Radioluminescence Response of Ce-, Cu-, and Gd-Doped Silica Glasses for Dosimetry of Pulsed Electron Beams

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Radioluminescence Response of Ce-, Cu-, and Gd-Doped Silica Glasses for Dosimetry of Pulsed Electron Beams © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Published version Söderström, Daniel; Kettunen, Heikki; Morana, Adriana; Javanainen, Arto; Ouerdane, Youcef; El Hamzaoui, Hicham; Capoen, Bruno; Bouwmans, Géraud; Bouazaoui, Mohamed; Girard, Sylvain Söderström, D., Kettunen, H., Morana, A., Javanainen, A., Ouerdane, Y., El Hamzaoui, H., Capoen, B., Bouwmans, G., Bouazaoui, M., & Girard, S. (2021). Radioluminescence Response of Ce-, Cu-, and Gd-Doped Silica Glasses for Dosimetry of Pulsed Electron Beams. Sensors, 21(22), Article 7523. https://doi.org/10.3390/s21227523 2021 sensors Article Radioluminescence Response of Ce-, Cu-, and Gd-Doped Silica Glasses for Dosimetry of Pulsed Electron Beams Daniel Söderström 1,* , Heikki Kettunen 1, Adriana Morana 2, Arto Javanainen 1,3 , Youcef Ouerdane 2, Hicham El Hamzaoui 4,*, Bruno Capoen 4, Géraud Bouwmans 4, Mohamed Bouazaoui 4and Sylvain Girard 2   Citation: Söderström, D.; Kettunen, H.; Morana, A.; Javanainen, A.; Ouerdane, Y.; El Hamzaoui, H.; Capoen, B.; Bouwmans, G.; Bouazaoui, M.; Girard, S. Radioluminescence Response of Ce-, Cu-, and Gd-Doped Silica Glasses for Dosimetry of Pulsed Electron Beams. Sensors 2021,21, 7523. https:// doi.org/10.3390/s21227523 Academic Editors: Flavio Esposito, Agostino Iadicicco, Stefania Campopiano, Andrei Stancalie and Flavio Esposito Received: 13 October 2021 Accepted: 8 November 2021 Published: 12 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Department of Physics, University of Jyväskylä, Survontie 9D, 40500 Jyväskylä, Finland; [email protected] (H.K.); [email protected] (A.J.) 2UJM, CNRS, IOGS, Laboratoire Hubert Curien, University of Lyon, UMR 5516, 18 rue Prof. B. Lauras, F-42000 Saint-Etienne, France; [email protected] (A.M.); [email protected] (Y.O.); [email protected] (S.G.) 3Department of Electrical and Computer Engineering, Vanderbilt University, Nashville, TN 37235, USA 4Univ-Lille, CNRS, UMR 8523-PhLAM-Physique des Lasers Atomes et Molécules, F-59000 Lille, France; [email protected] (B.C.); [email protected] (G.B.); [email protected] (M.B.) *Correspondence: [email protected] (D.S.); [email protected] (H.E.H.) Abstract: Radiation-induced emission of doped sol-gel silica glass samples was investigated under a pulsed 20-MeV electron beam. The studied samples were drawn rods doped with cerium, copper, or gadolinium ions, which were connected to multimode pure-silica core fibers to transport the induced luminescence from the irradiation area to a signal readout system. The luminescence pulses in the samples induced by the electron bunches were studied as a function of deposited dose per electron bunch. All the investigated samples were found to have a linear response in terms of luminescence as a function of electron bunch sizes between 10 −5 Gy/bunch and 1.5 × 10 −2 Gy/bunch. The presented results show that these types of doped silica rods can be used for monitoring a pulsed electron beam, as well as to evaluate the dose deposited by the individual electron bunches. The electron accelerator used in the experiment was a medical type used for radiation therapy treatments, and these silica rod samples show high potential for dosimetry in radiotherapy contexts. Keywords: dosimetry; electron accelerator; optical fiber; point dosimeter; pulsed electron beam; radiation-induced luminescence 1. Introduction A large part of the previous studies on radiation-induced luminescence (RIL) of doped silica glasses for ionizing radiation dosimetry has been done under X-ray irradiation. Regarding particle beams, most of the studies focused on proton beams. Examples of such studies are reported in [ 1 ], where Ce 3+ - and Cu + -doped samples were evaluated for proton therapy dosimetry purposes, and in [ 2 ], where Gd 3+ -doped silica glass was also studied. In these studies, a dose rate range of about 0.02–0.30 Gy/s from a continuous beam was used, and proton energies between 8 and 63 MeV were investigated. In [ 2 ], dose-depth profiles were also studied using Gd-, Cu-, and Ce-doped samples. A further review of optical fibre-based dosimetry for radiotherapy is reported in [3]. Ce-doped silica glass has been studied under X-ray irradiation in e.g., [ 4 ], where the optically stimulated luminescence (OSL) and RIL properties of the sample were investigated, and a linear RIL output for continuous dose rates between at least 26 and 1187 mGy/s was found. The electronic transition 4f–5d in Ce 3+ ions is the basis for RIL emission, which is discussed in