- Email: [email protected]
Comparison of thermoluminescence response of different sized Ge-doped flat fibers as a dosimeter
Radiation Physics and Chemistry 116 (2015) 155–159
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Comparison of thermoluminescence response of different sized Ge-doped flat fibers as a dosimeter Mahfuza Begum a,b,n, A.K.M. Mizanur Rahman a,b, H.A. Abdul-Rashid a, Z. Yusoff a, K.A. Mat-Sharif a, M.I. Zulkifli a, S.Z. Muhamad-Yasin a, N.M. Ung c, A.B.A. Kadir d, Y.M. Amin e, D.A. Bradley f a
Faculty of Engineering, Multimedia University, Cyberjaya, Malaysia Bangladesh Atomic Energy Commission, Dhaka, Bangladesh c Clinical Oncology Unit, Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur, Malaysia d SSDL, Malaysia Nuclear Agency, Bangi, Kajang, Malaysia e Physics, Faculty of Science, University of Malaya, Malaysia f Department of Physics, University of Surrey, Guildford, Surrey, UK b
H I G H L I G H T S
The thermoluminescences (TL) of different sizes of Ge-doped flat fibers were investigated. Ge-doped flat fibers were fabricated using the Modified Chemical Vapor Deposition (MCVD) process. The TL responses of the flat fibers have been compared with standard TLD-100 and TLD-700 chips. Among the flat fiber samples, the smallest dimension flat fiber (60 180) mm2 provided the best response.
art ic l e i nf o
a b s t r a c t
Article history: Received 7 October 2014 Received in revised form 19 February 2015 Accepted 31 March 2015 Available online 2 April 2015
Prime dosimetric properties, including dose–response, linearity with dose, energy response, fading and threshold doses were investigated for three different dimension Ge-doped flat fibers. The results of measurement were also compared with two of the more commonly used standard TLD media, TLD-100 (LiF:Mg,Ti 7.5%6LiF) and TLD 700 (7LiF:Mg,Ti 99.9%7LiF) chips. The flat cross-section samples (60 180) mm2, (100 350) mm2 and (200 750) mm2 were fabricated using the Modified Chemical Vapor Deposition (MCVD) process and pulled from the same “preform.” In the study, all flat fiber samples provided good linear dose–response for the photon and electron beams generated using a medical linear accelerator (LINAC), for doses in the range 0.5–8 Gy. Among the samples, the smallest dimension flat fiber provided the best response, with a sensitivity of some 61% and 54%, respectively of that of the TLD-100 and TLD-700 chips. The energy responses of the samples were studied for various photon (6 MV, 10 MV) and electron (6 MeV, 9 MeV) beam energies. TL fading of around 20% was observed over a period of thirty (30) days. These favorable TL characteristics point towards promising development of Ge-doped flat fibers for use in radiotherapy dosimetry. & 2015 Elsevier Ltd. All rights reserved.
Key words: Thermoluminescence Photon beam Linear accelerator (LINAC) Ge-doped flat fiber Threshold dose
1. Introduction Thermoluminescence dosimeters (TLDs) are widely used for measurement of absorbed doses in radiotherapy dosimetry, the TL being due to the presence of defects in the material (McKinlay et al., 1981; Yusoff et al., 2005; Gedam, 2013). Several commercial
n Corresponding at: Faculty of Engineering, Multimedia University, Cyberjaya, Malaysia. Fax: þ60 3 83125445. E-mail address: [email protected] (M. Begum).
http://dx.doi.org/10.1016/j.radphyschem.2015.03.038 0969-806X/& 2015 Elsevier Ltd. All rights reserved.
