Temperature response of several scintillator materials to light ions

Accepted Manuscript Temperature response of several scintillator materials to light ions M. Rodríguez-Ramos, M.C. Jiménez-Ramos, M. García-Muñoz, J. García López PII: S0168-583X(17)30536-0 DOI: http://dx.doi.org/10.1016/j.nimb.2017.04.084 Reference: NIMB 62551 To appear in: Nucl. Instr. and Meth. in Phys. Res. B Received Date: 18 January 2017 Revised Date: 20 March 2017 Accepted Date: 26 April 2017 Please cite this article as: M. Rodríguez-Ramos, M.C. Jiménez-Ramos, M. García-Muñoz, J. García López, Temperature response of several scintillator materials to light ions, Nucl. Instr. and Meth. in Phys. Res. B (2017), doi: http://dx.doi.org/10.1016/j.nimb.2017.04.084 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Temperature response of several scintillator materials to light ions M. Rodríguez-Ramos1,2*, M.C. Jiménez-Ramos1, M. García-Muñoz1,2,3, J. García López1,2 1 Dpto. Física Atómica, Molecular y Nuclear, Universidad de Sevilla. 41080 Sevilla, Spain. 2Centro Nacional de Aceleradores. (Universidad de Sevilla, Junta de Andalucía, CSIC), 41092 Sevilla, Spain. 3Max-Planck-Institut für Plasmaphysik, Garching, Germany Abstract Ion beam induced luminescence has been used to study the response of scintillator screens of Y2O3:Eu3+ (P56) and SrGa2S4:Eu2+ (TG-Green) when irradiated with light ions (protons, deuterium and helium particles). The absolute efficiency of the samples has been studied as a function of the ion energy (with energies up to 3.5 MeV), the beam current and the operating temperature. The evolution of the scintillator yield with ion fluence has been carried out for all the scintillators to estimate radiation damage. Finally, measurements of the decay time of these materials using a system of pulsed beam accelerated particles have been done. Among the screens under study, the TG-Green is the best suited material, in terms of absolute efficiency, temporal response and degradation with ion dose, for fast-ion loss detectors in fusion devices. Keywords Ionoluminescence ; Scintillators materials ; Absolute efficiency; Degradation ; Decay Time Abbreviations MHD, Magnetohydrodynamics; TFTR, Tokamak Fusion Test Reactor; Elms, Edge-Localized Modes; TAE, Toroidal Alfven Eigenmode; RBS, Rutherford backscattering spectrometry 1. Introduction In plasma fusion devices, fast ion losses caused by different mechanisms (prompt losses, coulomb collisions and MHD activity) represent a twofold problem for the reactor performance. Indeed, on the one hand, they do not contribute to the plasma heating leading to a decrease of the plasma power and, on the other hand, the plasma facing components can be damaged by the impinging particles [1]. The principal diagnostic to obtain information about the wave-particle interaction responsible for the fast ion losses in a magnetic fusion reactor is the scintillator based fast-ion loss detector (FILD) [2]. Based on the concept of the α-particle detector used in TFTR [3], probes are installed in several fusion devices like ASDEX Upgrade (AUG) [4], Joint European Torus (JET) [5], etc. The FILD system acts like a mass spectrometer collimating the incoming ion that reaches the scintillator. * *Corresponding author Tel. : +34 954460553, fax: +34 954460145, email: [email protected] The light produced by lost fast ions striking in the scintillator screen is imaged by a double system (camera + photomultipliers array) capable to provide time resolved measurements of the velocity space of escaping ions. The use of a scintillator material with a very short decay time makes it suitable to follow the frequency of the magnetohydrodynamics fluctuations (ELMs, TAEs, etc) present in fusion plasmas, but absolute measurements of the escaping ions are, however, not available due to the complex dependence of the detector response on the scintillator efficiency with varying temperature. During tokamak operation, heat load at first wall could make FILD operates in a hostile environment. This involves knowing the absolute efficiency in a wide range of temperatures to make them relevant for larger devices and reactors. The behaviour of scintillator material to irradiation with light ions is well reported in the literature at room temperature [6,7] but the absolute characterization of the scintillator response to charged particles at high temperature is not available. In this article, the absolute efficiency at different temperatures, the degradation of the scintillation yield and the characteristic decay time of two different scintillators used in FILD detectors have been carried out. After an introduction given in section 1, the experimental set-up is described in section 2, finally the main results are presented in section 3. 