20

CHAPTER-2THERMOLUMINESCENCE BASIC THEORY & APPLICATIONS

2.1 LUMINESCENCEThe term luminescence implies luminous emission which is not purely thermal in

origin i.e. luminescence is ‘cold light’, light from other sources of energy, which takes

place at normal and lower temperature. In luminescence, some energy sources kicks

an electron of an atom of its ground state (lowest energy) into an exited state (highest

energy) by supplying extra energy, then as this excited state is not stable electron

jumps back to its ground state by giving out this energy in form of light[1]. We can

observe the luminescence phenomenon in nature like during lightening, in

glowworms, fireflies, and in certain sea bacteria and deep-sea animals. This

phenomenon have been used in various fields by different scientist all over the world

like, Archaeology, Geology, Biomedical, Engineering, Chemistry, Physics, and

various Industrial Application for Quality Control, Research and Developments.

Luminescence is a rare phenomenon among inorganic compounds. This is due to the

predominance of nonradiative relaxation processes. An electronic excitation of a

complex or a metal center in a crystal usually ends up as vibrational energy and

eventually as heat. In those cases where spontaneous light emission does occur, its

spectral and temporal characteristics carry a lot of important information about the

metastable emitting state and its relation to the ground state. Luminescence

spectroscopy is thus a valuable tool to explore these properties. By studying the

luminescence properties we can gain insight not only into the light emission process

itself, but also into the competing nonradiative photophysical and photochemical

processes.

Luminescence is the emission of optical radiation (infrared, visible, or ultraviolet

light) by matter [2]. This phenomenon is to be distinguished incandescence, which is

the emission of radiation by a substance by virtue of its being at a high temperature

(>5000oC) (Black body radiation). Luminescence can occur in a wide variety of

matter and under many different circumstances. Thus, atoms, polymers, inorganic,

organic or organo metallic molecules, organic or inorganic crystals, and amorphous

substances all emit luminescence under appropriate conditions [3].

21

2.2 THERMOLUMINESCENCE

Thermoluminescence as mentioned by McKever and et al. is one of the processes in

Thermally Stimulated Phenomena [4]. In a general view, thermoluminescence is a

temperature stimulated light emission from a crystal after removal of excitation.

Nevertheless, microscopically, it is much more complicated. In this chapter, the

thermoluminescence mechanism will be discussed in detail. With the developing

technology, thermoluminescence has various application areas such as, radiation

dosimetry, age determination and geology.

2.2.1 History of Thermoluminescence

The studies on thermoluminescence go back to the seventeenth century, when

scholars like Johann Sigismund Elsholtz, Robert Boyle and Henry Oldenburg

conducted experiments on minerals to see their radiation due to heating. George

Kaspar Kirchmaier, who regarded the phosphorus as a green stone powdered and

mixed with water and glows when heated, and Nathaniel Grew, who used the name

Phosphorus metallorum, are other scientists who showed interest in the concept.

Among the eighteenth century researchers, Dufay is the first to be acknowledged for

his findings on thermoluminescence. He referred to lighting as a kind of burning. He

worked on many materials, primarily chlorophane, and found out that too much

heating would lead to loss of thermoluminescence of the material. A famous scientist,

Canton brought Dufay’s studies to a new level, by raising the temperature of

phosphorus even further and discovering a new type of light, which he referred to as

the thermoluminescence of artificial phosphorus.

Leading scientists, De Saussure and Thomas Wedgwood need to be mentioned in the

thermoluminescence studied of the eighteenth century. The former recognized three

types of stones which luminesced on heating (i) those containing sulphur or a hepar

(fois), a compound of sulphur, which burned in the free air, (ii) those which absorb

the light and then emit it, like the diamond, and (iii) those which do not require air

and will luminesce under hot water, like dolomite and fluorspar. He declared that the

intensity of the colour of the fluorspar is an indicator for the level of

22

thermoluminescence. The latter conducted a study on the thermoluminescence and

triboluminescence, lighting as a result of friction. His findings showed that it was not

possible to claim a solid relation between the patterns of two types of luminescence.