TL dosimeters are available, performance being evaluated on the basis of a number of prime dependencies, including the glow curve, sensitivity, dose–response and fading. In medical field, the most commonly used TL dosimeter is TLD-100 (LiF:Mg,Ti 7.5%6LiF). These well-established dosimeters also have several acknowledged limitations (Izewska and Rajan, 2005; McKinlay et al., 1981; Yusoff et al., 2005). Recently several research groups have investigated dosimetric characteristics of commercial optical fibers to develop TL material in ionizing radiation dosimetry (Hashim et al., 2009; Espinosa
156
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et al., 2006, Yaakob et al., 2011, Abdulla et al., 2001; Yusoff et al., 2005; Issa et al., 2011). It has further been established that TL responses of irradiated optical fiber depends on both radiation parameters and fiber types (Abdullah et al., 2011). Controlled amounts of defects, either intrinsic or extrinsic dopants in silica optical fiber can significantly enhance the sensitivity to radiation (Yaakob et al., 2011). Different doped-silica optical fibers have been considered by groups seeking to develop TL material, including the use of Germanium (Ge), Aluminum (Al) and Oxygen (O2) dopants. Among these, Ge-doped optical fibers have been noted to provide highly favorable TL response (Issa et al., 2011). Percentage depth dose for 6 MV and 10 MV X-ray beam from medical linear accelerator were measured using Ge-doped optical fiber dosimeter that provided good agreement with the results from a PTW 30001 cylindrical ionization chamber and standard TLD-100 (Abdulla et al., 2002). This new candidate also used in a variety of medical applications including interface radiation applications (Abdul Rahman et al., 2011), brachytherapy dosimetry (Issa et al., 2011), IMRT verification (Noor et al., 2010). TL sensitivity of the fiber was also investigated using Synchrotron microbeam over a wide range of doses from 1 Gy to 10 kGy, linearity being observed for doses from 1 Gy to 2 kGy (Abdul Rahman et al., 2010). In all such studies Ge-doped optical fibers have displayed promising responses that would support comprehensive dosimetric application in radiotherapy. These TL materials have additional benefits over conventional TLDs, including high spatial resolution (sub mm) and a water impervious nature, both convenient properties in regard to medical radiation dosimetry (Bradley et al., 2012; Ong et al., 2009). In present work key dosimetric properties of purpose-fabricated three different sized Ge-doped flat fiber have been investigated including photon and electron beam response linearity, energy response (6 MV, 10 MV photon beam and 6 MeV, 9 MeV electron beam), fading over a period of one month post-irradiation and also threshold doses. Comparison is made with two commonly used standard TL dosimeters, TLD-100 (LiF:Mg,Ti 7.5%6LiF) and TLD-700 (7LiF:Mg,Ti 99.9%7LiF) in the form of chips.
Fig. 1. Some of the flat fiber samples studied herein.
2. Materials and methods 2.1. Sample preparation Three different sized doped flat fibers, formed of silica doped with 3.07 wt% Ge-dopant were fabricated. Use was made of the Modified Chemical Vapor Deposition (MCVD) method in order to produce a doped-silica preform, with the process conducted at the “Preform Fabrication Laboratory” in Multimedia University, Cyberjaya, Malaysia. Subsequent pulling of the preform into fibers was carried out using the fiber drawing tower located at the Department of Electrical Engineering, University of Malaya. These flat fiber samples have both a doped core and a predominant silica cladding material, initiated from a hollow silica tube (Dambul et al., 2011). During the fiber drawing phase, a vacuum is applied in order to collapse down the hollow fiber into a rectangular geometry (Adikan et al., 2012). A collection of Ge-doped flat fiber samples and an SEM (Scanning Electron Microscope) image of the cross-section of one of these are shown in Figs. 1 and 2, respectively. The dimension of the flat fiber samples are respectively (60 180) mm2, (100 350) mm2 and (200 750) mm2. Each of the flat fiber samples was produced by cutting the drawn fibers into approximately 5 mm lengths using an optical fiber cleaver (Cl-03 Max, ILSIN, Korea). The mass of each sample was determined to normalize each data point using an electronic balance (Mettler Toledo, Switzerland), the fibers subsequently being cleaned with acetone. To avoid surface contamination, vacuum tweezers were
Fig. 2. SEM image of the cross-section of one of the flat fiber samples.
used for handling TL materials. 2.2. Annealing process Before irradiation the TLDs first need to be annealed to remove any residual TL resulting from the various potential background sources of TL excitation (Izewska and Rajan, 2005), including exposure to ambient light. In present study, each of the flat fiber samples were oven annealed at a temperature of 400 °C for one hour (the oven was supplied by Lindberg/Blue M, USA). Following annealing, the samples were slowly cooled to room-temperature to avoid thermal stress. Similarly, the standard TLD-100 chips were annealed for one hour at 400 °C and subsequently for two hours at 100 °C (Hashim et al., 2010; Hossain et al., 2013), then slowly allowed to cool to room-temperature. 2.3. Sample irradiation To examine photon and electron response, the Ge-doped flat fiber samples were irradiated at various photon (6 MV, 10 MV) and electron (6 MeV, 9 MeV) beam energies, delivering doses from 0.5 Gy to 8 Gy. During irradiation, the TL materials were retained in five separate labeled gelatin capsules and placed in a solid-waterTM phantom at the position of dose maximum Dmax. The
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source to surface distance (SSD) was set at 100 cm and field size was selected to be (10 10) cm2. 2.4. TL measurements Under a nitrogen gas (N2) atmosphere, the TL responses of the samples were measured using a Harshaw model-4500 TLD Reader. During readout the following parameters were used: preheat temperature 160 °C for 10 s; readout temperature of 300 °C for 13 s; heating ramp rate 25 °C/s and annealing temperature 300 °C for 10 s. The TL responses of the samples were normalized with respect to individual mass. For each dose measurement ten (10) flat fiber samples were readout and averaged; outlying, high and low response fibers had previously been discarded in a screening process. During handling and readout the samples had also been protected from exposure to direct light.
