- Email: [email protected]
Comparison of the TL fading characteristics of Ge-doped optical fibres and LiF dosimeters
Applied Radiation and Isotopes 70 (2012) 1384–1387
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Comparison of the TL fading characteristics of Ge-doped optical fibres and LiF dosimeters Noramaliza M. Noor a,n, Nasiha A. Shukor a, M. Hussein b, A. Nisbet a,b, D.A. Bradley a,c a
Centre for Nuclear and Radiation Physics, Department of Physics, University of Surrey, Guildford GU2 7XH, UK Department of Medical Physics, St. Luke’s Cancer Centre, Royal Surrey County Hospital NHS Foundation Trust, Edgerton Road, Guildford GU2 7XX, UK c Department of Radiological Sciences, King Saud University, P.O. Box 10219, Riyadh 11432, Saudi Arabia b
a r t i c l e i n f o
a b s t r a c t
Available online 25 November 2011
Fading is important in choosing appropriate thermoluminescence (TL) materials for particular applications. Comparison is made herein of changes due to fading in the TL yield of Ge-doped fibres and lithium fluoride (LiF) dosimeters, for varying temperature and dose. The fading is independent of dose for all investigated dosimeters while the loss in TL yield reduces for lower storage temperatures. At room temperature and for 133 days of storage, a maximum signal loss of 5% has been observed for both forms of LiF dosimeter, while 9 and 50 mm core diameter Ge-doped fibres produced a loss of 11% and 8%, respectively. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Fading Ge-doped optical fibres LiF dosimeter
1. Introduction
2. Materials and methods
Fading is the loss in TL (thermoluminescence) yield as a result of a delay in readout time following irradiation (post-irradiation fading) or loss of signal sensitivity before irradiation (pre-irradiation fading). It is one of the important dosimetric characteristics of such TL material and has been studied extensively, both theoretically and experimentally for LiF:Mg,Ti (TLD-100, TLD600, TLD-700) (Biran et al., 1996), CaSO4:Dy (TLD-900) (Wang et al., 1987) LiF:Mg,Cu,P (TLD-100H, TLD-600H, TLD-700H) (John et al., 2010; Luo, 2008) and CaF2 (TLD-200, TLD-300, TLD-400) (Yazici et al., 2002). Fading results reported in the literature show considerable disparity, in the range 1–10% at 30 days, due to differences in experimental setup as exemplified by storagetemperature and glow-curve peaks used to determine dose and annealing cycles (Rah et al., 2008). As is well established, fading can result from exposure to light (optical fading) and heat (thermal fading). Other causes of fading are much less well established, often referred to as anomalous fading. The present study focuses on post irradiation fading of Ge-doped optical fibres, supporting efforts to establish the fibres as mailed dosimeters for audit purposes, examining delay in readout time subsequent to irradiation. Results for Ge-doped fibres (9 and 50 mm core diameter) have been compared against those of lithium fluoride thermoluminescent dosimeters (TLD-100 and TLD-700).
2.1. Ge-doped optical fibres dosimeters
n
Corresponding author. E-mail address: [email protected] (N.M. Noor).
