Defect Engineering in SrI2:Eu2+ Single Crystal Scintillators - Crystal

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Defect Engineering in SrI2:Eu2+ Single Crystal Scintillators Yuntao Wu,*,†,‡ Lynn A. Boatner,⊥ Adam C. Lindsey,†,‡ Mariya Zhuravleva,†,‡ Steven Jones,§ John D. Auxier, II,§ Howard L. Hall,§ and Charles L. Melcher†,‡ †

Scintillation Materials Research Center, ‡Department of Materials Science and Engineering, and §Department of Nuclear Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States ⊥ Center for Radiation Detection Materials and Systems, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ABSTRACT: Eu2+-activated strontium iodide is an excellent single crystal scintillator used for gamma-ray detection, and significant effort is currently focused on the development of large-scale crystal growth techniques. A new approach of molten-salt pumping or so-called melt aging was recently applied to optimize the crystal quality and scintillation performance. Nevertheless, a detailed understanding of the underlying mechanism of this technique is still lacking. The main purpose of this paper is to conduct an in-depth study of the interplay between microstructure, trap centers, and scintillation efficiency after melt aging treatment. Three SrI2:2 mol % Eu2+ single crystals with 16 mm diameter were grown using the Bridgman method under identical growth conditions with the exception of the melt aging time (e.g., 0, 24, and 72 h). Using energy-dispersive X-ray spectroscopy, it is found that the matrix composition of the finished crystal after melt aging treatment approaches the stoichiometric composition. The mechanism responsible for the formation of secondary phase inclusions in melt-aged SrI2:Eu2+ is discussed. Simultaneous improvement in light yield, energy resolution, scintillation decay-time and afterglow is achieved in melt-aged SrI2:Eu2+. The correlation between performance improvement and defect structure is addressed. The results of this paper lead to a better understanding of the effects of defect engineering in control and optimization of metal halide scintillators using the melt aging technique.

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INTRODUCTION Scintillation single crystals are widely used in the field of medical imaging, security inspection, and high-energy-physics.1 In general, the scintillation performance of single crystals is directly influenced by the presence of micro- and macroscale defects.2,3 Many factors contribute to the single crystal quality, such as raw material purity, ampule design, seed orientation, and crystal growth parameters. For the latter three, there is a collection of research papers available.4−7 The quality control of the raw material for single crystal growth, such as removing unwanted impurities and residual moisture, is commonly regarded as a vital factor that determines the optical quality and scintillation performance, especially for halides. One purification method available is the zone refining technique.8,9 Because of the segregation effect of impurities with kseg > 1 or kseg < 1, they can be pushed to the ends of the refined rod of raw materials after multiple heating passes. Thereafter, the middle section can be used for high-purity crystal growth. Residual moisture, also a degrading effect in halide scintillator crystals, can be removed by baking the raw materials in a vacuum at temperatures much lower than melting points. An often overlooked question is whether the initial raw materials provided by the supplier conform to the assumed stoichiometric composition. Despite the confidentiality of raw material production process of the manufacturers, the main commercial batch preparation in large quantities can be roughly divided into three categories. Rare earth halides serve as good examples: © XXXX American Chemical Society

(i) Dehydration of corresponding rare earth halide hydrates. To avoid the hydrolysis of the anhydrous halides, the dehydration is carried out in HX (X = F, Cl, Br, I) gas;10 (ii) Using rare earth oxides and NH4X (X = F, Cl, Br, I) as raw materials through a solid state reaction to yield anhydrous rare earth halide;11 (iii) Dehydration of corresponding rare earth halide hydrates mixed with NH4X (X = F, Cl, Br, I).12 NH4X (X = F, Cl, Br, I) can react with rare earth halide hydrates to form some intermediate complexes which are thermally stable during the dehydration. Thus, hydrolysis of rare earth halides during dehydration can be effectively restrained by adding proper amounts of NH4X (X = F, Cl, Br, I). For the three methods above, an excess of halogen ions will most likely remain in the final raw material in order to complete conversion of rare earth oxides or hydrates into anhydrous halides. Similar treatments were also applied to alkaline earth halide raw materials.13 In this way, there may be varying degrees of halogen ions excess in different batches even for the same compound. It is impossible to optimize this situation by using traditional baking process, e.g., heating the raw materials below its melting point under vacuum. Boatner et al.14 report that the excess halogen ions in raw materials could cause coloring and cracking in the finished crystal. Consequently, Received: April 20, 2015 Revised: May 13, 2015