2. Experimental Setup 2.1 Characterization of scintillator efficiency and degradation in a tandem accelerator FILD systems are normally equipped with thin scintillator coatings that show a high efficiency to charged particles and low to other radiation fluxes present in a tokamak such as neutrons and gammas. The scintillators investigated in this work for ion beam irradiation experiments were selected according to their high efficiency, radiation hardness, heat resistance, fast response, and/or prior use in plasma diagnostics. The TG-Green phosphor (so called by the manufacturer, SRI International, USA) is an Eu doped SrGa2S4:Eu2+ powder substrate with a density of 3.65 g/cm3, physical thickness ≈ 9 µm and presents an emission at 535 nm with a very short decay time ≈ 490 ns [8] suitable to identify the frequency of the MHD fluctuations [9]. The P56 scintillator is an Eu doped Y2O3 powder substrate, manufactured by AST Corporation, England with a physical thickness around 28 µm. Although this material has a high efficiency, its light emission has a long decay time of 2 ms [10]. The characterization of the scintillator response to charged particles has been performed using the 3 MV tandem accelerator of the National Accelerator Centre (CNA) [11]. In order to measure the scintillation properties at different temperatures, a new vacuum chamber with black coating (to avoid reflections on the chamber wall) and equipped with a sample heater (ceramic resistance), a temperature control monitor (thermocouple) to cover the operation temperature range expected in fusion devices and a photonic diagnostic system have been installed at CNA in one dedicated beam line. The samples under study were placed in an electrically isolated holder. Movement of the sample holder was controlled remotely through stepping motors, capable of providing a linear motion in X-Y directions. This permits a fine control of the beam spot positioning as well as the study of several samples without venting the chamber. A collimator located at the entrance of the chamber permits adjusting the size of the ion beam between 1-3 mm. The position of the beam was monitored using a videocamera located outside of the chamber. The excitation and simultaneous measurement of the scintillation yield have been made by irradiation at normal incidence with light ions H+, D+ and He++ with energies between 1-3.5 MeV and different temperatures up to 500 ºC. Electrons emitted by thermionic effect in the holder at high temperature operations do not allow to measure directly the beam current at the vacuum chamber (Fig.1). Instead, the incident beam current has been determined using a calibrated beam chopper connected to a digital current integrator (model 439 by Ortec). The beam currents intensity used to irradiate the scintillator were kept constant at around 1-2 nA for the yield determination and 25-30 nA for the degradation tests. During the irradiation experiments, the vacuum pressure has been kept constant at ≈ 10-4 – 10-5 mbar. The emitted light from the scintillator was collected with a silica optical fiber of 1 mm diameter fixed to a side port of the vacuum chamber and connected to a high sensitive spectrometer, QE65000 (Ocean Optics Inc) which allows to measure the spectra of the scintillator in the range of 200-1100 nm with a spectral resolution ≈ 1-2 nm. The absolute calibration of the optical system was carried out with a source HL-2000-CAL Tungsten Halogen Calibration Standard light source. 2.2 Characterization of decay time with a beam pulser The decay time of both scintillators has been experimentally determined at different temperatures using a 1 MeV proton pulsed beam. The light emitted from scintillator has been recorded with a photomultiplier (PMT, Hamamatsu, Model H10721) that converts the photons into electrical pulses. The connection of the PMT with a fast oscilloscope (LeRoy 204MXi-A 2 GHz) (Fig.2) allows us to study the time evolution of the scintillator response. Different pulse widths and periods according to manufacturer decay time specifications have been used in these tests. 