0969-8043/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2011.11.026
The optical fibre dosimeters used in the present investigations are single and multi mode commercial Ge-doped optical fibre manufactured by INOCORP (Canada), with core diameters of 9 and 50 mm, respectively. The cladding diameter for both fibres was 11670.1 mm. Four preparatory steps are required prior to the use of the fibres, as detailed by Mohd Noor et al. (2010). In confronting variation in dopant concentration along the length of manufactured fibres (Ramli et al., 2009), a screening procedure has been carried out, establishing individual performance against ionisation chamber measurements positioned at the same depth as the fibres in a perspex phantom. This was performed by irradiating the fibres to a dose of 2 Gy using 6 MV nominal photon energy, with 1 m focus to surface distance (FSD), 0.1 0.1 m2 field size, delivered at a rate of 2 Gy/min. In the present study, fibres with 3% of coefficient variation and reproducibility of better than 5% were used. Finally, fibres providing the same individual sensitivity were retained in groups of ten to be loaded into black plastic capsules, ready to be used. 2.2. Lithium fluoride dosimeters Comparison of response of Ge-doped optical fibres dosimeters was made using the Harshaw lithium fluoride dosimeters TLD-100 and TLD-700 produced in the form of chips, with dimensions 3.2 mm 3.2 mm 0.89 mm. TLD-100 has a 6Li content of 7.5% and a 7Li content of 92.5%, whereas the more expensive TLD-700
N.M. Noor et al. / Applied Radiation and Isotopes 70 (2012) 1384–1387
has respective contents of 0.01 and 99.99%. All LiF dosimeter readings were calibrated against a Farmer-type ionisation chamber. Prior to irradiation, the LiF dosimeters were annealed using a furnace (Pickstone, UK) in order to standardise thermal history and sensitivity, removing residual TL signal. The LiF dosimeters were placed on a flat glass arrangement accommodating up to 100 chips and were annealed at 400 1C for 1 h, followed by 16 h at 80 1C. Finally they were left in an oven to cool down to room temperature. 2.3. Dose levels
1385
(Whatman Hotplate 420, UK) was used to keep other dosmeters at the elevated temperature. The fibres and LiF dosimeters were read out everyday up to seven days and thereafter at once per week for up to 31 days, also reading out the control dosimeters at those same times. 2.5. Reader The Ge-doped fibres and lithium fluoride were assessed using a MODEL 645 E ‘UNIVERSAL TOLEDO’ TLD reader. Nitrogen gas at
A total of 1520 fibres (152 capsules with 10 fibres in each) and 350 chips of LiF dosimeters were irradiated to a series of dose levels, 0.5 Gy, 2 Gy and 10 Gy, delivered at a dose rate of 2 Gy/min using a 6 MV photon beam. Ten of each type of Ge-doped optical fibre dosimeter and five chips of each type of LiF dosimeter were read out immediately after irradiation to obtain data at 0 h. Further fibres and LiF dosimeters were kept in a black box at room temperature (2172) 1C, these control fibres being used to account for background signal, also being stored together with the irradiated dosimeters. Samples of these were read out everyday up to seven days and thereafter at periods of once per week for up to 70 days. Control TLDs were readout at the same time as the other dosimeters. Fading response was also obtained for dosimeters over a more protracted period of time of 133 days, irradiated to a dose of 2 Gy and maintained throughout at room temperature (2172) 1C. Readouts were repeated on new sets of fibres, LiF and control dosimeters at day 112 and finally at day 133. 2.4. Temperature change A total of 440 fibre dosimeters (comprising 44 capsules with 10 fibres in each) and 220 chips of LiF dosimeters were used. As before, these were irradiated to a dose of 2 Gy under the same previous conditions. One capsule for each type of fibre dosimeter and five chips of each type of LiF dosimeter were read out immediately after irradiation, while the remaining capsules of fibres and LiF dosimeters were stored at a temperature of ( 2572) 1C, at room temperature (2172) 1C, and at an elevated temperature of (3572) 1C. This accounts for a possible range of temperature experienced by the fibres during air transportation in use in warm climates. A room freezer at the Faculty of Health and Medical Sciences, University of Surrey, was used to achieve the low temperature while a microprocessor controlled hotplate
Fig. 2. Post-irradiation fade at different dose levels for: (a) Ge-doped fibres with 9 mm core; (b) Ge-doped fibres with 50 mm core; (c) TLD-100; (d) TLD-700. The dosimeters were all stored at room temperature. The lines linking points are not fits but are provided as a guide to the eye.
Fig. 1. (a) The characteristic glow curves of Ge-doped dosimeters obtained with the heating profile mentioned in Section 2.6 and shown as the curve whose scale is depicted on the right-hand y-axis; (b) the characteristic glow curves of lithium fluoride dosimeters obtained with the heating profile outlined in Section 2.6 and shown as the curve whose scale is depicted on the right-hand y-axis.
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N.M. Noor et al. / Applied Radiation and Isotopes 70 (2012) 1384–1387
0.5 bar was used to reduce the oxidation of the heating element and TL dosimeters. During readout the following heating profile was used: preheat temperature of 160 1C for 10 s; readout temperature of 300 1C for 25 s with a linear heating rate of 25 1C/s. An annealing temperature of 300 1C was used for 10 s to remove any residual signal in the fibres.