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DOI: 10.1021/acs.cgd.5b00552 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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developing a technique to remove excess halogen ions seems quite necessary and worthwhile. Initially in 1968 the scintillation light yield of SrI2:Eu2+, the research target of this work, was reported to be only 35 300 photons/MeV.15 However, since 2008, a stepwise development of the Bridgman crystal growth technique and the optimization of Eu2+ dopant concentration increased the light yield to 90 000 photons/MeV or more, with an energy resolution of about 2.6% at 662 keV.7,14,16−19 Its characteristics can rival or exceed those of LaBr3:Ce, the high figure-of-merit commercial halide scintillator widely used in gamma-ray detection, in terms of light yield, energy resolution, nonproportionality, and lack of radioactive components.16 Recently, Boatner reviewed the optimum materials-processing and crystal-growth techniques that are specific to the Bridgman growth of SrI 2 :Eu 2+ scintillators: (a) the use of a porous quartz frit to physically filter the molten salt; (b) the use of a bent capillary design to suppress multiple grain growth; and (c) the use of melt aging to remove excess iodine.14 In fact, the former two techniques are quite familiar to the crystal grower, but the later one was addressed for the first time in 201414 and later in 201520 for the crystal quality optimization of SrI2:Eu2+ and CaI2/CaI2:Eu2+. Nevertheless, the specific influences of this technique on the composition, microstructure, point defects, and scintillation properties were not investigated in detail. Consequently, it must be considered that this technique may be beneficial to the optimization of other metal halide scintillators. Thus, it motivates us to construct a melt aging station for raw materials optimization, carry out the crystal growth of SrI2:Eu2+ by using the melt-aged raw materials, and fully characterize their performance with an aim toward a fundamental understanding of the beneficial effects of the melt aging technique. In this work, three SrI2:2 mol %Eu2+ single crystals with 16 mm diameter were grown using the Bridgman technology under identical growth condition except for the difference in melt aging time. Second, the optical quality and microstructure of melt-aged SrI2:Eu2+ single crystals were evaluated by Epifluorescence microscope, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Then, a systematic study of the optical and scintillation properties was carried out. A possible mechanism responsible for the formation of second phase inclusion caused by the melt aging treatment is discussed, and the correlation between the performance improvement and the change of defect structure is addressed.

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Figure 1. Schematic drawing of a melt aging station: an ampule loaded with halide raw materials, a muffle furnace, heat-resistant valves, a liquid nitrogen cold trap, and a roughing pump. microstructure, scintillation properties, and point defects are the primary aim of this work. Crystal Growth. We used the vertical Bridgman technique to grow three SrI2:Eu2+ single crystals at the SMRC. Alekhin et al.21 reported that 2 mol % Eu is the optimum concentration for SrI2:Eu2+ with the thickness close to or larger than 10 mm. Thus, we chose (Sr0.98Eu0.02)I2 as the initial composition. High-purity anhydrous SrI2 beads (5N) from Sigma-Aldrich were used. High-purity anhydrous EuI2 beads (4N) were also obtained from Sigma-Aldrich, but the purity level of EuI2 was found to be generally unsuitable for the growth of high quality single crystals.9 Thus, zone refining purification of the EuI2 raw materials was carried out at the SMRC. These SrI2 beads and zonerefined EuI2 were loaded and mixed in the quartz ampules according to the stoichiometric ratio. Before crystal growth, the samples received the following treatment: (i) One ampule loaded with raw materials was baked at a temperature of 250 °C for 24 h to remove the residual water without melt aging treatment (its finished crystal is defined as MA0). (ii) One ampule loaded with raw materials was heated to a molten state of the charge and held under vacuum for 24 h (its finished crystal is defined as MA24). (iii) One ampule loaded with raw materials was heated to a molten state of the charge and held under a vacuum for 72 h (its finished crystal is defined as MA72). All three presynthesized samples were grown under identical conditions, e.g., growth station, temperature gradient, pulling rate, and ampule geometry. A temperature gradient of 30 °C/cm and a translation speed of 1 mm/h were used. The grown crystals were cooled down to room temperature (RT) at a speed of 6 °C/h. The resulting boules were 16 mm in diameter and a length of cylinder ∼60 mm. All the crystals were fully transparent, colorless, and crackless except MA72 with some cracks (Figure 1). The actual dopant concentrations in the crystals were measured by the inductively coupled plasma time-of-flight mass spectrometry (ICP-TOF-MS) method. The samples were all cut at the same position along the boules into ϕ16 × 8 mm3 for MA0 and MA24 and 10 × 5 × 3 mm3 for MA72. Two end surfaces were mechanically polished. They were used for photoluminescence excitation and emission spectra, photoluminescence lifetime, pulse height spectra, X-ray radioluminescence spectra, and scintillation decay-time measurements. Before the measurements, all of the wrapped crystals were stored in the dark for at least 24 h. The crystal plates with a size of about ϕ16 × 2 mm3 were used for optical absorption and transmission spectra, as well as microstructural studies. The 5 × 5 × 2 mm3 crystal plates were used for thermoluminescence measurements. Microstructure Analysis. The microstructure was measured by using a stereomicroscope with an epi-fluorescence illuminator. Excitation of Eu2+ activators with 365 nm light from a mercury arc lamp was used to image emission uniformity in polished cross sections of SrI2:Eu2+ with varying melt-aging time. Bright-field images via transmitted white light can be overlaid with a corresponding Epi-

