Thermoluminescence Dosimetry, Medical Physics, glow curve, Lithium fluoride. radiation exposure, Radiation Physics, Medicine, radiation therapy, treatment, biomedical technology.

2. PRESENTED BY: Zermeen
Fatima
(2k-19-PHY-MS-08)

3. OUTLINE
• Introduction
• Definition
• Basic Phenomena of TL
• Mechanism of TL
• TL Dosimetry
• Instrumentation
• Materials used in TLD
• The Glow Curve
• Lithium Fluoride in TLD
• Calibration of TLD
• Example of LiF
• Conclusion

4. INTRODUCTION
• Thermoluminescent Dosimetry or TLD is one of the unique Radiation
Dosimeter.
• TLD measures the absorbed dose of radiation that is irradiated on the
specific area being treated.
• It is being used in many areas and fields of science because of its
versatility.
• TLD can also be used as a direct inserted dosimeter into the body
cavity.

5. DEFINITION
• TLD is a type of a dosimeter that measures the “absorbed dose” when
the patient is irradiated by the specific radiations for the treatment of
some particular disease.
• TLD has a Thermoluminescent material fixed inside it that works on
the principle of Thermoluminescence (TL) and thus gives the exact
value of radiation exposure.

6. BASIC PHENOMENA OF TL
• For the understanding of TLD, let’s go through the basic phenomena
of Thermoluminescence (TL).
• Some of the materials like minerals, when are irradiated by Alpha,
Gamma, Beta and even the cosmic rays, the atoms of those materials
absorb the incoming energy.
• This absorbed energy excite the electrons which leave their Ground
states and get trapped into different energy states.
• It is not wrong to say that those excited electrons roam within the
imperfect crystal lattice structure.

7. • When this irradiated material having such excited electrons is heated,
this heat acts as a stimulating agent and so the stimulated emission of
photons, having a specific frequency value and wavelength, occurs as
a consequence to the heat energy stimulating the excited electrons to
drop back down into their Ground state or Equilibrium state.
• The photons thus released is basically the Luminescence. This is called
TL.

8. THE MECHANISM OF TL
• The inorganic materials are used for the purpose of TL dosimetry. As
we know, there is a Band Gap between the Valence Band and the
Conduction Band. This Band Gap basically represents the amount of
energy E which is required to excite an electron to move it up to the
conduction band E2 from its valence band E1.

9. • The materials that are used for TL dosimetry are not used in their Pure
states. Impurities are added or incorporated in the lattice of such
inorganic materials.
• The incorporated impurities consequently, become the reason for the
formation of various Energy Traps within the Band Gaps. These
Energy Traps are always near to the valence or conduction bands. They
are divided into two categories:
Shallow Traps (near to the valence or conduction bands)
Deep Traps (near or close to the middle of a band gap)
• When the ionization radiation is fallen upon the TL crystal having an
impure lattice, then the electrons leave their ground or valence state by
absorbing the energy. Spontaneously, they lose some of the energy and
get trapped into the Energy Traps.

10. • The Energy Traps act as the Trapping States which means that they are
able to capture the carriers that maybe electrons or holes.
• These Trapping states have a definite value and are localized thus the
carrier that is trapped there will remain there.
• The deep traps require the external stimulus which may be heat energy
in order to release those trapped carriers which can be holes or
electrons. While the shallow traps need some definite time to release
the carriers.
• Hence TL occurs when the trapped carriers are stimulated by the heat
and they release a specific photon and go back to their ground state.
• The dosimeters are therefore always designed with the lattices that
have such an ability to imprison the carriers in the Energy Traps and
this is only possible when there are impurities present in the lattice
making it more vulnerable for the Energy Traps to build up.

11. The following figure illustrates the idea of the Energy Traps.

12. • First the radiation that is fallen
on the lattice excites the
electrons and electron hole
pairs are formed.
• After some time, they are
trapped into the Trapping
states. The holes can also
migrate into the trapping states.
• Then the heat is given which
stimulates the electrons or the
holes to come back to their
equilibrium states.
• This is represented in the
diagram.

13. • When the heat is applied, the holes represented in the above figure,
recombine with the electrons resulting into a photon in form of TL.
• Another possibility of recombination exists when the hole goes back
to its initial position and then is filled with the electron trapped.
• This phenomena is illustrated in the following figure that shows both
of the possibilities of TL photon released owing to the transition of
both holes and electrons.

15. TL DOSIMETRY
• TL dosimetry works on the principle of TL explained earlier.
• TL dosimeters have the inorganic crystals and thus they measure
exposure rate.
• The electron hole pairs are created when they are exposed to radiation.
These electron hole pairs release the photons which are obviously
equal to the number of electron hole pairs that were created in the first
place by radiation exposure.
• In this way radiation exposure can be measured by the photons that are
released in the same amount as the electron hole pairs.