Studies on thermoluminescence continued in the nineteenth century. Researcher,

Heinrich claimed that almost all substances could emit light, provided that they are in

powder form and subject to moderate heating. Another researcher Theodor von

Grotthus dealt specifically with the fluorspar, and showed resemblance between

thermoluminescence and essence; both are made of positive and negative parts. Later,

scientist David Brewster opposed to Grotthus, arguing that the luminescence property

cannot always be regained on exposing the minerals to light.

Other researchers who studied thermoluminescence in the nineteenth century are

Pearsall, who tried to find a relation between colour and thermoluminescence; Specia,

who invalidated Pearsall’s findings; Napier, who experimented on the chalks;

Wiedmann and Schmitt, who attributed the thermoluminescence characteristic to

cathode rays [6].

2.2.2 Thermoluminescence Mechanism

In sum, thermoluminescence can be described by two stages. First stage is the change

of the system from equilibrium to metastable state by absorption of energy from UV

or ionizing radiation. Then the second stage is relaxation of the system back to the

equilibrium by energy release such as light with the help of thermal stimulation. Thus,

thermoluminescence (TL) is the thermally stimulated emission of light following the

previous absorption of energy from radiation [6]. In this chapter, these stages and

output of this light emission will be discussed briefly.

A-Energy Storage

There are two ways for the stabilization of this absorbed energy: electronic excitation

and displacement damage. At the end of both processes, radiation-induced defects are

formed in the material structure. Radiation-induced defects are localized electronic

states occupied by non-equilibrium concentration of electrons [7]. Alias, before

irradiation, materials have localized electronic energy states and after irradiation,

some of these states are occupied by a non-equilibrium concentration of electrons.

Therefore, these occupied states are called radiation-induced defects. According to

23

McKeever, when investigations are taken into consideration, they show that the cause

of defect creation is electronic excitation rather than non-ionizing displacement

damage.

Energy storage caused by electronic excitation takes place by the electron-hole pair

production and excitation creation. Electron-hole pair production is the formation of

mobile holes and electrons in the crystal structure of the material after radiation.In

addition, there exists a mid gap state caused by defects which may be created by pre-

existing impurities or radiation induced defects. This gap is found between the two

energy bands; called conduction band and valence band. The valence band is the outer

most energy level and contains electron-hole pairs in ground state of the solid. On the

other hand; in the conduction band, electrons are free to move and have ability to

produce electric current.

According to thermoluminescence phenomena it is assumed that there are two kinds

of imperfections called electron trap and hole trap in the crystal which are localized at

mid gap states [4,5,6]. In the mid gap, the electron trap is believed close to the

conduction band and the hole trap is far from the valence band.

Figure 2.1 illustrates the energy storage mechanism. After irradiation, the electrons

pass from valence band to conduction band and hole becomes positively charged area

in the valence band. When electron reaches the conduction band, electron find its way

into an electron trap and hole occupies its associated trap. Hole traps are called

luminescence center in this process [4,5,6,8].

B-Energy Release

Excitation with an increase in temperature or giving light, results in release of the

stored energy. Furthermore, state of the material changes from metastable to ground.

When heat is increased, the electron trapped in the electron trap is released to

conduction band. After that electron is free to retrap or recombine with the hole found

in the hole trap. The recombination of the electron with the hole in hole trap results in

the emission of photons. In this case hole trap is called as recombination center [6].

This process is illustrated in the figure 2.2.