EXPERIMENTAL METHODS

Construction of Melt Aging Station. We constructed a nonferrous melt aging station at the Scintillation Materials Research Center (SMRC), University of Tennessee, Knoxville. In this station, metal halide raw materials are held above their melting point under continuous evacuation for a number of hours, during which excess of the halogen element is removed via volatilization from the melt surface and subsequently captured in a cold trap. A schematic diagram is shown in Figure 1. The whole system consists of several parts: an ampule loaded with halide raw materials; a muffle furnace to melt the halide raw materials; heat-resistant valves; a liquid nitrogen (L-N2) cold trap to collect the halogen gas/other impurities; and a roughing pump. The critical operation parameters for the melt aging station are vacuum level, melt aging temperature, and time. A low vacuum level of 10−2 mbar was used to avoid serious evaporation effects of the molten salt when pumping at a higher vacuum level. The melt aging temperature was set slightly above the melting point of SrI2 to avoid overheating. The effects of melt aging time on crystal quality, B



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fluorescence image, which allows us to observe the inclusions in the crystals. The scanning electron microscope (SEM) and energy-dispersive Xray spectroscopy (EDS) analyses for the microstructural characterization of the melt-aged SrI2:Eu2+ single crystals were performed with a Gemini Leo 1525 scanning electron microscope operated at 15 and 20 keV. The incident X-ray beam energy and collecting energy range for EDS are identical for all the samples. The actual Eu concentrations in the crystals were measured using a GBC Scientific Optimass 9500 ICP-TOF-MS. Samples were prepared by dissolving between 1 and 2 mg of crystal in 9 mL of high purity water. The samples were then diluted with ultrapure water to 3.52 μg/ mL. All samples also contained Ultra Trace 2% HNO3 to ensure complete dissolution of the analytes (JT Baker). A set of standards was created from Inorganic Ventures Rare Earth CCS-1 ICP−MS and IVStock-21 Multi-Element standards to quantify the signal using a linear least-square analysis. The nebulizer flow rate was set to 0.975 L/min and the skimmer voltage to −1100 V. The data were collected in three replicates, each using a 30 s acquisition time. Optical Property Measurements. Optical absorption spectra were measured with a Varian Cary 5000 UV−VIS−NIR spectrophotometer in the 200−800 nm range. Photoluminescence emission and excitation spectra were measured with a HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer. The excitation light went through an excitation monochromator with a 1 nm bandpass to ensure monochromaticity. Similarly, the emission monochromator was set at 1 nm bandpass to select emission light of a specific wavelength. In the case of emission and excitation spectra, a 450W continuous xenon lamp was used as the excitation source. Photoluminescence (PL) decay was measured on the same spectrofluorometer using a time-correlated-single-photon counting module. HORIBA Jobin Yvon NanoLEDs (pulsed light-emitting diodes) were used as the excitation source. The duration of the light pulse was shorter than 2 ns and therefore was not deconvoluted from the much longer decay profiles. Scintillation Property Measurements. Scintillation decay times were measured using a time-correlated single-photon counting setup under 137Cs source excitation. Absolute light yield measurements were recorded by using a pulse processing chain consisting of a Hamamatsu R2059 photomultiplier tube (PMT) operated at −1500 Vbias, an Ortec 672 Amp, a Canberra model 2005 pre-Amp and a Tukan 8k multichannel analyzer. Each sample was directly coupled to the PMT using mineral oil, and a PTFE-lined dome-shaped reflector with a 50 mm radius was used to maximize the collection of light. The photoelectron yields were estimated by using the single photoelectron peak method. Measurements on the samples were made with 10 μs shaping time to provide light integration. Each sample was measured under irradiation with a 15 μCi 137Cs source. The reproducibility of the LY measurements is ±5%. The energy resolution was measured by using a 2-in. diameter high quantum efficiency Hamamatsu R6231−100 PMT. This PMT was operated at −900 Vbias. Nonproportionality measurements were also measured by using this PMT. We used 133Ba, 241Am, 57Co, 137Cs, and 22 Na γ-ray sources to excite the crystals at energies from 31 to 662 keV. The energy resolution (E.R.) was calculated as the full width at half-maximum (fwhm) divided by the channel number. An X-ray tube operated at 35 kV and 0.1 mA was used as the excitation source for X-ray excited luminescence measurements. For the afterglow measurements, the crystals were coupled to a Hamamatsu R2059 photomultiplier tube with Dow Corning Q23067 optical couplant and covered with Tetratex TX3104 PTFE membrane. The crystals were irradiated with X-rays using an X-ray tube (35 kV, 0.1 mA) at room temperature for 15 min, after which a Uniblitz XRS6S2P1−040 shutter was used to cut off the X-ray beam within 3 ms and the luminescence emitted from crystal was recorded as a function of time. For each thermoluminescence (TL) measurement, a sample was mounted on a coldfinger of the cryostat. The pressure was reduced to 20 mTorr, and the sample was then heated to 600 K in order to ensure

