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Ge-doped silica fibre for proton beam dosimetry
Radiation Physics and Chemistry 165 (2019) 108390
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Ge-doped silica fibre for proton beam dosimetry a
b
c
d
e
f,g
M.F. Hassan , W.N.W.A. Rahman , T. Tominaga , M. Geso , H. Akasaka , D.A. Bradley , N.M. Noora,*
T
a
Department of Imaging, Faculty of Medicine and Health Sciences, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Medical Radiation Programme, School of Health Sciences, Universiti Sains Malaysia, 16150 Kubang Kerian, Kelantan, Malaysia Department of Clinical Radiology, Faculty of Health Sciences, Hiroshima International University, Hiroshima 739-2695, Japan d Discipline of Medical Radiations, School of Medical Sciences, Royal Melbourne Institute of Technology University, Bundoora, Victoria 3083, Australia e Division of Radiation Oncology, Kobe University Hospital, Hyogo 650-0017, Japan f Department of Physics, University of Surrey, Guildford, GU2 7XH, United Kingdom g Centre for Biomedical Physics, School of Healthcare and Medical Sciences, Sunway University, 46150 Petaling Jaya, Selangor, Malaysia b c
ARTICLE INFO
ABSTRACT
Keywords: Fabricated optical fibre Linearity index Minimum detectable dose Reproducibility Signal fading Proton beam
An investigation has been made of nominal 2.3 mol% and 6.0 mol% germanium (Ge) doped cylindrical optical fibres as novel radiation dosimeters for 150-MeV proton beam measurements. These optical fibres were locally fabricated using a modified chemical vapour deposition technique with a subsequent pulling process. Combined scanning electron microscope and energy dispersive X-ray spectroscopy analyses were performed to map the relative presence of the germanium and other elements in the optical fibres. Prior to irradiation, a thermal annealing process was carried out to erase any pre-irradiation signals potentially existing in the samples. Results were compared against nanoDot™, TLD-100, and commercial optical fibres to allow for a relative comparison of the response. For radiation dose in the range 1 up to 10 Gy, the fabricated optical fibres exhibit excellent radiation dose response (R2 > 0.99), with a linearity index that remains close to one (indicating a linear response). In terms of minimum detectable dose, these optical fibres are able to detect relatively low radiation dose (for the present batch of fibres down to 10.7 mGy). After repeated various irradiation campaigns, the fabricated optical fibres have been shown to provide consistent response, effectively without noticeable change in thermoluminescence (TL) yield (ANOVA, p > 0.05), suggesting excellent reproducibility. In regard to signal fading, 96 days post-irradiation the fabricated optical fibres showed minimal signal loss, at 19% at the most. These dosimetric characteristics confirm the potential of the fabricated optical fibres as TL dosimeters, specifically for present studies in conducting proton beam measurements.
1. Introduction Silica-based optical fibres have shown promise as thermoluminescence dosimetry (TLD), in particular for radiotherapy applications. It has been established at modest cost that optical fibres provide excellent spatial resolution, retaining excellent response to ionising radiation, are of a non-hygroscopic nature and offer a good degree of flexibility. One of the earliest investigation was that of Abdulla et al., 2001a, utilising commercially available germanium (Ge) doped optical fibres. Thus said, commercial optical fibres can display ambiguous behaviour, potentially leading to sub-optimal TL response that may influence the overall dosimetric sensitivity (Bradley et al., 2012; Entezam et al., 2016; Mahdiraji et al., 2015). Among the possibilites leading to this are non-uniform distribution of dopant concentration along the
*
length of the commercial optical fibres. Another reason lies in their fabrication process for telecommunication fibres, intended to provide optimum communication performance, not radiation dosimetry. In recent years, researchers have made efforts to design and fabricate doped silica fibres offering optimum performance for TL dosimetry (TLD). Such tailor-made fibres, customised via a modified chemical vapour deposition technique following a particular fabrication ‘recipe’ detailed in previous publications of this group and others, have been performance-tested based on dopant concentration (Begum et al., 2017; Fadzil et al., 2017; Noor et al., 2016a), dopant uniformity (Entezam et al., 2016), geometry (i.e. flat and cylindrical shapes) (Ghomeishi et al., 2015) and dimension (Begum et al., 2015; Nawi et al., 2015). It is known that undoped optical fibre (i.e. in the absence of doping elements) will give limited TL yield upon exposure to radiation.
Corresponding author. E-mail address: [email protected] (N.M. Noor).
https://doi.org/10.1016/j.radphyschem.2019.108390 Received 21 February 2019; Received in revised form 24 June 2019; Accepted 30 June 2019 Available online 03 July 2019 0969-806X/ © 2019 Elsevier Ltd. All rights reserved.
Radiation Physics and Chemistry 165 (2019) 108390
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for external beam radiation have been extensively studied, their applications in measuring proton beams have not been widely investigated. Today, the use of proton beams in radiation therapy treatment have attracted considerable and growing attention. According to the Particle Therapy Co-Operative Group (PTCOG), as of April 2019, there are 81 proton therapy facilities in operation worldwide; the number is forecast to increase from the current figure to 142 by 2025 including those under construction or in planning stage (PTCOG, 2019). This rapid growth in the number of proton beam therapy centres is mainly due to the physical characteristics of the proton beam Bragg curve, which offers distinct dosimetric advantages over conventional radiotherapy (i.e. photon and electron beam therapy). Despite these advantages, the main drawback in proton dose measurement is the under response of radiation dosimeters when the ionization density of the proton beam increases, particularly in the Bragg peak region, thus potentially leading to underestimation of the actual radiation dose (Rah et al., 2012). For example, Bradley et al. (2016) found that the commercial Ge-doped optical fibres showed a quenching effect in the distal fall-off region of the proton Bragg peak; the response was compared with the measurements obtained from the ionization chamber dosimeter (gold standard). This issue also has been reported for other types of dosimeter such as film, polyacrylamide gels, Fricke gels, PRESAGE and alanine (Baldock et al., 2010; Doran et al., 2015; Gustavsson et al., 2004). It is therefore with these restrictions in mind that the present study seeks to evaluate the performance of the fabricated Ge-doped fibre as a potential candidate for TLD, particularly in measuring proton beams. These locally fabricated Ge-doped fibres are studied in terms of their key dosimetric characteristics, including radiation dose response, linearity index, minimum detectable dose, reproducibility and signal fading effect.
Fig. 1. Fabrication process of the Ge-doped preform using MCVD technique.
Conversely, due to the presence of dopants, irradiated optical fibres have been shown to give rise to appreciable TL yield. To-date, optical fibres doped with germanium (the dopant) are found to exhibit superior sensitivity to radiation as compared to optical fibres doped with other elements such as aluminium (Ramli et al., 2009; Yaakob et al., 2011; Wagiran et al., 2012), neodymium (Refaei et al., 2014; Saeed et al., 2013), erbium (Abdulla et al., 2001b), ytterbium (Sahini et al., 2014), thulium (Alawiah et al., 2015), oxygen (Hashim et al., 2010a) and phosphorus (Girard et al., 2011), to name a few. The performance of Ge-doped fibres, either commercially available or locally fabricated, have been widely studied in radiation detection contexts, including via irradiation to protons (Hassan et al., 2017, 2018), diagnostic X-rays (Ramli et al., 2015), photons (Issa et al., 2011), alpha particles (Ramli et al., 2009), electrons (Rahman et al., 2010a), synchrotron radiation (Rahman et al., 2010b) and fast neutrons (Hashim et al., 2010b). Their performance has also been investigated in various radiation-based disciplines, such as in environmental monitoring (Bradley et al., 2012), quality dose audits (Fadzil et al., 2014; Noor et al., 2017), intensity modulated radiation therapy (Noor et al., 2010), the food irradiation industry (Noor et al., 2016b), small field stereotactic radiosurgery (Lam et al., 2018) and positron emission tomography-computed tomography myocardial perfusion examination (Salasiah et al., 2013). In all such studies, the fibres have yielded promising responses. Although the performance of the fabricated Ge-doped optical fibres
2. Materials and methods 2.1. Fabrication of Ge-doped optical fibre The Ge-doped fibres employed herein result from fabrication using a standard modified chemical vapour deposition (MCVD) technique (carried out at Multimedia University, MMU, Malaysia). The process involved several key stages, as in Fig. 1.
