Proceedings of the Second International Symposium on Radiation Detectors and Their Uses (ISRD2018)
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Proc. 2nd Int. Symp. on Radiation Detectors and Their Uses (ISRD2018)
JPS Conf. Proc. 24, 011036 (2019)
https://doi.org/10.7566/JPSCP.24.011036
Evaluation on Thermoluminescence Kinetic
Parameters of Ge-doped Cylindrical Fibre
Dosimeter by Computerised
Glow Curve
Deconvolution Technique
Muhammad S. A. FADZIL1, Ung N. MIN2, Alawiah ARIFFIN3, David A.
BRADLEY4,5, Noramaliza M. NOOR1*
1
Department of Imaging, Faculty of Medicine and Health Sciences, Universiti Putra
Malaysia, 43400 Serdang, Selangor, Malaysia
2
Clinical Oncology Unit, Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur,
Malaysia
3
Fiber Optic Research Center, Faculty of Engineering, Multimedia University, 63000
Cyberjaya, Selangor, Malaysia
4
Centre for Nuclear and Radiation Physics, Department of Physics, University of Surrey,
Guildford, GU2 7XH, UK
5
Sunway University, Institute for Health Care Development, Jalan Universiti, 46150, PJ,
Malaysia
*E-mail: noramaliza@upm.edu.my
(Received March 16, 2018)
The shapes of the glow curve obtained from computerized glow curve deconvolution
(CGCD) of Ge-doped cylindrical fibres (CF) were evaluated to determine the properties of
kinetic parameters of Ge-doped CF as a potential thermoluminescence (TL) dosimeter using
WinGCF software. The Ge-doped CF was irradiated to 6 MV and 10 MV photon beams
generated by medical linear accelerator with dose ranging from 100 cGy to 300 cGy. It was
observed that the maximum intensity of the glow curve for CF was located between 259 °C
to 271 °C for 6 MV and 253 °C to 269 °C for a 10 MV irradiation. After deconvolution was
done, the Ge-doped CF glow curve appeared as five individual glow peaks; indicate that the
peak height is highly dependent on the dose. As the dose increases, area under the curve
increases, suggesting the increment in number of electron ejected from the traps.
KEYWORDS: Cylindrical Fibre, Ge-doped, Glow Curve Deconvolution, Kinetic
Parameter
1. Introduction
Thermoluminescence (TL) dosimetry is a most famous and well-established system
in radiotherapy dosimetry. Most of the TL dosimeters will come with some impurities
which will form an electron trap in TL band models. The impurities or dopants are a
chemical substance which deliberately introduced into the TL base materials to increase
the defects and also helps to improve the dosimetric characteristics of the dosimeters
[1-3]. Lithium fluoride (LiF) is one of the most widely used dosimeters in medical
which comes with various impurities such as magnesium and titanium. However,
researchers are looking for other alternative materials for LiF based dosimeters due to
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Proceedings of the Second International Symposium on Radiation Detectors and Their Uses (ISRD2018)
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JPS Conf. Proc. 24, 011036 (2019)
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several limitations possessed by LiF such as potential hygroscopic problems and low
spatial resolutions, thus induced research interest on silica optical fibres as an
alternative material to be used in medical dosimetry. The properties of luminescence
exhibited by silica optical fibres including linearity dose response, good reproducibility
and low fading [4, 5] were observed to enhance with the introduction of a proper
amount of dopant concentrations in the core of fibres [6].
There has been an increasing amount of research works on fabricated Germanium
(Ge) doped optical fibre as radiation detector with improved dosimetric performances.
Studies on Ge-doped optical fibres involved in multiple aspects including different Ge
dopant concentrations [7, 8], various optical fibre sizes [9, 10] and the introduction of a
new geometry of optical fibre, i.e., flat shapes [11, 12]. One of the major advantages of
using Ge as an impurity in silica optical fibre dosimeter is that Ge possessed similar
chemical and physical properties demonstrated by silica. The fusion of Ge inside of the
silica-based optical fibre produced numerous defects center, thus significantly increase
the TL sensitivity of the Ge-doped optical fibre. Without any doubts, it is clearly
indicated that Ge is one of the competent dopant candidates for silica-based optical
fibre dosimeter.
However, none of the investigations mentioned above went into detail on the
kinetic parameters of the Ge-doped optical fibre. Research on these subjects has been
mostly restricted to limited comparisons of glow curve formation and kinetic
parameters such as glow curve formation, glow peak response, linearity behaviour with
dose, activation energy, and maximum temperature. TL kinetic parameters assessment
is necessary to accurately analyse the performance of the new dosimeter before be
introduced as a potential dosimeter and commissioning in clinical practice [13]. To
extend that latest investigation and to clarify the role of Ge-doped optical fibre as a
radiation detector, we conducted a study which is purposely to explore the glow curve
formation of Ge-doped cylindrical fibre (CF) and their kinetic parameters using
computerized glow curve deconvolution analysis software.