24

Figure 2.1 Energy-level diagram of the energy storage stage for Thermo-luminescence processes

Figure 2.2 Energy-level diagram of the energy release stage for Thermo-luminescence processes

25

Figure 2.3 Some representative examples of glow curves of some of the mainTLD materials.(a)LiF:Mg,Ti;(b)LiF:Mg,Cu,P; (c) CaF2:Mn; (d) CaF2:Dy;(e)AI2O3:C; (f) CaSO4: Dy.(4)

26

C-Glow Curve

After the energy release, the output of the emitted light as a function of temperature is

called thermoluminescence glow curve [6]. Shape of the glow curves is one or more

peaks of emitted light and some of them may overlap. Magnitudes and looks of the

glow curves may change depending on the spectral response of the light sensitive

device, different filter usage between the sample and the detector and heating rate. In

addition, when the sample is irradiated it has only one shot effect. A second

thermoluminescence emission cannot be recorded by cooling and reheating it unless it

is not irradiated again. Figure-2.3 shows glow curve examples of some

thermoluminescent materials.

2.2.3 Applications of Thermoluminescence

The thermoluminescent materials used in the industry have three major areas;

radiation dosimetry, age determining and geology. The radiation dosimetry measures

the dose that is absorbed by the sample that is exposed to irradiation. Radiation

dosimetry has three subgroups; personnel dosimetry, medical dosimetry and

environmental dosimetry. The Schematic representation applications of

thermoluminescence are in Figure 2.4.

Personnel dosimetry is used in areas where the personnel are exposed to radiation;

nuclear reactors, radiotherapy wings in hospitals and nuclear powered submarines or

such. Therefore, the purpose of using personnel dosimetry is to keep track of the

radiation exposure level of the individual to avoid averts radiation based effects. The

safe limits are determined by organizations such as International Commission on

Radiological Protection (IRCP). Besides, from the constant radiation exposure, there

are accidental or incidental radiation exposures, which are also measured by personnel

dosimetry.

Personnel dosimetry has three sub categories; extremity dosimetry, whole-body

dosimetry and tissue dosimetry. The first one focuses on body parts that are exposed

to radiation such as hands, arms or feet while the whole-body focuses on the tissue

below the surface of the body or the critical organs. It measures the dose absorbed in

these parts of the body by dealing with gamma and X- rays (greater than 15 keV) and

neutrons which are penetrating rays. Tissue dosimetry, which is also called skin dose,

27

measures the dose absorbed by skin. However rather than dealing with penetrating

radiation, it focuses on non-penetrating radiation such as beta particles or <15 KeV X-

rays. In order for these measurements to be done, a thermoluminescence dosimetry

(TLD) material that is equivalent to the human tissue is needed. The TLD material

should absorb the same dose or amount of radiation as the human tissue would do in

the same area within the same radiation levels.

Medical dosimetry intends to measure the effects of a TLD that is placed into the

appropriate places within human body. By doing so, before exposing the patient, to

ionizing radiations for treatment procedures or diagnosis, measurements can be made

upon these TLD. From the data obtained, possible additional treatments or dose

control can be implemented. It is impossible to do so by means of other than radiation

dosimeter. The major variables that determine patient doses include imaging

modality, technical factors and in the case of fluoroscopy, beam time [9]. In addition

to these factors, the size of the patient is also a determining factor. Medical dosimetry

has two categories; diagnostic radiology and radiotherapy. The radiation used here

may be X-rays (near 10 KeV level), gamma rays, beta particles, protons and other

heavy particles and neutrons [4].

Again, the TLD material needs to be tissue equivalent and highly sensitive. The latter

is needed for measurements done in laboratory conditions that require the possible

smallest size of TLD material. Other than these properties, the TLD should not be

toxic. Recommended diagnostic reference levels for medical imaging modalities have

been published by the ICRP [9].

Environmental dosimetry deals with the radiation present in the environment due to

humankind. Due to applications like nuclear power stations, waste disposals, usage or

processing of nuclear fuels and disastrous nuclear power plant malfunctions introduce

high levels of radiation into the environment. Therefore, it becomes essential to

monitor the radiation released to the environment continuously.

