Characterization of scintillator screens under irradiation of low energy 133Cs ions

Journal of Instrumentation PAPER • OPEN ACCESS Characterization of scintillator screens under irradiation of low energy 133Cs ions To cite this article: J.J. Toledo-Garrido et al 2022 JINST 17 P02026 View the article online for updates and enhancements. You may also like 3D structure of density fluctuations in the T-10 tokamak and new approach for current profile estimation V.A. Vershkov, M.A. Buldakov, G.F. Subbotin et al. - Microtearing mode (MTM) turbulence in JIPPT-IIU tokamak plasmas Y. Hamada, T. Watari, A. Nishizawa et al. - ECRH effect on the electric potential and turbulence in the TJ-II stellarator and T-10 tokamak plasmas A V Melnikov, L I Krupnik, E Ascasibar et al. - This content was downloaded from IP address 161.111.10.228 on 31/08/2022 at 10:47 2022 JINST 17 P02026 Published by IOP Publishing for Sissa Medialab Received:December 25, 2021 Revised:January 26, 2022 Accepted:January 30, 2022 Published:February 18, 2022 Characterization of scintillator screens under irradiation of low energy 133Cs ions J.J. Toledo-Garrido,𝑎,𝑏 J. Galdon-Quiroga,𝑎,𝑐,∗E. Viezzer,𝑎,𝑏 G. Birkenmeier,𝑐,𝑑 V. Olevskaia,𝑑M. Balden,𝑐J. Garcia-Lopez,𝑎,𝑏 M.C. Jimenez-Ramos,𝑎,𝑏 M. Rodriguez-Ramos,𝑒G. Anda, 𝑓M. Videla-Trevin,𝑏M. Garcia-Munoz𝑎,𝑏 and the ASDEX Upgrade Team 𝑎 Departamento de Física Atómica, Molecular y Nuclear, Universidad de Sevilla, Facultad de Física, Avda. Reina Mercedes s/n 41012, Seville, Spain 𝑏Centro Nacional de Aceleradores (Universidad de Sevilla, Junta de Andalucia, CSIC), C/ Thomas Alva Edison 7, 41092, Seville, Spain 𝑐Max Planck Institute for Plasma Physics, Boltzmannstrasse 2, 85748, Garching bei Muenchen, Germany 𝑑Physics Department E28, Technical University of Munich, James-Franck-Str. 1, 85748, Garching bei Muenchen, Germany 𝑒Laboratory for Ion Beam Interaction, Ruder Boskovic Institute, Bijenicka cesta 54, Zagreb, Croatia 𝑓Fusion Technology Department, Centre for Energy Research, Konkoly-Thege Miklós út, 1121, Budapest, Hungary E-mail: [email protected] Abstract: An imaging heavy ion beam probe (i-HIBP) diagnostic, for the simultaneous measurement of plasma density, magnetic field and electrostatic potential in the plasma edge, has been installed at ASDEX Upgrade. Unlike standard heavy ion beam probes, in the i-HIBP the probing (heavy) ions are collected by a scintillator detector, creating a light pattern or strike-line, which is then imaged by a camera. Therefore, a good characterization of the scintillator response is needed. Previous works focused on the scintillator behaviour against irradiation with light ions such as hydrogen and alpha particles. In this work we present the characterization of several scintillator screens — TG-Green (SrGa 2 S 4 :Eu 2+ ), YAG-Ce (Y 3 Al 5 O 12 :Ce 3+ ) and P11 (ZnS:Ag) — against irradiation with 133 Cs + ions, in an energy range between 5 and 70 keV and ion currents between 10 5 and 10 7 ions/(s · cm 2 ). Three main properties of the scintillators have been studied: the ionolumenescence efficiency or yield, the linearity and the degradation as a function of the fluence. The highest yield was delivered by the TG-Green scintillator screen with > 8 · 10 3 photons/ion at 50 keV. All the samples showed a linear response with increasing incident ion flux. The degradation was quantified in terms of the fluence ∗Corresponding author. c 2022 The Author(s). Published by IOP Publishing Ltd on behalf of Sissa Medialab. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. https://doi.org/10.1088/1748-0221/17/02/P02026 2022 JINST 17 P02026 𝐹1/2 , which leads to a reduction of the emissivity by a factor of 2. TG-Green showed the lowest degradation with 𝐹1/2= 5 . 4 · 10 14 ions/cm 2 . After the irradiation the samples were analyzed by Scanning Electron Microscopy (SEM), Rutherford Backscattering Spectrometry (RBS) and Particle Induced X-ray Emission (PIXE). No trace of Cs was found in the irradiated regions. These results indicate that, among the tested materials, TG-Green is the best candidate for the i-HIBP detector. Keywords: Scintillators, scintillation and light emission processes (solid, gas and liquid scintillators); Heavy-ion detectors; Plasma diagnostics - probes 2022 JINST 17 P02026 Contents 1 Introduction 1 2 Scintillator screens 2 2.1 Description of scintillators 2 2.2 Scintillator coating by sedimentation process at CNA 2 3 Experimental set-up 3 4 Results 4 4.1 Ionoluminiscence degradation of the scintillators 4 4.2 Calibrated photon detection yield of scintillators as a function of incident ion energy 5 4.3 Linearity of ionoluminiscence with ion current 7 4.4 Composition and thickness of the scintillators 8 5 Summary and conclusions 9 1 Introduction