2.6. Glow curves Glow curves, obtained during readout, provide a plot of TL yield with variation in temperature, being important in gaining a better understanding of the complex nature and properties of the TL medium. As such, each medium will produce characteristic glow curves, the detailed shape also reflecting the choice of temperature ramp-rate. In this study, crystalline (LiF dosimeters) and amorphous (Ge-doped dosimeters) materials have been employed. As illustrated in Fig. 1, the Ge-doped dosimeters show a broad peaked glow curve (Fig. 1a) while LiF dosimeters (TLD100 and TLD-700) show a narrow peaked glow curve (Fig. 1b). Both of these sets of glow curves were obtained as a result of irradiation to a photon-mediated dose of 2 Gy, at a dose rate of 2 Gy/min using a 6 MV photon beam.
3. Results and discussion All the dosimeters responses were normalised to the response at day two post-irradiation. Each data point indicates mean value together with error bars that represent percentage uncertainty for ten Ge-doped optical fibres and five LiF dosimeters. 3.1. Dose levels
Fig. 3. Post-irradiation fading of Ge-doped fibres and LiF dosimeters for storage times up to a maximum of 133 days. As before, the lines linking points are not fits but are provided as a guide to the eye.
Fig. 2a–d shows post irradiation fading for Ge-doped optical fibre dosimeters (of 9 and 50 mm core diameter) and LiF TLD-100 and TLD-700 dosimeters, respectively, at dose levels of 0.5 Gy, 2 Gy and 10 Gy. The dosimeters were all stored at room
Fig. 4. Post-irradiation fade of: (a) Ge-doped fibres with 9 mm core; (b) Ge-doped fibres with 50 mm core; (c) TLD-100; (d) TLD-700 at different temperatures. Again, the lines linking points are not fits but are provided as a guide to the eye.
N.M. Noor et al. / Applied Radiation and Isotopes 70 (2012) 1384–1387
1387
temperature. Within uncertainties, the rate of fading for all investigated TL dosimeter types, across the range of doses, are practically indistinguishable, reducing by some 15% over a period of 70 days. Exceptions are obtained for TLD-700 and 50 mm Ge-doped SiO2 for a dose of 10 Gy, these two TL forms being the most sensitive of dosimeter in each category of TL media. For these, the loss in TL signal after 50 days is particularly pronounced. Fig. 3 shows post irradiation fading for all four forms of dosimeter for storage at room temperature (2172) 1C, for storage times up to at 133 days. TLD-700 gave rise to the least loss in TL signal at 4%, followed by TLD-100 at 5%. A maximum signal loss of 5% has been observed for both forms of LiF dosimeters, being less than the 6% reported elsewhere (Biran et al., 1996). For Ge-doped optical fibre dosimeters of 50 and 9 mm core diameters, the TL signal decreased with storage time by 8% and 11%, respectively.
Ge-doped optical fibres and LiF dosimeters the rate of post-irradiation fade is independent of dose but systematically dependent upon storage temperature. The maximum signal loss, 11%, was obtained for 9 mm core diameter Ge-doped fibres dosimeters following 133 days of storage at room temperature, while for the more sensitive 50 mm core diameter fibres (by a factor of 12), under the same conditions, the fading was significantly less, at 8%.
3.2. Temperature change
References
Fig. 4a–d illustrates the normalised TL signal for Ge-doped optical fibre dosimeters of 9 and 50 mm core diameter and LiF TLD-100 and TLD-700, respectively, as a function of storage time, up to a maximum of 31 days, stored at low temperature ( 2572) 1C, room temperature (2172) 1C and an elevated temperature (35 72) 1C. TL signal loss (fading) is observed to increase with temperature. At low temperature 25 1C, TL signal loss is o4% for Ge-doped optical fibres and is a maximum of 6% for lithium fluoride dosimeters. At room temperature and the elevated temperature, after a storage time of 31 days TL signal decreased by 6% for both types of LiF dosimeter and by o6% for Ge-doped optical fibre dosimeters. The findings for LiF dosimeters are similar to that reported elsewhere (Yazici et al., 2002; Burgkhardt et al., 1976), although not in accord with (Sprunck et al., 1996).
