Characterization of scintillator materials for Fast-Ion Loss detectors in nuclear fusion reactors

Accepted Manuscript Characterization of scintillator materials for Fast-Ion Loss detectors in nuclear fusion reactors M.C. Jiménez-Ramos, J. García-López, M. García-Muñoz, M. RodríguezRamos, M. Carmona Gázquez, B. Zurro PII: S0168-583X(14)00321-8 DOI: http://dx.doi.org/10.1016/j.nimb.2014.02.064 Reference: NIMB 60211 To appear in: Nucl. Instr. and Meth. in Phys. Res. B Please cite this article as: M.C. Jiménez-Ramos, J. García-López, M. García-Muñoz, M. Rodríguez-Ramos, M. Carmona Gázquez, B. Zurro, Characterization of scintillator materials for Fast-Ion Loss detectors in nuclear fusion reactors, Nucl. Instr. and Meth. in Phys. Res. B (2014), doi: http://dx.doi.org/10.1016/j.nimb.2014.02.064 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. Characterization of scintillator materials for Fast-Ion Loss detectors in nuclear fusion reactors. M. C. Jiménez-Ramos1*, J. García-López1,2, M. García-Muñoz1,2, M. Rodríguez-Ramos1, M. Carmona Gázquez1 and B. Zurro3 1Centro Nacional de Aceleradores (U. Sevilla, J. Andalucia, CSIC). Av. Thomas A. Edison 7, Isla de la Cartuja, 41902 Seville, Spain. 2Dpto. Física Atómica, Molecular y Nuclear, Universidad de Sevilla. 41080 Sevilla, Spain 3Laboratorio Nacional de Fusión, Asociación Euratom-CIEMAT, Av. Complutense 22, E-28040, Madrid, Spain. Abstract In fusion plasma reactors, fast ion generated by heating systems and fusion born particles must be well confined. The presence of magnetohydrodynamic (MHD) instabilities can lead to a significant loss of these ions, which may reduce drastically the heating efficiency and may cause damage to plasma facing components in the vacuum vessel. In order to understand the physics underlying the fast ion loss mechanism, scintillator based detectors have been installed in several fusion devices. In this work we present the absolute photon yield and its degradation with ion fluence in terms of the number of photons emitted per incident ion of several scintillators thin coatings: SrGa2S4:Eu2+ (TG-Green), Y3Al5O12:Ce3+ (P46) and Y2O3:Eu3+ (P56) when irradiated with light ions of different masses (deuterium ions, protons and α-particles) at energies between approximately 575 keV and 3 MeV. The photon yield will be discussed in terms of the energy deposited by the particles into the scintillator. For that, the actual composition and thickness of the thin layers were determined by Rutherford Backscattering Spectrometry (RBS). Keywords Plasma Fusion Reactors, Escaping ions, Scintillators, Ionoluminiscence, Birk model * *Corresponding author Tel. : +34 954460553, fax: +34 954460145, email: [email protected] Introduction In magnetically confined fusion plasmas, fast ion generated by external heating systems and fusion born particles must be kept well confined until their thermalization. The presence of magnetohydrodynamic (MHD) instabilities can lead to a significant loss of these ions, which may reduce drastically the heating efficiency and may cause damage to plasma facing components of the vacuum vessel [1, 2]. In order to understand the physics underlying the fast ion loss mechanisms, scintillator based Fast Ion Loss Detector (FILD) [3] have been installed in most of the large fusion devices. However, absolute measurements of the escaping ions are not available due to the complex dependence of the scintillator photon yield on the escaping ion species, energy, operation temperature and background emission. In order to characterize the response of the scintillator screens used for fast ion detection in most of the current fusion devices, an ionoluminiscence (IL) system consisting of an optical fiber and a high resolution spectrometer has been installed into the multipurpose chamber of the National Accelerator Center (CNA). Materials and Methods The characterization of the scintillator materials was carried out in the Spanish National Accelerator Centre, in Seville, Spain [4, 5], in particular, in a 3 MV tandem machine (model 9SDH-2 by National Electrostatic Corp., Middleton, WI) with three ions sources which provide ions like protons, deuterons and alpha particles with an energy range very well suited to simulate the ions present in a fusion plasma. These tests were made on one of the seven beam-lines linked to the accelerator, specifically in a scattering chamber located at the end of the multipurpose or 0º line. The acquisition system to obtain the absolute IL yields consists in four main elements: 1) A collimator with 1 mm of diameter, which defines the beam size for the experiments, placed at the entrance of the chamber. 2) An electrically isolated sample holder biased to +300 volts to collect the secondary electrons, connected to a digital current integrator (model 439 by Ortec) to measure the incident beam current. A home made device has been used to store the real-time evolution of the beam current in a computer file allowing the correction of the IL yields due to the current fluctuations. The target holder is a rectangle of 150 x 112 mm2 and can be tilted. The X and Y movements are controlled through stepping motors, which permits a fine control of the beam spot positioning as well as the study of several samples without venting the chamber. 