Thermo Luminescent Dosimetry-Advanced Physics-Lab Report, Exercises for Advanced Physics. Alliance University
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sankaraa16 July 2012

Thermo Luminescent Dosimetry-Advanced Physics-Lab Report, Exercises for Advanced Physics. Alliance University

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This is lab report for Advanced Physics Course. It was submitted to Prof. Dhirendra Kapoor at Alliance University. Its main points are: Thermo, Luminescent, Advance, Dosimetry, Electron, Heating, Material, Process, Syste...
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Abstract

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.

Introduction

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.

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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

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Figure 2 shows a typical glow curve from TLD-100, having six peaks at 60 o , 120

o , 170

o , 190

o , 210

o and

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)

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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

equation:

Where

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

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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

Discussion

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.

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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.

Conclusion

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.

References

1. Knoll, G.F. ; Radiation Detection and Measurement, John Wiley & Sons (1999)

2. Nasir Ahmad ; Experimental Radiation Detection, CNS-20, (1987)

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