Table 1 Details of research samples. Sample a
Fabricated 2.3 mol% Ge-CF Fabricated 6.0 mol% Ge-CFa Commercial 4.0 mol% Ge-CF nanoDot™ (with casing) TLD-100 a
Outer dimension
Core dimension
Length
Manufacturer
481 μm (diameter) 486 μm (diameter) 125 μm (diameter) 10 × 10 × 2 mm3 3.2 × 3.2 × 0.89 mm3
124 μm (diameter) 78 μm (diameter) 50 μm (diameter) – –
6 mm 6 mm 6 mm – –
MMU-UM consortium (Malaysia) MMU-UM consortium (Malaysia) CorActive High-Tech Inc. (Canada) Landauer Inc. (USA) Thermo Fisher Scientific Inc. (USA)
Dopant concentrations are based on values in the preforms.
Table 2 Summary of parameters used for samples irradiation. Parameters
Proton
Gammaa
Photona
Electrona
Energy Radiation dose Dose rate Source-to-surface distance Beam field size Build-up thickness
150 MeV 1–10 Gy 4 Gy/min 100 cm 10 × 10 cm2 From 11 cm to 14 cm (for 3 cm SOBP width) 10 cm Synchrotron (Mitsubishi Electric)
1.25 MeV 5 Gy 0.04 Gy/min 100 cm 10 × 10 cm2 –
6 MV, 10 MV 5 Gy 600 cGy/min 100 cm 10 × 10 cm2 1.5 cm bolus for 6 MV, 2.5 cm bolus for 10 MV 10 cm Varian 2100C linear accelerator
6 MeV 5 Gy 600 cGy/min 100 cm 10 × 10 cm2 (applicator) 2.0 cm (bolus)
Backscatter thickness Model of the accelerator/irradiator a
10 cm ELDORADO 8 with cobalt-60 source
Additional irradiation beams used for reproducibility study. 2
10 cm Varian 2100C linear accelerator
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desired cross-sectional dimensions, producing conventionally shaped cylindrical fibre (hereinafter referred to as ‘Ge-CF’). The detailed procedures of the pulling process (carried out at the University of Malaya, UM) have been elaborated in Noor et al., 2016a, 2016b. Table 1 describes the details of the research samples employed in this study.
Table 3 Time-temperature profile used for samples readout. Sample
Phase
Temperature (°C)
Time (sec)
Ramp-rate (°C/sec)
Fibres
Preheat Acquisition Annealing Preheat Acquisition Annealing
80 400 400 50 260 260
10.0 13.3 10.0 0.0 26.7 0.0
– 30.1 – – 9.8 –
TLD-100
2.2. Samples preparation The outer plastic coating layer of the commercial Ge-CF (manufactured by CorActive High-Tech Inc., Canada) was carefully stripped away, the remaining fibre being cleaned using isopropyl alcohol solution to remove any resin residues. All fibres, including those locally fabricated, were cut into a nominal length of 6.0 ( ± 1.0) mm to accommodate the fibres within the dimension of the planchet (the heating plate in the TLD reader) during the readout process. For ease of handling and storage, samples were prepared in groups of a minimum of 10 according to their types and retained in thin-walled gelatine capsules. Prior to irradiation, thermal annealing was carried out in order to erase any pre-irradiation signals potentially existing in the samples. 2.3. Irradiation
Fig. 2. Horizontal orientation of the fibres with respect to the PMT and planchet during the readout process.
Table 2 summarises the parameters used for proton and other irradiations. For reproducibility, the same group of samples were studied for various irradiation conditions with subsequent annealing after each occasion. A water-equivalent phantom was used to serve as the build-up medium and to provide maximum absorbed radiation dose (dmax). All samples were placed horizontally at dmax, with the irradiation beams delivered perpendicularly (zero beam angulation) to the samples.
Utilising the MCVD technique outlined in Fig. 1, two types of collapsed-Ge-doped preform rods were produced with respect to dopant concentration (i.e. 2.3 mol% and 6.0 mol% of Ge, nominal preform values). Both preform rods were then pulled (drawn-down) to the final
Fig. 3. Fig. 3a and b are the SEM images for the 2.3 mol% and 6.0 mol% Ge-CF, respectively. Fig. 3c and d are the respective EDXS spectra (the inset images are zoomed-in views of the fibre core used for elemental mapping). 3
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9.89 keV; the characteristic 1.74 keV Kα1 line of Si and the 0.53 keV Kα1 line of O2 are also apparent. 3.2. Radiation dose response Radiation dose response is of particular interest in seeking a good dosimeter system, ideally with the response being linear over the full range of radiation doses, thus allowing for reliable calibration and utilisation. In Fig. 4, it is observed that all the fabricated Ge-CF offer excellent linear relationships throughout the investigated radiation dose range (1–10 Gy), with coefficient of determination (R2) values of better than 0.99. A similar behaviour was also observed for the TLD-100 dosimeter and commercial Ge-CF (R2 > 0.99), albeit with the TLD-100 providing much lower sensitivity. From Fig. 4, one can observe that optical fibres doped with higher germanium concentration at 6.0 mol% gave the greatest response compared to those that doped with lower concentration at 2.3 mol%. This is mainly due to the increase density of trapping/impurities centres in the optical fibre when the concentration level of germanium increases.
Fig. 4. Response of the investigated samples irradiated with a 150 MeV proton beam, for doses from 1 to 10 Gy.
2.4. Signal acquisition (readout) Prior to readout, the irradiated optical fibres and TLD-100 were kept in a light-tight box for about 72 h to allow uniform control of thermal fading. A Harshaw™ 3500 TLD reader (Thermo Scientific™, USA) was used to acquire TL signals from the samples. The TL signals were acquired based on the time-temperature profile as detailed in Table 3. Throughout the readout process, nitrogen gas atmosphere of 0.5 bar was used to prevent contributions of TL signal from the chemiluminescence phenomena. The gross TL yield was normalised per unit volume to mitigate against uncertainty in TL yield (the volume was made to relate to the diameter of the fibre core, a region doped with Ge). The fibres have been read-out with their long dimension in contact with the planchet (the heating plate of the TL reader), not least allowing good thermal contact (Fig. 2). Each data point presented in this study was obtained by taking a mean of at least ten measurements/readings.
3.3. Linearity index The linearity index, denoted as f(D), is defined as the deviation of TL response from linearity. The f(D) shown in Fig. 5 has been obtained over the dose range 1–10 Gy, use being made of Eq. (1) (Chen and McKeever, 1994)
f(D) =
S(D) SO D
/
S(D1) SO D1
(1)
where S(D) is the measured TL signal obtained from the maximum intensity, as a function of the dose D, whereas S(D1) is the measured TL signal at low dose (D1) which is somewhere within the linear region of f (D) and So is the intercept on the TL intensity axis from the extrapolation of the linear region in the curves (McKeever et al., 1995) obtained from Fig. 4. From Eq. (1), f(D) = 1 indicates linear, f(D) > 1 supralinear and f(D) < 1 sublinear responses. In Fig. 5, the 2.3 mol% Ge-CF shows minimal nonlinearity, typically < 2%, with a maximum deviation from f(D) = 1 of 9% at 1 Gy. The TL response of the 6.0 mol% Ge-CF again shows minimal nonlinearity deviation from f(D) = 1, with a maximum deviation of 4% at 7 Gy. Results from this study are consistent with previous reports, the Ge-doped fibres trending towards a sublinear response in the low radiation dose region (in particular, below 2 Gy) (Begum et al., 2017; Noor et al., 2014; (Noor et al., 2016a, b). The commercial Ge-CF and TLD-100 show near linear fits for radiation doses ranging from 2 to 10 Gy with at most 3% deviation from f(D) = 1. Also observed is a prominent sublinear response for the TLD-100 particularly at 1 Gy, with a 12% deviation from f(D) = 1.