A new diagnostic, the Imaging Heavy Ion Beam Probe (i-HIBP), has been installed in the ASDEX Upgrade (AUG) tokamak at the Max Planck Institute for Plasma Physics (IPP) [ 1 – 3 ]. Unlike standard heavy ion beam probes, in the i-HIBP the probing (heavy) ions are collected by a scintillator detector, creating a light pattern or strike-line, which is then imaged by a camera. Information about the magnetic field and electrostatic potential fluctuations can be retrieved from the strike-line shape and displacement. The plasma density profile can be retrieved from the signal intensity, for which a comprehensive knowledge of the scintillator response is needed. A more detailed description of the measurement principle of the i-HIBP can be found in [1,3]. Ionoluminescence consists in the emission of light by a material when it is irradiated by ions. This process could be described as the de-excitation of stimulated electronic states, related either to valence electrons of particular atoms or to defects inside crystalline structures [ 4 ]. Previous work focused on the study of the ionoluminescence of several scintillators when they are irradiated by light ions, for its application in Fast Ion Loss Detectors (FILD) [ 5 , 6 ]. In this work we focus on the study of scintillator response to irradiation with heavy ions. In particular, we will use 133 Cs which corresponds to the species of the primary beam selected for the i-HIBP at AUG. In this study we have characterized the ionoluminescence of four scintillators screens; two TG-Green plates (SrGa 2 S 4 :Eu 2+ ) (one provided by Sarnoff Corporation and another coated at Centro Nacional de Aceleradores (CNA)), YAGCe (Y 3 Al 5 O 12 :Ce 3+ ) and P11 (ZnS:Ag), to estimate which one is the most appropriate for the diagnostic. The ionoluminescence of these materials should meet the following requirements: –1– 2022 JINST 17 P02026 • A high efficiency or yield, i.e. number of emitted photons per irradiated ion (expected current densities at the i-HIBP scintillator plate of mA/m 2 [ 1 ]). A high yield means a high i-HIBP signal. •A linear behaviour of the emission with incident current is needed for signal interpretation. • Low scintillator degradation: ions create defects in the structure of the material that degrade the ionoluminescence, i.e. inhibit light emission. This characterization is related with the i-HIBP signal intensity and the “life-time” of the scintillator. In section 2the composition and emission of the scintillators is described. Besides, the sedimentation process, for which TG-Green-A screen was coated, is detailed. Section 3deals with a brief description of the experimental set up. In section 4the results of the measurements of ionoluminescence efficiency, degradation and linearity with incident current of the scintillators are discussed. Furthermore, a composition and thickness analysis of the materials was set. Finally, section 5summarizes and concludes this investigation. 2 Scintillator screens 2.1 Description of scintillators TG-Green is used in ASDEX Upgrade FILD detectors due to its high efficiency and fast response [ 7 , 8 ]. YAGCe is a solid candidate to be used in i-HIBP diagnostic due to its high efficiency when it is ionized by light particles [ 7 ]. P11 has a long decay time of 3 ms, which is unfavorable, but serves as a benchmark material for comparison to the others. In table 1some information about the scintillators used in this study is collected: its composition, the wavelength range of the emissions and the suppliers of the materials. Table 1 . Scintillator information. a See section 4. b The difference between emissions of TG-Green-A and TG-Green-B could be due to their different compositions (see section 4). Scintillator Composition Provided by Wavelength Range (nm) FWHM (nm) TG-Green-A —𝑎Coated in CNA 470–720𝑏80 TG-Green-B SrGa2S4:Eu2+Sarnoff Corporation 475–620𝑏45 YAGCe Y3Al5O12:Ce3+CRYTUR 470–730 108 P11 ZnS:Ag Coated in CNA 380–620 64 In figure 1the emission spectrum of the different scintillator materials is shown. It can be seen that TG-Green-A and TG-Green-B show some differences in their emission spectrum. It could be explained by the composition of the materials (see section 4.4). 2.2 Scintillator coating by sedimentation process at CNA One of the TG-Green plates (4 . 4 × 3 . 