Thermoluminescence response of flat optical fiber subjected to 9 MeVelectron irradiations

S. Hashim a,b,n, S.S. Che Omar a, S.A. Ibrahim a, W.M.S. Wan Hassan a, N.M. Ung c,G.A. Mahdiraji d, D.A. Bradley e,f, K. Alzimami g

a Department of Physics, Universiti Teknologi Malaysia, 81310, Skudai, Johor Darul Takzim, Malaysiab Oncology Treatment Centre, Sultan Ismail Hospital, 81100 Johor Bahru, Malaysiac Clinical Oncology Unit, Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur, Malaysiad Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiae Department of Physics, University of Surrey, Guildford GU2 7XH, UKf Department of Physics, Universiti Malaya, 50603 Kuala Lumpur, Malaysiag Department of Radiological Sciences, Applied Medical Sciences College,King Saud University,P.O. Box 10219, Riyadh 11433 Saudi Arabia

H I G H L I G H T S

� TL performance of pure silica flat optical fiber (FF) and TLD-100 rod to electron.� TL glow curve with a single prominent peak between 230 and 255 1C.� The sensitivity of FF is approximately 16% than TLD-100.� The minimum detectable dose was 0.09 mGy for TLD-100 rod and 8.22 mGy for FF.

a r t i c l e i n f o

Article history:Received 14 November 2013Accepted 30 June 2014Available online 9 July 2014

Keywords:ThermoluminescentFlat optical fiberSensitivityMinimum detectable dose

a b s t r a c t

We describe the efforts of finding a new thermoluminescent (TL) media using pure silica flat optical fiber(FF). The present study investigates the dose response, sensitivity, minimum detectable dose and glowcurve of FF subjected to 9 MeV electron irradiations with various dose ranges from 0 Gy to 2.5 Gy. Theabove-mentioned TL properties of the FF are compared with commercially available TLD-100 rods. TheTL measurements of the TL media exhibit a linear dose response over the delivered dose using a linearaccelerator. We found that the sensitivity of TLD-100 is markedly 6 times greater than that of FF opticalfiber. The minimum detectable dose was found to be 0.09 mGy for TLD-100 and 8.22 mGy for FF. Ourwork may contribute towards the development of a new dosimeter for personal monitoring purposes.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Many radiation detectors have been developed over the lastfew decades and some are being used routinely for environmentaland personnel dose control. Some detectors make use of materialsthat emit light when heated after exposure to radiation. Thistechnique is known as thermoluminescence dosimetry (TLD).Because of its simplicity and suitability for automation muchresearch and development work has been put into this type ofdosimetry, which has also turned out to be useful in fields otherthan radiation protection (Bradley et al., 2012).

In radiotherapy the aim of dosimetry is to make sure that thedose to the tumour is as prescribed while minimizing the dose tothe surrounding normal tissue. The most commonly used energyrange for electrons in radiotherapy is 6–20 MeV. At these parti-cular energies, the electron beams are used to treat superficialtumors that are located down to 5 cm in depth. For instance, thetreatment of skin or lip cancers, chest wall irradiation for breastcancer and the treatment of head and neck cancers. Electron beamirradiation is preferably compared to superficial X-rays, brachy-therapy or tangential photon beams to treat these tumors, since itoffers several advantages such as dose uniformity in the targetvolume and the ability to minimize dose to deeper tissues. (Khan,2003, Wagiran et al., 2012).

The potential use of commercially available single mode dopedSiO2 optical fibers has been investigated by a number of workers, for

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Radiation Physics and Chemistry

http://dx.doi.org/10.1016/j.radphyschem.2014.06.0280969-806X/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author.E-mail address: suhairul@utm.my (S. Hashim).