that all traps were empty in the temperature range of interest. The samples were cooled to 5 K and irradiated by an X-ray generator (Xray model; CMX003) at 35 kV and 0.1 mA for 3 min. Subsequently, the sample was heated to 600 K at a rate of 9 K/min; noise due to thermionic emissions precluded the acquisition of good quality data above this temperature. A Hamamatsu H3177 PMT optically coupled to the cryostat’s light transport interface was used to measure the sample’s emission. The PMT current signal was transformed into a voltage signal using standard NIM electronics. A National Instruments 6002-E data acquisition card was then used to digitize this voltage signal. Software developed in-house was used to correlate the sample temperature with the signal intensity.

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RESULTS AND DISCUSSION Optical Quality and Microstructure. Figure 2a−c shows finished SrI2:Eu single crystals treated with different melt-aging

Figure 2. Pictures of SrI2:Eu2+ single crystal boules treated with different melt aging time: (a) MA0, (b) MA24, and (c) MA72; (d) the polished SrI2:Eu single crystal slabs: ϕ16 × 8 mm3 for MA0 and MA24, 10 × 5 × 3 mm3 for MA72 (from left to right).

times. Neither cracking nor visible-inclusions were observable in MA0 and M24, but a cracking plane along the crystal growth direction can be observed in MA72, in which half of the total volume is polycrystalline. Figure 2d shows polished crystal slabs of MA0, MA24, and MA72 (from left to right) used for the evaluation of light yield, energy resolution, nonproportionality, and scintillation decay-time. To investigate the microstructure, all three crystals were cut into 2 mm thick plates. The pictures of crystal plates under daylight and a UV lamp are shown in Figure 3a,b. Negligible differences in transparency and brightness among these crystals can be distinguished from initial inspection. Therefore, epi-fluorescence microscope through excitation of Eu2+ activators by using 365 nm light was utilized to observe the differences in microstructure. No significant emission nonuniformities or inclusions are observed in the MA0 and MA24 samples, Figure 3c,d, but the MA72 sample contains inclusions that appear brighter in the epiC



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constitutional supercooling process. In order to clarify the inclusion formation mechanism, the SEM and EDS techniques are utilized to investigate the practical compositions of matrix and inclusions for each sample. The scanning electron microscopic (SEM) images of inclusions in SrI2:Eu2+ MA72 plates are shown in Figure 5a−

Figure 3. Pictures of SrI2:Eu2+ single crystal thin plates under daylight (a) and UV lamp (b); the epi-fluorescence images for each plate: (a) MA0, (b) MA24, and (c) MA72.

fluorescence images, Figure 3e. To have a closer look at the inclusions in MA72, the morphology of the inclusion is magnified by ten times and shown in Figure 4. It is found that

Figure 5. Scanning electron microscopic (SEM) images of SrI2:Eu2+ MA72 plates at three different areas: (a, d, c); (d) the matrix composition as a function of melt aging time and the inclusion composition in MA72 sample. The practical compositions are listed in Table 1.

c. The practical compositions of the inclusions and matrix in the MA72 sample are measured using energy-dispersive X-ray spectroscopy (EDS). The matrix and inclusion compositions are all listed in Table 1. By using SEM, we could not observe Table 1. Composition of Matrix and Inclusion in the MeltAged SrI2:Eu2+ Single Crystals Analyzed by the EDS Methoda

Figure 4. Microscopic pictures of a 2 mm thick SrI2:Eu2+ MA72 plate under different magnification scales and observation modes: (a, b) a transmission mode at different magnification scales, (c) a fluorescence mode, and (d) a combination of transmission and fluorescence modes.