3) A silica optical fiber of 1 mm diameter fixed to the vacuum chamber, which collects the light from the scintillators. The solid angle subtended by the fiber is ~ 2.2 x 10-5 sr. 4) The final element is a compact and high sensitive spectrometer, QE6500 (Ocean Optics Inc.) with a 2D area detector which allow us to measure simultaneously in the range of 2001100 nm with a spectral resolution ~ 1-2 nm. The measured signals were analyzed and stored with the SpectraSuite software [6]. The absolute calibration of the optical system described above was carried out with a HL-2000-CAL Tungsten Halogen Calibration Standard light source which provides absolute intensity values (in µW/cm2/nm) at the fiber port at wavelengths from 360-1050 nm. The beam fluxes used to irradiate the phosphors were ~ 1012 p/cm2sfor the IL yields determination, and up to ten times higher for the degradation analyses. The Rutherford Backscattering Spectrometry (RBS) measurements of the screens were accomplished in the same vacuum chamber using protons at 3 MeV and 5 MeV. Two different energies were employed due to the large difference between the thicknesses of the samples. The proton beam intensity was 10 nA and the beam size 1 mm of diameter. The analysis were performed with a Passivated Implanted Planar Silicon (PIPS) detector of 300 mm2, positioned at 150º and with a 10 µm thick aluminized mylar foil placed at the detector surface to avoid the light emitted by the scintillators. The RBS spectra were analyzed using the SIMNRA code [7]. The scintillators investigated in this work were selected according to their availability, radiation hardness, fast response, and/or prior use in plasma diagnostics. In this paper, three different kinds of materials have been analyzed. The TG-Green (so called by the manufacturer, Sarnoff Corporation, USA) is a Eu doped SrGa2S4 powder substrate with density of 3.65 g/cm3, and presents an emission at 540 nm with a very short decay time.≈ 490 ns [8]. A TG-Green scintillator coating has been applied, for the first time, to a fusion plasma diagnostics for the detection of fast-particle losses on the AUG tokamak [9, 10]. The same material supplied by other manufacturer (CIEMAT) has been used to compare the yields for both samples. We will refer to these screens as TGa and TGb for the corresponding to Sarnoff Co. and CIEMAT, respectively. The P46 is a rare earth oxide Y3Al5O12 (YAG) doped with Ce by 0.15% CeO2, manufactured by Proxitronic GmbH, Germany. The luminescence emission consists in a broad peak, centered at 550 nm with a stated decay time constant of 70 ns. [11]. The P46 has been widely applied to fusion plasma diagnostic and in particular to fast-ion loss detection on several devices such as TFTR and NSTX [12, 13]. Finally, the P56 scintillator is a Eu doped Y2O3 powder substrate, Y2O3:Eu3+, manufactured by AST Corporation, England. Although this material has a high efficiency, its light emission has a long decay time of 2 ms [14], making the P56 unsuitable to follow the frequency of the MHD fluctuations. The samples were deposited using different processes directly by the manufacturers on 2 mm thick stainless steel plates. It is important to remind that reflections on the substrate may contribute to a luminescence enhancement of the thin scintillator screens. Therefore, the screens under study here as well as the experimental set-up were designed to mimic the real operation of a fast-ion loss detector. Results Figures 1, 2 and 3 show the experimental absolute IL yield of the different materials under study as a function of the ion energy for protons, deuterons and alpha particles, respectively. The TGb phosphor shows the highest efficiency for protons and deuterons while for α-particles the yields are very similar for TGa, TGb and P56. On the other hand, the P46 is the scintillator with the lower efficiency having response one or two orders of magnitude lower compared with the other materials. The IL yields for the three types of ions are in the range from 4 x1014 to 1x 1015 photons/ion for TGa, TGb and P56. However, the behavior with beam energy is not the same for all the species, for α-particles the yields always increase with energy while for protons and deuterons the response decreases at the higher energies. In Fig. 4 it is depicted the experimental RBS spectra, together with the simulation obtained from the SIMNRA code, for the samples TGa and P56. The thicknesses found by RBS (in units of 1015 at/cm2) for all the coatings are shown in Table 1. The corresponding physical thickness was calculated using the theoretical density of the materials provided by the suppliers. Scintillator Thickness from RBS (1015 at/cm2) Physical thickness (μm) P46 34,000 4.3 P56 165,000 27.7 TGa 35,000 9 TGb 154,000 40 Table 1. Thickness of different scintillators found by RBS. To analyze the results, the energy deposited in the screens by the impinging ions was calculated using the composition and thickness found by RBS together with the SRIM code [15]. It was found that all the samples are thick enough to fully stop the alpha particles within the film, which explains the continuous increase of IL yields vs. alpha particle energy. However, for protons and deuterons, depending on their energy, they can penetrate through the whole film