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Thermo luminescent dosimetry (TLD) was studied using phosphor material LiF (TLD-100). TLD-100
chips were annealed in the furnace followed by their irradiation under known exposure and then were
read in the TLD reader to measure the TL output which was used to determine the average calibration
constant. The glow curve obtained from an annealed TLD-100 chip irradiated under unknown exposure
was analyzed and TL reading was measured to determine the exposure.
Luminescence describes the process of emission of optical radiation (visible light) from a material from
causes other than heating it to incandescence. Luminescent materials can absorb energy, store a fraction
of it and convert it to optical radiation which is then emitted. Phosphorescence, fluorescence and thermo
luminescence are some particular forms of luminescence, which differ not by means of excitation but to
the time scale over which the emission of luminescence after the absorption of energy takes place. After
excitation, if the electrons return immediately to their original energy state (ground state) with the
emission of light then the process is called fluorescence. However, if, due to the presence of electron traps
(metastable states), the return of electrons to the ground state is delayed, the process is termed as
phosphorescence. In this case, the transition of electron from metastable state to the ground state is
forbidden. The metastable state represents shallow electron trap and electrons returning from it to the
excited state require energy. This energy can be supplied in the form of heat (thermal stimulation) and the
probability of escape of an electron from a metastable state depends on heating temperature and increases
with rise in temperature. This process is called thermo luminescence (TL).
Thus thermo luminescent (TL) materials are the materials which after exposure to radiation emit light on
heating and the total light output is proportional to the amount of radiation absorbed. In thermo
luminescent dosimetry (TLD), this property is used to measure the radiation dose. This is done by using a
TLD reader, consisting of a controlled heating element and a photomultiplier system which determines
the amount of light emitted during the heating of dosimeter material. In most TLD systems, the integrated
light intensity is measured as a function of heating temperature cycle. The range of the heating cycle
depends upon the nature of the TL material.
The most common TL materials are lithium florid (LiF) and calcium florid (CaF2). In this experiment, LiF
was used to study the different parameters of a TLD system and to determine the dose. LiF is currently
the most commonly used family of thermo luminescent phosphors having the effective atomic number 8.2
and thus for most application it can be considered to be approximately air or tissue equivalent. In its
purest form, it exhibits relatively little TL. The presence of impurities in LiF appears to be necessary for
the appearance of radiation induced TL. Thus Mg and Ti are added as impurities in LiF. Thermo
luminescent grade LiF is commercially available as TLD-100, TLD-600 and TLD-700, differing only in
the relative isotopic abundance of Li 6 and Li
7 . TLD-100 containing Li in its natural isotopic ratio is the
least costly and the most widely used TLD phosphor.
The graph of light emitted as a function of time or temperature during heating is called the glow curve.
The usual procedure is to plot the light emitted by the phosphor vs. the temperature of the bulk material.
The curve is obtained by recording on a plotter the light signal from a photomultiplier tube viewing the
material and the temperature signal from a thermocouple in close contact with the TL material.
A typical glow curve would show one or more peaks (maxima) as electrons trapped at various energy
levels are released. The relative amplitudes of the peaks indicate approximately the relative population of
the electrons in the various traps. Either the total light emitted during part or all of the glow curve or the
height of one or more peaks may be used as a measure of absorbed dose, the heating cycle must be
reproducible to avoid peak height fluctuations.
To prepare the dosimeter material for reuse, it must be heated again or annealed after reading. The exact
heat treatment procedure of the crystal depends on the material itself and on its intended use such as the
exposure level range. The reusability of TLD is one major advantage over other dosimeters such as film
badges. On the other hand reading or annealing process causes a loss of information stored in the
dosimeter and thus the loss of permanent record of the dose.
There are a number of factors which may affect the shape of the glow curve. These include the heating
rate and its uniformity, the size, history of the sample, the recording instrument selected for use and some
spurious effects which may appear. If all these factors are held constant then doubling the heating rate
will double the height of the glow peak. This effect is illustrated in figure 1. For measurements of thermo
luminescence by integration of the peak or by peak height, the heating rate need not be uniform; the
heating rate need only be reproducible.
Another parameter of importance is fading. Fading is the apparent loss of TL signal between exposure
and evaluation. This is especially important when using the phosphor for personnel or environmental
monitoring, where low doses are to be measured.