sample

matrix composition

inclusion composition

SrI2:Eu MA0 SrI2:Eu MA24 SrI2:Eu MA72

SrI2.21±0.03 SrI2.16±0.09 SrI2.10±0.04

N.O. N.O. SrI1.42 (inclusion #1, Figure 5a) SrI1.68 (inclusion #2, Figure 5b) SrI1.29 (inclusion #3, Figure 5c)

a

The average matrix composition for each sample is calculated based on the six selected areas on its measured surface. The composition deviation is also listed. The inclusion composition is measured by selecting the central area of the inclusion. N.O. represents “not observed”.

the second-phase inclusions propagate inward (from rim to the center), similar to the shape of inclusions observed in LaB3:Ce.22 There are two prevailing theories for melt inclusion formation, namely, the constitutional supercooling with resulting interface breakdown23,24 and solute supersaturation theory.22 As for the former theory, during solidification, solute (impurities and/or secondary phase) concentration is piled up in front of the solidification interface due to segregation. The pileup of a solute-rich boundary layer leads to the formation of a constitutionally supercooled region, causing instability of the planar growth front. The later theory is related to nucleation of bubble/particle from dissolved gases and the diffusion of solute into bubble/particle, resulting in an increase of bubble/particle volume to form inclusions. In our case, we used the same growth furnace with identical growth parameters and capillary design with only the initial composition altered by the melt aging treatment. Thus, the melt inclusion in this mechanism should not be related to the latter one, but more likely to the

any visible inclusions in the MA0 and MA24 samples, but indeed did observe inclusions with a 10 μm size in MA72. These phenomena are consistent with the epi-fluorescence images (see Figure 3c−e). The EDS analysis of the matrix in all three crystals (MA0, MA24, and MA72) shown in Figure 5d illustrates that, as the melt aging time increases from 0 to 72 h and excess iodine is removed, the matrix composition in the finished crystal gradually approaches the stoichiometric strontium/iodine ratio of 1:2. It corroborates the effectiveness of the original motivation.14 The EDS analysis of inclusions in MA72 shown in Figure 5d has pinpointed their approximate compositions to be Sr-rich, suggesting a lack of iodine at the localized crystal growth interface. This might be explained from the perspectives of treatment for raw materials and the phase D



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diagram: (i) it is demonstrated that continuous melt aging treatment can gradually remove iodine from the matrix. In fact, although the matrix composition shows excess of iodine ions, the halogen deficiency is still likely to form. According to theoretical calculation results for LaBr3, even at the Br-rich end, the VBr is still the dominant point defect rather than the others, like VLa, Brint;25 (ii) based on the phase diagram of SrI2−Sr,26 a sharp liquidus slope toward the pure SrI2 phase will cause the lower melting point by shifting the melt composition to a few molar percentage rich in Sr end member, and in turn forming a precipitate of Sr leading to a SrI2−Sr eutectic with a decreased melt point. Consequently, the inclusions are more likely to be the combination of clusters of elemental Sr and SrI2 with different ratios due to the localized constitutional supercooling. An appropriate melt aging time for SrI2:Eu2+ should be considered as a balance between approaching matrix stoichiometry and suppression of the formation of secondary phase inclusions. Optical Properties. Figure 6 presents the optical absorption and transmission spectra in the range of 200−800

Figure 7. (a) Photoluminescence excitation (λem = 427 nm) and emission (λex = 370 nm) spectra; (b) photoluminescence decay profiles of SrI2:Eu2+ single crystals, monitoring at λem = 427 nm, λex = 370 nm.

83.7%. An overall transmittance from 435 to 800 nm for MA0 is within 80−88%, which reveals its good optical quality. The MA24 shows a comparable transmittance with MA0, although a slightly lower value in the region of 435−635 nm. However, for MA72, an obvious reduction of transmittance by 4−9% in the whole concerned wavelength region indicates its worse optical quality. This trend is consistent with the microstructure analysis in the previous section, in which the MA72 sample is overspread with Sr-rich inclusions. The photoluminescence excitation and emission spectra of SrI2:Eu2+ single crystals treated with different melt aging time are shown in Figure 7a. Directly excited at 370 nm, an emission band peaking at 427 nm could be observed, which is assigned to the interconfigurational radiative 5d → 4f transition in the Eu2+ ions.29 The broad excitation bands from 250 to 420 nm correspond to the Eu2+ 4f → 5d transitions. The photoluminescence decay profiles of Eu2+ are presented in Figure 7b. They all can be fitted by a single-exponential function. The decay constant decreases from 1.06 μs for MA0, 1.03 μs for MA24, to 0.86 μs for MA72. Because of the absence of Eu3+ 4f → 4f emission features in the RL spectra (Figure 8), the

Figure 6. Optical absorption (right axis) and transmittance (left axis) spectra of 2 mm thickness SrI2:Eu2+ single crystals treated with different melt aging time.