Biran, T.I., Machi, S., Shamai, Y., 1996. Low pre- and post-irradiation fading of LiF:Mg,Ti (TLD-100, TLD-600, TLD-700) using a preheat technique. Radiat. Prot. Dosim. 64, 269–274. Burgkhardt, B., Herrera, R., Piesch, E., 1976. Fading characteristics of different thermoluminescent dosimeters. Nucl. Instrum. Methods 137, 41–47. John, A.H., Nathan, P.H., Kimberlee, J.K., 2010. Characterization of the glow-peak fading properties of six common thermoluminescent materials. Appl. Radiat. Isot. 68, 1988–2000. Luo, L.Z., 2008. Entensive fade study of Harshaw LiF TLD materials. Radiat. Meas. 43, 365–370. Mohd Noor, N., Hussein, M., Bradley, D.A., Nisbet, A., 2010. Investigation of the use of Ge-doped optical fibre for in vitro IMRT prostate dosimetry. Nucl. Instrum. Methods Phys. Res. Sec. A: Accel., Spectrom., Detect. Assoc. Equip. 619, 157–162. Rah, J.E., Hong, J.Y., Kim, G.Y., Kim, Y.L., Shin, D.O., Suh, T.S., 2008. A comparison of the dosimetric characteristics of a glass rod dosimeter and a thermoluminescent dosimeter for mailed dosimeter. Radiat. Meas. 44, 18–22. 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, 428–432. Sprunck, M., Regulla, D., Fill, U., 1996. TL measurements with computerised glow curve analysis in clinical dosimetry. Radiat. Prot. Dosim. 66, 269–271. Wang, T.K., Hsu, P.C., Weng, P.S., 1987. Feasibility study of using CaSO4:Dy phosphor for simultaneous estimation of exposure and time elapsed postexposure. Radiat. Prot. Dosim. 18, 157–161. Yazici, A.N., Chen, R., Solak, S., Yagingil, Z., 2002. The analysis of thermoluminescent glow peaks of CaF2:Dy (TLD-200) after b-irradiation. J. Phys. D: Appl. Phys. 35, 2526–2535.
4. Conclusion Results obtained in this study demonstrate accountable systematic fading of Ge-doped fibres dosimeters, making them suitable for trans-national dose audit programmes. For both
Acknowledgements The first author would like to acknowledge the University of Science of Malaysia and the Ministry of Higher Education, Malaysia for sponsorship under the Academic Staff Training Scheme (ASTS).
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Comparison of the TL fading characteristics of Ge-doped optical fibres and LiF dosimeters Noramaliza M. Noor a,n, Nasiha A. Shukor a, M. Hussein b, A. Nisbet a,b, D.A. Bradley a,c a
Centre for Nuclear and Radiation Physics, Department of Physics, University of Surrey, Guildford GU2 7XH, UK Department of Medical Physics, St. Luke’s Cancer Centre, Royal Surrey County Hospital NHS Foundation Trust, Edgerton Road, Guildford GU2 7XX, UK c Department of Radiological Sciences, King Saud University, P.O. Box 10219, Riyadh 11432, Saudi Arabia b
a r t i c l e i n f o
a b s t r a c t
Available online 25 November 2011
Fading is important in choosing appropriate thermoluminescence (TL) materials for particular applications. Comparison is made herein of changes due to fading in the TL yield of Ge-doped fibres and lithium fluoride (LiF) dosimeters, for varying temperature and dose. The fading is independent of dose for all investigated dosimeters while the loss in TL yield reduces for lower storage temperatures. At room temperature and for 133 days of storage, a maximum signal loss of 5% has been observed for both forms of LiF dosimeter, while 9 and 50 mm core diameter Ge-doped fibres produced a loss of 11% and 8%, respectively. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Fading Ge-doped optical fibres LiF dosimeter
1. Introduction
2. Materials and methods
Fading is the loss in TL (thermoluminescence) yield as a result of a delay in readout time following irradiation (post-irradiation fading) or loss of signal sensitivity before irradiation (pre-irradiation fading). It is one of the important dosimetric characteristics of such TL material and has been studied extensively, both theoretically and experimentally for LiF:Mg,Ti (TLD-100, TLD600, TLD-700) (Biran et al., 1996), CaSO4:Dy (TLD-900) (Wang et al., 1987) LiF:Mg,Cu,P (TLD-100H, TLD-600H, TLD-700H) (John et al., 2010; Luo, 2008) and CaF2 (TLD-200, TLD-300, TLD-400) (Yazici et al., 2002). Fading results reported in the literature show considerable disparity, in the range 1–10% at 30 days, due to differences in experimental setup as exemplified by storagetemperature and glow-curve peaks used to determine dose and annealing cycles (Rah et al., 2008). As is well established, fading can result from exposure to light (optical fading) and heat (thermal fading). Other causes of fading are much less well established, often referred to as anomalous fading. The present study focuses on post irradiation fading of Ge-doped optical fibres, supporting efforts to establish the fibres as mailed dosimeters for audit purposes, examining delay in readout time subsequent to irradiation. Results for Ge-doped fibres (9 and 50 mm core diameter) have been compared against those of lithium fluoride thermoluminescent dosimeters (TLD-100 and TLD-700).
