal temperatures). During the measurements of

the TLDs, the reader also measures the reference light and the PMT noise, which are

the light produced from the background and the PMT respectively.

A Quality Assurance profile is also displayed to monitor the accuracy of the

operating parameters of the Reader. The option of the electronics Quality Control, in

this menu, monitors a series of electronic measurements from the Card Reader to

identify if the reader is properly adjusted [8].

Instrument specifications [8]

Dynamic Range: 7 decades

TTP reproducibility: 1 C

Light Stability: Less than 0.5% variation

Linearity: Less than 1% deviation

High Voltage: 0.005%

Dark Current (background

noise):

Less than 1Gy 137Cs equivalent dark current

Warm-up time: Less than 5 minutes

Tissue Equivalent: Nearly tissue equivalent

Throughput: 4 chip dosimeters 60 per hour

2 chip dosimeters 100 per hour

34

TTP Capabilities: Preheat temperature 20 to 200 C

Preheat time 0 to 300 seconds

Acquisition time 10 to 300 seconds

Temperature rate 1 to 50 C/sec

Acquisition temperature to 300 C

Post-read anneal temperature to 300 C

Specification of mammography unit (PLANMED SOPHIE classic) [22]

Generator Potential 80 kKz (constant)

20-35 kV output

10-500 mAs

120 mA large focus

42 mA fine focus

Computer controlled

X ray tube Rotating anode 300,000 HU Mo target

0.1/0.3 mm foci

Biangular anode

High speed

Be window(1.0 mm)

Mo filter (+ Rh optional filter)

35-11 mA focus

Air and oil cooled

Bult-in spot collimator

C Arm Isocentric movement within -135 to 180 degrees

SID 65 cm

Dual Control panels

Digital display of projection angle

35

Bucky Manual cassette loading and unloading

18 x 24 and 24 x 30 cm cassettes

5:1 grid ratio

Exposure Control Modes Advanced AEC with Auto-kV Density control in 15 steps

Compression Efficient patient compression Easy lock-in facility

Two foot controls plus hand controls

Base Free standing base

Specifications of Medcal EIDOS (radiography unit) [23-24]

DR system Advanced system with grid equipped

and auto-focusing device

Detector Technology a-Si Resolution 143 m Detector size 43 x 43 cm Height 270 cm from the floor Filtration 1.7 mm Al Active Area of the detector 43 x 43 (3000 x 3000 pixels 14 bit) Image Immediately image availability

No film or cassettes

Carbon-fiber tabletop For 90 rotation Motorized movements Full automated control Mode Manual

36

Other features High efficiency DICOM 3 interface

Anatomical programming

Anatomical Tissue Harmonization

DICOM Modality Performance

Procedures Step

Image Stitching

Specifications of 137Cs source [25]

The 137Cs irradiation unit used is of the Irradiation type DCI-01-I Institute of

Hungarian Academy of Science. Installed in 1992, with the source activity 1900 GBq

on 01/07/1990. The TL dosimeters were irradiated on the 23/05/2001 in horizontal

beam geometry.

Decay chain of the 137Cs source [26]

37

Specifications of ion chamber IONEX 2511/3 [27]

Volume 600 cc

Dimensions Height 112 mm (4.4) Dia. 130 mm (5.1)

Charge sensitivity Coulomb/Rontgen

1.8 x 10-7

Weight 850 g (1 Ib 14 oz) Cable Length 1 m (39.6) Useful Energy range 40 to 3000 kV Measuring ranges Provided Exposure in R

0-1 mR

0-10 mR

0-100 mR

0-1 R

Maximum leakage rate 10 R/min Maximum exposure rate for greater than 99% saturation

0.2 R/ min continuous

Energy response Calibration Certificates supplied with each Ionization chamber specifies the

measured correction factors at

various x and - rays energies. Connector Precision Electronic Terminations

Limited, Tri-axial, Double screened,

Plug Free Type 201-DS-T3329-

P.T.F.E.

Specifications for Radcall ion chambers [28]

For the low energies (mammography unit) a 6cc Radcall ion chamber was

used (Model 100X9-6M) and for higher energies (50-110 kV radiography unit) a

38

60cc Radcall chamber was used (Model 10X9-6O). Both ion chambers have

calibration factors 1.04 at 98.3 kPa and 25.6 C. Their specifications are identical.

Operating temperature 15 to 35 C Storage Temperature -20 to 50 C Humidity Up to 80% Pressure 60 to 105 kPa Accuracy 4 % of reading, 1 digit Repeatability 1 % of reading, 1 digit 40-160 kV Accuracy 40-160 kV Repeatability 22-40 kV Accuracy 22-40 kV Repeatability Width ( 2ms 5s)

1kV or 1%

0.2 kV

0.5 kV

0.1 kV

0.1 % 0.2 ms measured at 75% of

peak kV

Sensitivity 20 mV/kV 0.5 mV/mA

Output 100 Sample Rate 77 s 3 db frequency 2.3 Hz

39

Appendix II Row Data

Raw Data for the Selection of CDs and FDs and their calibration factors

40

41

42

43

Due to the large number of measurements only a small sample of data is given

below. The rest of the data records can be found at the Department of Medical Physics

in the Nicosias General Hospital.