nm for all crystals with 2 mm thickness. The abrupt absorbance increase below 420 nm is due to the onset of the 4f-5d1 absorption band of the Eu2+ center. However, limited information can be obtained from absorption spectra below 400 nm because of the underestimation of the measured value of absorbance easily occurred at strongly luminescent samples and high absorbance in the excitation bands, like SrI2:Eu2+. The luminescence from the sample can add the false light signal to the measurement beam of the spectrometer, because there is no monochromator between the sample and detection photomultiplier. In this sense the increase of absorbance signal below 250 nm is a sign that luminescence of sample becomes weaker which perfectly coincides with the excitation spectra shown in Figure 7a. In consideration of the multiple light bouncing between two end surfaces, the theoretical limits of the transmittance (Ttheo) can be calculated using the follow equations:27

Figure 8. X-ray radioluminescence spectra of SrI2:Eu2+ single crystals treated with different melt aging time by using the reflection mode. Inset shows the close-up for the region between 470 and 550 nm.

fraction of Eu3+ ions could be regarded as zero or negligible. It is analogous to the other SrI2-based halides, e.g., CsSrI3:Eu. The europium valence state in CsSrI3:Eu grown in the same way was demonstrated to be entirely divalent by X-ray absorption spectroscopy.30 Scintillation Properties. The X-ray excited radioluminescence spectra of SrI2:Eu2+ single crystals are displayed in Figure 8. All present an emission peak centered at 436 nm related to the 5d → 4f de-excitation of Eu2+. According to ref 31, the

Ttheo = (1 − R )2 + R2(1 − R )2 + ... = (1 − R )/(1 + R ) (1) 2

2

R = (ncrystal − nair ) /(ncrystal + nair )

(2)

where ncrystal and nair are the refractive index of crystal and air, respectively. The refractive index of SrI2 at 435 nm is 1.85.28 By using eqs 1 and 2, the Ttheo at 435 nm can be estimated to be E



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Table 2. Physical and Scintillation Characteristics of SrI2:Eu2+ Single Crystals Treated with Different Melt Aging Time energy resolutionc (%) samples

size (mm3)

Eu concentration in crystala (mol %)

RL emission wavelength (nm)

light yield at 662 keVb (photons/MeV)

122 keV

662 keV

scintillation decay timed (μs)

MA0 MA24 MA72

ϕ16 × 8 ϕ16 × 8 10 × 5 × 3

0.9230 ± 0.0020 0.9398 ± 0.0002 1.0263 ± 0.0001

436 436 436

71,600 ± 3580 87,000 ± 4350 71,600 ± 3580

7.6 6.7 7.4

4.6 3.4 4.9

1.19 1.11 0.91

a c

Measured by ICP-TOF-MS method. bMeasured under 137Cs excitation by Hamamatsu R2059 PMT with known quantum efficiency curve; Measured under 57Co and 137Cs excitation by high quantum efficiency Hamamatsu R6231 PMT; dUnder 137Cs gamma-ray source irradiation.

decreases down to below 1 μs for samples of comparable size and Eu2+ concentration. This shortening should be related to the optimization of the scintillation process prior to free charge carrier capture by the Eu2+ ions. The conversion process in which the energy of the incoming radiation or particles converting into a large number of electron−hole pairs only takes less than 0.001 ns.35 During the transfer process, shallow traps acting as a secondary reservoir can provide a delayed feed of carriers to the activator centers.3 Thus, the optimization of trap centers could positively contribute to decay-time shortening phenomenon. Based on the scintillation decay time and the lifetime of Eu2+ itself (see Figure 7b), the difference between these two time responses, corresponding to the migration time before free charge carriers captured by activators after irradiation, can be estimated to be 130 ns for MA0, 80 ns for MA24, and 50 ns for MA72 (Figure 9 inset). In other words, the (shallow) trapping-detrapping effect becomes weaker in this scenario. The evidence will be presented in following section. Pulse height spectra of SrI2:Eu2+ single crystals under 137Cs (662 keV) excitation at room temperature are plotted in Figure 10. Considering the wavelength-weighted quantum efficiency of

