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Response of plastic scintillators to low-energy photons
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Phys. Med. Biol. 59 4621 (http://iopscience.iop.org/0031-9155/59/16/4621) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 132.239.1.231 This content was downloaded on 02/06/2017 at 22:42 Please note that terms and conditions apply.
You may also be interested in: A systematic characterization of the low-energy photon response of plastic scintillation detectors Jonathan Boivin, Sam Beddar, Chris Bonde et al. Review of plastic and liquid scintillation dosimetry for photon, electron, and proton therapy Luc Beaulieu and Sam Beddar Backscatter factors and mass energy-absorption coefficient ratios for diagnostic radiology dosimetry Hamza Benmakhlouf, Hugo Bouchard, Annette Fransson et al. Determination of the energy dependence of the BC-408 plastic scintillation detector in medium energy x-ray beams H Yücel, Çubukçu, E Uyar et al. Monte Carlo modelling of radiotherapy kV x-ray units F Verhaegen, A E Nahum, S Van de Putte et al. Advances in kilovoltage x-ray beam dosimetry Robin Hill, Brendan Healy, Lois Holloway et al. Thimble ionization chambers in medium-energy x-ray beams F Ubrich, J Wulff, R Kranzer et al.
Institute of Physics and Engineering in Medicine Phys. Med. Biol. 59 (2014) 4621–4633
Physics in Medicine & Biology doi:10.1088/0031-9155/59/16/4621
Response of plastic scintillators to low-energy photons Luis Peralta1,2 and Florbela Rêgo1 1
Laboratório de Instrumentação e Física Experimental de Partículas, Lisboa, Portugal 2 Universidade de Lisboa, Faculdade de Ciências, Departamento de Física, Portugal and Laboratório de Instrumentação e Física Experimental de Partículas, Lisboa, Portugal Received 2 March 2014, revised 21 June 2014 Accepted for publication 25 June 2014 Published 31 July 2014 Abstract
Diagnostic radiology typically uses x-ray beams between 25 and 150 kVp. Plastic scintillation detectors (PSDs) are potentially successful candidates as field dosimeters but careful selection of the scintillator is crucial. It has been demonstrated that they can suffer from energy dependence in the low-energy region, an undesirable dosimeter characteristic. This dependence is partially due to the nonlinear light yield of the scintillator to the low-energy electrons set in motion by the photon beam. In this work, PSDs made of PMMA, PVT or polystyrene were studied for the x-ray beam range 25 to 100 kVp. For each kVp data has been acquired for additional aluminium filtrations of 0.5, 1.0, 2.0 and 4.0 mm. Absolute dose in the point of measurement was obtained with an ionization chamber calibrated to dose in water. From the collected data, detector sensitivities were obtained as function of the beam kVp and additional filtration. Using Monte Carlo simulations relative scintillator sensitivities were computed. For some of the scintillators these sensitivities show strong energy-dependence for beam average energy below 35 keV for each additional filtration but fair constancy above. One of the scintillators (BC-404) has smaller energy-dependence at low photon average energy and could be considered a candidate for applications (like mammography) where beam energy has small span. Keywords: plastic scintillators, light collection, dosimetry, radiology (Some figures may appear in colour only in the online journal)
0031-9155/14/164621+14$33.00 © 2014 Institute of Physics and Engineering in Medicine Printed in the UK & the USA 4621
L Peralta and F Rêgo
Phys. Med. Biol. 59 (2014) 4621
1. Introduction Plastic scintillators have been used in dosimetry for a long time (Beddar et al 1992a,b). Extensive studies have been conducted on their properties and their applications to megavoltage photon beams are now well established (Beaulieu et al 2013). Its use for low energy photon beams (in the range useful for radiology) is more problematic and their behaviour less well-known. In 1999 Williamson et al (1999) demonstrated that plastic scintillators exhibit a dependence of the ‘inherent sensitivity’ as a function of effective energy. The observed effect was explained as a consequence of a quenching process being the scintillation light yield not proportional to the