2. Materials and Methods
2.1 Fabrication of cylindrical optical fibre
The CF was originated from collapsed preform made by a standard modified
chemical vapour deposition (MCVD) process. Then, the collapsed preform was drawn
into desired shape and diameter. In this study, CF with a diameter of 483 µm was pulled
out and later was cut into 6 mm length. A scanning electron microscope with energy
dispersive x-ray fluorescence (SEM-EDX) was used to evaluate the elemental
composition of the CF.
2.2 Experimental irradiation setup
The samples were prepared according to Fadzil et al. [14] prior to photon
irradiations. A Varian Clinac linear accelerator located at University Malaya Medical
Centre was used to deliver 6 MV and 10 MV nominal energies. A group of 10 CF was
placed into plastic capsule in order to get an average result for each type of irradiation.
The plastic capsules were positioned on top of 10 cm solid–water™ phantom (Gammex,
U.S.A) with dimension of 30 cm × 30 cm to accommodate the backscattered photons.
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JPS Conf. Proc. 24, 011036 (2019)
Plastic capsules were sandwiched with 1.5 cm and 2.5 cm bolus for 6 MV and 10 MV
irradiations respectively to provide maximum dose to the samples. Dose ranging from
100 cGy to 300 cGy were used to irradiate the samples and make use of 600 MU/min
dose rate with focus to surface distance of 100 cm and field size of 10 cm × 10 cm.
The reader Harshaw 3500 (Thermo Fisher Scientific Inc, U.S.A) located at
Secondary Standard Dosimetry Laboratory, Malaysian Nuclear Agency was used as a
TL readout system. The glow curves were measured by Window-based Radiation
Evaluation and Management System (WinREMS) software. The TL yield of the optical
fibres was obtained in the presence of nitrogen gas to reduce the oxidation of the
heating element. Table I showed the timetemperature profile (TTP) used in this
Table I. TTP setups for TL yield acquisition.
study for TL acquisition. For establishment
Parameters
Setting
of glow curve analysis, a computerized
Preheat
temperature
80 °C
glow curve fitting software known as
Windows®–based Glow Curve Fitting
Readout temperature
400 °C
(WinGCF) was used. In order to assess the
Acquisition time
13.3 s
glow curve kinetic parameters, the
measured glow curves were deconvoluted
Heating rate
30 °Cs-1
into individual peaks. The fitting
Annealing temperature
400 °C
assessment was done by checking the FOM
(figure of merit) parameter, which is defined as follows:
FOM (%) =
∑𝑖 |𝑦𝑖 − 𝑦(𝑥𝑖 )|
∑𝑖 𝑦𝑖
× 100% ,
(1)
where 𝑦𝑖 is the measured signal within the 𝑖 channel and 𝑦(𝑥𝑖 ) is the value of the fitted
function in the middle of the 𝑖 channel. The fitted function fits perfectly to the
measured data when the FOM value reaches zero. This means that the fit quality is
better when the FOM value is lower. The FOM employed in this research works were
between 1.5% and 5.0%.
3. Results and Discussion
3.1 Optical fibre structure
Figure 1(a) showed the scanning
Table II. Element composition of Ge-doped
electron microscopy with energy dispersive
cylindrical fibre.
X-ray spectroscopy (SEM-EDX) image of
Composition Composition
Ge-doped CF used in present work. It is Element by weight % by atomic %
clearly seen that the concentric ring
Si
85.12
93.67
formation happen in the core region (Fig.
Ge
14.88
6.33
1(b)) is due to the MCVD fabrication
Total
100.00
100.00
process, with layer by layer deposition
techniques. The distribution of the Ge was concentrated at the central region of the
optical fibre while the cladding was mainly dominated by the silica. The elemental
composition of CF was summarized in Table II.
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JPS Conf. Proc. 24, 011036 (2019)
(a)
(b)
Fig. 1. A cross sectional image of Ge-doped CF (a) and core (b) obtained using SEM-EDX.
3.2 Dose response
TL signal per unit volume
(nC/mm³)
All the TL signals were
y = 1.804x + 17.675
700
normalized per unit volume of the
R² = 0.9911
600
fibre. The results revealed that
500
Ge-doped CF was linear over the
400
y = 1.7591x + 18.451
entire dose range explored for
300
R² = 0.993
both 6 MV and 10 MV
200
irradiations, with r2 value of 0.993
100
0
and 0.991 respectively (Fig. 2).
0
100
200
300
400
The error bar represents the
Dose
(cGy)
variation from 10 optical fibres
Cylindrical Fibre 6 MV
used. The TL signal produced by
Cylindrical Fibre 10 MV
CF irradiated by 10 MV is
slightly higher compared to 6 MV
Fig.2. Linearity response of Ge-CF subjected to 6 MV and
photon irradiation due to deeper
10 MV beams.
penetration for higher photon
beam. These results are in agreement with previous study on Ge-doped CF [15].