3. Results 3.1 Absolute efficiency of scintillators as a function of the temperature The ionoluminescence spectra were acquired by the spectrometer exposure time set to 1 s and were normalized by the beam current intensity. The measurements were carried out when the scintillator was stabilized to the operation temperature (± 5 ºC) and before the light yield started to degrade. Fig.3 and Fig.4 show the measured spectra for the TG-Green scintillator and P56, respectively, excited with a deuterium beam of 3 MeV for different temperatures. The signals are normalized with respect to the emission at room temperature. For the TG-Green, the light output of the scintillator quickly decreases with the temperature operation. The spectra present a broad peak centered at 535 nm and no shift was observed in the wavelength between RT and 500 ºC, indicating that the temperature does not affect the gap energy configuration of the sample. Similar results have been observed for protons and alpha particles. The P56 spectra present different emission bands with the main emission centered at 611 nm. Its light yield also presents a decrease with the temperature for all the bands, but the diminution is less pronounced than for the TG-Green scintillator. It has been found that the quenching mechanism is identical in the most intense bands. In both cases, the absolute yield was calculated by integrating the spectra in the ROI for each scintillator. One important parameter found in this work is the experimental ratio (κ = εT/εRT) between the yield at different temperatures (εT) with respect to room temperature (εRT). It has been proven that, within the energy range explored in this work, the ratio is approximately independent of the energy of the beam but slightly rests on the ion mass. The overall findings for the chosen scintillator are summarized in Table I for the TG-Green and in Table II for the P56 when irradiated with protons and deuterium at energies between 0.5-3 MeV and helium particles with energy of 3.5 MeV. The results correspond to the average measured ratios and the errors include the standard deviation and the experimental uncertainty. As observed, the temperature at which the efficiency decreases by factor of two compared to the yield at room temperature is about 200 ºC and 400 ºC for TGGreen and P56, respectively. In general, the luminescence thermal quenching of a phosphor is a phenomenon that depends on a complex way on the material analyzed and the ionizing radiation (electrons, UV, x-rays, ions, …) used to excite the light emission [12,13]. However, a comparison of our results with the photoluminescent properties of TG-Green [14], P56 [15] and different Eu2+ doped phosphors [16] in a temperature range up to 200 ºC shows a very similar trend for the thermal dependence of the emitted spectrum, independently of the excitation source (ions or photons), suggesting a similar quenching mechanism for the bands in the wavelength range of 450-720 nm. It is also important to highlight that the use of TG-Green at temperatures around 400 ºC or higher can be seriously hindered by the very important drop in efficiency, while the light yield of P56 remains at a considerable level even at 500 ºC. 3.2 Deterioration by ion beams Tests to determine the ionoluminescent efficiency degradation of the two scintillator screens have been done to estimate the damage caused by large ion fluxes as a function of the temperature. Fig.5 shows an example of the measured spectra for the TG-Green scintillator excited by an alpha beam of 3.5 MeV for an operation temperature of 100 ºC. The spectra were taken during 1 s of exposure time for all the measurements, with a beam diameter of 1 mm and were normalized by the beam current intensity (≈ 28 nA). As can be seen, the scintillator spectra present a gradual degradation of the luminescence intensity induced by the irradiation damage that can be significant after a certain fluence. Our experimental results, illustrated in Fig. 6, show that the degradation rate of the scintillator light is a function of the temperature of the samples. Scintillators exhibit much higher resistance to ion radiation damage at high temperatures. The experimental data have been fitted to the Black-Birk model [17]: 2 1 0 1 )( F F L FL + = Where, L(F) is the observed luminescence intensity after an absorbed fluence F and L0 is the initial luminescence. This model is employed to get the value of the half-brightness radiation fluence F1/2 for the TG-Green and P56 from their measured emissions. This parameter gives information about the sensitivity of the scintillators photon yield to the radiation-induced damage. Table III shows the fluence needed to reduce the normalized yield to half of the initial value for each temperature. The error in the Table corresponds to the fitting of the experimental data to the Black-Birk model. For both scintillators, we found a good agreement between the calculated and the experimental F1/2 value. From a microscopic description of this model [18], F1/2 can be related to: σ k F1 2 1= where, σ is the effective damage cross section and k is the ratio between non radiative and radiative transition rate. Supposing that the effective cross section does not change with the temperature, as indicated in [19], our results imply that the parameter k should decrease as the temperature increases. This is indicative of the capacity of a given material to anneal or self-repair the damage in the emission centers caused by the ionizing radiation. Therefore, from Table III we can deduce that the annealing mechanism is slightly more efficient in the case of P56 compared to TG-Green. 