Thermoluminescence characteristics of flat optical fiber in radiation dosimetry under different electron irradiation conditions

Thermoluminescence characteristics of flat optical fiber in radiation dosimetry under different electron irradiation conditions

A. Alawiah*a, A. M. Intana, S. Baukb, H. A. Abdul-Rashidc, Z. Yusoffc, M. R. Mokhtarc, W. S.

Wan Abdullahd, K. A. Mat Sharife, G. A. Mahdirajif, F. R. Mahamd Adikanf, N. Tamchekg,N. M. Noorh and D. A. Bradleyi,j

aFaculty of Engineering and Technology, Multimedia University, 75450 Malacca, MALAYSIA;

bPhysics Section, School of Distance Education, Universiti Sains Malaysia, 11800 Penang, MALAYSIA; cFaculty of Engineering, Multimedia University, 20100 Cyberjaya, Selangor,

MALAYSIA; d Department of Non-Ionizing Radiation, Nuclear Malaysia Agency, 43000 Bangi, Selangor, MALAYSIA; eTMR&D Photonics Lab, Telekom Malaysia, Research & Development Sdn. Bhd, 20100 Cyberjaya, Selangor, MALAYSIA; fPhotonics Research Group, Department of

Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, MALAYSIA; g Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400

Serdang, Selangor, MALAYSIA; hDepartment of Medical Imaging, Faculty of Medicine and Health Sciences, Universiti Putra Malaysia, 43400 Serdang, Selangor, MALAYSIA, iDepartment of Physics, University of Surrey, Guildford, Surrey, GU2 7XH, UK; jDepartment of Physics,

University of Malaya, 50603 Kuala Lumpur, MALAYSIA.

ABSTRACT

The thermoluminescence (TL) provided by flat optical fibers (FF) have been proposed as the basis for a novel radiation sensor, for use in medical dosimetry for both diagnostic and radiotherapy applications. A flat optical fiber with nominal dimensions of (3.226 x 3.417 x 0.980) mm3 and made of pure silica SiO2 was selected for this research. The FF was annealed at 400oC for 1 h before being irradiated. Using a linear accelerator (LINAC), delivering doses in the range 2.0-10.0 Gy, the kinetic parameters and dosimetric glow curve representing TL response of the FF were studied with respect to electron irradiation of 6 MeV, 15 MeV and 21 MeV. The TL response was read out using a TLD reader Harshaw Model 3500. The Time-Temperature-Profile (TTP) of the reader used includes; initial preheat temperature of 80oC, maximum readout temperature of 400oC and a heating rate of 30oCs-1. The proposed FF shows excellent linear radiation response behavior within the clinical relevant dose range for all of these energies, good reproducibility, independence of radiation energy, independence of dose rate and exhibits a very low thermal fading. From these results, the proposed FF can be used as radiation dosimeter and favorably compares with the widely used LiF:MgTi dosimeter, for application in medical radiotherapy application. Keywords: Thermoluminescence, flat optical fiber, dosimetry, glow curve, kinetic parameters

*[email protected]; phone +60125963159; fax +6062316552; http://www.mmu.edu.my

Micro-structured and Specialty Optical Fibres II, edited by Kyriacos Kalli, Jiri Kanka, Alexis Mendez, Proc. of SPIE Vol. 8775, 87750S · © 2013 SPIE

CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2017209

Proc. of SPIE Vol. 8775 87750S-1

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1. INTRODUCTION

Radiation dosimeters are used in for instance, environmental monitoring, routine personnel monitoring and patient dose measurement, the latter for imaging studies and radiotherapy. Other applications include in studies of radiobiology, radiation chemistry, nuclear reactor shielding and space radiation sciences. Given the diverse needs, each application places its own set of demands on the capability of the system of dosimetry. Desirable characteristics to be considered in choosing a suitable dosimetric system include: long-term stability of the signal (pointing to the ability to measure integrated dose), linearity between radiation dose and response, the ability to distinguish between different types of radiation, acceptable accuracy and precision, and a well characterized relationship between the dosimeter response in the medium in which dose is to be measured and the calibration radiation field. A present interest is in thermoluminescent (TL) media, as discussed below. Thermoluminescence is the emission of light resulting from the heating of insulating or semiconductor materials subsequent to radiation exposure. For TL material, the characteristics independent of the readout instruments are the kinetics parameters, i.e. the activation energy, the frequency factor and the kinetics order of the glow curve1. The TL yield versus temperature is produced as one or more TL peaks2. Under favorable conditions, the emitted TL yield is proportional to the absorbed dose, allowing determination of the radiation field using an appropriate calibration. These properties are used in TL materials for dosimetry applications, hence, the term TLD. The main features required of a good TLD medium are reproducibility, stability, dose-rate independence and linearity. Of note is fading, the spontaneous escape of charge carriers from traps at ambient temperature. The fading factor assesses how much of the TL yield loses over a given period of time. The TL characteristics of silica (SiO2) optical fibers have been widely investigated. In one study the TL response of commercially available telecommunication Ge-doped fibers were found to be some several times that of O2 doped fibers (SiO2 fibers ion implanted with O2) and very much greater than the TL response of Al-doped SiO2 fibers3. Screening in response has lead to Ge-doped fibers with good reproducibility, also enjoying the linear response to dose over a wide dynamic range; for photons in the MeV range, linearity of response was found from 1 to 120 Gy, with a minimum detectable dose of 20 mGy4. For photons of tens of keV range, from a synchrotron source, linear response was obtained from 1 Gy to 2 kGy5. In a wider context, the potential of doped SiO2 optical fibers in measuring photon and charged particle beams is to be found in a number of reports6-10. Ge-doped fibers have also been demonstrated to offer excellent TL yield and spatial resolution, sufficient for use in high dose gradient situations as in Intensity Modulated Radiation Therapy (IMRT) 11. The same optical fibers also demonstrate reusability and low residual signal and fading12. Herein, pure silica flat optical fibers13 were subjected to electron irradiation, determining the TL characteristics and kinetic parameters from the respective TL glow curves. The fiber form used herein is shown in Figure 1, the comparison being done with the capillary fiber from which the flat fiber is formed and a doped circular fiber, as conventionally used in optical fiber communications and previously studied as a radiation dosimeter. Kinetic parameters include the number of glow curve peaks, activation energy Ea and maximum temperature Tmax. The influence of irradiation dose and energy on the kinetic parameters for the flat optical fiber were also investigated.

2. MATERIALS AND METHOD

2.1 Details of materials and sample preparation

An unclad pure silica flat optical fiber has been used as shown in Figure 1 and Figure 2, with a mass of 19.33 mg (Table 1), use being made of a Bleutronic electronic balance. The flat fiber is fabricated using a conventional 5 m fiber drawing tower located at the Flat Fiber Laboratory, Department of Electrical Engineering, University of Malaya, Malaysia14.15. A scanning electron microscope with energy dispersive x-ray fluorescence (SEM-EDXRF) capability was used to determine elemental composition (Table 2). The TLD material LiF:MgTi (TLD-100) has been used as a reference material in this study. TLD 100 crystal chips (3.0 mm x 3.0 mm x 0.89 mm) were obtained from the Harshaw-Bicron Company. The study involved annealing, sample irradiation, TL measurement and glow curve analysis.



,,,,....._..-.

Figure 1. From left to right are depicted: (a) Capillary fiber; (b) flattened capillary fiber, referred to as flat fiber, and; (c) the typical form of doped single mode telecommunication fiber.

Figure 2. The schematic diagram of pure silica flat optical fiber.

Table 1. Dimension details of pure silica flat optical fiber

Dimension Pure silica (mm) a 0.980 b 0.880 W 3.417 L 3.226

Weight (mg) 19.330

a a b

L

W

(a) (b) (c)



Jr

Table 2. Elemental data of pure silica flat optical fiber

% composition

Element Weight Atomic

O 36.37 50.09

Si 63.63 49.91

Total 100.00 100.00

2.2 Annealing

Thermal annealing is carried out in order to stabilize the sensitivity and background of the medium such that dosimetric properties remain constant in re-use, re-obtaining the same glow curves after similar irradiations given that annealing procedures are maintained. In this study, the optical fibers were annealed using a Nabertherm Program Controlled S27 Furnace (Nabertherm, Germany). The fibers were wrapped in aluminum foil and annealed at 400 oC for 1 h. The fibers were then removed and allowed to cool to room temperature at the rate of 14.52 oC/min, rapid cooling in air having been shown to produce greater sensitivity12.