3. Results and discussion 3.1. Elemental mapping Fig. 3a and b shows topographic images of the 2.3 mol% and 6.0 mol % Ge-CFs obtained from SEM analysis. The brighter area corresponds to the Ge deposited at the core of the fibre, the darker to the surrounding silica. The concentric formation is a result of the MCVD fabrication process, the dopant being deposited layer by layer. The core diameter for the 2.3 mol% Ge-CF is ~1.6 times larger than the core of the 6.0 mol % Ge-CF (details of the measurements are shown in Table 1). In regard to the EDXS spectra of Fig. 3c and d, in use of the present analytical system the characteristic X-rays are dominated by the Lβ1 peak of Ge at 1.22 keV, with lesser appearance of the Kα1 peak at
Fig. 5. Linearity index curves of the (a) 2.3 mol% Ge-CF, (b) 6.0 mol% Ge-CF, (c) commercial 4.0 mol% Ge-CF, and (d) TLD-100 after irradiation using a proton beam energy of 150 MeV, for a range of radiation doses from 1 to 10 Gy. 4
Radiation Physics and Chemistry 165 (2019) 108390
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3.4. Minimum detectable dose Minimum detectable dose (MDD), demarcated as Do, represents the realm of applicability of any dosimeter materials in a dosimetric system, not least setting limits on use for low radiation dose evaluations. Here, the MDD is determined based on Eq. (2), in accord with the procedure of Furetta et al. (2001). 10.7 31.0 41.6 1.4 0.7 Gradient, α (nC m−3 Gy−1)
410.77 538.95 528.14 14.96 67.08
Standard deviation of background, σ
0.62 1.70 2.04 0.00 0.01
3.5. Reproducibility of fabricated fibres to different types of radiation In Table 5, each sample type is seen to exhibit relatively consistent response (essentially showing the same CV) albeit being repeated for various irradiation conditions, thus indicating excellent reproducibility (ANOVA, p > 0.05). In assessing the homogeneity of signal variances within ten measurements/readings of the same sample type, the fibres demonstrate a CV of 8% at the most, exceeding the ± 5% standard requirement for radiotherapy clinical applications as recommended by the International Commission on Radiation Units and MeasurementsICRU (ICRU, 1976). This is almost certainly a result of the absence of carrying out radiation sensitivity selection (i.e. screening process) of the fibres prior to their use in the present irradiations. While such selection can be expected to improve the situation, one can nevertheless conclude that the dopant uniformity in the fabricated fibres is generally well distributed along the fibre length. In respect of the TLD-100 dosimeters, carefully selected prior to sale, the CV is found to be better than 4% of the mean value, as a result surpassing the performance of the fibres. Also observed is that the nanoDot™ easily outperforms the fibres and TLD-100, with a CV of better than 1% of the mean value. 3.6. Post-irradiation signal fading
3.15 13.30 17.88 0.02 0.04
Throughout the post-irradiation period (96 days), all the irradiated samples were stored in a light-tight black plastic container at room temperature until the particular read-out day to avoid thermally or optically stimulated release of the trapped electrons (signals). Fig. 6 shows signal fading effect following to 96 days of samples storage, with response normalised to day three post-irradiation. The optical fibres and TLD-100 were readout after a set delay of 72 h (3 days) to allow more uniform control of thermal fading, assuming that the signal is more stable compared to 24 h delay. The analysis of variance (ANOVA) test shows the results to be statistically significant (p < 0.05), all samples experiencing signal loss throughout the storage period. Among the fibres, the commercial Ge-CF exhibited the least TL signal loss with 12% loss, followed by 2.3 mol% Ge-CF at 13%, and 6.0 mol% Ge-CF at about 19%. Concerning the locally fabricated Ge-CF, the 2.3 mol% GeCF exhibits superior minimal signal fading to that of the nanoDot™ (at 17% signal loss.
Fabricated 2.3 mol% Ge-CF Fabricated 6.0 mol% Ge-CF Commercial 4.0 mol% Ge-CF nanoDot™ TLD-100
Mean of background, B (nC m−3)
(2)
where B indicates a mean of background readings obtained from a set of measurements of annealed but unirradiated samples, σB is a standard deviation of the mean of the background readings, while F is the reciprocal of the gradient determined from the radiation dose linearity curves as presented in Fig. 4. From Table 4, the MDD of the fabricated 2.3 mol% and 6.0 mol% Ge-CFs were found to be 10.7 and 31.0 mGy respectively, having potential for applications in low-dose dosimetry, as in for instance in fluoroscopy. Also observed is that these fibres are capable of detecting radiation doses lower than that of the commercial Ge-CF (i.e. 41.6 mGy). The TLD-100 and nanoDot™ dosimeters still outperform the fibres in detecting very low radiation dose. We intend to make further investigation of the MDD to verify these findings particularly in the lower radiation dose range (i.e. below 100 mGy) using the same irradiation setups.
0.002 0.002 0.002 0.067 0.015
Reciprocal of gradient, F (Gy nC−1 m3) Normalised per unit volume × 1010 Sample
Table 4 Minimum detectable dose after 150 MeV proton beam irradiation.
Do = (B + 2 B) × F
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Table 5 Sample reproducibility following repeat use under various irradiation conditions for a constant radiation dose of 5 Gy. Sample
Fabricated 2.3 mol% Ge-CF
Fabricated 6.0 mol% Ge-CF
Commercial 4.0 mol% Ge-CF
nanoDot™
TLD-100
Irradiation condition
Number of sample reading
Irradiation beam
Energy
Gamma Electron Proton Photon Photon Gamma Electron Proton Photon Photon Gamma Electron Proton Photon Photon Gamma Electron Proton Photon Photon Gamma Electron Proton Photon Photon
1.25 MeV 6 MeV 150 MeV 6 MV 10 MV 1.25 MeV 6 MeV 150 MeV 6 MV 10 MV 1.25 MeV 6 MeV 150 MeV 6 MV 10 MV 1.25 MeV 6 MeV 150 MeV 6 MV 10 MV 1.25 MeV 6 MeV 150 MeV 6 MV 10 MV
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
Normalised per unit volume Mean × 1010
Standard deviation
Coefficient of variation (%)
2632.66 2878.27 2093.09 3673.84 3340.12 3808.13 4231.11 2868.23 5001.34 4998.17 2508.25 3707.63 2598.08 4204.55 4193.69 52.43 71.18 71.77 90.35 77.46 509.64 812.19 322.11 467.74 486.20
143.01 168.39 132.53 237.36 205.93 274.95 334.20 129.20 383.78 293.60 123.82 244.79 200.46 148.71 313.84 0.40 0.69 0.45 0.58 0.38 15.95 21.52 12.20 15.86 14.88
5.43 5.85 6.33 6.46 6.17 7.22 7.90 4.50 7.67 5.87 4.94 6.60 7.72 3.54 7.48 0.77 0.97 0.62 0.64 0.49 3.13 2.65 3.79 3.39 3.06
Note: Mean for optical fibres and TLD-100 are expressed in nC/m3, and for nanoDot™ is Gy/m3.
4. Conclusion Results of this study represent the dosimetric characteristics of novel fabricated 2.3 mol% and 6.0 mol% Ge-CF, including radiation dose response, linearity index, minimum detectable dose (MDD), reproducibility and signal fading. All of the fibres provide excellent dose response, albeit with some sign of minimal nonlinearity in the linearity index curve. Among the fabricated Ge-CFs, the 2.3 mol% Ge-CF exhibited the lowest MDD, at 10.7 mGy, suggesting potential for measurement of relatively low radiation doses. In regard to repeat irradiations under various conditions, all of the fibres have shown relatively consistent response without noticeable change in TL signal. As far as post-irradiation signal fading is concerned, the fabricated optical fibres experience minimal signal loss, 19% at the most, measured after 96 days of storage. The performance characteristics of the fabricated Ge-doped fibres show promise for TL dosimetry applications, in particular in respect of present interests in proton beam measurements. We intend further investigating of the TL response of these optical fibres in terms of measuring depth-dose distribution, especially for the proton Bragg peak. Acknowledgements The authors are greatly indebted to technical members of the MCVD Laboratory at Multimedia University (Malaysia) as well as the Flat Fibre Laboratory, University of Malaya (Malaysia), for their assistance during fabrication and pulling of the fibres. The authors also would like to extend their gratitude to staff members at Hyogo Ion Beam Medical Center (Japan) for assisting in performing the proton irradiation. Use of the Harshaw™ 3500 TLD reader at the Malaysian Nuclear Agency is acknowledged. This work was supported by the High Impact Putra Grant (grant number 9521800) from Universiti Putra Malaysia; and Fundamental Research Grant Scheme (grant number 5540112) from the Ministry of Education, Malaysia.