44

Data for the Repeatability

Position ii

(Sv) Position iii

(Sv) Position ii

(Sv) Position iii

(Sv) Position ii

(Sv) Position iii

(Sv) 4396.3 4461.2 4493.6 4576.2 4442.1 4469.9

Exposure 1

4430.1 4514.5

Exposure 4

4437.8 4474.5

Exposure 8

4455.5 4629.7

4471.9 4510.7 4417.8 4557.1 4479.3 4505.8

4459.0 4499.0 4502.2 4530.3 4432.0 4456.9

4346.8 4405.5 4409.1 4397.9 4415.9 4433.9

4417.4 4487.3 4339.7 4455.9 4536.6 4585.7

4370.4 4467.9 4373.1 4390.6 4438.0 4460.1

4400.7 4452.7 4419.2 4482.2 4528.0 4551.7

4456.6 4471.3 4487.7 4552.7 4424.0 4415.9

4398.4 4421.5 4409.2 4473.8 4517.4 4482.1

4454.2 4477.0 4437.8 4475.3 4458.9 4458.6

4277.8 4307.2 4358.5 4439.2 4455.0 4434.3

4390.2 4434.5 4473.8 4410.9 4475.0 4389.6

4344.4 4489.7 4426.3 4374.9 4484.2 4393.3

4361.2 4399.2 4424.7 4460.9 4440.7 4428.6

4297.6 4401.4 4402.3 4398.2 4399.6 4347.2

4390.6 4460.4 4469.3 4459.3 4499.2 4471.8

4472.2 4493.0 4422.8 4367.5 4423.9 4375.0

4491.8 4491.2 4360.4 4474.3 4429.8 4453.3

4385.4 4409.4 4430.6 4466.4 4442.6 4428.6

45

Data for the Energy Response

28 keV (Mo) 33 keV (Mo)

Position ii

(Sv) Position iii

(Sv) Position ii

(Sv) Position iii

(Sv) 6513.6 6706.0 6222.4 6719.0

6486.0 6669.2 6843.0 6948.8

5848.5 6312.9 6034.6 6525.7

5747.9 6314.9 6845.8 7015.1

5706.8 6279.2 6054.3 6573.4

6517.8 6597.7 6060.9 6505.4

6396.4 6663.9 6865.5 6946.8

5819.8 6253.3 6136.6 6602.4

6595.2 6770.6 6809.9 6997.8

5859.2 6295.9 6109.2 6479.5

28 keV (Rh) 33 keV (Rh)