deposited energy. The dependence of scintillation light yield on electron/ photon energy was measured by Frelin et al (2008) for some PSDs reporting a strong variation in the low energy (less than 100 keV) limit. On the other hand it was argued by Lessard et al (2012) that the BCF-60 plastic scintillators can be used for dosimetry in the 80 to 150 kVp beam range. Other authors (Rego 2010, Rego and Peralta 2008, Jones and Hintenlang 2008) reported energy dependence of several dosimeters based on PSDs for beams in the 20 to 100 kVp range. For a given beam kVp, PSDs present a very good linearity with air-kerma (Moutinho et al 2014) or dose in water (Rego 2010, Rego and Peralta 2012). PSDs present some interesting features that make them attractive for low-energy kilovoltage x-ray beam dosimetry. Their image is hard to differentiate from soft tissue on an x-ray image. They have small temperature dependence (Nowotny and Taubeck 2009, Buranurak et al 2013, Wootton and Beddar 2013), they can assume different shapes and sizes. A number of photodetectors (PMT’s, photodiodes, SiPM, etc) can be used as readout devices, adding flexibility to the system. In the low-energy region, below the Cherenkov threshold, background light (stem effect) has its main origin in fluorescence processes in the optical cable transmitting the signal to the photodetector. This background light can be reduced using low-luminescence PMMA optical fibers (Moutinho et al 2014) or by carefully-choosing the photodetector’s sensitivity window outside the fluorescent light spectrum region (Lessard et al 2012). We study scintillators made of three different materials: PMMA (Polymethyl Methacrylate), PVT (Polyvinyltoluene) and Polystyrene. The scintillators were coupled to PMMA optical cables and read by a PMT. The sensitivity of the dosimeters thus build has been measured for x-ray beams in the range 25 to 100 kVp and for 4 additional aluminium filtrations (0.5, 1.0, 2.0 and 4.0 mm). 2. The experimental setup A schematic of the experimental setup is presented in figure 1. A Philips PW2184/00 x-ray tube (Panalytical 2014) with a 25 μm Be window and with accelerating potentials between 25 and 100 kV was used to generate the x-ray beams. The beam was collimated using lead bricks. Additional Al filtration of 0.5, 1.0, 2.0 and 4.0 mm was used to harden the x-ray beam. The scintillators (Sc) to be measured were placed on the surface of a 20 × 15 × 10 cm3 PMMA phantom, in such a way that the scintillator face was at a distance of 70.0 cm from the x-ray beam window. The light produced at the scintillator was collected by an optical cable made of an ESKA SK-80 PMMA fibre 2.0 mm in diameter (Mitsubishi 2014) and 120 cm-long. The fibre was protected from daylight by a black plastic jacket (a 2.4 mm in diameter heat-shrinkable type sleeve). The optical cable was read by a Hamamatsu R647P PMT (Hamamatsu 2014). The PMT was biased with negative voltage by a high-stability (
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Contact us
My IOPscience
Response of plastic scintillators to low-energy photons
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Phys. Med. Biol. 59 4621 (http://iopscience.iop.org/0031-9155/59/16/4621) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 132.239.1.231 This content was downloaded on 02/06/2017 at 22:42 Please note that terms and conditions apply.
You may also be interested in: A systematic characterization of the low-energy photon response of plastic scintillation detectors Jonathan Boivin, Sam Beddar, Chris Bonde et al. Review of plastic and liquid scintillation dosimetry for photon, electron, and proton therapy Luc Beaulieu and Sam Beddar Backscatter factors and mass energy-absorption coefficient ratios for diagnostic radiology dosimetry Hamza Benmakhlouf, Hugo Bouchard, Annette Fransson et al. Determination of the energy dependence of the BC-408 plastic scintillation detector in medium energy x-ray beams H Yücel, Çubukçu, E Uyar et al. Monte Carlo modelling of radiotherapy kV x-ray units F Verhaegen, A E Nahum, S Van de Putte et al. Advances in kilovoltage x-ray beam dosimetry Robin Hill, Brendan Healy, Lois Holloway et al. Thimble ionization chambers in medium-energy x-ray beams F Ubrich, J Wulff, R Kranzer et al.