3.3 Decay Time Fig.7 shows the time evolution of the signal emitted for both materials, which were measured using different pulsed beam conditions according to their expected decay times (20 µs width/100 µs period for TG-Green and 2ms width/20 ms period for P56). The temporal response is about four orders of magnitude faster for TG-Green compared to P56 in the temperature range explored (between RT and 500 ºC). Due to the efficiency drop at 450 ºC, the gain of the PMT was adapted in each of the measurements to improve the signal to noise ratio. In fig.7, the PMT signal at 450 ºC has been multiplied by a factor of 30 for a better comparison of the two signals. The light output as a function of the time was fitted to a single exponential to determine the decay time for different operation temperatures. The results, shown in Fig. 8, indicate that the possible change of the time constants at different temperatures is small and lies within our experimental error. The decay time of the TG-Green ranges between 350-500 ns and is in a good agreement with the literature [20], but the P56 presents a decay time between 0.8-1.0 ms, about half of the decay time reported by S. Bäumel et al [21] but very similar to that measured by Z. Antic et al [22]. 4. Conclusions In this paper we report a pioneer study about luminescence properties of the TG-Green and P56 scintillator screens under ion beam irradiation at temperatures of interest for their use as diagnostic element in fusion devices. For both materials the absolute photon yield decreases with the temperature, but for P56 the drop in efficiency is acceptable even at 500 ºC, while for TG-Green the light output becomes too small at T> 400 º C. The progressive damage induced with protons, deuterons and alpha particles has been studied using the Black-Birk model. It is observed that the ion beam fluence needed to reach the half-brightness increases with temperature, indicating the occurrence of a beneficial annealing which is more pronounced for the P56 material. For both scintillators, we did not measure any significant change in the decay time for the considered temperature range. From these results it can be stated that P56 is the best suited material for high temperature operation, although unfortunately its decay time is too long (~ 1 ms) to follow the frequency of the MHD instabilities in fusion plasmas. This is the main reason why the TG-Green phosphor is the most appropriated scintillator to be used in FILD in the actual fusion devices. The knowledge of the scintillator efficiency at different temperatures is being applied to provide, for the first time, the absolute measurements of the number of fast ions that are lost in the fusion plasma reactors [23] and will serve as a critical test bed for fast ion stability and transport codes. In all, the finding of this work could help for choosing the best suitable material to be employed in the next fusion device generation. Acknowledgements This research was supported in part by the Spanish Ministry of Economy and Competitiveness through the projects RYC-2011-09152, ENE2012-31087 and FIS2015-69362-P (MINECO/FEDER, UE) and the Marie Curie FP7 Integration Grant (No.PCIG11-GA-2012-321455) and the V Plan propio de la Universidad de Sevilla (VPPI-US).We are grateful to P. Medina for the set-up model in CAD. References [1] H. Duong et al. Confinement of fusion produced MeV ions in the DIII-D tokamak. Nucl. Fusion, 33 749 (1993). [2] M. Garcia - Munoz et al. Fast Ion Loss Diagnostic in ASDEX Upgrade. 2nd EPS Conference on Plasma Phys. Tarragona, 27 June - 1 July 2005 ECA Vol.29C, P-5.085 (2005) [3] D. S. Darrow et. al. Measurement of loss of DT fusion products using scintillator detectors in TFTR. Rev. Sci. Instrum., 66 476 (1995). [4] M. Garcia - Munoz et al. Scintillator based detector for fast-ion losses induced by magnetohydrodynamic instabilities in the ASDEX upgrade tokamak. Rev. Sci. Instrum., 80 053503 (2009). [5] V. G. Kiptily et al. Fast ion JET diagnostics: confinement and losses. AIP Conf. Proc. 988, 283 (2008); [6] M.C. Jimenez-Ramos et al. Characterization of scintillator materials for fast-ion loss detectors in nuclear fusion reactors. Nucl. Instr. Meth. Phys. Res., 55 124014 (2014). [7] M. Garcia - Munoz et al. Characterization of scintillator screens for suprathermal ion detection in fusion devices. JINST. 