Fig. 6. Signal fading over 96 days of storage of: (a) 2.3 mol% Ge-CF, (b) 6.0 mol % Ge-CF, (c) commercial 4.0 mol% Ge-CF, (d) nanoDot™, and (e) TLD-100. The bar chart represents the remaining signal at particular readout days (with response normalised to day 3).
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Appendix A. Supplementary data
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Mahdiraji, G.A., Ghomeishi, M., Dermosesian, E., Hashim, S., Ung, N.M., Adikan, F.R.M., Bradley, D.A., 2015. Optical fiber based dosimeter sensor: beyond TLD-100 limits. Sens. Actuators A Phys. 222, 48–57. https://doi.org/10.1016/j.sna.2014.11.017. McKeever, S.W.S., Moscovitch, M., Townsend, P.D., 1995. Thermoluminescence Dosimetry Materials: Properties and Uses. Nuclear Technology Publishing, Ashford. Nawi, S.N.M., Wahib, N.F., Zulkepely, N.N., Amin, Y.M., Min, U.N., Bradley, D.A., Nor, R.B., Maah, M.J., 2015. The thermoluminescence response of Ge-doped flat fibers to gamma radiation. Sensors 15 (8), 20557–20569. https://doi.org/10.3390/ s150820557. Noor, N.M., Fadzil, M.S.A., Ung, N.M., Maah, M.J., Mahdiraji, G.A., Abdul-Rashid, H.A., Bradley, D.A., 2016a. Radiotherapy dosimetry and the thermoluminescence characteristics of Ge-doped fibres of differing germanium dopant concentration and outer diameter. Radiat. Phys. Chem. 126, 56–61. https://doi.org/10.1016/j.radphyschem. 2016.05.001. 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. 619 (1–3), 157–162. https://doi.org/10.1016/j.nima. 2010.01.013. Noor, N.M., Hussein, M., Kadni, T., Bradley, D.A., Nisbet, A., 2014. Characterization of Ge-doped optical fibres for MV radiotherapy dosimetry. Radiat. Phys. Chem. 98, 33–41. https://doi.org/10.1016/j.radphyschem.2013.12.017. Noor, N.M., Maryam, A.J., Hassan, S., Abdullah, W.S.W., Nizam, T., Faizal, M., Bradley, D.A., Razis, A.F.A., 2016b. Characterization of fabricated optical fiber for food irradiation dosimetry. Int. Food Res. 23 (5), 2125–2129. Noor, N.M., Nisbet, A., Hussein, M., Sarene, C.S., Kadni, T., Abdullah, N., Bradley, D.A., 2017. Dosimetry audits and intercomparisons in radiotherapy: a Malaysian profile. Radiat. Phys. Chem. 140, 207–212. https://doi.org/10.1016/j.radphyschem.2017. 03.046. PTCOG, 2019. Particle therapy centers. Retrieved June 19, 2019, from. https://www. ptcog.ch/index.php/ptcog-patient-statistics. Rah, J., Oh, D.H., Shin, D., Kim, D., Ji, Y.H., Kim, J.W., Park, S.Y., 2012. Dosimetric evaluation of a glass dosimeter for proton beam measurements. Appl. Radiat. Isot. 70, 1616–1623. https://doi.org/10.1016/j.apradiso.2012.04.007. Rahman, A.T.A., Nisbet, A., Bradley, D.A., 2010a. Dose-rate and the reciprocity law: TL response of Ge-doped SiO2 optical fibers at therapeutic radiation doses. Nucl. Instrum. Methods Phys. Res. 652 (1), 891–895. https://doi.org/10.1016/j.nima. 2010.08.048. Rahman, A.T.A., Bradley, D.A., Doran, S.J., Thierry, B., Bräuer-Krisch, E., Bravin, A., 2010b. The thermoluminescence response of Ge-doped silica fibres for synchrotron microbeam radiation therapy dosimetry. Nucl. Instrum. Methods Phys. Res. 619 (1–3), 167–170. https://doi.org/10.1016/j.nima.2009.10.173. Ramli, A.T., Bradley, D.A., Hashim, S., Wagiran, H., 2009. The thermoluminescence response of doped SiO2 optical fibres subjected to alpha-particle irradiation. Appl. Radiat. Isot. 67 (3), 428–432. https://doi.org/10.1016/j.apradiso.2008.06.034. Ramli, N.N.H., Salleh, H., Mahdiraji, G.A., Zulkifli, M.I., Hashim, S., Bradley, D.A., Noor, N.M., 2015. Characterization of amorphous thermoluminescence dosimeters for patient dose measurement in X-ray diagnostic procedures. Radiat. Phys. Chem. 116, 130–134. https://doi.org/10.1016/j.radphyschem.2015.01.016. Refaei, A., Wagiran, H., Saeed, M.A., Hosssain, I., 2014. Thermoluminescence characteristics of Nd-doped SiO2 optical fibres irradiated with the 60Co gamma rays. Appl. Radiat. Isot. 94, 89–92. https://doi.org/10.1016/j.apradiso.2014.07.012. Saeed, M.A., Hossain, I., Hida, N., Wagiran, H., 2013. Thermoluminescent sensitivity of single clad neodymium doped SiO2 optical fibres measured with 6 MeV photons. Radiat. Phys. Chem. 91, 98–100. https://doi.org/10.1016/j.radphyschem.2013.05. 032. Sahini, M.H., Hossain, I., Wagiran, H., Saeed, M.A., Ali, H., 2014. Thermoluminescence responses of the Yb- and Yb–Tb-doped SiO2 optical fibres to 6-MV photons. Appl. Radiat. Isot. 92, 18–21. https://doi.org/10.1016/j.apradiso.2014.05.024. Salasiah, M., Nordin, A.J., Fathinul, F.A.S., Hishar, H., Nizam, T., Taiman, K., Kadir, A.B.A., Abdul-Rashid, H.A., Mahdiraji, G.A., Rahman, A.K.M.M., Noor, N.M., 2013. Radiation dose to radiosensitive organs in PET/CT myocardial perfusion examination using versatile optical fibre. SPIE Conf. Proc. 8775 (87750U), 1–8. https://doi.org/ 10.1117/12.2017113. Wagiran, H., Hossain, I., Bradley, D., Yaakob, A.N.H., Ramli, T., 2012. Thermoluminescence responses of photon and electron irradiated Ge-and Al-doped SiO2 optical fibres. Chin. Phys. Lett. 29 (2), 027802. https://doi.org/10.1088/0256307X/29/2/027802. Yaakob, N.H., Wagiran, H., Hossain, M.I., Ramli, A.T., Bradley, D.A., Ali, H., 2011. Lowdose photon irradiation response of Ge and Al-doped SiO2 optical fibres. Appl. Radiat. Isot. 69 (9), 1189–1192. https://doi.org/10.1016/j.apradiso.2011.03.039.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.radphyschem.2019.108390. References Abdulla, Y.A., Amin, Y.M., Bradley, D.A., 2001b. The effect of dose and annealing on TL sensitivity of germanium and erbium doped optical fibres. J. Fiz. Malays. 22, 49–53. Abdulla, Y.A., Amin, Y.M., Bradley, D.A., 2001a. The thermoluminescence response of Ge-doped optical fibre subjected to photon irradiation. Radiat. Phys. Chem. 61 (3–6), 409–410. https://doi.org/10.1016/S0969-806X(01)00282-1. Alawiah, A., Alina, M.S., Bauk, S., Abdul-Rashid, H.A., Gieszczyk, W., Noramaliza, M.N., Marashdeh, M.W., 2015. The thermoluminescence characteristics and the glow curves of thulium doped silica fibre exposed to 10MV photon and 21MeV electron radiation. Appl. Radiat. Isot. 98, 80–86. https://doi.org/10.1016/j.apradiso.2015.01. 