3cm 2 ) and the P11 scintillator was deposited at the Centro Nacional de Aceleradores (Seville) using the sedimentation process [9]. This technique consists in making a solution of the scintillator powder and an adhesive and pouring it over a stainless steel –2– 2022 JINST 17 P02026 Figure 1 . Ionoluminiscence spectrum of the following scintillators irradiated by 133 Cs at 50 keV: TG-Green-A (red), TG-Green-B (blue), YAGCe (black) and P11 (magenta). substrate. We choose the sedimentation process due to its easy approach and the uniformity of the layers, which is achievable with this technique. The sedimentation of the layers was performed using a mixture of 1.5 g of the scintillator powder dispersed in 40 ml of a 0.1% liquid solution of sodium hexametaphosphate (NaPO 3 ) which acts as adhesive. The solution is then mixed at 300 rpm and heated at 60 ◦ C during 1 hour. Once the mix is ready, it is poured over the metal substrate. When the solution is cooled down and the powder mix has formed an uniform layer over the substrate, we remove the water using a pipette and dry the metal plates in an oven at 140◦C for 10 minutes. 3 Experimental set-up The characterization of the scintillator response to irradiation with 133 Cs + ions was performed using the ASDEX Upgrade i-HIBP beam injector. The injector is based on the design described in [ 10 ] and has been provided by the Centre for Energy Research (Budapest, Hungary). The main elements of the injector are a high-voltage cage that contains the ion source, the emitter and extractor electrodes, an electron suppression ring and a pair of deflection plates. These plates allow poloidal and toroidal beam steering as well as fast beam chopping. A collimator with a diameter of 1 mm is placed at the end of the injector to define the beam size and position on the scintillator. The samples under study were placed in an electrically isolated holder inside a vacuum chamber with a black coating to minimize light reflection with an inclination of 45 ◦ in the vertical axis. The vertical movement of the sample holder was controlled manually by a worm drive, perpendicular to the ion beam axis. The light emission was collected by a silica optical fiber of a diameter of 440 μ m fixed to a port of the vacuum chamber. This fiber was connected to a high-sensitivity spectrometer (QE65000, Ocean Optics Inc. Quantum Efficiency of 90% peak) which allows making –3– 2022 JINST 17 P02026 simultaneous measurements in the range of 199.05–1000.04 nm with a spectral resolution of 1–2 nm. The measured signals were analyzed and stored with the SpectraSuite software [ 11 ]. The calibration of the optical system has been done with a HL-2000-CAL tungsten halogen standard calibration light source. This calibration gives the spectrometer software the relation between the energy measured by the CCD camera and the number of photons emitted by the tungsten light source. On the other hand, the spectrometer takes the data for a certain amount of time (given by the user), so by integrating the calibrated spectra of the spectrometer one could obtain the total amount of photons that was measured during the time set by the user (photons/s). In order to get the total amount of photons emitted by the scintillators (and not only the one collected by the optical fiber) a solid angle correction was applied. This method was done with the same spectrometer and calibration light source as in [8]. The incident ion current was measured by a Faraday cup which is within a secondary electron suppression ring and is placed behind the sample holder. Due to the position of the Faraday Cup, simultaneous measurement of the incident current and the scintillator emission was not possible. Instead, the ion current was measured in a first shot with the sample holder in a low position. Then, the sample holder was brought into position and a second shot was performed in which the scintillator was irradiated, thus producing the light emission. 4 Results 4.1 Ionoluminiscence degradation of the scintillators The ionoluminescence of a scintillator suffers degradation with accumulated ion fluence. To investigate this effect, we carried out a sequence of shots with a duration of ∼ 10 s at a fixed beam energy of 55 keV and beam currents of ∼ 10 6 ions/(s · cm 2 ). During each sequence the exact same point of the scintillator sample was irradiated. Figure 2shows the degradation of the ionoluminescence emission as a function of incident fluence. The Black-Birk model (eq. (4.1) ) [ 12 ] describes the degradation of the ionoluminescence of a scintillator when it suffers by ionization with a certain energy. 𝐿0 𝐿−1= 𝐹 𝐹1/2 (4.1) Here, 𝐿0 is the initial emission of the scintillator, 𝐿 is the emission of the scintillator after a fluence 𝐹 has damaged it and 𝐹1/2 is the fluence needed to reduce its emission to 50% of its initial emission. A linear fit between 𝐿0/𝐿− 1and 𝐹 gives a slope equal to 1 /𝐹1/2 (see figure 2), so we could compare this result with 𝐹1/2 by reading the fluence value at which each curve crosses the normalized emission value of 0.5 (table 2). TG-Green-A has the slowest degradation and P11 has the fastest degradation of the studied scintillators, all of them showing an 𝐹1/2 of the order of 10 14 ions/cm 2 These values can be compared to previous work [ 13 ] where a 𝐹1/2 of 3 . 