3. Results and discussion 3.1. Dose response The Ge-doped flat fiber samples have all been observed to provide good linear dose–response for both photon and electron beam irradiation, as shown in Figs. 3 and 4 respectively. For comparison, the figures also show the TL response of the standard media, TLD-100 and TLD-700, in the form of chips. Among the flat fibers, the smallest dimension samples (60 180) mm2 showed the greatest TL yield. In particular, for photon beams these provided 3.8 times and 2.2 times the yield of that of the (100 350) mm2 and (200 750) mm2 flat fibers respectively, some 61% and 54% of that of the TLD-100 and TLD-700 chips. For electron beam irradiation, it was also apparent that the TL yield improved with reduction in cross section of the flat fibers. Here it can be noted that during flat fiber fabrication process vacuum pressure is applied at the furnace hot zone area in order to collapse down the circular shaped fibers into a flat shape (Adikan et al., 2012). This deformation produces stress on the circumference of the neck-down region of the preform causing additional structural defects (Abdul Sani et al., 2014). The diameter of the initial fiber from which the flat fiber is produced also reduces with increasing vacuum pressure (Dambul et al., 2011), as does the cladding thickness. As a result the inner surface contact length become progressively thin for larger sized flat fiber. Added to this is the relatively greater cladding thickness of the larger cross-sectional flat fibers, with commensurate attenuation of the luminescence that exits the fiber
Fig. 3. Comparison TL response of three different sizes of Ge-doped flat fiber and standard TLD-100 and TLD-700 chips for 6 MV photon beam irradiations. For each dose measurement 10 samples were readout using fibers that had previously been screened to eliminate outlier data as noted in the text. The uniform sensitivity samples produced uncertainties in TL yield that were of the same order as the size of the data points.
Fig. 4. Comparison TL response of three different sizes of Ge-doped flat fiber and standard TLD-100 and TLD-700 chips for 6 MeV electron beam irradiations.
towards the TL reader photomultiplier tube whose function it is to collect the TL light yield. These are the notable influences on the recorded TL yields, supporting an expected greater yield in the smallest dimension flat fibers.
3.2. Energy response As shown in Fig. 5, the flat fiber samples provided essentially identical TL responses for photon beams generated at accelerating potentials of 6 MV and 10 MV. Fig. 6 represents the same situation for the 6 MeV and 9 MeV electron beam energies. The result indicates one of the desirable dosimetric characteristics providing flat energy response over a wide range of the incident radiation.
3.3. Fading Fading, the random release of trapped electrons prior to readout, can occur due to thermally or optically stimulated release of electrons. As such, study of the effect is essential for accurate evaluation of absorbed radiation dose (IAEA Safety Standard Series, 1999). For the measurements, several samples were annealed and exposed to a dose of 4 Gy using a 6 MV photon beam. Postirradiation, the samples were kept inside a light-tight box at roomtemperature and measured at various intervals over a period of thirty days. For the flat fiber samples, fading of 19.5%, 19% and 18.6% were found for the (60 180) mm2, (100 350) mm2 and (200 750) mm2 samples respectively, as shown in Fig. 7, being insignificantly different from each other.
Fig. 5. Differential TL response of the Ge-doped flat fiber samples irradiated at photon energies generated at 6 MV and 10 MV.
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Fig. 6. Differential TL response of the Ge-doped flat fiber samples for 6 MeV and 9 MeV electron irradiations.
progressively thin for larger sized flat fibers, with commensurate reduced TL yield, to which can be added the further influence of the relatively greater cladding thickness of the larger cross-sectional flat fibers, commensurate with greater attenuation of the luminescence. These notable influences on the recorded TL yields, supporting an expected greater yield in the smallest dimension flat fibers. The samples also provided practically the same TL response for the 6 MV and 10 MV photon beams; similarly, no practical difference in response was observed as a result of electron beam irradiations at 6 MeV and 9 MeV. For photon beams, the flat fiber threshold dose was 4 mGy, while for electron beams a marginally lower threshold dose was observed. Over a period of thirty days post-irradiation, fading of the samples was found to be less than 20%. TL performance of the Ge-doped flat fibers show these to have considerable potential for dosimetric applications in radiotherapy.