Institute of Physics and Engineering in Medicine Phys. Med. Biol. 59 (2014) 4621–4633
Physics in Medicine & Biology doi:10.1088/0031-9155/59/16/4621
Response of plastic scintillators to low-energy photons Luis Peralta1,2 and Florbela Rêgo1 1
Laboratório de Instrumentação e Física Experimental de Partículas, Lisboa, Portugal 2 Universidade de Lisboa, Faculdade de Ciências, Departamento de Física, Portugal and Laboratório de Instrumentação e Física Experimental de Partículas, Lisboa, Portugal Received 2 March 2014, revised 21 June 2014 Accepted for publication 25 June 2014 Published 31 July 2014 Abstract
Diagnostic radiology typically uses x-ray beams between 25 and 150 kVp. Plastic scintillation detectors (PSDs) are potentially successful candidates as field dosimeters but careful selection of the scintillator is crucial. It has been demonstrated that they can suffer from energy dependence in the low-energy region, an undesirable dosimeter characteristic. This dependence is partially due to the nonlinear light yield of the scintillator to the low-energy electrons set in motion by the photon beam. In this work, PSDs made of PMMA, PVT or polystyrene were studied for the x-ray beam range 25 to 100 kVp. For each kVp data has been acquired for additional aluminium filtrations of 0.5, 1.0, 2.0 and 4.0 mm. Absolute dose in the point of measurement was obtained with an ionization chamber calibrated to dose in water. From the collected data, detector sensitivities were obtained as function of the beam kVp and additional filtration. Using Monte Carlo simulations relative scintillator sensitivities were computed. For some of the scintillators these sensitivities show strong energy-dependence for beam average energy below 35 keV for each additional filtration but fair constancy above. One of the scintillators (BC-404) has smaller energy-dependence at low photon average energy and could be considered a candidate for applications (like mammography) where beam energy has small span. Keywords: plastic scintillators, light collection, dosimetry, radiology (Some figures may appear in colour only in the online journal)
0031-9155/14/164621+14$33.00 © 2014 Institute of Physics and Engineering in Medicine Printed in the UK & the USA 4621
L Peralta and F Rêgo
Phys. Med. Biol. 59 (2014) 4621
1. Introduction Plastic scintillators have been used in dosimetry for a long time (Beddar et al 1992a,b). Extensive studies have been conducted on their properties and their applications to megavoltage photon beams are now well established (Beaulieu et al 2013). Its use for low energy photon beams (in the range useful for radiology) is more problematic and their behaviour less well-known. In 1999 Williamson et al (1999) demonstrated that plastic scintillators exhibit a dependence of the ‘inherent sensitivity’ as a function of effective energy. The observed effect was explained as a consequence of a quenching process being the scintillation light yield not proportional to the deposited energy. The dependence of scintillation light yield on electron/ photon energy was measured by Frelin et al (2008) for some PSDs reporting a strong variation in the low energy (less than 100 keV) limit. On the other hand it was argued by Lessard et al (2012) that the BCF-60 plastic scintillators can be used for dosimetry in the 80 to 150 kVp beam range. Other authors (Rego 2010, Rego and Peralta 2008, Jones and Hintenlang 2008) reported energy dependence of several dosimeters based on PSDs for beams in the 20 to 100 kVp range. For a given beam kVp, PSDs present a very good linearity with air-kerma (Moutinho et al 2014) or dose in water (Rego 2010, Rego and Peralta 2012). PSDs present some interesting features that make them attractive for low-energy kilovoltage x-ray beam dosimetry. Their image is hard to differentiate from soft tissue on an x-ray image. They have small temperature dependence (Nowotny and Taubeck 2009, Buranurak et al 2013, Wootton and Beddar 2013), they can assume different shapes and sizes. A number of photodetectors (PMT’s, photodiodes, SiPM, etc) can be used as readout devices, adding flexibility to the system. In the low-energy region, below the Cherenkov threshold, background light (stem effect) has its main origin in fluorescence processes in the optical cable transmitting the signal to the photodetector. This background light can be reduced using low-luminescence PMMA optical fibers (Moutinho et al 2014) or by carefully-choosing the photodetector’s sensitivity window outside the fluorescent light spectrum region (Lessard et al 2012). We study scintillators made of three different materials: PMMA (Polymethyl Methacrylate), PVT (Polyvinyltoluene) and Polystyrene. The scintillators were coupled to PMMA optical cables and read by a PMT. The sensitivity of the dosimeters thus build has been measured for x-ray beams in the range 25 to 100 kVp and for 4 additional aluminium filtrations (0.5, 1.0, 2.0 and 4.0 mm). 2. The experimental setup A schematic of the experimental setup is presented in figure 1. A Philips PW2184/00 x-ray tube (Panalytical 2014) with a 25 μm Be window and with accelerating potentials between 25 and 100 kV was used to generate the x-ray beams. The beam was collimated using lead bricks. Additional Al filtration of 0.5, 1.0, 2.0 and 4.0 mm was used to harden the x-ray beam. The scintillators (Sc) to be measured were placed on the surface of a 20 × 15 × 10 cm3 PMMA phantom, in such a way that the scintillator face was at a distance of 70.0 cm from the x-ray beam window. The light produced at the scintillator was collected by an optical cable made of an ESKA SK-80 PMMA fibre 2.0 mm in diameter (Mitsubishi 2014) and 120 cm-long. The fibre was protected from daylight by a black plastic jacket (a 2.4 mm in diameter heat-shrinkable type sleeve). The optical cable was read by a Hamamatsu R647P PMT (Hamamatsu 2014). The PMT was biased with negative voltage by a high-stability (