e.g., [4–6]. Ce-activated silica glass was also studied in [ 5 ]. The doped glass was tested with a continuous X-ray beam up to a dose rate of 50 Gy/s, with a linear output up to 30 Gy/s. At dose rates higher than 30 Gy/s, a luminescence response over the linear trend was observed. Sensors 2021,21, 7523. https://doi.org/10.3390/s21227523 https://www.mdpi.com/journal/sensors Sensors 2021,21, 7523 2 of 16 Silica glass doped with Gd 3+ -ions has been studied in [ 7 ], where the RIL response under steady-state X-ray irradiation was found to be linear between at least 125 µGy (SiO 2 )/s and 12.25 Gy (SiO 2 )/s. The RIL of Gd 3+ ions is ascribed to the transition between the 6 P 7/2 and 8S7/2 levels [2,7,8]. Cu-doped silica samples were studied in e.g., [ 9 ], in the shape of a photonic crystal fibre (PCF) under UV light excitation, and in [ 10 ], under X-ray irradiation. In [ 10 ], a linear trend of the luminescence response was also reported up to a dose rate of 30 Gy/s, and then a response over the linear trend above 30 Gy/s, as was the case for Ce-doped samples in [ 5 ]. In Cu + ions, the transitions responsible for the RL emission are those from the state 3d94s to the ground state 3d10 [9,11]. Very limited results of doped silica glass RIL responses to pulsed electron beams exist in the literature. In [ 12 ], a scintillating material (terbium-activated gadolinium oxysulfide) was placed in contact with a light-guiding fibre. The sample was then subjected to a beam of pulsed X-rays from a clinical linear accelerator (Clinac ® ). Studies of electron beams include those of thermoluminescence (TL) of Ge-doped optical fibers, such as in [ 13 , 14 ], and the same type of samples has been tested with other particles in e.g., [ 15 ]. The scintillation and OSL response of a Cu + -doped quartz glass was investigated in electron and X-ray beams from a Clinac in [ 16 , 17 ], where the sample was used to measure the total dose deposited during radiation runs. In this paper, the RIL responses of Ce-, Cu-, and Gd-doped sol-gel silica glasses under a pulsed electron beam are investigated. The emission properties, and possibilities of monitoring the beam pulse-by-pulse with these samples are presented. The interest and possibility of using these types of samples for dosimetry in the context of radiation therapy [ 1 , 2 ] makes the investigation of their responses to pulsed electron beams highly relevant. The particle accelerator used for irradiation tests in this study is a Clinac, and a characterization of the doped sol-gel silica rods in the pulsed Clinac electron beam opens the prospect of using the doped rods for dosimetry in a wider range of radiation therapy contexts. 2. Materials and Methods 2.1. Tested Samples The tested materials were sol-gel glass rods. Further information regarding the production and fabrication of these samples can be found in previous publications, e.g., in [ 18 , 19 ]. Each sample, consisting of a rod drawn from doped silica glass, was fusion-spliced to 500- µm core multimode pure-silica core optical fibers (here referred to as transport fibers) to guide the induced RIL to the read-out electronics. The radioluminescent rods were approximately 1 cm long and 0.5 mm thick. Information about the tested samples and their doping concentrations are listed in Table 1. Table 1. Summary of tested samples Sample Dopant Dopant Concentration (wt%) Ce-rod Ce3+0.07 Cu-rod Cu+0.07 Gd-rod Gd3+0.1 2.2. Test Setup At the end of the transport fiber, the signal readout system was located. For the tests of the response of the fibers as a function of electron pulse size, the readout system consisted of a photomultiplier tube (PMT) to convert the incident light to a voltage pulse, which was collected in an oscilloscope with high input impedance (1 MΩ). The PMT, a Hamamatsu H9305-13 [ 20 ], was encased in a dark metal casing where the luminescent light from the sample could be collected from the transport fiber in the PMT window without background light contamination. A schematic of the PMT setup and a sample is shown in Figure 1, where the whole system was kept in darkness to Sensors 2021,21, 7523 3 of 16 shut out parasitic light. The transport fibers were running through a black tube, and the samples were covered with dark tape. Between the transport fiber and the PMT, an optical band-pass filter was placed to select a relevant wavelength span that included the RIL. The used filter was thus specific for each sample. Doped rod 1 cm Transport fiber ~ 5 m PMT Optical filter Gd: no filter Ce: 500±40 nm Cu: 550±40 nm 20 MeV electrons Figure 1. Schematic overview of the setup, where radiation-induced luminescence (RIL) in the doped rods are transported to a photomultiplier tube (PMT) through an optical filter. To measure the optical emission spectra from the samples, an Ocean Optics USB2000+ UV-VIS-ER spectrometer [ 21 ] was used. The transport fiber was then placed directly against the spectrometer window, which thus replaces the optical filter and PMT in Figure 1. 