Acknowledgment The project was carried out with support from Grants University of Malaya- Ministry of Higher Education of Malaysia UMMOHE High Impact Research Grant UM.C/625/1/HIR/33 and Multimedia University Malaysia Fundamental Research Grant Scheme FRGS/1/2012/SG02/MMU/02/2.
References Fig. 7. Fading characteristics of three different sizes flat fiber samples over thirty days post photon irradiation.
Table 1 Threshold dose for the three different sizes of Ge-doped flat fibers studied herein, for photon and electron beam irradiations. Flat fiber dimension (mm2)
60 180 100 350 200 750
Threshold dose (mGy) Photon
Electron
4.07 3.68 3.50
3.91 3.46 3.10
3.4. Threshold dose The threshold doses D0 for the three different sized Ge-doped flat fibers were calculated using the following equation (Furetta et al., 2001): D0 ¼(B þ 2 sB) F where B is the mean TL background signal of 15 flat fiber samples, measured from samples that were annealed but not irradiated, sB is the standard deviation of the average background and F is the calibration factor in Gy/TL. The measured values for the samples are detailed in Table 1.
4. Conclusion From the results it was found that over the dose range 0.5–8 Gy the Ge-doped flat fiber samples all provided a linear dose–response, for both photon and electron beams. The smallest size flat fiber (60 180) mm2 provided the greatest TL yield, at some 61% and 54% of that of standard TLD-100 and TLD-700 chips respectively. This can be traced back to the fact that the inner surface contact length (the Ge-doped component of the fiber) become
Abdulla, Y.A., Amin, Y.M., Khoo, H.B., 2002. Determination of percentage depth dose for 6 and 10 MV x-rays using Ge-dopped optical fiber and TLD-100. J. Radiol. Prot. 22, 417–421. Abdulla, Y.A., Amin, Y.M., Bradley, D.A., 2001. The thermoluminescence response of Ge-doped optical fibre subjected to photon irradiation. Radiat. Phys. Chem. 61, 409–410. Abdul Rahman, A.T., Bradley, D.A., Doran, S.J., Thierry, B., Brauer-Krisch, E., Bravin, A., 2010. The thermoluminescence response of Ge-doped silica fibers for synchrotron microbeam radiation therapy dosimetry. Nucl. Instrum. Methods Phys. Res. Sect. A: Accel. Spectrom. Detect. Assoc. Equip. 619, 167–170. Abdul Rahman, A.T., Hugtenburg, R.P., Abdul Sani, Siti Fairus, Alalawi, A.I.M., Issa, Fatma, Thomas, R., Barry, M.A., Nisbet, A., Bradley D.A., 2011. An investigation of the thermoluminescence of Ge-doped silica optical fibers for application in interface radiation dosimetry. In: 8th Industrial Radiation and Radioisotope Measurement Applications (IRRMA) Kansas City, USA. Sani, Abdul, Alalawi, Siti F., Hairul Azhar, AmaniI., Mahdiraji, A.R.,, Ghafour, Amouza, Tamchek, Nizam, Nisbet, A., Maah, M.J., Bradley, D.A., 2014. High sensitivity flat SiO2 fibers for medical dosimetry. Radiat. Phys. Chem. 104, 134–138. Adikan, F.R.M., Sandoghchi, S.R., Wu, Yi, C., Simpson, R.E., Mahdi, M.A., Webb, A.S., Gates, C.J., Holmes, C., 2012. Direct UV written optical waveguides in flexible glass flat fiber chips. IEEE J. Sel. Top. Quantum Electron. 18 (5), 1534–1538. Bradley, D.A., Nisbet, A., Abdul Rahman, A.T., Issa, F., Mohd Noor, N., Alalawi, A., Hugtenburg, R.P., 2012. Review of doped silica glass optical fiber: their properties and potential applications in radiation therapy dosimetry. Appl. Radiat. Isot. 71, 2–11. Dambul, K.D., Tamchek, T., Sandoghchi, S.R., Abu Hassan, M.R., Tee, D.C., Adikan, F.R. M., 2011. Fabrication and Characterization of Flat Fibers. IEEE 2nd International Conference on Photonics (ICP). Espinosa, G., Golzarri, J.I., Bogard, J., Garcia-Macedo, J., 2006. Commercial optical fiber as TLD material. Radiat. Prot. Dosim. 119, 197–200. Furetta, C., Prokic, M., Salamon, R., Prokic, V., Kitis, G., 2001. Dosimetric characteristics of tissue equivalent thermoluminescent solid TL detectors based on lithium borate. Nucl. Instrum. Methods Phys. Res. Sect. A 456, 411–417. Gedam, S.C., 2013. Thermoluminescence (TL) study of CeSO4Cl: Dy phosphor for γradiation dosimetr. Res. J. Eng. Sci. 2 (3), 28–31. Hashim, S., Al-Ahbabi, S., Bradley, D.A., Webb, M., Jeynes, C., Ramli, A.T., Wagiran, H., 2009. The thermoluminescence response of doped SiO2 optical fibres subjected to photon and electron irradiations. Appl. Radiat. Isot. 67, 423–427. Hashim, S., Bradley, D.A., Saripan, M.I., Ramli, A.T., Wagiran, H., 2010. The thermoluminescence response of doped SiO2 optical fibres subjected to fast neutrns. Appl. Radiat. Isot. 68, 700–703. Hossain, I., Wagiran, H., Yaakob, N.H., 2013. Thermoluminescence of Ge- and Aldoped SiO2 optical fibers subjected to 0.2-4.0 Gy external photon radiotherapeutic dose. J. Appl. Spectrosc. 80 (4), 635–638. IAEA Safety Standards series, 1999. RS-G-1.3, Assessment of Occupational Exposure Due to External Sources of Radiation. International Atomic Energy Agency and International Labour office, Vienna.