6 P04002 (2011). [8] S. Yang et all. Green phosphor for low-voltage cathodoluminescent applications: SrGa2S4:Eu2+. 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Luminescence Properties of Green-emitting Phosphors-Sr4Al14O25:Eu2+ for LED Applications , J. Chem. Chem. Eng. 5 (2011) 638-643 [17] N.Markovic et al. Ion beam induced luminescence (IBIL) system for imaging of radiation induced changes in materials., Nucl. Instr. Meth. Phys. Res., 343 167-172 (2015). [18] J.B. Birks, Deterioration of Anthracene under a-Particle Irradiation., 1950a, Proc.Phys.Soc. A, 63, 1294; 195ob, Ibtd., 63, 1044. [19] P. A. Sullivan et al. Ion beam induced luminescence in natural diamond., J. Appl. Phys. 76 (8), 15 October 1994. [20] F.D. McDaniel et al. Ionoluminescence decay measured with single ions, Nuclear Instruments and Methods in Physics Research B 190 (2002) 1–10 [21] S. Bäumel et. al. Scintillator probe for lost alpha measurements in JET. Rev. Sci. Instrum., (2004). [22] Z. Antic et al. Optical properties of Y2O3:Eu3+ red emitting phosphor obtained via spray pyrolysis, Acta Physica Polonica A, vol. 116, Issue 4, p.622. [23] M. Rodríguez-Ramos et al. First absolute measurements of fast-ion losses in the ASDEX Upgrade tokamak, 21ST Topical Conference on high-temperature plasma diagnostics. (2016) Figure Captions Figure 1: A schematic diagram of the experimental IBIL (ion beam induced luminescence) set-up at CNA showing the scintillator and the photon detector as well as the incident ion beam and light acquisition systems. Figure 2: Overview of the experimental setup for the decay time measurements: Light from the scintillator reaches to the PMT and is recorded by the oscilloscope. Figure 3: Normalized ionoluminescence spectra of the TG-Green scintillator for different operation temperatures between room temperature and 500 ºC. Figure 4: Normalized ionoluminescence spectra of the P56 scintillator for different operation temperatures between room temperature and 500 ºC. Figure 5: Temporal evolution of the spectrum emission of the TG-Green scintillator heated to 100 ºC and irradiated with alpha particles at 3.5 MeV and a beam current of I ≈ 28 nA. Figure 6: Normalized efficiency as a function of the ion fluence for the two phosphors under study (TGGreen and P56) and different operating temperatures. The samples were irradiated with He++ ions at energy E= 3.5 MeV. Figure 7: Scintillation light pulse readout by a PMT and recorded by a fast oscilloscope for different temperatures: a) TG-Green scintillator decay time signal at room temperature (blue) and 450 ºC (red). b) P56 scintillator decay time signal at room temperature (blue) and 450 ºC (red). Figure 8: (Left) Light output (normalized) as a function of time for different temperatures for the TGGreen scintillator. The response signals are compared with two single exponential (dashed lines) with decay of τ=350 ns and τ=500 ns. (Right) Light output (normalized) as a function of time for different temperatures for the P56 scintillator. The response signals are compared with two single exponentials (dashed lines) with decay of τ=0.8 ms and τ=1.0 ms. For T>200 ºC a smoothing filter has been applied to reduce fluctuations in the raw data. Tables Ion ε εε ε RT (photons/ion) κ κκ κ 200ºC (%) κ κκ κ 300ºC (%) κ κκ κ 400ºC (%) κ κκ κ 500ºC (%) H + ++ + 35000 47±12 10±3 1.3±0.3 0.10±0.03 D + ++ + 48000 41±10 10±3 1.8±0.4 0.20±0.05 Η ΗΗ Η e + ++ + + ++ + 87000 30±7 10±3 1.7±0.4 0.20±0.05 Table I. Dependence of the yield at different temperatures and ion species for the TG-Green phosphor. Ion ε εε ε RT (photons/ion) κ κκ κ 200ºC (%) κ κκ κ 300ºC (%) κ κκ κ 400ºC (%) κ κκ κ 500ºC (%) Η ΗΗ Η + ++ + 72000 80±20 80±20 50±13 20±5 D + ++ + 96000 80±20 70±18 50±13 40±10 Η ΗΗ Η e + ++ + + ++ + 135300 52±13 42±11 40±9 30±7 Table II. Dependence of the yield at different temperatures and ion species for the P56 phosphor. Temperature TG - Green F 1/2 (ions/cm 2 ) P56 F 1/2 (ions/cm 2 ) Room Temperature (1.05±0.06)x10 16 (1.09±0.05)x10 16 100 ºC (1.28±0.03)x10 16 (1.43±0.03)x10 16 300 ºC (1.75±0.09)x10 16 Not Available 400 ºC (3.1±0.3)x10 16 (4.20±0.24)x10 16 Table III: Half-brightness radiation fluence F1/2 for the TG-Green and P56 at different temperatures. -200 -100 100 200 0 Time (ms) 30 20 10 010 20 30 Time (ms) -6 -4 -2 0 20.5 0.0 -1.0 -1.5 -0.5 TG-GREEN P56 I (a.u.) PMT I (a.u.) x 30 PMT 450 ºC I (a.u.) PMT I (a.u.) x 30 PMT 450 ºC Figure_7 Room Temperature 100 ºC 200 ºC 300 ºC 400 ºC -t/1.0 e -t/0.8 e Time (ms) Normalized PMT Signal 124 5 3 06 0.4 0.6 0.0 0.8 1.0 0.2 -0.2 Normalized PMT Signal 0.4 0.6 0.0 0.8 1.0 0.2 1000 2000 3000 0 Time (ns) -0.2 -t/500 e -t/350 e Room Temperature 100 ºC 200 ºC 400 ºC Figure_8