2.3 Sample Irradiation

The fibers were irradiated at Mount Miriam Cancer Hospital using a Siemens Mevatron MD2 (Siemens, Germany) linear accelerator (LINAC). The LINAC operates at accelerating potentials of 6 MV and 10 MV, providing for high-energy x-rays with a target in place and in the absence of the target electron energies ranging from 6- to 21 MeV. The source to sample surface distance (SSD) was at 100 cm, with a field size of 15 × 15 cm2 selected for electron irradiations. In all cases, the fibers were placed at the center of the field. A Gammex RMI solid waterTM phantom (Gammex, U.S.A) (30 cm length x 30 cm width) was used to ensure calibration conditions. The applicator size was 15 × 15 cm2. To provide for charged particle equilibrium at the sample position, the samples were located at the depth at which, in use of a single stationary beam, the maximum dose is deposited, Dmax; another 10 cm slab of the solid waterTM phantom was placed below the sample fibers to provide for full backscattering conditions, both as satisfied in calibration (Figure 3).

Figure 3. The buildup structure of dosimeter and the experimental set-up using the medical LINAC, for electron irradiations, for doses up to 10 Gy.

Solid water phantom

Applicator

Sample, placed at Dmax

LINAC beam generator

Solid water phantom

TL dosimeter sample placement

Field size

Ion chamber



2.4 Dose response, reproducibility and energy dependence

At the various electron energies, the flat fibers were irradiated to doses ranging from 2.0 Gy to 10.0 Gy. For each dose a minimum of 5 fibers were irradiated, allowing assessment of statistical variation and reproducibility. The energy dependence test determines response of the fibers in relation to the energy of the incident radiation.

2.5 Glow curve analysis using computerized glow curve deconvolution (CGCD)

Deconvolution of the glow curve into its component glow peaks was carried out using a first order kinetics algorithm and WinREMs, under the windows CGCD software WinGCF. Unresolved glow curves of the FF studied herein are shown in Figures 4 and 5. The CGCD program allows region-of-interest (ROI) analysis of the glow curve. The channel number is proportional to the temperature. The area under each glow peak is associated with the number of traps, both for electrons and holes and hence the amount of charge involved in the transition of electrons from conduction band to the trap center. Three types of trap can be identified: shallow traps, dosimetry traps and deep seated traps. Shallow traps, those nearest to the conduction band, are easily emptied at room temperature (RT), leading to measurable fading of the TL signal. The dosimetric traps require somewhat more energy to release trapped electrons, normally forming the peak within which maximum TL yield is obtained and hence are used as the principal peak in dosimetric evaluation. The deep seated traps require appreciable energy in order to be emptied, obtained by high temperature annealing. 2.6 TL readout

The TL yield as a function of temperature, referred to as the TL glow curve, were carried out herein using a TLD reader, model Harshaw 3500 (Thermo Fisher Scientific Inc, U.S.A). The time-temperature profile (TTP) of the reader was obtained to a preheat temperature of 80 oC, the output signal being acquired at a temperature ramp-rate of 30 oC s-1, acquisition time of 13.33 s and a maximum temperature of 400 oC for the pure fiber. All readings were taken under N2 gas flow, suppressing oxidation and potential triboluminesence. The TL readings were normalized to the mass of fiber, to obtain results in Cµ /mg.

3. RESULTS AND DISCUSSION

3.1 Consolidated TL glow curve

The Region of Interest (ROI) at channel 51-100, gives the highest contribution of electrical charge produced during TL readout, shown by the maximum intensity (Imax) of TL peak located in this channel. This observation was for the electron energies ranging from 6- to 21 MeV. With the temperature increase from 100- to 220 OC rapid increase in TL intensity was observed. Then when the temperature was further increased to in excess of 210 oC, TL intensity was found to decrease rapidly as shown in Figure 4. The area under the glow curve relates to the number of electrons released from traps. At 209 oC, the number of electrons released from the trap is maximum, commensurate with a glow peak maximum at channel no 83, obtained at 5.53 s, see Figure 4.



1.205 µC

.0210 R011

.7881 R012

.3343 R013

.0616 R014

ECC

RCF

328 n4

0 - 50

51 - 100101 - 150

151 - 200

Channel: 83

Intensity: 328.276 net

Temperature: 209 T

Time: 5.53 sec

300.0

200.0

cN

100.0

0.0

oi

--51

41

-31

-.21

r :11

K-n50 100

Channel

150 200

D

0

D m

D

3.260 µC

.0556 R0112.112 R012.9300 R013.1624 R014

ECC

RCF

871 n4

0 -- 50

51 100

101 -- 150151 -- 200

Channel: 80Intensity: 871.144 n4

Temperature: 203 °C

Time: 5.33 sec

800.0

700.0

600.0

a 500.0Ñ

400.0

300.0

200.0

600

--500

400 ma

300C(D

200 ri

100

di 100

Channel

150o

200

Figure 4. TL glow curve for a pure silica flat fiber subjected to 21 MeV electron irradiation to give a dose of 1.0 Gy. Imax appears at channel no 83, commensurate with a peak temperature Tmax of 209 oC.