7 · 10 15 ions/cm 2 ,9 . 82 · 10 14 ions/cm 2 and 4 . 36 · 10 14 ions/cm 2 , was found for TG-Green when irradiated by light ions (H + , D + and He ++ ) at 1 MeV, and a 𝐹1/2 of 2 . 56 · 10 16 ions/cm 2 and 7 . 3 · 10 15 ions/cm 2 for YAGCe and TG-Green respectively when irradiated by D+at 2 MeV. Different mechanisms can play a role in the degradation of the scintillator light yield when irradiated by ions [ 14 ]. Amongst them, the most common mechanism is the radiation induced absorption, which leads to the formation of color centers that effectively attenuate the light output emitted by the scintillation process [ 15 ]. Our observations, together with the results reported in –4– 2022 JINST 17 P02026 Figure 2 . Black-Birk model fit of the scintillators: TG-Green-A (red), TG-Green-B (blue), YAGCe (black) and P11 (green, multiply by a factor 0.1). previous work [ 13 ], suggest that heavy ion irradiation inhibits light emission faster than light ions in TG-Green and YAGCe scintillators. We speculate that this might be due to a larger fraction of the incident energy being transferred through nuclear collisions, and thus favouring the formation of defects, when irradiating with heavy ions as compared to light ions. However, this deserves a more in depth analysis which is out of the scope of this paper and left for future work. Table 2.𝐹1/2of the scintillators calculated using 2 different methods for 55 keV ion impact. Scintillator Black-Birk Model (1014 ions/cm2) Fluence at the emission value of 50% (1014 ions/cm2) TG-Green-A 5.4±0.01 5.4±0.1 TG-Green-B 4.64 ±0.02 4.9±0.1 YAGCe 3.81 ±0.02 3.6±0.1 P11 0.27 ±0.01 0.69 ±0.01 4.2 Calibrated photon detection yield of scintillators as a function of incident ion energy The ionoluminescence efficiency or yield is expressed as the ratio between the number of emitted photons and incident ions [ 7 , 8 ]. Figure 3shows the scintillator efficiency as a function of incident 133 Cs + ions energy. Two different scintillator positions were irradiated at TG-Green-A (TG-Green-A-I and TG-Green-A-II). For TG-Green-A-I and TG-Green-B measurements, a set of consecutive shots was done starting with a beam at 55 keV and then following the order: 5-10-20-30-40-50 keV and finally a repetition at 55 keV. Also, YAGCe measurements shots at 60, 65 and 70 keV were conducted. Only three measurements were performed on P11: 40, 50 and 60 keV. The investigated range of –5– 2022 JINST 17 P02026 energies is the relevant one for the i-HIBP operation at ASDEX Upgrade, i.e. from 30–70 keV, which would correspond to plasma operation with magnetic fields on axis ranging from 1.5 to 2.5 T approximately [2]. Figure 3 . Ionoluminescence yield of the studied scintillators: TG-Green-A (red & dark green), TG-Green-B (blue), YAGCe (black) and P11 (green). Two measurements of the incident number of ions were set by the Faraday Cup (see section 2). On the other hand, the number of emitted photons at certain energy is the mean value of a distribution integrated by SpectraSuit during the measurement. The yield errorbars are obtained by propagating the errors from the Faraday Cup measurements and the standard deviation of the number of photons distribution. As we can see in figure 3, TG-Green-A and TG-Green-B have a very similar efficiency, followed by YAGCe and at last P11, which presents the lowest yield of the scintillators under study. The final measurement at 55 keV for TG-Green-A-I and TG-Green-B delivers a much lower yield than the first one. This indicates that the scintillator degradation during the shot sequence needs to be taken into account. This ionoluminescence degradation is a consequence of the ion fluencies the scintillators have received: 4 · 10 13 ions/cm 2 (TG-Green-A-I and TG-Green-A-II), 7 · 10 13 ions/cm 2 (TG-Green-B) and 9·1013 ions/cm2(YAGCe) during the measurements. The three measurements at P11 were set at different scintillators position, where the fluence could be considered null. The efficiency of P11 increases linearly with energy. By looking at the lower energy points (5, 10 and 20 keV), where the fluence ( ∼ 2 · 10 13 ions/cm 2 ) is still not high, the behaviour is also approximately linear for TG-Green-A, TG-Green-B and YAGCe. The linear behaviour at higher energies is weak, due to the degradation of the scintillators. The yield dependence with incident energy has also been measured at the same scintillator position at which the degradation study was carried out (figure 4). We have irradiated the scintillators after this study until a decrease of the ionoluminescence with irradiation was not noticed anymore –6–