016. Baldock, C., De Deene, Y., Doran, S., Ibbott, G., Jirasek, A., Lepage, M., McAuley, K.B., Oldham, M., Schreiner, L.J., 2010. Polymer gel dosimetry. Phys. Med. Biol. 55 (5), 1–63. https://doi.org/10.1088/0031-9155/55/5/R01. Begum, M., Rahman, A.K.M.M., Abdul-Rashid, H.A., Yusoff, Z., Mat-Sharif, K.A., Zulkifli, M.I., Muhamad-Yasin, S.Z., Ung, N.M., Kadir, A.B.A., Amin, Y.M., Bradley, D.A., 2015. Comparison of thermoluminescence response of different sized Ge-doped flat fibers as a dosimeter. Radiat. Phys. Chem. 116, 155–159. https://doi.org/10.1016/j. radphyschem.2015.03.038. Begum, M., Rahman, A.K.M.M., Zubair, H.T., Abdul-Rashid, H.A., Yusoff, Z., Begum, M., Alkhorayef, M., Alzimami, K., Bradley, D.A., 2017. The effect of different dopant concentration of tailor-made silica fibers in radiotherapy dosimetry. Radiat. Phys. Chem. 141, 73–77. https://doi.org/10.1016/j.radphyschem.2017.06.008. Bradley, D.A., Hugtenburg, R.P., Nisbet, A., Rahman, A.T.A., Issa, F., Noor, M.N., Alalawi, A., 2012. Review of doped silica glass optical fibre: their TL properties and potential applications in radiation therapy dosimetry. Appl. Radiat. Isot. 71, 2–11. https://doi. org/10.1016/j.apradiso.2012.02.001. Bradley, D.A., Shaharuddin, S.S.A., Zahariman, S.R., Sabtu, S.N., Sani, S.F.A., Alanazi, A.H., Jafari, S.M., Mahdiraji, G.A., Adikan, F.R.M., Maah, M.J., Nisbet, A.N., Tamchek, N., Abdul-Rashid, H.A., Alkhorayef, M., Alzimami, K., 2016. Developments in production of silica-based thermoluminescence dosimeters. Radiat. Phys. Chem. 137, 37–44. https://doi.org/10.1016/j.radphyschem.2016.01.013. Chen, R., McKeever, S.W.S., 1994. Characterization of nonlinearities in the dose dependence of thermoluminescence. Radiat. Meas. 23 (4), 667–673. https://doi.org/10. 1016/1350-4487(94)90002-7. Doran, S.J., Gorjiara, T., Adamovics, J., Kuncic, Z., Kacperek, A., Baldock, C., 2015. The quenching effect in PRESAGE® dosimetry of proton beams: is an empirical correction feasible? J. Phys. Conf. Ser. 573 (012043), 1–5. https://doi.org/10.1088/1742-6596/ 573/1/012043. Entezam, A., Khandaker, M.U., Amin, Y.M., Ung, N.M., Bradley, D.A., Maah, J., Safari, M.J., Moradi, F., 2016. Thermoluminescence response of Ge-doped cylindrical-, flatand photonic crystal silica-fibres to electron and photon radiation. PLoS One 11 (5), 1–15. https://doi.org/10.1371/journal.pone.0153913. Fadzil, M.S.A., Ramli, N.N.H., Jusoh, M.A., Kadni, T., Bradley, D.A., Ung, N.M., Suhairul, H., Noor, N.M., 2014. Dosimetric characteristics of fabricated silica fibre for postal radiotherapy dose audits. J. Phys. Conf. Ser. 546 (012010), 1–7. https://doi.org/10. 1088/1742-6596/546/1/012010. Fadzil, M.S.A., Min, U.N., Bradley, D.A., Noor, N.M., 2017. Different germanium dopant concentration and the thermoluminescence characteristics of flat Ge-doped optical fibres. Pertanika J. Sci. Technol. 25 (1), 327–336. 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. 456 (3), 411–417. https://doi.org/10.1016/ S0168-9002(00)00585-4. Ghomeishi, M., Mahdiraji, G.A., Adikan, F.R.M., Ung, N.M., Bradley, D.A., 2015. Sensitive fibre-based thermoluminescence detectors for high resolution in-vivo dosimetry. Sci. Rep. 5 (13309), 1–10. https://doi.org/10.1038/srep13309. Girard, S., Ouerdane, Y., Marcandella, C., Boukenter, A., Quenard, S., Authier, N., 2011. Feasibility of radiation dosimetry with phosphorus-doped optical fibers in the ultraviolet and visible domain. J. Non-Cryst. Solids 357 (8–9), 1871–1874. https://doi. org/10.1016/j.jnoncrysol.2010.11.113. Gustavsson, H., Back, S., Lepage, M., Rintoul, L., Baldock, C., 2004. Development and optimization of a 2-hydroxyethylacrylate MRI polymer gel dosimeter. Phys. Med. Biol. 49 (2), 227–241. https://doi.org/10.1088/0031-9155/49/2/004. Hashim, S., Bradley, D.A., Peng, N., Ramli, A.T., Wagiran, H., 2010a. The thermoluminescence response of oxygen-doped optical fibres subjected to photon and electron irradiations. Nucl. Instrum. Methods Phys. Res. 619 (1–3), 291–294. https:// doi.org/10.1016/j.nima.2009.10.109. Hashim, S., Bradley, D.A., Saripan, M.I., Ramli, A.T., Wagiran, H., 2010b. The thermoluminescence response of doped SiO2 optical fibres subjected to fast neutrons. Appl. Radiat. Isot. 68 (4–5), 700–703. https://doi.org/10.1016/j.apradiso.2009.10.027. Hassan, M.F., Rahman, W.N.W.A., Fadzil, M.S.A., Tominaga, T., Geso, M., Akasaka, H., Bradley, D.A., Noor, N.M., 2017. The thermoluminescence response of Ge-doped flat fibre for proton beam measurements: a preliminary study. J. Phys. Conf. Ser. 851 (012034), 1–6. https://doi.org/10.1088/1742-6596/851/1/012034. Hassan, M.F., Rahman, W.N.W.A., Kadir, A.B.A., Isa, N.M., Tominaga, T., Geso, M.,
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Ge-doped silica fibre for proton beam dosimetry a
b
c
d
e
f,g
M.F. Hassan , W.N.W.A. Rahman , T. Tominaga , M. Geso , H. Akasaka , D.A. Bradley , N.M. Noora,*
T
a
Department of Imaging, Faculty of Medicine and Health Sciences, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Medical Radiation Programme, School of Health Sciences, Universiti Sains Malaysia, 16150 Kubang Kerian, Kelantan, Malaysia Department of Clinical Radiology, Faculty of Health Sciences, Hiroshima International University, Hiroshima 739-2695, Japan d Discipline of Medical Radiations, School of Medical Sciences, Royal Melbourne Institute of Technology University, Bundoora, Victoria 3083, Australia e Division of Radiation Oncology, Kobe University Hospital, Hyogo 650-0017, Japan f Department of Physics, University of Surrey, Guildford, GU2 7XH, United Kingdom g Centre for Biomedical Physics, School of Healthcare and Medical Sciences, Sunway University, 46150 Petaling Jaya, Selangor, Malaysia b c
ARTICLE INFO
ABSTRACT
Keywords: Fabricated optical fibre Linearity index Minimum detectable dose Reproducibility Signal fading Proton beam
An investigation has been made of nominal 2.3 mol% and 6.0 mol% germanium (Ge) doped cylindrical optical fibres as novel radiation dosimeters for 150-MeV proton beam measurements. These optical fibres were locally fabricated using a modified chemical vapour deposition technique with a subsequent pulling process. Combined scanning electron microscope and energy dispersive X-ray spectroscopy analyses were performed to map the relative presence of the germanium and other elements in the optical fibres. Prior to irradiation, a thermal annealing process was carried out to erase any pre-irradiation signals potentially existing in the samples. Results were compared against nanoDot™, TLD-100, and commercial optical fibres to allow for a relative comparison of the response. For radiation dose in the range 1 up to 10 Gy, the fabricated optical fibres exhibit excellent radiation dose response (R2 > 0.99), with a linearity index that remains close to one (indicating a linear response). In terms of minimum detectable dose, these optical fibres are able to detect relatively low radiation dose (for the present batch of fibres down to 10.7 mGy). After repeated various irradiation campaigns, the fabricated optical fibres have been shown to provide consistent response, effectively without noticeable change in thermoluminescence (TL) yield (ANOVA, p > 0.05), suggesting excellent reproducibility. In regard to signal fading, 96 days post-irradiation the fabricated optical fibres showed minimal signal loss, at 19% at the most. These dosimetric characteristics confirm the potential of the fabricated optical fibres as TL dosimeters, specifically for present studies in conducting proton beam measurements.