Figure 1.Effect of Heating Rate on The Glow Curve
Figure 2 shows a typical glow curve from TLD-100, having six peaks at 60 o , 120
o , 170
o , 190
o , 210
285 o C. After irradiation, peak 1 through 6 decays at room temperature with approximate half lives as
given in figure 2. Consequently, peaks 4 and 5 are the most suitable for dosimetry. The height of the peak
6 depends on the linear energy transfer (LET) of radiation and increases with increase in LET. Therefore,
the ratio of the height of the peak 6 to that of the peak 5 has been used for the mixed field dosimetry.
Peaks 1 and 2 having very low stability can be removed by various combinations of pre and post-
irradiation thermal annealing procedures. The annealing procedure also has pronounced effect on the
relative height of the peaks 2 through 5. The pre-irradiation thermal annealing procedure of TLD-100 at
400 o C for 1 hour followed by fast cooling to room temperature enhances the height of peaks 2 and 3. If
the phosphor is then annealed for 1 or 2 hours at 100 o C or 16-24 hours at 80
o C (standard annealing
procedure), peaks 2 and 3 are almost entirely eliminated. The same effect can also be achieved by post-
irradiation annealing for 10 minutes at 100 o C. This necessary and somewhat complicated annealing is a
major disadvantage of this phosphor.
Figure 2.Typical Glow Curve of LiF (TLD-100)
Experimental setup and results
After annealing the TLD-100 chips for 1 hour at 400 o C temperature, all of the chips were inserted in the
TLD reader one by one in order to determine the amount of light B emitted by them before irradiation.
That amount B was found to be zero for all the chips. Then about 6 or 7 chips were selected for irradiation
under known exposure of 100mR. After that all chips were read in the TLD reader and respective TL
readings were noted and calibration constant f was determined for all the chips using the following
B = 0
D = 100mR = (5mR per revolution)*(20 revolutions)
Chip # B TL f
1 0 1079 10.79
2 0 1022 10.22
3 0 994 9.94
4 0 1021 10.21
5 0 1049 10.49
6 0 1060 10.60
Figure 3.Major Components of A TLD System
The average value of f was calculated to be 10.375. This value was used then to determine the dose
received by a selected annealed TLD chip irradiated under unknown exposure. The glow curve from that
TLD chip is shown below:
Figure 4.The Glow Curve
TL = 2835
B = 0
D = TL/f = 2835/10.375 = 273.25mR
TLD-100 (LiF) was used to study the dosimetry process because LiF is currently the most commonly
used family of thermo luminescent phosphors having the effective atomic number 8.2 and thus for most
applications it can be considered to be approximately air or tissue equivalent and TLD-100 containing Li
in its natural isotopic ratio is the least costly and the most widely used TLD phosphor.
The annealing procedure was done in order to empties the phosphor before exposure to radiation and to
minimize the effect of the dose already received by it from background radiation or from previous
exposure to radiation if used for dosimetry so that the error in the results might be reduced.
Since the light output of the phosphor is proportional to the dose received, calibration constant f was
determined in order to estimate the dose absorbed when exposed to unknown exposure of radiation. This
was done by irradiating the TLD-100 under known exposure of 100mR and then by reading it in the
reader and observing the glow curve formed.
The glow curve obtained from a TLD-100 chip irradiated under unknown exposure has been shown above
in the figure 4. It can be observed that the curve has only two main peaks which are surely the peaks 4
and 5 of the typical glow curve discussed in the introduction part and is shown in figure 2. All the other
four peaks out of six have almost been eliminated. Thus some of the TL information was lost. Same was
the case when calculating the calibration constant f and thus an error was introduced in calculations
relating the determination of f and unknown dose. Another source of error is the contribution of the dose
received by the chip from exposure to background radiation when it remains in the atmosphere after
annealing till the time of irradiation and after irradiation until reading.
The average calibration constant for TLD-100 chips was found to be 10.375. This was then used to
estimate the unknown exposure and dose received. The unknown exposure was found to be 273.25mR. A
TLD chip must be thermally annealed before irradiation either to do calibration or to determine the
unknown exposure. This is a major disadvantage of thermo luminescent dosimetry. However, since LiF
can be approximated to be tissue equivalent and it is not very costly and is reusable, it has been found to
be a very useful dosimeter especially in case of personnel monitoring. It can also be concluded that this
dosimeter falls in the category passive type of detectors.
1. Knoll, G.F. ; Radiation Detection and Measurement, John Wiley & Sons (1999)
2. Nasir Ahmad ; Experimental Radiation Detection, CNS-20, (1987)