2.1. Ge-doped optical fibres dosimeters
n
Corresponding author. E-mail address: [email protected] (N.M. Noor).
0969-8043/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2011.11.026
The optical fibre dosimeters used in the present investigations are single and multi mode commercial Ge-doped optical fibre manufactured by INOCORP (Canada), with core diameters of 9 and 50 mm, respectively. The cladding diameter for both fibres was 11670.1 mm. Four preparatory steps are required prior to the use of the fibres, as detailed by Mohd Noor et al. (2010). In confronting variation in dopant concentration along the length of manufactured fibres (Ramli et al., 2009), a screening procedure has been carried out, establishing individual performance against ionisation chamber measurements positioned at the same depth as the fibres in a perspex phantom. This was performed by irradiating the fibres to a dose of 2 Gy using 6 MV nominal photon energy, with 1 m focus to surface distance (FSD), 0.1 0.1 m2 field size, delivered at a rate of 2 Gy/min. In the present study, fibres with 3% of coefficient variation and reproducibility of better than 5% were used. Finally, fibres providing the same individual sensitivity were retained in groups of ten to be loaded into black plastic capsules, ready to be used. 2.2. Lithium fluoride dosimeters Comparison of response of Ge-doped optical fibres dosimeters was made using the Harshaw lithium fluoride dosimeters TLD-100 and TLD-700 produced in the form of chips, with dimensions 3.2 mm 3.2 mm 0.89 mm. TLD-100 has a 6Li content of 7.5% and a 7Li content of 92.5%, whereas the more expensive TLD-700
N.M. Noor et al. / Applied Radiation and Isotopes 70 (2012) 1384–1387
has respective contents of 0.01 and 99.99%. All LiF dosimeter readings were calibrated against a Farmer-type ionisation chamber. Prior to irradiation, the LiF dosimeters were annealed using a furnace (Pickstone, UK) in order to standardise thermal history and sensitivity, removing residual TL signal. The LiF dosimeters were placed on a flat glass arrangement accommodating up to 100 chips and were annealed at 400 1C for 1 h, followed by 16 h at 80 1C. Finally they were left in an oven to cool down to room temperature. 2.3. Dose levels
1385
(Whatman Hotplate 420, UK) was used to keep other dosmeters at the elevated temperature. The fibres and LiF dosimeters were read out everyday up to seven days and thereafter at once per week for up to 31 days, also reading out the control dosimeters at those same times. 2.5. Reader The Ge-doped fibres and lithium fluoride were assessed using a MODEL 645 E ‘UNIVERSAL TOLEDO’ TLD reader. Nitrogen gas at
A total of 1520 fibres (152 capsules with 10 fibres in each) and 350 chips of LiF dosimeters were irradiated to a series of dose levels, 0.5 Gy, 2 Gy and 10 Gy, delivered at a dose rate of 2 Gy/min using a 6 MV photon beam. Ten of each type of Ge-doped optical fibre dosimeter and five chips of each type of LiF dosimeter were read out immediately after irradiation to obtain data at 0 h. Further fibres and LiF dosimeters were kept in a black box at room temperature (2172) 1C, these control fibres being used to account for background signal, also being stored together with the irradiated dosimeters. Samples of these were read out everyday up to seven days and thereafter at periods of once per week for up to 70 days. Control TLDs were readout at the same time as the other dosimeters. Fading response was also obtained for dosimeters over a more protracted period of time of 133 days, irradiated to a dose of 2 Gy and maintained throughout at room temperature (2172) 1C. Readouts were repeated on new sets of fibres, LiF and control dosimeters at day 112 and finally at day 133. 2.4. Temperature change A total of 440 fibre dosimeters (comprising 44 capsules with 10 fibres in each) and 220 chips of LiF dosimeters were used. As before, these were irradiated to a dose of 2 Gy under the same previous conditions. One capsule for each type of fibre dosimeter and five chips of each type of LiF dosimeter were read out immediately after irradiation, while the remaining capsules of fibres and LiF dosimeters were stored at a temperature of ( 2572) 1C, at room temperature (2172) 1C, and at an elevated temperature of (3572) 1C. This accounts for a possible range of temperature experienced by the fibres during air transportation in use in warm climates. A room freezer at the Faculty of Health and Medical Sciences, University of Surrey, was used to achieve the low temperature while a microprocessor controlled hotplate
Fig. 2. Post-irradiation fade at different dose levels for: (a) Ge-doped fibres with 9 mm core; (b) Ge-doped fibres with 50 mm core; (c) TLD-100; (d) TLD-700. The dosimeters were all stored at room temperature. The lines linking points are not fits but are provided as a guide to the eye.