M. Begum et al. / Radiation Physics and Chemistry 116 (2015) 155–159 Issa, F., Latip, N.A.A., Bradley, D.A., Nisbet, A., 2011. Ge-doped optical fibres as thermoluminescence dosimeters for kilo voltage X-ray therapy irradiations. Nucl. Instrum. Methods Phys. Res. Sect. A 652 (1), 834–837. Izewska, J., Rajan, G., 2005. Radiation dosimeters In: Podgorsak, E.B. (Ed.), Radiation Oncology Physics: A Handbook for Teachers and Students.. International Atomic Energy Agency (IAEA), Vienna Ong, C.L., Kandaiya, S., Kho, H.T., Chong, M.T., 2009. Segments of a commercial Gedoped optical fiber as a thermoluminescent dosimeter in radiotherapy. Radiat. Meas. 44, 158–162. McKinlay, A.F., 1981. Thermoluminescence Dosimetry (Medical Physics Handbooks, 5 ISSN 0143-0203). Adam Hilger Ltd.
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Noor, N.M., Hussein, M., Bradley, D.A., Nisbet, A., 2010. The potential of Ge-doped optical fibre TL dosimetry for 3D verification of high energy IMRT photon beams. Nucl. Instrum. Methods Phys. Res. Sect. A 619, 157–162. Yaakob, Nor A., Wagiran, H., Hossain, M.I., Ramli, A.T., 2011. Low-dose photon irradiation response of Ge and Al-doped SiO2 optical fibres. Appl. Radiat. Isot. 69, 1189–1192. Yusoff, A.L., Hugtenburg, R.P., Bradley, D.A., 2005. Review of development of a silicabased thermoluminescence dosimeter. Radiat. Phys. Chem. 74, 459–481.
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Comparison of thermoluminescence response of different sized Ge-doped flat fibers as a dosimeter Mahfuza Begum a,b,n, A.K.M. Mizanur Rahman a,b, H.A. Abdul-Rashid a, Z. Yusoff a, K.A. Mat-Sharif a, M.I. Zulkifli a, S.Z. Muhamad-Yasin a, N.M. Ung c, A.B.A. Kadir d, Y.M. Amin e, D.A. Bradley f a
Faculty of Engineering, Multimedia University, Cyberjaya, Malaysia Bangladesh Atomic Energy Commission, Dhaka, Bangladesh c Clinical Oncology Unit, Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur, Malaysia d SSDL, Malaysia Nuclear Agency, Bangi, Kajang, Malaysia e Physics, Faculty of Science, University of Malaya, Malaysia f Department of Physics, University of Surrey, Guildford, Surrey, UK b
H I G H L I G H T S
The thermoluminescences (TL) of different sizes of Ge-doped flat fibers were investigated. Ge-doped flat fibers were fabricated using the Modified Chemical Vapor Deposition (MCVD) process. The TL responses of the flat fibers have been compared with standard TLD-100 and TLD-700 chips. Among the flat fiber samples, the smallest dimension flat fiber (60 180) mm2 provided the best response.