2.3. Test Methodology To investigate the RIL response of the samples when subjected to a pulsed beam, a large number of RIL pulses were collected at a fixed beam setting. Then, the properties of the collected pulses in the oscilloscope were investigated after irradiation. The experimental procedure was the following for the different samples: 1. Fix a constant electron bunch size and frequency in the accelerator; 2. Start irradiation and keep on for circa 30 s, by irradiating until a fixed dose value; 3. Save the collected trace from the oscilloscope containing RIL pulses from the 30 s of irradiation; 4. Tune to a different electron bunch size and repeat. During irradiation, a Si-diode detector was also located in the beam. The signal from this detector was saved as well, and used to identify electron bunches also in certain beam configurations where the signal in the tested sample was small. Such configurations consist, for example, in a shielded sample, in a sample positioned outside of the beam, or for very small bunch sizes. The saved traces of pulses were analyzed post-irradiation in terms of the height and area of the separate pulses. One pulse from a run with the Ce-rod sample is shown in Figure 2, where the separate procedures for determining the height and area of the pulses are presented. The baseline for the pulse was calculated as the average signal level immediately before the pulse, and the height of the pulse was recorded as the absolute difference between the pulse maximum and the calculated baseline, as shown in orange in the figure. The area was calculated as the absolute value of the integral of the pulse with respect to the calculated baseline, so that effectively the pulse area below the zero level was counted as positive, and the area above the zero level as negative. Sensors 2021,21, 7523 4 of 16 5.000 5.001 5.002 5.003 5.004 Time of run (s) 10 8 6 4 2 0 PMT voltage (V) Ce-rod pulse Pulse height Pulse area positive Pulse area negative 0 25 50 75 Time of pulse trace ( s) 0.2 0.0 0.2 0.4 (V) Figure 2. Example of one recorded pulse from a test using the Ce-rod, where the methods of calculating the height and area of the pulses are presented. 2.4. Irradiation Facility, RADEF The irradiation experiments presented in this paper were performed at the radiation effects facility (RADEF) at the accelerator laboratory of the University of Jyväskylä, Finland. A Varian Clinac 2100C/D [ 22 ] was used to generate the electron beam that was used in the experiments. At the facility, 6, 9, 12, 16, and 20 MeV electrons are available, with dose rates between 1 and 10 Gy(H 2 O)/min in standard operation. The dose rates mentioned here correspond to the dose rate at maximum dose depth in water. The machine was, however, not utilized in the standard mode of operation during the experiments presented in this paper, but instead used in a manner allowing for manual tuning of the amount of electrons present in the separate electron bunches from the machine. In the standard mode of operation (which was not utilized here), 5- µs long electron bunches are delivered at a frequency of up to 200 Hz when the machine is set to a dose rate of 10 Gy(H 2 O)/min. When the dose rate is lowered, an increasing number of 5- µs electron bunches are removed, so that when, e.g., running at 1 Gy(H 2 O)/min, 10 times fewer bunches are present than in the 10 Gy(H 2 O)/min operation, but the sizes of the individual electron bunches stay the same. This is shown for a few dose rate settings in Figure 3a, and is also discussed in e.g., [ 12 ]. In Figure 3, the PMT signal from consecutive electron bunches collected in an oscilloscope are shown (see Section 2.2) for different Sensors 2021,21, 7523 5 of 16 operating modes and different dose rates. The figures are made using the signal from a Cu-doped rod. In this work, 20-MeV electrons were used at different dose rates, where the dose rate was modulated in a different manner than described above. Here the automatic dose rate regulation of the machine was turned off, and a certain bunch frequency was fixed. The bunch frequencies that were used were 20 and 200 Hz, corresponding to electron bunches delivered every 50 ms and 5 ms. Then the sizes of the electron bunches at the fixed frequency could be manually tuned by changing the current to the electron gun. This way, the responses of the samples to different sizes of electron bunches could be investigated. The dose rate tuning in this operating mode is shown in Figure 3b. 