28

Fig: 2.4 Schematic representation applications of thermoluminescence

Archaeological sciencesDating of- Pottery

-Burnt flint,chert,brick- testing

Provenance studiesThermal historyFiring temperature

Geology aiding physicsPhototransferLET dependenceNon thermal quantummechanical fadingPhonon assisted opticalbleaching

Material sciencesMaterial characterizationHigh Tc superconductors

Detection of surfaceimpurity phases

Environmentaldegradation monitoring

TLAPPLICATIONS

Health physicsDosimetry

-Personal-Environmental-Fall out from

nuclear explosionsForensic sciencesRadiation therapy

Earth SciencesDating of

-QuaternarySediments

-Volcanic Lava Flows-Authegenic Minerals

Uranium ProspectionWeathering History ofSolids

Space sciencesThermal andirradiation history ofmeteorites

-Terrestrial agesDating of impactcratersThermal & irradiationhistory of Lunarsamples

29

TLD’s are used for environmental dosimetry applications. However, the performance

criteria for TLDs in this application are different from those required for personnel

monitoring. They are still needed to be highly sensitive, more preferably extremely

sensitive and this time it is not essential for TLDs to be tissue equivalent. Because the

environment is exposed to radiation for a long time continuously, environmental

dosimetry measures the values within a long period. Therefore, the TLDs should be

structurally intact and stable in long term.

In recent years, with the help of cutting edge technological innovations, space flight

with astronauts has been possible. Radiation exists in space and since there is no more

an atmosphere to protect from galactic cosmic rays, it is important to measure the

radiation at these space flights. Besides, from the astronauts on board, the radiation is

also harmful for the digital equipment of the vehicles. The radiation sources are

galactic cosmic rays of which the main component is high-energy protons and heavy

charged particles from the solar wind [8]. In order to measure the effects of these

radiation sources, TLDs are used.

Thermoluminescence is used in age determining processes of materials, as it became

an established method of age determination. A famous scientist, Daniels and his co-

workers are the first to suggest the use of thermoluminescence for this purpose. They

argued that there are already radioactive elements within the rocks, such as uranium,

thorium and potassium and these elements assigned a natural thermoluminescence to

the rocks. From this radioactivity an accumulation, which is called ‘geological’ dose,

takes place in the material. If the rate of irradiation from the radioactive minerals is

established, and if the rate of thermal release of the thermoluminescence during the

rock’s irradiation can be shown to be negligible, then the length of time over which

the rock has been irradiated (i.e., its ‘geological age’) can be determined from the

ratio of absorbed dose over dose rate [9,10].

Thermoluminescence was used for age determination of rock formations however; it

was not used for archeological dating until natural thermoluminescence was found in

ancient samples. Due to the effects of heat on thermoluminescence,

thermoluminescence of the pots diminished to zero during its bakery. Nevertheless,

the surroundings of the pot does not change and is naturally radioactive itself with

elements like uranium, thorium and potassium. Therefore, the pot continues to be

30

exposed to radioactivity and will absorb a certain amount of it, which will be

measured to get the archeological age of the pottery. Thermoluminescence is now an

established way of age determination.

Minerals tend to give off different glow curves according to their location of

extraction. Since thermoluminescence is a fine way of radiation trace detection, this

enables to identify the source of these minerals by using thermoluminescence in

geology. Another aspect of geology that uses thermoluminescence is in examining

meteorites and materials originating from the moon. It is possible to tell the distance

of meteorite to the sun while it was travelling in the space as well as how long the

meteorite was on earth.

Thermoluinescent properties of materials enable them to be used in dosimeters, which are

used in measuring doses of radiation. Since characteristics of materials differ from each

other, different materials with different thermoluminescent properties are preferred for

different purposes. LiF and CaF2 are the most common thermoluminescence materials,

which are followed by sulfates [11].

2.3 RADIATION DOSIMETERS

A radiation dosimeter is a device, instrument or system that measures or evaluates,

either directly or indirectly, the quantities exposure, kerma, absorbed dose or

equivalent dose, or their time derivatives (rates), or related quantities of ionizing

radiation [12]. A dosimeter along with its reader is referred to as a dosimetry system.