Fig. 1. (a) The characteristic glow curves of Ge-doped dosimeters obtained with the heating profile mentioned in Section 2.6 and shown as the curve whose scale is depicted on the right-hand y-axis; (b) the characteristic glow curves of lithium fluoride dosimeters obtained with the heating profile outlined in Section 2.6 and shown as the curve whose scale is depicted on the right-hand y-axis.
1386
N.M. Noor et al. / Applied Radiation and Isotopes 70 (2012) 1384–1387
0.5 bar was used to reduce the oxidation of the heating element and TL dosimeters. During readout the following heating profile was used: preheat temperature of 160 1C for 10 s; readout temperature of 300 1C for 25 s with a linear heating rate of 25 1C/s. An annealing temperature of 300 1C was used for 10 s to remove any residual signal in the fibres.
2.6. Glow curves Glow curves, obtained during readout, provide a plot of TL yield with variation in temperature, being important in gaining a better understanding of the complex nature and properties of the TL medium. As such, each medium will produce characteristic glow curves, the detailed shape also reflecting the choice of temperature ramp-rate. In this study, crystalline (LiF dosimeters) and amorphous (Ge-doped dosimeters) materials have been employed. As illustrated in Fig. 1, the Ge-doped dosimeters show a broad peaked glow curve (Fig. 1a) while LiF dosimeters (TLD100 and TLD-700) show a narrow peaked glow curve (Fig. 1b). Both of these sets of glow curves were obtained as a result of irradiation to a photon-mediated dose of 2 Gy, at a dose rate of 2 Gy/min using a 6 MV photon beam.
3. Results and discussion All the dosimeters responses were normalised to the response at day two post-irradiation. Each data point indicates mean value together with error bars that represent percentage uncertainty for ten Ge-doped optical fibres and five LiF dosimeters. 3.1. Dose levels
Fig. 3. Post-irradiation fading of Ge-doped fibres and LiF dosimeters for storage times up to a maximum of 133 days. As before, the lines linking points are not fits but are provided as a guide to the eye.
Fig. 2a–d shows post irradiation fading for Ge-doped optical fibre dosimeters (of 9 and 50 mm core diameter) and LiF TLD-100 and TLD-700 dosimeters, respectively, at dose levels of 0.5 Gy, 2 Gy and 10 Gy. The dosimeters were all stored at room
Fig. 4. Post-irradiation fade of: (a) Ge-doped fibres with 9 mm core; (b) Ge-doped fibres with 50 mm core; (c) TLD-100; (d) TLD-700 at different temperatures. Again, the lines linking points are not fits but are provided as a guide to the eye.
N.M. Noor et al. / Applied Radiation and Isotopes 70 (2012) 1384–1387
1387
temperature. Within uncertainties, the rate of fading for all investigated TL dosimeter types, across the range of doses, are practically indistinguishable, reducing by some 15% over a period of 70 days. Exceptions are obtained for TLD-700 and 50 mm Ge-doped SiO2 for a dose of 10 Gy, these two TL forms being the most sensitive of dosimeter in each category of TL media. For these, the loss in TL signal after 50 days is particularly pronounced. Fig. 3 shows post irradiation fading for all four forms of dosimeter for storage at room temperature (2172) 1C, for storage times up to at 133 days. TLD-700 gave rise to the least loss in TL signal at 4%, followed by TLD-100 at 5%. A maximum signal loss of 5% has been observed for both forms of LiF dosimeters, being less than the 6% reported elsewhere (Biran et al., 1996). For Ge-doped optical fibre dosimeters of 50 and 9 mm core diameters, the TL signal decreased with storage time by 8% and 11%, respectively.