Radiation Physics and Chemistry 106 (2015) 46–49

photons (Abdulla et al., 2001, Abdul Rahman et al., 2011; Issa et al.,2011), electrons (Hashim et al., 2009; Yaakob et al., 2011; AbdulRahman et al., 2011 and Alawiah et al., 2013), protons (Hashim et al.,2006), alpha particles (Ramli et al., 2009), fast neutrons (Hashim et al.,2010) and synchrotron radiation (Abdul Rahman et al., 2010). In allsuch studies the TL performances of irradiated fibers have shownconsiderable potential for dosimetric applications. The Ge-dopedoptical fibers show linear dose response for 6, 9 and 12MeV electronirradiations up to 4 Gy, encompassing the range of fractionated dosesnormally used in radiotherapy (Hashim et al., 2009). The Ge-dopedoptical fiber exhibits sensitivity 23 times higher than the Al-dopedfiber (Yaakob et al.,2011). The electron response of both Al- and Ge-doped fibers was found to be about 1.3 times that of photon irradiationover the dose range 0.2–4.0 Gy, due to the greater linear energytransfer (LET) of the electrons in the doped fibers compared tophotons.(Wagiran et al., 2012). With regard to energy response, aslight reduction in TL yield is observed for the electron beams,decreasing by 11% when comparing the TL yield at 16 MeV with thatat 9 MeV for a dose rate of 400 cGy min�1 (Abdul Rahman et al.,2011). The optical fibers also demonstrated good reproducibility(7 1.5%), low residual signal and minimal fading (Abdul Rahmanet al., 2011).

The pure silica FF preform is a hollow silica tube that contains thecore and cladding deposited layers. FF has been fabricated to combinethe structural advantages of optical fibers and the functional benefitsof planar devices. The ribbon-like planar shapes, with an extendedlength and flexibility, can be used to develop multiple functionaloptical components such as splitters, couplers and multiplexers. Onthe other hand, FF offers a platform for the multi-functionality ofintegrated optics, while trying to retain many mechanical, chemicaland optical properties characteristic of the optical fiber (Dambul et al.,2012).

Herein, we report the efforts made by our group in introducing theFF as a new TL media. The comparison being made with thecommercially available radiation dosimeter i.e. TLD-100. The presentwork describes several attractive features of the proposed dosimeterssuch as dose response, TL glow curve and minimum detectable dosesubjected to 9 MeV electron irradiations.

2. Materials and methods

2.1. Sample preparation

The dosimetry system herein was based on TLD-100 rods size withthe dimension of 1 mm �6mm and FF with dimension of approxi-mately 3.9 mm �3.9 mm �0.9 mm (Table 1). The FF is fabricatedusing a conventional 5 m fiber drawing tower located at the Flat FiberLaboratory, Department of Electrical Engineering, Universiti Malaya(UM), Malaysia. Unlike normal optical fiber preform that resemblesa solid rod, the FF preform is a hollow silica tube that contains the coreand cladding deposited layers (Fig. 1(i)). Table 1 shows the dimensionsof the FF used in this study shown in Fig. 1(ii). In this work, thepreform (outer diameter¼26.5 mm, inner diameter¼22.8 mm andlength¼60mm) used is a commercially available uncollapsed silicatube. The temperature of the furnace was initially set at 2100 1C, whichis silica's melting temperature. When the temperature of the furnace

reaches silica's melting temperature, the preform starts to drop. Then,a capillary cane with a diameter of 2-3 mm was pulled from thebottom of the furnace. The capillary cane was then re-pulled to formthe desired FF by applying a low vacuum pressure from the top of thecapillary, while the furnace temperature was maintained at 2000 1C.A review of the facilities offered by the UM fiber drawing tower iscovered in Chow et al., 2012 and Alawiah et al., 2013.

The mass of FF and TLD-100 were determined using ananalytical balance BSA224S-CW, (Germany) obtaining values equalto 18.74 mg and 23.62 mg (7 0.05 mg), respectively. The mea-sured TL yield was normalized to unit mass of the TL media.Vacuum tweezers were used for handling and grouping of TLmaterials (Hashim et al., 2009).

2.2. Annealing

The fibers were annealed in a furnace (Harshaw) in order tostandardize their sensitivities and background. The fibers, retainedinside an alumina container were oven annealed at 400 1C for 1 h.Following the annealing cycle fibers were kept inside the ovenallowing their natural cooling down for 24 h to avoid thermalstress, finally the samples were equilibrated at room temperature(Hashim et al., 2010). After cooling, the samples were placed insidean opaque plastic container in order to minimize exposure topotentially high ambient light levels (which could promote de-trapping, otherwise referred to as bleaching), both prior to andafter irradiation.