Measurement of a dosimetric quantity is the process of finding the value of the

quantity experimentally using dosimetry systems. The result of a measurement is the

value of a dosimetric quantity expressed as the product of a numerical value and an

appropriate unit.

To function as a radiation dosimeter, the dosimeter must possess at least one physical

property that is a function of the measured dosimetric quantity and that can be used

for radiation dosimetry with proper calibration. In order to be useful, radiation

dosimeters must exhibit several desirable characteristics. For example, in radiotherapy

exact knowledge of both the absorbed dose to water at a specified point and its spatial

31

distribution are of importance, as well as the possibility of deriving the dose to an

organ of interest in the patient. In this context, the desirable dosimeter properties will

be characterized by accuracy and precision, linearity, dose or dose rate dependence,

energy response, directional dependence and spatial resolution [13].

Obviously, not all dosimeters can satisfy all characteristics. The choice of a radiation

dosimeter and its reader must therefore be made judiciously, taking into account the

requirements of the measurement situation; for example, in radiotherapy ionization

chambers are recommended for beam calibrations and other dosimeters are suitable

for the evaluation of the dose distribution (relative dosimetry) or dose verification.

2.3.1. Properties of dosimeters(i) Accuracy and precisionIn radiotherapy dosimetry the uncertainty associated with the measurement is often

expressed in terms of accuracy and precision. The precision of dosimetry

measurements specifies the reproducibility of the measurements under similar

conditions and can be estimated from the data obtained in repeated measurements.

High precision is associated with a small standard deviation of the distribution of the

measurement results. The accuracy of dosimetry measurements is the proximity of

their expectation value to the ‘true value’ of the measured quantity. Results of

measurements cannot be absolutely accurate and the inaccuracy of a measurement

result is characterized as ‘uncertainty’ [14].

The uncertainty is a parameter that describes the dispersion of the measured values of

a quantity; it is evaluated by statistical methods or by other methods, has no known

sign and is usually assumed to be symmetrical.

The error of measurement is the difference between the measured value of a quantity

and the true value of that quantity.

● An error has both a numerical value and a sign.

● Typically, the measurement errors are not known exactly, but they are estimated in

the best possible way, and, where possible, compensating corrections are introduced.

● After application of all known corrections, the expectation value for errors should

be zero and the only quantities of concern are the uncertainties.

32

(ii) Linearity

Ideally, the dosimeter reading M should be linearly proportional to the dosimetric

quantity Q. However, beyond a certain dose range a non-linearity sets in. The linearity

range and the non-linearity behaviour depend on the type of dosimeter and its

physical characteristics.

In general, a non-linear behaviour should be corrected. A dosimeter and its reader

may both exhibit non-linear characteristics, but their combined effect could produce

linearity over a wider range.

(iii) Dose rate dependence

Integrating systems measure the integrated response of a dosimetry system. For such

systems the measured dosimetric quantity should be independent of the rate of that

quantity.

Ideally, the response of a dosimetry system M/Q at two different dose rates ((dQ/dt)1

and (dQ/dt)2) should remain constant. In reality, the dose rate may influence the

dosimeter readings and appropriate corrections are necessary, for example

recombination corrections for ionization chambers in pulsed beams.

(iv) Energy dependence

The response of a dosimetry system M/Q is generally a function of radiation beam

quality (energy). Since the dosimetry systems are calibrated at a specified radiation

beam quality (or qualities) and used over a much wider energy range, the variation of

the response of a dosimetry system with radiation quality (called energy dependence)

requires correction.

Ideally, the energy response should be flat (i.e. the system calibration should be

independent of energy over a certain range of radiation qualities). In reality, the

energy correction has to be included in the determination of the quantity Q for most

measurement situations. Ιn radiotherapy, the quantity of interest is the dose to water

(or to tissue). As no dosimeter is water or tissue equivalent for all radiation beam

qualities, the energy dependence is an important characteristic of a dosimetry system.