art ic l e i nf o
a b s t r a c t
Article history: Received 7 October 2014 Received in revised form 19 February 2015 Accepted 31 March 2015 Available online 2 April 2015
Prime dosimetric properties, including dose–response, linearity with dose, energy response, fading and threshold doses were investigated for three different dimension Ge-doped flat fibers. The results of measurement were also compared with two of the more commonly used standard TLD media, TLD-100 (LiF:Mg,Ti 7.5%6LiF) and TLD 700 (7LiF:Mg,Ti 99.9%7LiF) chips. The flat cross-section samples (60 180) mm2, (100 350) mm2 and (200 750) mm2 were fabricated using the Modified Chemical Vapor Deposition (MCVD) process and pulled from the same “preform.” In the study, all flat fiber samples provided good linear dose–response for the photon and electron beams generated using a medical linear accelerator (LINAC), for doses in the range 0.5–8 Gy. Among the samples, the smallest dimension flat fiber provided the best response, with a sensitivity of some 61% and 54%, respectively of that of the TLD-100 and TLD-700 chips. The energy responses of the samples were studied for various photon (6 MV, 10 MV) and electron (6 MeV, 9 MeV) beam energies. TL fading of around 20% was observed over a period of thirty (30) days. These favorable TL characteristics point towards promising development of Ge-doped flat fibers for use in radiotherapy dosimetry. & 2015 Elsevier Ltd. All rights reserved.
Key words: Thermoluminescence Photon beam Linear accelerator (LINAC) Ge-doped flat fiber Threshold dose
1. Introduction Thermoluminescence dosimeters (TLDs) are widely used for measurement of absorbed doses in radiotherapy dosimetry, the TL being due to the presence of defects in the material (McKinlay et al., 1981; Yusoff et al., 2005; Gedam, 2013). Several commercial
n Corresponding at: Faculty of Engineering, Multimedia University, Cyberjaya, Malaysia. Fax: þ60 3 83125445. E-mail address: [email protected] (M. Begum).
http://dx.doi.org/10.1016/j.radphyschem.2015.03.038 0969-806X/& 2015 Elsevier Ltd. All rights reserved.
TL dosimeters are available, performance being evaluated on the basis of a number of prime dependencies, including the glow curve, sensitivity, dose–response and fading. In medical field, the most commonly used TL dosimeter is TLD-100 (LiF:Mg,Ti 7.5%6LiF). These well-established dosimeters also have several acknowledged limitations (Izewska and Rajan, 2005; McKinlay et al., 1981; Yusoff et al., 2005). Recently several research groups have investigated dosimetric characteristics of commercial optical fibers to develop TL material in ionizing radiation dosimetry (Hashim et al., 2009; Espinosa
156
M. Begum et al. / Radiation Physics and Chemistry 116 (2015) 155–159
et al., 2006, Yaakob et al., 2011, Abdulla et al., 2001; Yusoff et al., 2005; Issa et al., 2011). It has further been established that TL responses of irradiated optical fiber depends on both radiation parameters and fiber types (Abdullah et al., 2011). Controlled amounts of defects, either intrinsic or extrinsic dopants in silica optical fiber can significantly enhance the sensitivity to radiation (Yaakob et al., 2011). Different doped-silica optical fibers have been considered by groups seeking to develop TL material, including the use of Germanium (Ge), Aluminum (Al) and Oxygen (O2) dopants. Among these, Ge-doped optical fibers have been noted to provide highly favorable TL response (Issa et al., 2011). Percentage depth dose for 6 MV and 10 MV X-ray beam from medical linear accelerator were measured using Ge-doped optical fiber dosimeter that provided good agreement with the results from a PTW 30001 cylindrical ionization chamber and standard TLD-100 (Abdulla et al., 2002). This new candidate also used in a variety of medical applications including interface radiation applications (Abdul Rahman et al., 2011), brachytherapy dosimetry (Issa et al., 2011), IMRT verification (Noor et al., 2010). TL sensitivity of the fiber was also investigated using Synchrotron microbeam over a wide range of doses from 1 Gy to 10 kGy, linearity being observed for doses from 1 Gy to 2 kGy (Abdul Rahman et al., 2010). In all such studies Ge-doped optical fibers have displayed promising responses that would support comprehensive dosimetric application in radiotherapy. These TL materials have additional benefits over conventional TLDs, including high spatial resolution (sub mm) and a water impervious nature, both convenient properties in regard to medical radiation dosimetry (Bradley et al., 2012; Ong et al., 2009). In present work key dosimetric properties of purpose-fabricated three different sized Ge-doped flat fiber have been investigated including photon and electron beam response linearity, energy response (6 MV, 10 MV photon beam and 6 MeV, 9 MeV electron beam), fading over a period of one month post-irradiation and also threshold doses. Comparison is made with two commonly used standard TL dosimeters, TLD-100 (LiF:Mg,Ti 7.5%6LiF) and TLD-700 (7LiF:Mg,Ti 99.9%7LiF) in the form of chips.