2.3. Irradiation

The electron irradiations were conducted at Universiti MalayaMedical Centre (UMMC) using a Linear Accelerator (LINAC) VarianModel 2100C. The samples were irradiated using the most com-monly used electron energies for radiotherapy at this centre i.e.9 MeV; doses in the range 1 to 4 Gy were delivered to the TLmaterials. These samples were sandwiched between slabs ofa solid water phantom. The solid water phantom is a soft mattermedium that can be considered to be a standard tissue-equivalentmedium for use in radiation dosimetry. The field and applicatorsize was set to 10 � 10 cm2 and positioned at the standardSource-Surface Distance (SSD) of 100 cm. The dose delivered byLINAC is set at a constant dose rate of 600 MU min�1, where MUsignifies Monitor Units with 1 MU being equivalent to 1 cGy.

2.4. TL measurements

In the present TL measurements, a Harshaw 3500 TL reader(USA) was used together with WinREMS software, enabling onedosimeter to be read out per loading. During readout the followingparameters were used: preheat temperature of 50 1C for 10 s

Table 1Dimension details of pure silica FF in Fig. 1(ii).

Parameter Dimension (mm)

Outer thickness (a) 1.0Inner thickness (b) 0.8Length (L) 3.9Width (W) 3.9

Fig. 1. (i) The cross-sectional area of FF (ii) The FF is a hollow silica tube thatcontains the core and cladding deposited layers (Alawiah et al., 2013).

S. Hashim et al. / Radiation Physics and Chemistry 106 (2015) 46–49 47

(Yaakob et al., 2011b); acquisition temperature 300 1C for 14 s anda heating rate cycle of 25 1C s�1. These settings have been shownto provide an optimal glow curve, free of the effects of superficialtraps i.e. flushed out by the pre-heat cycle (Hashim et al., 2014).Finally, in the present investigations, an annealing temperature of300 1C was applied for 10 s to sweep out any residual signal(Hashim et al., 2014).

3. Results and discussions

3.1. TL glow curve

The important physical factor in order to determine the stability ofthe TL material for dosimetry is the temperature at which the peak ofthe glow curve occurs. Most commercially available TL dosimetershave high glow peak temperatures (i.e. deep electron traps), so thatthey are stable for several months or years. An example of a TL glowcurves obtained from FF after 9 MeV electron irradiations is shown inFig. 2.

This typical pattern of the glow curves obtained from FF is therepresentation of all those measured fiber samples. It is composed ofa broad, dominant peak located between 230–255 1C, which issensitive to the amount of absorbed dose. This convenient positionat relatively high temperature allows us to measure absorbed dose,both at room and high temperature. The general structure of the TL

glow curve remains unchanged by repeating the cycles of annealingand irradiation at various doses. By comparison, the TLD-100 has fivetypical peaks at 65 1C, 120 1C, 160 1C, 195 1C and 210 1C (Portal, 1981).Our findings agrees closely with the TL glow peak of SiO2 optical fibersat 230 1C subjected to 60Co gamma radiation (Espinosa et al., 2006),Ge-doped fibers broad component peaking at 257 1C after X-rayirradiation (Benabdesselam et al., 2013) and the unknown dopedINO-optical fiber had one well-defined glow peak at 32772 1C from aMeV electron beams linear accelerator (Ong et al., 2009).

3.2. Dose response

The sensitivity of a TLD material can be expressed in one of twoways, namely, by the TL yield normalized to the mass of dosimeter orby the TL yield normalized to the mass of the dosimeter and itsabsorbed dose (McKeever et al., 1995). In this work, the latter approachwas utilized to determine the TL sensitivity (TL response.mg�1 Gy �1).Fig. 3 shows the TL dose response of TL materials to electronirradiations.

The TL response of the FF was relatively small compared to that ofTLD-100 i.e. the change in TL yield per unit absorbed dose or slope forFF and TLD-100 are 21.43 mg�1 Gy�1 and 132.20 mg�1 Gy�1,,respec-tively. We found that the FF sensitivity is approximately 16% of TLD-100 media. This clearly indicates that the TLD-100 rod has a bettersensitivity and capability in producing higher TL signals compared toFF (by a factor of 6). Our result was comparable with the TL sensitivityof TLD-100 subjected to electron irradiation of 6-, 15- and 21MeVi.e.�5 times that of FF (Alawiah et al., 2013).

Fig. 4 shows the f (D) supralinearity function of FF where f (D) isa measure of deviation from linearity, given by Eq. (1):

f ðDÞ ¼ FðDÞ=DFðDlÞ=Dl

ð1Þ

where F(D) is the TL signal intensity as a function of dose D and F(Dl) ismeasured at low dose Dl, i.e. somewhere in the linear region of F(D)(Mahajna and Horowitz, 1997). By using Eq. (1), the linear behavior isachieved when f (D)¼1; supralinearity f (D)41 and in sublinearity thef (D)o1.