3.3 Glow curve deconvolution
The glow curves for Ge-doped CF appeared as a broad single peak for both 6 MV
(Fig. 3(a)) and 10 MV (Fig. 3(b)) photon beams. As the dose increases, the shape of the
glow curves remains constant. The glow curve maximum temperature (Tmax) and peak
height are absolutely dependent on the dose given. As the dose increases, the area under
the curve increases, suggesting an increasing in number of electrons being ejected from
its traps. Tmax of the glow curve defines as the glow peak highest intensity. It is
observed that the maximum intensity of the glow curve is located at 259 to 271 °C for 6
MV and 253 to 269 °C for 10 MV irradiation.
This result is found to be in a good agreement with previous study [16] which
demonstrated the maximum TL intensity for Ge-doped optical fibre is around 256 °C.
These findings suggested that in general the Ge-doped CF demonstrated the secondorder kinetic peak [17], indicated by the shape of the glow curve which is nearly
symmetric, where the high temperature half of the peak is slightly broader than the low
temperature one.
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300
200
100
0
0
50
100
150
500
140
120
100
80
60
40
20
0
200
400
300
200
100
100 cGy
125 cGy
150 cGy
175 cGy
200 cGy
225 cGy
250 cGy
275 cGy
300 cGy
TTP
Temperature (°C)
400
TL signal x 103 (nC)
500
140
120
100
80
60
40
20
0
Temperature (°C)
TL signal x 103 (nC)
JPS Conf. Proc. 24, 011036 (2019)
0
0
50
100
150
200
Channel
Channel
(a)
(b)
Fig. 3. The variations of TL glow curve of CF, following 6 MV (a) and 10 MV (b) photon beam
irradiations from 100 to 300 cGy.
Based on the computerized
Table III. Tmax of glow peaks subjected to dose range of
glow curve deconvolution, the
100 cGy to 300 cGy.
glow curve of the Ge doped CF
Tmax (°C)
Peak
consists of five individual glow
number
6 MV
10 MV
peaks which are consistent with
P1
186.3 to 191.5
177.7 to 187.8
previous study [18]. It is observed
P2
230.8
to
241.3
219.0 to 234.6
that all peaks tend to overlap with
P3
257.4 to 269.4
253.2 to 269.4
each other throughout the studied
P4
286.7
to
298.2
284.1 to 298.1
dose range for both 6 MV (Fig.
P5
303.4 to 314.9
297.6 to 318.1
4(a)) and 10 MV (Fig. 4(b))
irradiation. The first peak (P1) is found to be a dominant individual peak with the range
of Tmax were between 186.3 to 191.5 °C for 6 MV and 177.7 to 187.8 °C for 10 MV
irradiations. The Tmax of the glow peaks (P1 to P5) is unvarying throughout the dose
range used. The summary of Tmax of each glow peak across the dose delivered shown in
Table III.
400
3.8e+04
P2P3
P4P5
9.5e+03
0
200
160
50
100
Channel
(a)
150
40
200
P1
6.7e+04
280
P2
3.8e+04
9.5e+03
0
200
160
P3
P4 P5
50
100
Channel
150
Temperature (°C)
280
TL intensity (nC)
P1
6.7e+04
400
9.5e+04
Temperature (°C)
TL intensity (nC)
9.5e+04
40
200
(b)
Fig. 4. Glow curve deconvolution of CF subjected to 6 MV (a) and 10 MV (b) beams.
Channel
Channel
Figure 5 shows activation energy (Ea) of individual deconvoluted peak affected by
increasing of the dose. Activation energy refers to the amount of energy needed in order
to excite the electrons from valence band to conduction band [19]. Peak 1 shows the
lowest of Ea whereas peak 4 required the highest Ea. These results suggested that the
electrons in peak 1 were occupied at low energy trap whereas at peak 4, the electrons
were trapped in deeper trapping level.
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Activation Energy
(eV)
Activation Energy
(eV)
JPS Conf. Proc. 24, 011036 (2019)
2.5
2
1.5
1
0.5
2.5
2
P1
P2
P3
P4
P5
1.5
1
0.5
0
0
100 125 150 175 200 225 250 275 200
100 125 150 175 200 225 250 275 300
Dose (cGy)
(a)
Dose (cGy)
(b)
Fig. 5. Influence of various dose of 6 MV (a) and 10 MV (b) irradiation on the Ea.
4. Conclusion
The glow curve of Ge-doped CF appeared as a single broad peak with five
individual deconvoluted peaks. Kinetic parameters analysis revealed that Ge-doped CF
obeys the second order kinetic model due to the shape of the glow curve is nearly
symmetric where it was characterized by high temperature half of the glow curve is
slightly broader than the low temperature half of the glow curve. These results indicate
a possibility of electron retrapping and thus cause a delay in luminescence emission.
Acknowledgment
This research is supported mainly by the Malaysia Fundamental Research Grant
Scheme (FRGS) no: 5524789 and Universiti Putra Malaysia Geran Putra Incentive
Putra Siswazah (UPM-GP-IPS) no 9433970. Special thanks to ISRD 2018 organizing
committee for providing the student travel grant to the first author.
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