Ge-doped optical fibres and LiF dosimeters the rate of post-irradiation fade is independent of dose but systematically dependent upon storage temperature. The maximum signal loss, 11%, was obtained for 9 mm core diameter Ge-doped fibres dosimeters following 133 days of storage at room temperature, while for the more sensitive 50 mm core diameter fibres (by a factor of 12), under the same conditions, the fading was significantly less, at 8%.
3.2. Temperature change
References
Fig. 4a–d illustrates the normalised TL signal for Ge-doped optical fibre dosimeters of 9 and 50 mm core diameter and LiF TLD-100 and TLD-700, respectively, as a function of storage time, up to a maximum of 31 days, stored at low temperature ( 2572) 1C, room temperature (2172) 1C and an elevated temperature (35 72) 1C. TL signal loss (fading) is observed to increase with temperature. At low temperature 25 1C, TL signal loss is o4% for Ge-doped optical fibres and is a maximum of 6% for lithium fluoride dosimeters. At room temperature and the elevated temperature, after a storage time of 31 days TL signal decreased by 6% for both types of LiF dosimeter and by o6% for Ge-doped optical fibre dosimeters. The findings for LiF dosimeters are similar to that reported elsewhere (Yazici et al., 2002; Burgkhardt et al., 1976), although not in accord with (Sprunck et al., 1996).
Biran, T.I., Machi, S., Shamai, Y., 1996. Low pre- and post-irradiation fading of LiF:Mg,Ti (TLD-100, TLD-600, TLD-700) using a preheat technique. Radiat. Prot. Dosim. 64, 269–274. Burgkhardt, B., Herrera, R., Piesch, E., 1976. Fading characteristics of different thermoluminescent dosimeters. Nucl. Instrum. Methods 137, 41–47. John, A.H., Nathan, P.H., Kimberlee, J.K., 2010. Characterization of the glow-peak fading properties of six common thermoluminescent materials. Appl. Radiat. Isot. 68, 1988–2000. Luo, L.Z., 2008. Entensive fade study of Harshaw LiF TLD materials. Radiat. Meas. 43, 365–370. Mohd Noor, N., Hussein, M., Bradley, D.A., Nisbet, A., 2010. Investigation of the use of Ge-doped optical fibre for in vitro IMRT prostate dosimetry. Nucl. Instrum. Methods Phys. Res. Sec. A: Accel., Spectrom., Detect. Assoc. Equip. 619, 157–162. Rah, J.E., Hong, J.Y., Kim, G.Y., Kim, Y.L., Shin, D.O., Suh, T.S., 2008. A comparison of the dosimetric characteristics of a glass rod dosimeter and a thermoluminescent dosimeter for mailed dosimeter. Radiat. Meas. 44, 18–22. 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, 428–432. Sprunck, M., Regulla, D., Fill, U., 1996. TL measurements with computerised glow curve analysis in clinical dosimetry. Radiat. Prot. Dosim. 66, 269–271. Wang, T.K., Hsu, P.C., Weng, P.S., 1987. Feasibility study of using CaSO4:Dy phosphor for simultaneous estimation of exposure and time elapsed postexposure. Radiat. Prot. Dosim. 18, 157–161. Yazici, A.N., Chen, R., Solak, S., Yagingil, Z., 2002. The analysis of thermoluminescent glow peaks of CaF2:Dy (TLD-200) after b-irradiation. J. Phys. D: Appl. Phys. 35, 2526–2535.
4. Conclusion Results obtained in this study demonstrate accountable systematic fading of Ge-doped fibres dosimeters, making them suitable for trans-national dose audit programmes. For both
Acknowledgements The first author would like to acknowledge the University of Science of Malaysia and the Ministry of Higher Education, Malaysia for sponsorship under the Academic Staff Training Scheme (ASTS).