0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 Time (s) 0.5 0.4 0.3 0.2 0.1 0.0 Sample PMT signal (V) Machine dose rate: 10.0 Gy/min 5.0 Gy/min 1.0 Gy/min (a) 0.000 0.001 0.002 0.003 0.004 0.005 Time (s) 4 3 2 1 0 Sample PMT signal (V) Tuned rate: 7.0 Gy/min 5.0 Gy/min 3.0 Gy/min 1.0 Gy/min 0.0 0.2 0.4 4 2 0 (b) Figure 3. Dose rate tuning of the electron machine in different operating modes, recorded using a Cu-doped rod. ( a ) The standard dose rate tuning scheme is shown in the figure, where the bunch frequency is automatically modulated while the bunch sizes are kept constant. ( b ) The operation mode used in the experiments, where the bunch frequency is kept constant (shown in the figure inset) while the electron bunch size is tuned (see the varying size of the PMT pulses). Changing the electron bunch sizes, so they become larger than normal, affects the functionality of the built-in dosimetry system of the electron accelerator. In the accelerator, there are ionization chambers which monitor the outgoing accelerator beam, which are meant to handle electron bunches of a certain size as shown in Figure 3a. As the bunches get larger, non-linearities in the built-in dosimeters are observed. This can be seen in Figure 4, where the nominal bunch size of the machine corresponds to a dose rate of 1 Gy(H 2 O)/min. The saturation of the Clinacs internal ionization chambers at large bunch sizes is shown in Figure 4a, comparing the dose recorded by the machine with an external dosimeter (IBA PPC40 dosimeter [ 23 ]) at a maximum dose depth in water. Comparing this external dosimeter with a second one (IBA FC65-P [ 23 ]) located in the beam periphery in air, results in a linear relationship as seen in Figure 4b. The external dosimeters were used to ensure that the correct values of dose and dose rate were recorded, and they were used as the reference dosimeters during the experiments in the tests where electron bunches larger than nominally was used. The values of dose and dose per electron bunch that are reported in this study refers to the electron fluence, which corresponds to said dose at maximum dose depth in water, and not the absorbed dose in the tested samples. During irradiation, the samples were located under a thin layer of darkening material (a black plastic sheet and a layer of black tape), but it can be approximated as the samples being located in air and being subjected to the immediate electron beam. Sensors 2021,21, 7523 6 of 16 0 2 4 6 8 10 12 14 Reference dosimeter at CPE (Gy(H20)) 0 2 4 6 8 10 Machine reading (Gy(H20)) Calibration values Ideal relationship (y = x) (a) 0.0 0.5 1.0 1.5 2.0 Peripheral dosimeter in air (Gy) 0 2 4 6 8 10 12 14 16 Dosimeter at CPE (Gy(H20)) Calibration values Linear fit (b) Figure 4. Response of the built-in dosimetry of the accelerator and external dosimeters when the electron bunch sizes are changed. The data points are each taken at a fixed dose rate (bunch size) for one minute of irradiation at a bunch frequency of 20 Hz. At this setting, the bunch size corresponding to the nominal machine value is 1 Gy/min. ( a ) Accelerator internal dosimetry against an external dosimeter at maximum dose depth in water. ( b ) Dosimeter at maximum dose depth in water against a peripheral dosimeter in air. 3. Results and Discussion 3.1. Emission Spectra of the Samples The measured RIL emission spectra of the samples are shown in Figure 5for the different types of samples under 20-MeV electron irradiation. The emission spectra for the three different dopants correspond well to previously reported RIL emission spectra in the literature where X-rays were used as the excitation source. The reported spectra are all dominated by the expected RIL wavelengths without visible contamination from other sources such as ˇ Cerenkov radiation. No optical filters were used while obtaining the spectra presented in Figure 5. The emission spectrum of the Gd-doped sample is a narrow peak at 314 nm. This is the same result as was discussed in [ 7 ], where a narrow emission peak at 314 nm was found under both X-ray and 275-nm UV excitation. The emission spectra of a Cu-doped sample under X-ray and 325-nm UV excitation was compared in e.g., [ 24 ], where the UV excited spectra was seen to be slightly broadened. This was ascribed to an increased emission from non-bridging oxygen hole centers (NBOHC) in the UV excitation case. The corresponding spectrum in Figure 5peaks at 543 nm, and does not show this broadening. It is similar to the reported X-ray excited spectrum in [24]. This same comparison was done for a Ce-doped sample in [ 4 ] between X-ray and a 351-nm UV excited emission spectra. The X-ray emission spectrum in that study corresponds well to the one in Figure 5, however