and be stopped in the stainless steel substrate, therefore leading to lower IL efficiencies at higher energy. However, it is important to remark that the experimental yields do not present a linear response with the deposited energy in the phosphors. Actually, we have observed that for a given energy deposition within the material, the higher yields are obtained for the particles with the lower stopping power, which means that not only is important how much energy is delivered to the phosphors but also how this energy is deposited. The fact that larger stopping powers let to lower yields indicates that for highly ionizing particles some molecules of the scintillator can be damaged and they may produce quenching, and therefore they do not participate in the emission of light. Taken into account the previous results and in order to determine the absolute IL general curves, for all the ions and all the energies, we tried to fit the experimental results to some of the ionoluminesce models reported in the literature. The most popular and simple model in this area is Birk’s rule [16]: dx dE k dx dE S dx dY ⋅+ ⋅ = 1 (1) In this expression, dY/dx is the photon yield per unit length, dE/dx is the stopping power of the incoming particles, S is a constant representing the scintillator efficiency and k is also a constant related to the degree of quenching. Although this method was defined originally for organic scintillators, several works support the validity of this rule to inorganic scintillators [17, 18]. To find the appropriate constants S and k for each material we have performed the following approach: On one hand, when the stopping power is small, eq. (1) can be simplified to dY/dx≈S·dE/dx which, after integration, relates the photon yield with the energy lost into the screen. In our experimental conditions the particles with the lower stopping power are protons at 3 MeV, so that the constant S is obtained in first approximation using the IL values for these ions. On the other hand, for highly ionizing particles (He at 575 keV in this work), Birk’s formula can be written as dY/dx≈S/k, and the ratio S/k can be found by knowing the range of the ions into the materials, which was calculated using SRIM program. The final S and k values are obtained by an iterative process using the full eq. 1. In Table 2 are compiled the results for S and k for all the materials analyzed. P46 P56 TGa TGb S (γ/ion·eV) 0.051 0.083 0.100 0.179 K ((1015 at/cm2)/eV) 0.13 0.03 0.03 0.05 Table 2. Birk’s parameter calculated for all the scintillators. As an example, we present in Fig. 5 the experimental results and calculated values for the TGb phosphor. In general, for P46, TGa y TGb the IL yields for all the ions and energies can be reproduced by Birk’s formula with a difference lower than 10%. For the P56 material, Birk’s law does not reproduce with accuracy all the experimental values. Although for protons and deuterons the agreements is similar to the other scintillators, for α-particles the calculated data are ~30% higher than the experimental results. In addition, preliminary studies of the scintillator efficiencies degradation have been carried out. Figure 6 shows the degradation as a function of the ion fluence (ion/cm2) for all the screens irradiated with deuterons at 2 MeV and an ion flux of 1013 p/cm2·s. It is clear that there are important differences between the phosphors, being the P56 the material that degrades faster and the P46 the most resistant. Both, the absolute IL yield and the efficiency degradation are relevant properties of a scintillator for its use as a fast-ion loss detector in a fusion plasma reactor. Conclusions The absolute ionoluminiscence response in terms of photons per ion has been determined for the P46, P56 and TG-Green phosphors for protons, deuterons and alpha particles in the energy range between 575 keV and 3 MeV. It was found that the photon yield is not linear with the energy deposited into the materials and that the experimental results can be well reproduced using the Birk’s law. Therefore, this model can be employed to predict the sample’s IL response for other ions and/or energies of interest, as for instance beams with energy around or below 100 keV that are frequently used in the Neutral Beam Injectors of fusion reactors and that are not accessible in our accelerator. By comparing the performance of the scintillator screens analyzed in this work in terms of absolute IL yield and efficiency degradation it is concluded that the TG-Green is the best suited material for its use in FILD for fusion reactors. References [1] A. Fasoli et al., Chapter 5: physics of energetic ions, Nucl. Fusion 47 (2007) S264. [2] M. Garcia-Munoz et al., Nucl. Fusion 49 (2009) 085014. [3] M. Garcia-Munoz et al., REVIEW OF SCIENTIFIC INSTRUMENTS 80, 053503 (2009). [4] J. 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Acknowledgements This research was supported in part by the Spanish Ministry of Economy and Competitiveness (RYC2011-09152 and ENE2012-31087), the Marie Curie FP7 Integration Grant, PCIG11-GA-2012-321455, within the 7th EU Framework Program and the European Fusion Development Agreement (EFDA WP11, 12 and 13). Figures Figure 1 Absolute IL yield versus the ion beam energy at a beam current of 1 nA for protons.