1. Introduction Silica-based optical fibres have shown promise as thermoluminescence dosimetry (TLD), in particular for radiotherapy applications. It has been established at modest cost that optical fibres provide excellent spatial resolution, retaining excellent response to ionising radiation, are of a non-hygroscopic nature and offer a good degree of flexibility. One of the earliest investigation was that of Abdulla et al., 2001a, utilising commercially available germanium (Ge) doped optical fibres. Thus said, commercial optical fibres can display ambiguous behaviour, potentially leading to sub-optimal TL response that may influence the overall dosimetric sensitivity (Bradley et al., 2012; Entezam et al., 2016; Mahdiraji et al., 2015). Among the possibilites leading to this are non-uniform distribution of dopant concentration along the
*
length of the commercial optical fibres. Another reason lies in their fabrication process for telecommunication fibres, intended to provide optimum communication performance, not radiation dosimetry. In recent years, researchers have made efforts to design and fabricate doped silica fibres offering optimum performance for TL dosimetry (TLD). Such tailor-made fibres, customised via a modified chemical vapour deposition technique following a particular fabrication ‘recipe’ detailed in previous publications of this group and others, have been performance-tested based on dopant concentration (Begum et al., 2017; Fadzil et al., 2017; Noor et al., 2016a), dopant uniformity (Entezam et al., 2016), geometry (i.e. flat and cylindrical shapes) (Ghomeishi et al., 2015) and dimension (Begum et al., 2015; Nawi et al., 2015). It is known that undoped optical fibre (i.e. in the absence of doping elements) will give limited TL yield upon exposure to radiation.
Corresponding author. E-mail address: [email protected] (N.M. Noor).
https://doi.org/10.1016/j.radphyschem.2019.108390 Received 21 February 2019; Received in revised form 24 June 2019; Accepted 30 June 2019 Available online 03 July 2019 0969-806X/ © 2019 Elsevier Ltd. All rights reserved.
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for external beam radiation have been extensively studied, their applications in measuring proton beams have not been widely investigated. Today, the use of proton beams in radiation therapy treatment have attracted considerable and growing attention. According to the Particle Therapy Co-Operative Group (PTCOG), as of April 2019, there are 81 proton therapy facilities in operation worldwide; the number is forecast to increase from the current figure to 142 by 2025 including those under construction or in planning stage (PTCOG, 2019). This rapid growth in the number of proton beam therapy centres is mainly due to the physical characteristics of the proton beam Bragg curve, which offers distinct dosimetric advantages over conventional radiotherapy (i.e. photon and electron beam therapy). Despite these advantages, the main drawback in proton dose measurement is the under response of radiation dosimeters when the ionization density of the proton beam increases, particularly in the Bragg peak region, thus potentially leading to underestimation of the actual radiation dose (Rah et al., 2012). For example, Bradley et al. (2016) found that the commercial Ge-doped optical fibres showed a quenching effect in the distal fall-off region of the proton Bragg peak; the response was compared with the measurements obtained from the ionization chamber dosimeter (gold standard). This issue also has been reported for other types of dosimeter such as film, polyacrylamide gels, Fricke gels, PRESAGE and alanine (Baldock et al., 2010; Doran et al., 2015; Gustavsson et al., 2004). It is therefore with these restrictions in mind that the present study seeks to evaluate the performance of the fabricated Ge-doped fibre as a potential candidate for TLD, particularly in measuring proton beams. These locally fabricated Ge-doped fibres are studied in terms of their key dosimetric characteristics, including radiation dose response, linearity index, minimum detectable dose, reproducibility and signal fading effect.
Fig. 1. Fabrication process of the Ge-doped preform using MCVD technique.
Conversely, due to the presence of dopants, irradiated optical fibres have been shown to give rise to appreciable TL yield. To-date, optical fibres doped with germanium (the dopant) are found to exhibit superior sensitivity to radiation as compared to optical fibres doped with other elements such as aluminium (Ramli et al., 2009; Yaakob et al., 2011; Wagiran et al., 2012), neodymium (Refaei et al., 2014; Saeed et al., 2013), erbium (Abdulla et al., 2001b), ytterbium (Sahini et al., 2014), thulium (Alawiah et al., 2015), oxygen (Hashim et al., 2010a) and phosphorus (Girard et al., 2011), to name a few. The performance of Ge-doped fibres, either commercially available or locally fabricated, have been widely studied in radiation detection contexts, including via irradiation to protons (Hassan et al., 2017, 2018), diagnostic X-rays (Ramli et al., 2015), photons (Issa et al., 2011), alpha particles (Ramli et al., 2009), electrons (Rahman et al., 2010a), synchrotron radiation (Rahman et al., 2010b) and fast neutrons (Hashim et al., 2010b). Their performance has also been investigated in various radiation-based disciplines, such as in environmental monitoring (Bradley et al., 2012), quality dose audits (Fadzil et al., 2014; Noor et al., 2017), intensity modulated radiation therapy (Noor et al., 2010), the food irradiation industry (Noor et al., 2016b), small field stereotactic radiosurgery (Lam et al., 2018) and positron emission tomography-computed tomography myocardial perfusion examination (Salasiah et al., 2013). In all such studies, the fibres have yielded promising responses. Although the performance of the fabricated Ge-doped optical fibres
2. Materials and methods 2.1. Fabrication of Ge-doped optical fibre The Ge-doped fibres employed herein result from fabrication using a standard modified chemical vapour deposition (MCVD) technique (carried out at Multimedia University, MMU, Malaysia). The process involved several key stages, as in Fig. 1.
Table 1 Details of research samples. Sample a
Fabricated 2.3 mol% Ge-CF Fabricated 6.0 mol% Ge-CFa Commercial 4.0 mol% Ge-CF nanoDot™ (with casing) TLD-100 a
Outer dimension
Core dimension
Length
Manufacturer
481 μm (diameter) 486 μm (diameter) 125 μm (diameter) 10 × 10 × 2 mm3 3.2 × 3.2 × 0.89 mm3
124 μm (diameter) 78 μm (diameter) 50 μm (diameter) – –
6 mm 6 mm 6 mm – –
MMU-UM consortium (Malaysia) MMU-UM consortium (Malaysia) CorActive High-Tech Inc. (Canada) Landauer Inc. (USA) Thermo Fisher Scientific Inc. (USA)
Dopant concentrations are based on values in the preforms.
Table 2 Summary of parameters used for samples irradiation. Parameters
Proton
Gammaa
Photona
Electrona
Energy Radiation dose Dose rate Source-to-surface distance Beam field size Build-up thickness
150 MeV 1–10 Gy 4 Gy/min 100 cm 10 × 10 cm2 From 11 cm to 14 cm (for 3 cm SOBP width) 10 cm Synchrotron (Mitsubishi Electric)
1.25 MeV 5 Gy 0.04 Gy/min 100 cm 10 × 10 cm2 –
6 MV, 10 MV 5 Gy 600 cGy/min 100 cm 10 × 10 cm2 1.5 cm bolus for 6 MV, 2.5 cm bolus for 10 MV 10 cm Varian 2100C linear accelerator
6 MeV 5 Gy 600 cGy/min 100 cm 10 × 10 cm2 (applicator) 2.0 cm (bolus)
Backscatter thickness Model of the accelerator/irradiator a
10 cm ELDORADO 8 with cobalt-60 source
Additional irradiation beams used for reproducibility study. 2
10 cm Varian 2100C linear accelerator
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desired cross-sectional dimensions, producing conventionally shaped cylindrical fibre (hereinafter referred to as ‘Ge-CF’). The detailed procedures of the pulling process (carried out at the University of Malaya, UM) have been elaborated in Noor et al., 2016a, 2016b. Table 1 describes the details of the research samples employed in this study.