For increase in absorbed dose from 1 Gy to 2 Gy, the electrical charge and by association the TL yield increases by in excess of a factor of 2. This indicates that with increase in irradiation dose, an increasing number of electrons are excited to the conduction band and commensurately more electrons are trapped at the various electron trap centers. During the thermal stimulation process, these electrons are released from the traps, reflecting in the higher TL yield with irradiation dose. Figure 5 shows the higher glow peak intensity compared to that obtained in Figure 4, for the indicated increase in dose. Also shown is a slight shift of the glow curve peak maximum position to lower channel number and hence lower temperature. At 2 Gy, the maximum peak appeared at channel number 80 whereas at 1 Gy the peak appeared at channel number 83. It was observed that at the higher irradiation dose, lesser time needed for emptying of the electron trap centers.

Figure 5. TL glow curve for a pure silica flat fiber subjected to 21 MeV electron irradiation to give a dose of 2.0 Gy. Imax appears at channel no 80, commensurate with a peak temperature Tmax of 203 oC.



28.61 µC

3.102 R01124.36 R0121.087 R013.0620 R014

ECC

RCF

15 pA

0 -- 50

51 -- 100101 -- 150151 -- 200

Channel: 78

Intensity: 15471.289

Temperature: 243 °C

Time: 5.20 sec

14000.0

12000.0

10000.0

8000.0

6000.0

4000.0

2000.0

0.0

or

-51

41

°31

--21

--11

I I I r .n50 100

Channel

150 200

o

0

0 maW

0

0

Figure 6. TL glow curve for a TLD 100 subjected to 21 MeV electron irradiation to give a dose of 2.0 Gy. Imax appears at channel no 78, commensurate with a peak temperature Tmax of 243 oC.

The shape of the glow curve of TLD 100, shown in Figure 6 is manifestly different from that for pure silica flat fiber obtained at the same irradiation energy and dose. The shape or peak produced by the glow curve indicates the characteristic of the electron trap centers, highly localized in crystalline media such as that of the LiF of TLD 100 and delocalized in amorphous media such as the present glass system.

3.2 Deconvolved TL glow peaks

A glow curve can be deconvolved into several peaks at different temperatures and each peak has a different thermal stability. See Table 3 and 4 for physical characterization of the component peaks of the flat fibers and TLD 100 respectively. Ea corresponding to the notional activation energy of electron traps at the deconvolved levels and Tmax to the associated peak temperatures. The consolidated glow curve of a pure silica flat fiber was found to consist of five component glow peaks at 198.5 oC, 238.4 oC, 279.6 oC, 309.8 oC and 334.30 oC (Figure. 7). The fibers were irradiated to doses in the range 2.0 Gy to 10.0 Gy and readout at a ramp-rate of 30 oCs-1. Peak I can be used as dosimetric peak for dose measurement in radiation dosimetry.



2.7e 05

24e105

2.1e 05

1.9e105

1.6e105

1.3e105

1.1c+05

8.0c+04

5.30+04

2. le+04

206

400.0

360.0

320.0

280.0

240.0

200.0

160.0 I-

120 0

80.0

40

400 600 800 1002 1220 1400 160Û 1800 2000

Channel

1.7e+07

1.5e+07

1.3e+07

1.2e+07

O.Oe+06

8.3e+06

6.6e+06

S.Oe+06

3.3e+06

1.7e+06

20.0 40.0 60.0 80.0 100.0

Channel

120.0 140.0

400.0

- 360.0

- 320.0

- 280.0

-240.0

-200.0E

-160.0

-120.0

- 80.0

-40.0

160.0 180.0 200.0

Figure 7. The glow curve of pure silica flat fiber following 10 Gy electron irradiation at an energy of 21MeV. The deconvolved TL glow curve consists of five glow peaks.

Figure 8. The glow curve of TLD 100 following 10 Gy electron irradiation at an energy of 21MeV. The deconvolved TL glow curve consists of five glow peaks.