6548.6 7140.5 6150.5 6552.8

7224.0 7411.5 6655.5 6687.2

6522.0 7008.3 5946.4 6309.0

7288.7 7495.7 6685.1 6832.0

6382.4 6930.4 6575.0 6656.6

6525.3 7096.3 6048.6 6520.0

6358.3 6921.0 6678.9 6767.8

7159.4 7341.6 5988.8 6338.8

7378.7 7505.2 6639.0 6622.0

6486.4 7031.8 6574.5 6671.7

47 keV 60 keV

5770.0 6367.3 5270.1 5806.7

6697.6 7132.4 6107.1 6513.4

6832.2 7186.6 5948.7 6427.9

46

5635.1 6277.2 5121.2 5803.5

5684.0 6212.5 5211.5 5759.9

6731.1 7060.9 5120.5 5681.9

5897.2 6321.2 5778.7 6301.9

6606.6 6938.6 5191.5 5741.1

5514.2 5969.9 5976.0 6509.4

5714.8 6189.2 5993.1 6431.0

80 keV 100 keV

5536.1 5928.0 4259.2 4751.3

4711.0 5281.8 4867.7 5373.6

4678.8 5302.5 4857.3 5334.4

5644.6 6007.1 4264.8 4717.7

4822.6 5332.1 4772.3 5245.4

5551.0 5903.8 4180.9 4747.6

4738.0 5218.6 4854.7 5269.1

4577.6 5063.4 4092.1 4672.2

5392.5 5768.7 4248.8 4847.7

5450.4 5813.6 4979.6 5424.1

120 keV

3995.1 4406.0

4546.0 5011.6

4553.8 4960.6

3927.2 4370.3

4682.2 4974.8

4592.6 4898.8

3957.6 4371.0

3905.3 4326.1

3934.5 4398.7

4631.8 5045.9

47

Data for the Angular Dependence

-90 -70

Position ii

(Sv) Position iii

(Sv) Position ii

(Sv) Position iii

(Sv) 2160.3 2572.2 3859.2 4439.6

1701.6 2025.9 3769.7 4296.0

1859.4 2281.9 4738.9 5078.7

2476.1 2419.8 4302.8 4788.0

1980.3 2498.4 3966.3 4451.5

3316.6 3454.3 3716.5 4175.4

2254.1 2449.8 4738.4 5006.0

2592.6 2415.4 4383.6 4705.2

1770.9 2037.6 4738.7 5094.0

2296.5 2427.2 4478.9 4825.4

-50 -30

4685.7 5055.2 3928.9 4477.8

4409.3 4920.6 4820.0 5063.5

3942.0 4453.7 4462.7 4998.5

4438.2 4785.4 3997.2 4463.7

4780.5 5132.5 4505.9 4836.3

4545.1 4890.5 4801.7 5159.4

3782.6 4271.1 4667.1 5041.2

4558.1 4925.6 4020.3 4484.2

4452.3 4808.9 4554.1 4916.2

4747.9 5133.6 4554.1 4916.2

-10 10

3838.8 4414.8 4101.7 4418.8

4545.4 4879.8 4650.6 4901.3

4415.0 4890.9 4607.0 4993.9

48

3839.8 4297.6 3930.0 4304.1

4575.2 4900.0 3899.2 4358.3

4625.8 4967.7 4691.3 4965.7

4604.9 4922.6 4703.5 4955.0

3861.8 4341.1 3902.9 4343.1

4518.8 4793.2 3837.8 4284.9

4445.1 4788.2 4572.0 4862.7

30 50

4070.4 4644.8 3764.8 4301.8

4602.3 4944.2 4109.6 4683.3

4635.6 5163.4 4570.6 4854.3

3859.8 4344.4 4682.5 5196.6

3966.6 4494.7 3826.4 4283.5

4676.6 5043.7 3972.0 4497.0

4833.0 5205.5 4655.7 4967.6

3879.4 4376.1 4850.5 5216.9

3932.1 4456.4 3970.4 4494.5

4464.9 4826.7 4436.7 4779.6

70 90

4112.7 4675.7 1784.1 1863.8

5411.5 4752.2 2979.9 3178.7

4743.8 5050.0 1905.9 1629.1

3921.4 4289.8 2781.6 3152.5

4058.7 4560.0 1506.9 1886.9

4634.9 4963.2 2483.0 2985.9

4924.5 5235.0 1505.7 2007.5

3970.2 4463.7 2838.1 3029.2

4363.3 4684.5 1520.3 1377.7

4717.1 5019.6 2301.2 2668.2

49

Acronyms and Abbreviations

ECC= Element Correction Coefficient

RCF= Reader Calibration Factor

CD= Calibration Dosimeter

FD= Field Dosimeter

TTP= Time Temperature Profile

CV= Coefficient of Variation

ISO= International Standard Organization

50

References

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AbstractAcknowledgements1. Introduction2. Theory2.1. Thermoluminescence Dosimetry - A general model2.2. TLD Reader2.3. Characteristics of TL materials2.4. TL properties of LiF2.5. Advantages and Disadvantages of TLD1002.6. Calibration2.6.1. Batch Calibration - Element Correction Coefficient2.6.2. Reader Calibration Factor

3. Materials and Methods3.1. Materials3.2. Methodology3.2.1. Selection of Time Temperature Profile (TTP)3.2.2. Selection of Calibration Dosimeters (CDs) 3.2.3. Calculation of Reader Calibration Factor (RCF) 3.2.4. Calculation of the Element Correction Coefficients (ECCs) and Selection of Field Dosimeters (FDs)3.2.5. Irradiation procedure3.2.6. Repeatability3.2.7. Energy dependence3.2.8. Angular dependence

4. Results and Discussion4.1. Selection of Calibration Dosimeters (CDs)4.2. Calculation of Reader Calibration Factor (RCF) 4.3. Calculation of Element Correction Coefficients (ECCs) and Selection of Field Dosimeters (FDs)4.4. Repeatability4.5. Energy dependence4.6. Angular dependence

5. ConclusionAppendix I Equipment SpecificationsHarshaw Bicron Model 6600E Automatic TLD WorkstationInstrument specifications [8]Specification of mammography unit (PLANMED SOPHIE classic) [22]Specifications of Medcal EIDOS (radiography unit) [23-24]Specifications of 137Cs source [25]Decay chain of the 137Cs source [26]Specifications of ion chamber IONEX 2511/3 [27]Specifications for Radcall ion chambers [28]

Appendix II Row DataRaw Data for the Selection of CDs and FDs and their calibration factors Data for the RepeatabilityData for the Energy ResponseData for the Angular Dependence

Acronyms and AbbreviationsReferences