increase of the Eu2+ concentration from 0.5% to 10% can cause a redshift of Eu2+ emission peak about 7 nm. The stability of Eu 2+ emission peak in our case indicates the Eu 2+ concentrations in melt-aged crystals are at the same level, consistent with the ICP-TOF-MS results shown in Table 2. After normalization, an interesting phenomenon can be observed: a gradual decrease of emission intensity of the shoulder peaking at around 490 nm with the increase of the melt aging time. This shoulder emission was proposed to originate from self-trapped or impurity-trapped or defecttrapped exciton like states, called trapped exciton emission (TE).21,32 Also in other metal halides, like barium-based halides, the emission at a similar wavelength was specifically assigned to the oxygen-related emission.33 In fact, aside from removing excess of iodine, dissociated oxygen impurities, one kind of stubborn impurities in commercial metal halide raw materials present in 10−100 ppm wt9 can also be continuously siphoned by floating to the top of the melt and pumped out. The declining tendency of oxygen content is in accordance with that of the emission at 490 nm. Although unsubstantiated, a possible assignment for this emission band is the oxygen-related TE emission. The scintillation decay time profiles of SrI2:Eu2+ single crystals treated with different melt aging time are plotted in Figure 9. All the decay curves could be fitted by a singleexponential method. The decay constant is 1.19 μs for MA0, 1.11 μs for MA24, and 0.91 μs for MA72. The decay constant for MA0 crystal is within the principal decay time range reported in ref 34, considering the size and Eu2+ concentration. As the melt aging time increases, the decay constant value

Figure 10. Pulse height spectra under 137Cs irradiation of SrI2:Eu2+ single crystals treated with different melt aging times detected using a Hamamatsu R2059 PMT at room temperature.

R2059 PMT for the SrI2:Eu2+ of about 24.1%, it is estimated that the light yield can reach 71,600 ± 3580 photons/MeV for MA0, 87,000 ± 4350 photons/MeV for MA24, and 71,600 ± 3580 photons/MeV for MA72. The enhancement of light yield from MA0 to MA24 achieved under the same size and Eu2+ concentration allows us to ascribe it to the removing/ decreasing certain traps responsible for reduced light yield because of the free-carrier trapping. The deterioration of light yield in MA72 could be associated with the formation of light

Figure 9. Scintillation decay profiles under 137Cs gamma-ray irradiation of SrI2:Eu2+ single crystals treated with different melt aging time. Inset is the migration time for free charge carriers to activators after irradiation in the melt-aged SrI2:Eu2+. F



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energy resolution at 662 keV is 4.6% for MA0, 3.4% for MA24, and 4.9% for MA72. The measured energy resolution R can be considered as having three main contributions:

scattering centers (Sr-rich secondary phase inclusions), which are shown in Figure 3 and Figure 4. The values of light yield are in good agreement with the reported values for SrI2:Eu2+.34 Scintillation nonproportional (nPR) response was derived from measurements of the light yield under γ-ray excitation using different γ-ray isotopes. The 22Na (511 keV), 137Cs (662 keV), 57Co (122 keV), 133Ba (33 and 356 keV), and 241Am (59.5 keV) were used as irradiation sources. From the measured position of the full energy peak and the known γray energy for each isotope, the channel number of the full energy peak at each γ-ray energy was determined. The data points were then normalized with respect to the response at 662 keV. The results obtained are shown in Figure 11. A

R2 = R2 stat + R2 nPR + R2 inh

(3)

where Rstat the statistical contribution, RnPR the contribution due to nPR, and Rinh the contribution due to inhomogeneity including inhomogeneous LY response across the crystal and inhomogeneous light collection. The Rstat can be expressed as −1/2 ⎡ R stat(%) = 235 × ⎢(1 + var(M )/Nphe PMT)⎤⎦ ⎣

(4)

where NphePMT is the number of the photoelectrons produced in the PMT per MeV of absorbed energy, var(M) is the fraction variance in the PMT gain, which is 0.28 for the Hamamatsu R6231−100 PMT.36 Using eq 4, the Rstat contribution is 2.19% for MA0, 2.05% for MA24, and 2.11% for MA72 at 662 keV. Using eq 3, the combined contribution of R2nPR + R2inh can be evaluated to be 16.36 for MA0, 7.36 for MA24, and 19.56 for MA72. Because of the comparable nPR in all three samples, the optimization of energy resolution in MA24 should be ascribed to the improvement of inhomogeneity as well as the light yield enhancement. The energy resolution at lower energy 122 keV is 7.6% for MA0, 6.7% for MA24, and 7.4% for MA72. The comparative relationship of these values is close to that at 662 keV. Atomic-Level Defects. The thermoluminescence (TL) technique is used to investigate the atomic-level defects in SrI2:Eu2+ single crystals. TL glow curves are presented in Figure 13. A highly concentrated electron trap at 58 K is exhibited in the MA0 sample. The featured TL peak was indeed identified in SrI2:Eu2+ single crystals before.37 The intensity of this peak drops at least 2 orders of magnitude compared to that in meltaged samples, e.g., MA24 and MA72 (See Figure 13 left inset). Besides, the intensity of another TL peak at round 400 K increases gradually from MA0 to MA72, but the TL peak above 500 K shows the completely opposite trend. To discuss the