16. INSTRUMENTATION
TL dosimetry consists of the following parts usually:
• A TLD sample
• Source of heat
• Source of voltage
• Photomultiplier tubes (PMTs)
• Amplifier
• Recorder
• Computer System

17. SCHEMATIC DIAGRAM

18. • The diagram shows the working of TLD. A TL sample is irradiated
first and then is heated.
• The PMTs catch and measure the TL. PMT convert the TL signal into
an electrical signal.
• This electrical signal is enhanced and amplified by an amplifier. The
amplifier is connected with a Recorder which records all of the data.
This data is then interpreted by a computer having CGA (computerized
Glow Curve Analysis) gives a complete information.
• A computer having CIEMAT code can also be used to generate and
study the first peak.

19. MATERIALS USED IN TLD
• There are numerous materials that show the phenomena of TL. In TLD
however, following materials are used. Each material has different
properties and thus each material show different characteristics.
 LiF
 LiF;Mg,Cu,P
 Li2B4O7;Mn
 LiF;MgTi
 CaF2;Mn
 CaF2;nat
 CaSO4;Mn
 Al2O3;C

20. • LiF has effective atomic number very close to the tissue due to which
it is used for clinical procedures.
• Calcium sulphate has an effective atomic number of 15.6 which is
close to the bone due to which it is useful in protection dosimetry.
• Calcium fluoride has the effective atomic number 16.9 and is very
sensitive which makes it useful for protection dosimetry as well.

21. Following are the characteristics that vary from material to material
mentioned in the previous slide.
 Range
 Maximum value of temperature for Glow Peak of TL
 Physical forms of the materials that are used in such states
 Useful range
 The fading
 Energy responses without the filters
 Efficiency

22. THE GLOW CURVE
• The Glow Curve is actually the result that TLD generates. The graph
is plotted between TL and the temperature. The increase in temperature
increases the TL which means that the irradiated sample when heated,
the electron hole pairs start to recombine or in other words they start to
go back to their ground states from the trapped states, therefore,
producing more and more TL.
• The glow curve consists of many peaks in orderly form. The reason
behind these peaks is the various Energy traps. First the glow curve
gives the maximum value which means that the TL is maximum. After
the maximum peaks, the curve starts to fall and ultimately reaches the
value zero because the phosphorescent materials possess various
energy levels in between the valence and conduction bands.

23. • Thus allowing an electron or hole to drop down to these levels just like
stairs and with each descent of energy level they emit TL. This is the
reason behind the glow curve that contains one maximum and other
peaks gradually decreasing to zero where the TL stops. Each peak in
the glow curve is characteristic of the energy traps in the material.
Glow Curve of LiF:

24. LiF IN TLD
• Lithium fluoride (LiF) is the most widely used material in TLD for
clinical purposes because the effective atomic number of LiF is very
near to the effective atomic number of a human soft tissue.
Effective atomic number of LiF: 8.2
Effective atomic number of a soft tissue: 7.4
• This allows one to measure the absorbed dose even by comparison.
This is done by comparing the value of absorbed dose by the tissue
with the LiF absorbance of the same dose.
• If the ratio of the absorbed doses in each of them is calculated then it
will give the same value as the value of the ratio of their mass energy
absorption coefficients. LiF is mostly used in the measurement of dose
in Radiotherapy.

25. • Bragg Gray formulism can also be used for such an approach where
the ratio between the absorbed doses will be equal to the ratio of
“Mass Stopping Powers”.

26. CALIBRATION OF TLD
• TLD must be calibrated before the use for measuring the absorbed
dose. There are two reasons:
The material used in TLD might affect the graph and the readings due
to its previous absorbed energy from the radiation and also the heat
that is given to it before.
The residual effects are also present that might change the new
readings.
• To overcome these problems, calibration is needed. Calibration is done
by the method which is called “Annealing”.
• Calibration is mostly done in ion chambers. Cobalt-60 gamma source
is therefore used for carrying out such a procedure.

27. LiF AS AN EXAMPLE
• Let’s discuss this method by the example of LiF.
• LiF is calibrated by heating it for one hour at a very high
temperature of 400oC. After this, LiF is then heated slowly at 80oC
for one complete day (24hours).
• This annealing results in the stable formation of glow curve
because it removes all of the prior peaks that occur due to the
exposure of radiation in previous use.

28. CONCLUSION
• PL dosimetry is thus very helpful for clinical uses. It is not just
easy to use but also has a lot of applications in the field of
Biology and Medicine.