(v) Directional dependence

The variation in response of a dosimeter with the angle of incidence of radiation is

known as the directional, or angular, dependence of the dosimeter. Dosimeters usually

exhibit directional dependence, due to their constructional details, physical size and

the energy of the incident radiation. Directional dependence is important in certain

applications, for example in vivo dosimetry while using semiconductor dosimeters.

33

Therapy dosimeters are generally used in the same geometry as that in which they are

calibrated.

(vi) Spatial resolution and physical size

Since the dose is a point quantity, the dosimeter should allow the determination of the

dose from a very small volume (i.e. one needs a ‘point dosimeter’ to characterize the

dose at a point). Τhe position of the point where the dose is determined (i.e. its spatial

location) should be well defined in a reference coordinate system.

Thermoluminescent dosimeters (TLDs) come in very small dimensions and their use,

to a great extent, approximates a point measurement. Film dosimeters have excellent

2-D and gels 3-D resolution, where the point measurement is limited only by the

resolution of the evaluation system. Ionization chamber type dosimeters, however, are

of finite size to give the required sensitivity, although the new type of pinpoint

microchambers partially overcomes the problem.

(vii) Readout convenience

Direct reading dosimeters (e.g. ionization chambers) are generally more convenient

than passive dosimeters (i.e. those that are read after due processing following the

exposure, for example TLDs and films). While some dosimeters are inherently of the

integrating type (e.g. TLDs and gels), others can measure in both integral and

differential modes (ionization chambers).

(viii) Convenience of useIonization chambers are reusable, with no or little change in sensitivity within their

lifespan. Semiconductor dosimeters are reusable, but with a gradual loss of sensitivity

within their lifespan; however, some dosimeters are not reusable (e.g. films, gels and

alanine). Some dosimeters measure dose distribution in a single exposure (e.g. films

and gels) and some dosimeters are quite rugged (i.e. handling will not influence

sensitivity, for example ionization chambers), while others are sensitive to handling

(e.g. TLDs).

2.3.2 Luminescence DosimetrySome materials, upon absorption of radiation, retain part of the absorbed energy in

metastable states. When this energy is subsequently released in the form of ultraviolet,

visible or infrared light, the phenomenon is called luminescence. Two types of

luminescence, fluorescence and phosphorescence, are known, which depend on the

time delay between stimulation and the emission of light [15, 16]. Fluorescence

occurs with a time delay of between 10–10 and 10–8 s; phosphorescence occurs with a

34

time delay exceeding 10–8 s. The process of phosphorescence can be accelerated with

a suitable excitation in the form of heat or light.

● If the exciting agent is heat, the phenomenon is known as thermoluminescence and

the material is called a thermoluminescent material, or a TLD when used for purposes

of dosimetry.

● If the exciting agent is light, the phenomenon is referred to as optically stimulated

luminescence (OSL).

The highly energetic secondary charged particles, usually electrons that are produced

in the primary interactions of photons with matter are mainly responsible for the

photon energy deposition in matter. In a crystalline solid these secondary charged

particles release numerous low energy free electrons and holes through ionizations of

atoms and ions. The free electrons and holes thus produced will either recombine or

become trapped in an electron or hole trap, respectively, somewhere in the crystal.

The traps can be intrinsic or can be introduced in the crystal in the form of lattice

imperfections consisting of vacancies or impurities. Two types of trap are known in

general: storage traps and recombination centers.

● A storage trap merely traps free charge carriers and releases them during the

subsequent (a) heating, resulting in the thermoluminescence process, or (b) irradiation

with light, resulting in the OSL process.

● A charge carrier released from a storage trap may recombine with a trapped charge

carrier of opposite sign in a recombination centre (luminescence centre). The

recombination energy is at least partially emitted in the form of ultraviolet, visible or

infrared light that can be measured with photodiodes or photomultiplier tubes (PMTs).