Table 3 Time-temperature profile used for samples readout. Sample
Phase
Temperature (°C)
Time (sec)
Ramp-rate (°C/sec)
Fibres
Preheat Acquisition Annealing Preheat Acquisition Annealing
80 400 400 50 260 260
10.0 13.3 10.0 0.0 26.7 0.0
– 30.1 – – 9.8 –
TLD-100
2.2. Samples preparation The outer plastic coating layer of the commercial Ge-CF (manufactured by CorActive High-Tech Inc., Canada) was carefully stripped away, the remaining fibre being cleaned using isopropyl alcohol solution to remove any resin residues. All fibres, including those locally fabricated, were cut into a nominal length of 6.0 ( ± 1.0) mm to accommodate the fibres within the dimension of the planchet (the heating plate in the TLD reader) during the readout process. For ease of handling and storage, samples were prepared in groups of a minimum of 10 according to their types and retained in thin-walled gelatine capsules. Prior to irradiation, thermal annealing was carried out in order to erase any pre-irradiation signals potentially existing in the samples. 2.3. Irradiation
Fig. 2. Horizontal orientation of the fibres with respect to the PMT and planchet during the readout process.
Table 2 summarises the parameters used for proton and other irradiations. For reproducibility, the same group of samples were studied for various irradiation conditions with subsequent annealing after each occasion. A water-equivalent phantom was used to serve as the build-up medium and to provide maximum absorbed radiation dose (dmax). All samples were placed horizontally at dmax, with the irradiation beams delivered perpendicularly (zero beam angulation) to the samples.
Utilising the MCVD technique outlined in Fig. 1, two types of collapsed-Ge-doped preform rods were produced with respect to dopant concentration (i.e. 2.3 mol% and 6.0 mol% of Ge, nominal preform values). Both preform rods were then pulled (drawn-down) to the final
Fig. 3. Fig. 3a and b are the SEM images for the 2.3 mol% and 6.0 mol% Ge-CF, respectively. Fig. 3c and d are the respective EDXS spectra (the inset images are zoomed-in views of the fibre core used for elemental mapping). 3
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9.89 keV; the characteristic 1.74 keV Kα1 line of Si and the 0.53 keV Kα1 line of O2 are also apparent. 3.2. Radiation dose response Radiation dose response is of particular interest in seeking a good dosimeter system, ideally with the response being linear over the full range of radiation doses, thus allowing for reliable calibration and utilisation. In Fig. 4, it is observed that all the fabricated Ge-CF offer excellent linear relationships throughout the investigated radiation dose range (1–10 Gy), with coefficient of determination (R2) values of better than 0.99. A similar behaviour was also observed for the TLD-100 dosimeter and commercial Ge-CF (R2 > 0.99), albeit with the TLD-100 providing much lower sensitivity. From Fig. 4, one can observe that optical fibres doped with higher germanium concentration at 6.0 mol% gave the greatest response compared to those that doped with lower concentration at 2.3 mol%. This is mainly due to the increase density of trapping/impurities centres in the optical fibre when the concentration level of germanium increases.
Fig. 4. Response of the investigated samples irradiated with a 150 MeV proton beam, for doses from 1 to 10 Gy.
2.4. Signal acquisition (readout) Prior to readout, the irradiated optical fibres and TLD-100 were kept in a light-tight box for about 72 h to allow uniform control of thermal fading. A Harshaw™ 3500 TLD reader (Thermo Scientific™, USA) was used to acquire TL signals from the samples. The TL signals were acquired based on the time-temperature profile as detailed in Table 3. Throughout the readout process, nitrogen gas atmosphere of 0.5 bar was used to prevent contributions of TL signal from the chemiluminescence phenomena. The gross TL yield was normalised per unit volume to mitigate against uncertainty in TL yield (the volume was made to relate to the diameter of the fibre core, a region doped with Ge). The fibres have been read-out with their long dimension in contact with the planchet (the heating plate of the TL reader), not least allowing good thermal contact (Fig. 2). Each data point presented in this study was obtained by taking a mean of at least ten measurements/readings.
3.3. Linearity index The linearity index, denoted as f(D), is defined as the deviation of TL response from linearity. The f(D) shown in Fig. 5 has been obtained over the dose range 1–10 Gy, use being made of Eq. (1) (Chen and McKeever, 1994)
f(D) =
S(D) SO D
/
S(D1) SO D1
(1)
where S(D) is the measured TL signal obtained from the maximum intensity, as a function of the dose D, whereas S(D1) is the measured TL signal at low dose (D1) which is somewhere within the linear region of f (D) and So is the intercept on the TL intensity axis from the extrapolation of the linear region in the curves (McKeever et al., 1995) obtained from Fig. 4. From Eq. (1), f(D) = 1 indicates linear, f(D) > 1 supralinear and f(D) < 1 sublinear responses. In Fig. 5, the 2.3 mol% Ge-CF shows minimal nonlinearity, typically < 2%, with a maximum deviation from f(D) = 1 of 9% at 1 Gy. The TL response of the 6.0 mol% Ge-CF again shows minimal nonlinearity deviation from f(D) = 1, with a maximum deviation of 4% at 7 Gy. Results from this study are consistent with previous reports, the Ge-doped fibres trending towards a sublinear response in the low radiation dose region (in particular, below 2 Gy) (Begum et al., 2017; Noor et al., 2014; (Noor et al., 2016a, b). The commercial Ge-CF and TLD-100 show near linear fits for radiation doses ranging from 2 to 10 Gy with at most 3% deviation from f(D) = 1. Also observed is a prominent sublinear response for the TLD-100 particularly at 1 Gy, with a 12% deviation from f(D) = 1.
3. Results and discussion 3.1. Elemental mapping Fig. 3a and b shows topographic images of the 2.3 mol% and 6.0 mol % Ge-CFs obtained from SEM analysis. The brighter area corresponds to the Ge deposited at the core of the fibre, the darker to the surrounding silica. The concentric formation is a result of the MCVD fabrication process, the dopant being deposited layer by layer. The core diameter for the 2.3 mol% Ge-CF is ~1.6 times larger than the core of the 6.0 mol % Ge-CF (details of the measurements are shown in Table 1). In regard to the EDXS spectra of Fig. 3c and d, in use of the present analytical system the characteristic X-rays are dominated by the Lβ1 peak of Ge at 1.22 keV, with lesser appearance of the Kα1 peak at
Fig. 5. Linearity index curves of the (a) 2.3 mol% Ge-CF, (b) 6.0 mol% Ge-CF, (c) commercial 4.0 mol% Ge-CF, and (d) TLD-100 after irradiation using a proton beam energy of 150 MeV, for a range of radiation doses from 1 to 10 Gy. 4
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3.4. Minimum detectable dose Minimum detectable dose (MDD), demarcated as Do, represents the realm of applicability of any dosimeter materials in a dosimetric system, not least setting limits on use for low radiation dose evaluations. Here, the MDD is determined based on Eq. (2), in accord with the procedure of Furetta et al. (2001). 10.7 31.0 41.6 1.4 0.7 Gradient, α (nC m−3 Gy−1)
410.77 538.95 528.14 14.96 67.08
Standard deviation of background, σ
0.62 1.70 2.04 0.00 0.01
3.5. Reproducibility of fabricated fibres to different types of radiation In Table 5, each sample type is seen to exhibit relatively consistent response (essentially showing the same CV) albeit being repeated for various irradiation conditions, thus indicating excellent reproducibility (ANOVA, p > 0.05). In assessing the homogeneity of signal variances within ten measurements/readings of the same sample type, the fibres demonstrate a CV of 8% at the most, exceeding the ± 5% standard requirement for radiotherapy clinical applications as recommended by the International Commission on Radiation Units and MeasurementsICRU (ICRU, 1976). This is almost certainly a result of the absence of carrying out radiation sensitivity selection (i.e. screening process) of the fibres prior to their use in the present irradiations. While such selection can be expected to improve the situation, one can nevertheless conclude that the dopant uniformity in the fabricated fibres is generally well distributed along the fibre length. In respect of the TLD-100 dosimeters, carefully selected prior to sale, the CV is found to be better than 4% of the mean value, as a result surpassing the performance of the fibres. Also observed is that the nanoDot™ easily outperforms the fibres and TLD-100, with a CV of better than 1% of the mean value. 3.6. Post-irradiation signal fading
3.15 13.30 17.88 0.02 0.04
Throughout the post-irradiation period (96 days), all the irradiated samples were stored in a light-tight black plastic container at room temperature until the particular read-out day to avoid thermally or optically stimulated release of the trapped electrons (signals). Fig. 6 shows signal fading effect following to 96 days of samples storage, with response normalised to day three post-irradiation. The optical fibres and TLD-100 were readout after a set delay of 72 h (3 days) to allow more uniform control of thermal fading, assuming that the signal is more stable compared to 24 h delay. The analysis of variance (ANOVA) test shows the results to be statistically significant (p < 0.05), all samples experiencing signal loss throughout the storage period. Among the fibres, the commercial Ge-CF exhibited the least TL signal loss with 12% loss, followed by 2.3 mol% Ge-CF at 13%, and 6.0 mol% Ge-CF at about 19%. Concerning the locally fabricated Ge-CF, the 2.3 mol% GeCF exhibits superior minimal signal fading to that of the nanoDot™ (at 17% signal loss.