Fig. 1. Some of the flat fiber samples studied herein.
2. Materials and methods 2.1. Sample preparation Three different sized doped flat fibers, formed of silica doped with 3.07 wt% Ge-dopant were fabricated. Use was made of the Modified Chemical Vapor Deposition (MCVD) method in order to produce a doped-silica preform, with the process conducted at the “Preform Fabrication Laboratory” in Multimedia University, Cyberjaya, Malaysia. Subsequent pulling of the preform into fibers was carried out using the fiber drawing tower located at the Department of Electrical Engineering, University of Malaya. These flat fiber samples have both a doped core and a predominant silica cladding material, initiated from a hollow silica tube (Dambul et al., 2011). During the fiber drawing phase, a vacuum is applied in order to collapse down the hollow fiber into a rectangular geometry (Adikan et al., 2012). A collection of Ge-doped flat fiber samples and an SEM (Scanning Electron Microscope) image of the cross-section of one of these are shown in Figs. 1 and 2, respectively. The dimension of the flat fiber samples are respectively (60 180) mm2, (100 350) mm2 and (200 750) mm2. Each of the flat fiber samples was produced by cutting the drawn fibers into approximately 5 mm lengths using an optical fiber cleaver (Cl-03 Max, ILSIN, Korea). The mass of each sample was determined to normalize each data point using an electronic balance (Mettler Toledo, Switzerland), the fibers subsequently being cleaned with acetone. To avoid surface contamination, vacuum tweezers were
Fig. 2. SEM image of the cross-section of one of the flat fiber samples.
used for handling TL materials. 2.2. Annealing process Before irradiation the TLDs first need to be annealed to remove any residual TL resulting from the various potential background sources of TL excitation (Izewska and Rajan, 2005), including exposure to ambient light. In present study, each of the flat fiber samples were oven annealed at a temperature of 400 °C for one hour (the oven was supplied by Lindberg/Blue M, USA). Following annealing, the samples were slowly cooled to room-temperature to avoid thermal stress. Similarly, the standard TLD-100 chips were annealed for one hour at 400 °C and subsequently for two hours at 100 °C (Hashim et al., 2010; Hossain et al., 2013), then slowly allowed to cool to room-temperature. 2.3. Sample irradiation To examine photon and electron response, the Ge-doped flat fiber samples were irradiated at various photon (6 MV, 10 MV) and electron (6 MeV, 9 MeV) beam energies, delivering doses from 0.5 Gy to 8 Gy. During irradiation, the TL materials were retained in five separate labeled gelatin capsules and placed in a solid-waterTM phantom at the position of dose maximum Dmax. The
M. Begum et al. / Radiation Physics and Chemistry 116 (2015) 155–159
157
source to surface distance (SSD) was set at 100 cm and field size was selected to be (10 10) cm2. 2.4. TL measurements Under a nitrogen gas (N2) atmosphere, the TL responses of the samples were measured using a Harshaw model-4500 TLD Reader. During readout the following parameters were used: preheat temperature 160 °C for 10 s; readout temperature of 300 °C for 13 s; heating ramp rate 25 °C/s and annealing temperature 300 °C for 10 s. The TL responses of the samples were normalized with respect to individual mass. For each dose measurement ten (10) flat fiber samples were readout and averaged; outlying, high and low response fibers had previously been discarded in a screening process. During handling and readout the samples had also been protected from exposure to direct light.
3. Results and discussion 3.1. Dose response The Ge-doped flat fiber samples have all been observed to provide good linear dose–response for both photon and electron beam irradiation, as shown in Figs. 3 and 4 respectively. For comparison, the figures also show the TL response of the standard media, TLD-100 and TLD-700, in the form of chips. Among the flat fibers, the smallest dimension samples (60 180) mm2 showed the greatest TL yield. In particular, for photon beams these provided 3.8 times and 2.2 times the yield of that of the (100 350) mm2 and (200 750) mm2 flat fibers respectively, some 61% and 54% of that of the TLD-100 and TLD-700 chips. For electron beam irradiation, it was also apparent that the TL yield improved with reduction in cross section of the flat fibers. Here it can be noted that during flat fiber fabrication process vacuum pressure is applied at the furnace hot zone area in order to collapse down the circular shaped fibers into a flat shape (Adikan et al., 2012). This deformation produces stress on the circumference of the neck-down region of the preform causing additional structural defects (Abdul Sani et al., 2014). The diameter of the initial fiber from which the flat fiber is produced also reduces with increasing vacuum pressure (Dambul et al., 2011), as does the cladding thickness. As a result the inner surface contact length become progressively thin for larger sized flat fiber. Added to this is the relatively greater cladding thickness of the larger cross-sectional flat fibers, with commensurate attenuation of the luminescence that exits the fiber
Fig. 3. Comparison TL response of three different sizes of Ge-doped flat fiber and standard TLD-100 and TLD-700 chips for 6 MV photon beam irradiations. For each dose measurement 10 samples were readout using fibers that had previously been screened to eliminate outlier data as noted in the text. The uniform sensitivity samples produced uncertainties in TL yield that were of the same order as the size of the data points.