2.3.3 Thermoluminescent dosimetryThermoluminescent dosimetry is used in many scientific and applied fields such as

radiation protection, radiotherapy clinic, industry, and environmental and space

research, using many different materials. The basic demands of a thermoluminescent

dosimeter (TLD) are good reproducibility, low hygroscopicity, and high sensitivity

for very low dose measurements or good response at high doses in radiotherapy and in

mixed radiation fields. LiF is used for dose measurements in radiotherapy since the

effective atomic number of 8.3 is close to that of water or tissue. Lithium tetraborate

is more tissue-equivalent than LiF, but it is deliquescent (absorbs moisture from the

atmosphere) and its stored signals fade rapidly. Its use is therefore only worthwhile

35

for x-rays, where the closeness of its effective atomic number of 7.3 to tissue

outweighs the disadvantages [17]. Calcium sulphate has an effective atomic number

of 15.6 and is therefore much less tissue-equivalent, but its effective atomic number

is quite close to that of bone. It is very sensitive and therefore can be used for

protection dosimetry. Calcium fluoride has an effective atomic number of 16.9 and is

also used for protection dosimetry, as it is also very sensitive.

TLDs are relative dosimeters and therefore have to be calibrated against absolute

dosimetry systems such as a calibrated ion chamber. A 60 Co gamma source is

generally used. Due to their small size, TLDs are convenient for dose-distribution

measurements in medicine and biology.

Thermoluminescent Dosimeter SystemsThe TLDs most commonly used in medical applications are LiF:Mg,Ti, LiF:Mg,Cu,P

and Li2B4O7:Mn, because of their tissue equivalence. Other TLDs, used because of

their high sensitivity, are CaSO4: Dy, Al2O3:C and CaF2:Mn.

● TLDs are available in various forms (e.g. powder, chips, rods and ribbons).

● Before they are used, TLDs need to be annealed to erase the residual signal. Well

established and reproducible annealing cycles, including the heating and cooling

rates, should be used.

A basic TLD reader system consists of a planchet for placing and heating the TLD, a

PMT to detect the thermoluminescence light emission and convert it into an electrical

signal linearly proportional to the detected photon fluence and an electrometer for

recording the PMT signal as a charge or current. A basic schematic diagram of a TLD

reader is shown in Fig. 2.5.

The thermoluminescence intensity emission is a function of the TLD temperature T.

Keeping the heating rate constant makes the temperature T proportional to time t, and

so the thermoluminescence intensity can be plotted as a function of t if a recorder

output is available with the TLD measuring system. The resulting curve is called the

TLD glow curve [18, 19]. In general, if the emitted light is plotted against the crystal

temperature one obtains a thermoluminescence thermogram (Fig. 2.6).

● The peaks in the glow curve may be correlated with trap depths responsible for

thermoluminescence emission.

● The main dosimetric peak of the LiF:Mg,Ti glow curve between 180ºC and 260ºC

is used for dosimetry. The peak temperature is high enough so as not to be affected by

36

room temperature and still low enough so as not to interfere with black body emission

from the heating planchet.

● The total thermoluminescence signal emitted (i.e. the area under the appropriate

portion of the glow curve) can be correlated to dose through proper calibration.

● Good reproducibility of heating cycles during the readout is important for accurate

dosimetry.

● The thermoluminescence signal decreases in time after the irradiation due to

spontaneous emission of light at room temperature. This process is called fading.

Typically, for LiF:Mg,Ti, the fading of the dosimetric peak does not exceed a few per

cent in the months after irradiation

● Good reproducibility of heating cycles during the readout is important for accurate

dosimetry.

● The thermoluminescence dose response is linear over a wide range of doses used in

radiotherapy, although it increases in the higher dose region, exhibiting superlinear

behavior before it saturates at even higher doses.