Fabricated 2.3 mol% Ge-CF Fabricated 6.0 mol% Ge-CF Commercial 4.0 mol% Ge-CF nanoDot™ TLD-100
Mean of background, B (nC m−3)
(2)
where B indicates a mean of background readings obtained from a set of measurements of annealed but unirradiated samples, σB is a standard deviation of the mean of the background readings, while F is the reciprocal of the gradient determined from the radiation dose linearity curves as presented in Fig. 4. From Table 4, the MDD of the fabricated 2.3 mol% and 6.0 mol% Ge-CFs were found to be 10.7 and 31.0 mGy respectively, having potential for applications in low-dose dosimetry, as in for instance in fluoroscopy. Also observed is that these fibres are capable of detecting radiation doses lower than that of the commercial Ge-CF (i.e. 41.6 mGy). The TLD-100 and nanoDot™ dosimeters still outperform the fibres in detecting very low radiation dose. We intend to make further investigation of the MDD to verify these findings particularly in the lower radiation dose range (i.e. below 100 mGy) using the same irradiation setups.
0.002 0.002 0.002 0.067 0.015
Reciprocal of gradient, F (Gy nC−1 m3) Normalised per unit volume × 1010 Sample
Table 4 Minimum detectable dose after 150 MeV proton beam irradiation.
Do = (B + 2 B) × F
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Table 5 Sample reproducibility following repeat use under various irradiation conditions for a constant radiation dose of 5 Gy. Sample
Fabricated 2.3 mol% Ge-CF
Fabricated 6.0 mol% Ge-CF
Commercial 4.0 mol% Ge-CF
nanoDot™
TLD-100
Irradiation condition
Number of sample reading
Irradiation beam
Energy
Gamma Electron Proton Photon Photon Gamma Electron Proton Photon Photon Gamma Electron Proton Photon Photon Gamma Electron Proton Photon Photon Gamma Electron Proton Photon Photon
1.25 MeV 6 MeV 150 MeV 6 MV 10 MV 1.25 MeV 6 MeV 150 MeV 6 MV 10 MV 1.25 MeV 6 MeV 150 MeV 6 MV 10 MV 1.25 MeV 6 MeV 150 MeV 6 MV 10 MV 1.25 MeV 6 MeV 150 MeV 6 MV 10 MV
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
Normalised per unit volume Mean × 1010
Standard deviation
Coefficient of variation (%)
2632.66 2878.27 2093.09 3673.84 3340.12 3808.13 4231.11 2868.23 5001.34 4998.17 2508.25 3707.63 2598.08 4204.55 4193.69 52.43 71.18 71.77 90.35 77.46 509.64 812.19 322.11 467.74 486.20
143.01 168.39 132.53 237.36 205.93 274.95 334.20 129.20 383.78 293.60 123.82 244.79 200.46 148.71 313.84 0.40 0.69 0.45 0.58 0.38 15.95 21.52 12.20 15.86 14.88
5.43 5.85 6.33 6.46 6.17 7.22 7.90 4.50 7.67 5.87 4.94 6.60 7.72 3.54 7.48 0.77 0.97 0.62 0.64 0.49 3.13 2.65 3.79 3.39 3.06
Note: Mean for optical fibres and TLD-100 are expressed in nC/m3, and for nanoDot™ is Gy/m3.
4. Conclusion Results of this study represent the dosimetric characteristics of novel fabricated 2.3 mol% and 6.0 mol% Ge-CF, including radiation dose response, linearity index, minimum detectable dose (MDD), reproducibility and signal fading. All of the fibres provide excellent dose response, albeit with some sign of minimal nonlinearity in the linearity index curve. Among the fabricated Ge-CFs, the 2.3 mol% Ge-CF exhibited the lowest MDD, at 10.7 mGy, suggesting potential for measurement of relatively low radiation doses. In regard to repeat irradiations under various conditions, all of the fibres have shown relatively consistent response without noticeable change in TL signal. As far as post-irradiation signal fading is concerned, the fabricated optical fibres experience minimal signal loss, 19% at the most, measured after 96 days of storage. The performance characteristics of the fabricated Ge-doped fibres show promise for TL dosimetry applications, in particular in respect of present interests in proton beam measurements. We intend further investigating of the TL response of these optical fibres in terms of measuring depth-dose distribution, especially for the proton Bragg peak. Acknowledgements The authors are greatly indebted to technical members of the MCVD Laboratory at Multimedia University (Malaysia) as well as the Flat Fibre Laboratory, University of Malaya (Malaysia), for their assistance during fabrication and pulling of the fibres. The authors also would like to extend their gratitude to staff members at Hyogo Ion Beam Medical Center (Japan) for assisting in performing the proton irradiation. Use of the Harshaw™ 3500 TLD reader at the Malaysian Nuclear Agency is acknowledged. This work was supported by the High Impact Putra Grant (grant number 9521800) from Universiti Putra Malaysia; and Fundamental Research Grant Scheme (grant number 5540112) from the Ministry of Education, Malaysia.
Fig. 6. Signal fading over 96 days of storage of: (a) 2.3 mol% Ge-CF, (b) 6.0 mol % Ge-CF, (c) commercial 4.0 mol% Ge-CF, (d) nanoDot™, and (e) TLD-100. The bar chart represents the remaining signal at particular readout days (with response normalised to day 3).
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Appendix A. Supplementary data
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Ramli, A.T., Bradley, D.A., Hashim, S., Wagiran, H., 2009. The thermoluminescence response of doped SiO2 optical fibres subjected to alpha-particle irradiation. Appl. Radiat. Isot. 67 (3), 428–432. https://doi.org/10.1016/j.apradiso.2008.06.034. Ramli, N.N.H., Salleh, H., Mahdiraji, G.A., Zulkifli, M.I., Hashim, S., Bradley, D.A., Noor, N.M., 2015. Characterization of amorphous thermoluminescence dosimeters for patient dose measurement in X-ray diagnostic procedures. Radiat. Phys. Chem. 116, 130–134. https://doi.org/10.1016/j.radphyschem.2015.01.016. Refaei, A., Wagiran, H., Saeed, M.A., Hosssain, I., 2014. Thermoluminescence characteristics of Nd-doped SiO2 optical fibres irradiated with the 60Co gamma rays. Appl. Radiat. Isot. 94, 89–92. https://doi.org/10.1016/j.apradiso.2014.07.012. Saeed, M.A., Hossain, I., Hida, N., Wagiran, H., 2013. Thermoluminescent sensitivity of single clad neodymium doped SiO2 optical fibres measured with 6 MeV photons. Radiat. Phys. 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