Fig. 4 shows the data point is clustered close to 1 for doses of 1 Gyup to 2.5 Gy. Then, supralinearity (f (D)41) appears to start at dosesgreater than 3 Gy. Each data point in Fig. 4 is obtained by taking anaverage of three individual fiber readings. The linear/supralinearbehavior may be attributed to the non-uniform spatial ionizationdensity following electron irradiation. This phenomenon have beensuccessfully introduced as the Unified Interaction Model (UNIM)(Horowitz, 2014). In TLD-100, f (D)¼ 1 only up to a few gray. Abovethis we have f (D)41 (supralinearity) reaching values as high as tenat approximately 100 Gy. At even higher doses f(D) decreases dueto recombination, saturation and/or radiation damage and reaches

0

5

10

15

20

25

30

35

40

45

0 50 100 150 200 250 300 350 400

TL In

tens

ity (n

orm

aliz

ed to

uni

t mas

s)

Temperature (°C)

4.0 Gy

3.0 Gy

2.0 Gy

1.0 Gy

Fig. 2. The TL glow curve of FF at various radiation exposures.

TLD-100: y = 132.2x -0.090R² = 0.996

FF: y = 21.43x + 1.117R² = 0.980

-50

0

50

100

150

200

250

300

350

400

0.0 0.5 1.0 1.5 2.0 2.5TL R

espo

nse

(nor

mal

ized

to u

nit m

ass)

Dose (Gy)

Linear ( TLD 100 ) Linear (FF)

Fig. 3. TL response of FF optical fibers and TLD-100 at 9 MeV electron versus dose.

0

0.5

1

1.5

0 0.5 1 1.5 2 2.5 3

f(D)

Dose (Gy)

Fig. 4. Supralinearity function of FF subjected to 9 MeV electron irradiations.

S. Hashim et al. / Radiation Physics and Chemistry 106 (2015) 46–4948

values substantially less than unity in the 103–105 Gy dose region(Horowitz, 1981).

3.3. Minimum detectable dose

The minimum detectable dose (MDD) is important in low dosemeasurements where the signal of an irradiated TLD is almost thesame as the background. The MDD was calculated from Eqs.(2) and (3) (Khan, 2003).

Do ¼ ðBmeanþ2σÞF ð2Þ

F ¼ 1=m ð3ÞThe Bmean is the TL background signal obtained from the TL

samples annealed but not irradiated, σ is the standard deviation ofthe mean background, and F is the TL system calibration factor foreach type of the TL media expressed in Gy nC�1. The value of F isthe reciprocal of the slope, m obtained from the dose responsegraph (Fig. 3) of each particular TL material. In the present work,the MDD was found to be 0.09 mGy and 8.22 mGy for TLD-100 andFF, respectively.

4. Conclusion

We have investigated the performance of FF and TLD-100subjected to 9 MeV electron irradiations including TL glow curve,dose response, sensitivity, linearity and minimum detectable dose.The present dosimeter shows a broad glow curve with a singleprominent peak between 230–255 1C i.e. depends on the radiationdoses being delivered to the FF. The sensitivity of FF is approxi-mately 16% than TLD-100. The TL yield versus dose dependencehas been observed to be linear over the dose range up to 2.5 Gy.The MDD value for FF and TLD-100 was 8.22 mGy and 0.09 mGy,respectively. The literature indicates that radiation dosimetry canbe divided into two regions, i.e. for personal dosimetry, demand-ing high sensitivity at very low doses, and a second region forradiation processing, which require dosimeters capable to mea-sure very high doses. The quest for new, improved TLD materialsmoves on in terms of dose response and the simple glow curveanalysis. In conclusion, the FF shows several promising dosimetricfeatures to be introduced as a new TL material.

Acknowledgments

The authors would like to acknowledge the MOHE HIR Grantnumbers A000007-50001, H-21001-00-F000033 and UniversitiTeknologi Malaysia for providing financial assistance throughResearch University Grant Scheme (RUGS), Project number(Q.J130000.2526.03H27). This research project was partially sup-ported by The Strategic Technologies Program, National Plan forScience, Technology and Innovation (NPSTI) in The Kingdom ofSaudi Arabia under Contract no. 11-MED1977-02.

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