Peak I

Peak II

Peak III

Peak IV

Peak V

Peak V

Peak III

Peak IV Peak VI Peak VII



Table 3. Kinetic Parameters of Flat Optical Fiber

Flat optical fiber

Peak No. Ea (eV) Tmax (oC)

Peak I 0.7010 198.45

Peak II 1.1024 238.42

Peak III 1.8522 279.60

Peak IV 2.6049 309.81

Peak V 3.6294 334.04

Table 4. Kinetic Parameters of TLD 100

TLD 100

Peak No. Ea (eV) Tmax (oC)

Peak III 1.922 145.37

Peak IV 1.609 173.03

Peak V 1.494 206.47

Peak VI 2.114 256.68

Peak VII 1.132 281.70

3.3 Reproducibility

The reproducibility of the flat fibers was evaluated by randomly taking 5 samples of the same batch, and irradiating with the same dose of 4 Gy. After TL readout and annealing treatment, the procedure was repeated 5 times. The mean peak integral of peak I for the selected electron energies are shown in Table 5. The pure silica fibers show good reproducibility with a standard deviation of less than 4.0 %.

Table 5. Dose reproducibility of flat fibers under electron irradiation.

Energy Dose (Gy)

Peak Integral (μ C)

Mean SD(%)

Pure Silica Pure Silica

6 MeV 4.0 3.776 2.21

21 MeV 4.0 14.46 2.84

3.4 Dose Response (linearity)

The flat fibers were irradiated to electrons of nominal energy of 6-, 15- and 21 MeV, for doses in the range 2.0- to 10 Gy; the associated dose response curves are shown in Figure 9. The flat fiber displays a linear TL-absorbed dose response from 2.0 Gy up to 10 Gy. A linear TL dose response of silica fiber has also been reported by other workers for the same dose range used in this work1-11. The TL measurements of TLD 100 show a linear dose response up to 10 Gy as shown in Figure 10.



1.80

1.60

1.40

1.20

1.00

0.60

0.40

0.20

0.00

o 2 4 6

Oose{Gy}

8

6 MeV15 MeV

21MeV

10 12

12.00

10.00

8.00

6.00

4.00

2.00

0.00

o 2 4 6

Dose (GO

8 10 12

6 McV

BMW21 MeV

Figure 9. TL response of the flat optical fiber to electron irradiation at energies of 6 MeV, 15 MeV and 21 MeV.

Fig. 10: TL response of the TLD 100 to electron irradiation at energies of 6 MeV, 15 MeV and 21 MeV

3.5 Energy dependence and sensitivity

To obtain the energy dependence, the sensitivity of the fiber in TL yield/Gy was calculated from the dose response relation for the different electron energies. The electron energy of 21 MeV is observed to give the greatest TL sensitivity for both types of dosimeter, compared to 6- and 15 MeV electrons for the dose range 2.0 Gy to 10 Gy (Figure 11). TL sensitivity curves are shown in Figure 12.



12.00

10.00

8.00

6.00

4.00

2.00

0.00

0 2 4 6

noselavl

8

TLD 100

Flat Fiber

10 12

1.40 -

1.20 -

c

x 0.40

0.20 A

0.00

0 2 4 6 8 10 12

Doe 00111

TLD 100

I Flat fiber

Fig. 11: TL response of the TLD 100 and Flat Fiber to electron irradiation at the energy of 21 MeV. At a dose of 8 Gy the TLD 100 shows a 5 times higher yield of TL emission compared to that of flat fiber.

Figure 12: The sensitivity curve of the TLD 100 and Flat Fiber to electron irradiation at an energy of 21 MeV. TLD 100 was observed to show a 5.5 times more sensitive response to the irradiation compared to flat fiber.

4. Conclusion Kinetic parameters evaluation has been carried out using TL glow curve analysis of the flat optical fibers, the results indicating that the flat fibers can be used as radiation sensors for dosimetry. The general characteristics: linearity, energy response, reproducibility, point to favorable applications within the dose ranges studied.



Acknowledgements We are indebted to the Medical Physics Group of the Radiotherapy Unit of Mount Miriam Cancer Hospital in Penang, Malaysia for use of a LINAC. This work was supported by FRGS Research Grant EP20120521004 under the Ministry of Higher Education (MOHE), Malaysia. In addition, the authors would like to acknowledge the University of Malaya fiber pulling group and the MOHE HIR Grant A000007-50001, which supported use of the fiber pulling tower system.

REFERENCES

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[15] Mahamd Adikan, F. R., Sandoghchi, S. R., Chong, W. Y., Robert, E. S., Mahdi, M. A., Andrew, S. W., James, C. G., and Christopher, H., “Direct UV written optical waveguides in flexible glass flat fiber chips,” IEEE Journal of Selected Topics in Quantum Electronics. 18, 1534-1539 (2012).



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Thermoluminescence characteristics of flat optical fiber in radiation dosimetry under different electron irradiation conditions