the knee structure around 450 nm is slightly less pronounced in [ 4 ] than it is here. Such differences can however be masked or amplified depending on the total transfer function of the detection system that was used (the combination of transport fiber and spectrometer), on calibrations of the spectrometer, and on potential post-processing of the data. For the following tests, optical band-pass filters at 500 ± 40 nm and 550 ± 40 nm were used for the Ce-rod and the Cu-rod respectively, in front of the PMT window. No optical band-pass filter was used for the Gd-rod tests, since none were available that could cover the 314-nm emission peak of Gd. Sensors 2021,21, 7523 7 of 16 200 300 400 500 600 700 800 900 Wavelength (nm) 0.0 0.2 0.4 0.6 0.8 1.0 Intensity (au, normalized) RIL emission spectrum source: Gd-rod Ce-rod Cu-rod Figure 5. RIL emission spectra taken from the different sample types. 3.2. Sample Response to Varying Electron Pulse Sizes 3.2.1. Variations of Output Pulse Height Examples of collected pulses in the oscilloscope are shown in Figure 6. All the resulting pulses from the PMT for half a minute of irradiation at a fixed electron bunch frequency of 20 Hz, and at constant electron bunch sizes of 9.7 × 10 −4 Gy/bunch impinging on the Ce-doped sample are displayed in the figure. The electron bunch that resulted in the pulse at 1.0 V in pulse height was the first recorded bunch of the run. This is a common behavior among all the runs in that the machine reaches the set bunch size after one or two smaller initial bunches. The relation between the height of the pulses and the size of the electron bunches is seen in Figure 7for different sample dopants, and for dose rates up to 1.5 × 10 −2 Gy/bunch. A note to keep in mind is that this dose rate corresponds to an instantaneous dose rate during a 5- µs pulse of 3 kGy/s. The data is based on 30-s irradiation runs at each electron bunch size, and the data points are located at the average pulse height (see Figures 2and 6). The error bars represent, in the y-axis direction, the standard deviation of the pulse height, and in the x-axis direction, 10% of the reported dose per pulse. The results in Figure 7show a linear trend over the whole tested range of bunch sizes for the Ce- and Cu-doped rods. For the Gd-doped rod, the point at 2 ×10−5Gy/bunch is slightly above the fitted linear slope. The signal light output was smaller from this sample than the others in terms of the pulse height, and at smaller electron bunches, the signal was influenced by noise. Thus a fit to the data containing a constant factor taking into account the background noise level in the signal gives a better representation of the sample response for small electron bunches. The constant factor (min the figure legend) in the dotted line was fitted to a value of 3.8 ×σnoise , where σnoise = 0.6 mV was the calculated standard deviation of the signal noise in the data point at 2 ×10−5Gy/bunch. Sensors 2021,21, 7523 8 of 16 Figure 6. Collected pulses in the oscilloscope from 20-Hz electron bunch irradiation of the Ce-doped rod sample at 9.7 ×10−4Gy/bunch. 10 510 410 310 2 Dose per electron bunch (Gy) 10 2 10 1 100 101 Average pulse height (V) Cu-rod Ce-rod Gd-rod 200 Hz bunch rate 20 Hz bunch rate Linear fit Fit y=kx + m Figure 7. Pulse height as a function of electron bunch size in the different samples. The average pulse height is shown for electron bunch rates of 20 Hz and 200 Hz. The actual magnitude of the data points in Figure 7depend on many parameters. Luminescence properties of the sample is one of these parameters, but PMT gain voltage, Sensors 2021,21, 7523 15 of 16 Funding: The results presented here have been conceived within the RADSAGA ITN, which has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie Grant Agreement No. 721624. This work was also supported by the European Space Agency (ESA) under contract 4000124504/18/NL/KML/zk, by the ANR: LABEX CEMPI (ANR-11- LABX-0007), the Equipex Flux (ANR-11-EQPX-0017), by the Ministry of Higher Education and Research, and the Hauts-de-France Regional Council and the European Regional Development Fund (ERDF) through the Contrat de Projets Etat-Region (CPER Photonics for Society P4S). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that were analyzed to produce this publication are available through the JYX database (https://jyx.jyu.fi/handle/123456789/78578, accessed on 7 November 2021). Acknowledgments: This work has been supported by the IRCICA institute and the FiberTech Lille platform (https://fibertech.univ-lille.fr/en/, accessed on 7 November 2021) of the University of Lille. Conflicts of Interest: The authors declare no conflict of interest. References 1. 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