Fig. 4. Comparison TL response of three different sizes of Ge-doped flat fiber and standard TLD-100 and TLD-700 chips for 6 MeV electron beam irradiations.
towards the TL reader photomultiplier tube whose function it is to collect the TL light yield. These are the notable influences on the recorded TL yields, supporting an expected greater yield in the smallest dimension flat fibers.
3.2. Energy response As shown in Fig. 5, the flat fiber samples provided essentially identical TL responses for photon beams generated at accelerating potentials of 6 MV and 10 MV. Fig. 6 represents the same situation for the 6 MeV and 9 MeV electron beam energies. The result indicates one of the desirable dosimetric characteristics providing flat energy response over a wide range of the incident radiation.
3.3. Fading Fading, the random release of trapped electrons prior to readout, can occur due to thermally or optically stimulated release of electrons. As such, study of the effect is essential for accurate evaluation of absorbed radiation dose (IAEA Safety Standard Series, 1999). For the measurements, several samples were annealed and exposed to a dose of 4 Gy using a 6 MV photon beam. Postirradiation, the samples were kept inside a light-tight box at roomtemperature and measured at various intervals over a period of thirty days. For the flat fiber samples, fading of 19.5%, 19% and 18.6% were found for the (60 180) mm2, (100 350) mm2 and (200 750) mm2 samples respectively, as shown in Fig. 7, being insignificantly different from each other.
Fig. 5. Differential TL response of the Ge-doped flat fiber samples irradiated at photon energies generated at 6 MV and 10 MV.
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Fig. 6. Differential TL response of the Ge-doped flat fiber samples for 6 MeV and 9 MeV electron irradiations.
progressively thin for larger sized flat fibers, with commensurate reduced TL yield, to which can be added the further influence of the relatively greater cladding thickness of the larger cross-sectional flat fibers, commensurate with greater attenuation of the luminescence. These notable influences on the recorded TL yields, supporting an expected greater yield in the smallest dimension flat fibers. The samples also provided practically the same TL response for the 6 MV and 10 MV photon beams; similarly, no practical difference in response was observed as a result of electron beam irradiations at 6 MeV and 9 MeV. For photon beams, the flat fiber threshold dose was 4 mGy, while for electron beams a marginally lower threshold dose was observed. Over a period of thirty days post-irradiation, fading of the samples was found to be less than 20%. TL performance of the Ge-doped flat fibers show these to have considerable potential for dosimetric applications in radiotherapy.
Acknowledgment The project was carried out with support from Grants University of Malaya- Ministry of Higher Education of Malaysia UMMOHE High Impact Research Grant UM.C/625/1/HIR/33 and Multimedia University Malaysia Fundamental Research Grant Scheme FRGS/1/2012/SG02/MMU/02/2.
References Fig. 7. Fading characteristics of three different sizes flat fiber samples over thirty days post photon irradiation.
Table 1 Threshold dose for the three different sizes of Ge-doped flat fibers studied herein, for photon and electron beam irradiations. Flat fiber dimension (mm2)
60 180 100 350 200 750
Threshold dose (mGy) Photon
Electron
4.07 3.68 3.50
3.91 3.46 3.10
3.4. Threshold dose The threshold doses D0 for the three different sized Ge-doped flat fibers were calculated using the following equation (Furetta et al., 2001): D0 ¼(B þ 2 sB) F where B is the mean TL background signal of 15 flat fiber samples, measured from samples that were annealed but not irradiated, sB is the standard deviation of the average background and F is the calibration factor in Gy/TL. The measured values for the samples are detailed in Table 1.
4. Conclusion From the results it was found that over the dose range 0.5–8 Gy the Ge-doped flat fiber samples all provided a linear dose–response, for both photon and electron beams. The smallest size flat fiber (60 180) mm2 provided the greatest TL yield, at some 61% and 54% of that of standard TLD-100 and TLD-700 chips respectively. This can be traced back to the fact that the inner surface contact length (the Ge-doped component of the fiber) become
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