TLDs need to be calibrated before they are used (thus they serve as relative

dosimeters). To derive the absorbed dose from the thermoluminescence reading a few

correction factors have to be applied, such as those for energy, fading and dose

response non-linearity.

Typical applications of TLDs in radiotherapy are: in vivo dosimetry on patients

(either as a routine quality assurance procedure or for dose monitoring in special cases,

for example complicated geometries, dose to critical organs, total body irradiation

(TBI), brachytherapy); verification of treatment techniques in various phantoms (e.g.

anthropomorphic phantoms); dosimetry audits (such as the IAEA–World Health

Organization (WHO) TLD postal dose audit programme); and comparisons among

hospitals.

37

Fig. 2.5 TLD Reader

Fig: 2.6. A typical thermogram (glow curve) of LiF: Mg, Ti measured with aTLD reader at a low heating rate.

38

REFERENCES[1]. Harvey, E.: A History of Luminescence: From the Earliest Times Until 1900.

Dover Phoenix Editions, USA,(2005)

[2]. Cees Ronda, Luminescence From Theory to Applications, WILEY-VCH Verlag

GmbH & Co. KGaA, Weinheim,(2008)

[3]. Blasse.G, Grabmair,B.C, Luminescent materials,Springer-Verlag,(1994)

[4]. McKeever, S., Moscovitch, M., Townsend, P,Thermoluminescence Dosimetry

Materials: Properties and Uses. Nuclear Technology Publishing, England.(1995)

[5]. Pekpak, E.: Synthesis and Characterization of Lithium Tetraborate Doped with

Metals. MS Thesis, METU, Ankara,(2009)

[6]. Chen,R., McKeever, S.: Theory of Thermoluminescence and Related Phenomena.

World Scientific Publishing Co. Pte. Ltd., Singapore,(1997)

[7]. Furetta,C, Handbook of Thermoluminescence. World Scientific Publishing Co.

Pte.Ltd.,Singapore,(2003)

[8]. Schauer D. Brodsky A. Sayeg,J, Handbook of Radioactivity Analysis, Second

Edition. Academic Press, Great Britan,(2003)

[9]. Natarajan,V.,Bhide,M.K.,Dhobale,A.R.,Godboleb,S.V.,Seshagirib,T.K.,Pageb,

A.G., Lua,C.H,Photoluminescence, thermally stimulated luminescence and electron

para- magnetic resonance of europium-ion doped strontium pyrophosphate. Materials

Research Bulletin,(39), 2065-2075, (2004)

[10].McKeever, S. W. S.: Thermoluminescence of solids. Cambridge University Press,

Cambridge (1983)

[11].Kortov, V.: Materials for thermoluminescent dosimetry: Current status and future

needs. Radiation Measurements(42), 576-581 (2007)

[12].Cameron,J.R.,Suntharalingam,N.,Kenney,G.K.,Thermoluminescent Dosimetry,

University of Wisconsin Press, Madison, WI,(1968).

[13].Attix,F.H.,Introduction to Radiological Physics and Radiation Dosimetry, Wiley,

New York (1986).

[14].Horton, J.,Handbook of Radiation Therapy Physics,Prentice Hall,New York,

(1987).

[15].Khan, F.M., The Physics of Radiation Therapy, Lippincott, Williams and

Wilkins,Baltimore, MD,(2003).

[16].Klevenhagen, S.C., Physics and Dosimetry of Therapy Electron Beams, Medical

Physics Publishing, Madison, WI,(1993).

39

[17].Vandyk, J. (Ed.), Modern Technology of Radiation Oncology: A Compendium

for Medical Physicists and Radiation Oncologists, Medical Physics Publishing,

Madison, WI (1999).

[18].Cameron, J, Radiation dosimetry,Environ Health Perspect. 1991 February;

91:45–48.

[19].International Atomic Energy Agency, Absorbed Dose Determination in Photon

and Electron Beams, Technical Reports Series No. 277, IAEA, Vienna (1987).