Figure 11. Nonproportionality of SrI2:Eu2+ single crystals with different melt aging treatments.

deviation from the flat line (ideal proportionality) occurring below about 122 keV appears in all three samples with the MA72 sample exhibiting slightly better proportionality than MA0 and MA24. The energy resolutions of all SrI2:Eu2+ crystals at 122 and 662 keV were measured under 57Co and 137Cs gamma-ray sources, respectively, by using a high quantum efficiency Hamamatsu R6231−100 PMT. Observed from Figure 12, the

Figure 12. Pulse height spectra under 137Cs and 57Co gamma-ray irradiation of SrI2:Eu2+ single crystals treated with different melt aging time detected by using a Hamamatsu R6231−100 PMT: (a1) MA0, 137Cs; (a2) MA0, 57Co; (b1) MA24, 37Cs; (b2) MA24, 57Co; (c1) MA72, 137Cs; (c2) MA72, 57Co. G



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Table 3. Trap Depth and Detrapping Time Parameters of SrI2:Eu2+ Single Crystals sample

Tmax (K)

Etrap (eV)

SrI2:Eu MA0

58 205 297 390 506 60 210 265 403 556 58 210 274 408 522

0.099 0.310 0.720 1.121 1.543 0.103 0.326 0.687 1.131 1.543 0.103 0.328 0.715 1.120 1.578

SrI2:Eu MA24

SrI2:Eu MA72

Figure 13. Thermoluminescence glow curves of SrI2:Eu2+ single crystals with different melt aging treatments. The left inset is the closeup of the region from 0 to 100 K. The right inset shows the fitting glow curves (red lines) by using eq 4 as well as the experimental glow curves (empty cycles).

⎛ E ⎞ I(T ) = sn0 exp⎜ − t ⎟ ⎝ κBT ⎠

∫T

T

0

⎤−l /(l − 1) ⎛ E ⎞ exp⎜ − t ⎟ dT + 1⎥ ⎥⎦ ⎝ κBT ⎠

8.99 1.19 1.55 1.11 1.98 6.50 0.72 1.90 1.09 1.07 6.70 0.90 1.46 6.66 2.07

× × × × × × × × × × × × × × ×

107 106 1011 1013 1014 107 105 1010 1013 1014 107 106 109 1012 1014

τ298 K (ns) 5.26 1.47 9.59 8.06 6.15 8.49 4.52 2.17 1.20 1.15 8.24 3.91 8.39 1.30 2.31

× × × × × × × × × × × × × × ×

102 108 109 1014 1020 102 109 1010 1015 1021 102 108 1011 1015 1021

the experimental data available, and further work is needed. The intensity reduction of the TL peak within 500 and 550 K should positively contribute to the enhancement of light yield from MA0 to MA24. The detrapping time τ of the trap at the temperature T can be calculated as42

possible origin of each TL peak, the parameters of the traps corresponding to TL peaks are analyzed first. The general-order kinetics expression describing TL intensity I as a function of temperature T is38

⎡ (l − 1)s ×⎢ ⎢⎣ β

s (s−1)

τ = s−1e E / kT

(7)

The detrapping time at room temperature is also listed in Table 3. The detrapping time related to the TL peak at about 58 K is about 500−900 ns, close to the value of 547 ns reported for this trap.37 Because removing shallow traps could efficiently enhance the energy migration of charge carriers to the recombination centers, the near absence of this trap may cause the scintillation decay-time shortening. It is in complete agreement with our previous deduction. It is well-known that the deep traps corresponding to high temperature TL peaks can cause the afterglow in the time frame of seconds, minutes, hours, or even longer.43 Such traps can also result in the socalled “bright burn” or radioluminescence sensitization.44,45 Specifically, as the accumulated dose increases, the progressive filling of traps can lower their competition with luminescence centers in free charge carrier capture and then give rise to an increase of radioluminescence intensity. It must be noted that, in our case, before light yield measurements, the analyzed SrI2:Eu2+ samples were kept in the dark for at least 24 h and without irradiation under UV or X-ray to prevent them from being influenced by this effect, and it is also worthwhile to investigate the afterglow profiles for all SrI2:Eu2+ samples, shown in Figure 14. The afterglow level after X-ray cutoff in the MA24 sample is about 1 order of magnitude lower throughout the entire measurement time range up to 2 h than that in the MA0 sample. The afterglow level before 500 s in MA72 drops at nearly the same rate along with MA24, but gradually increases thereafter until reaching the same level with the MA0 sample at 10000 s.

(5)

where n0 is the concentration of trapped charges at t = 0, Et the energy level of the trap, κB the Boltzmann constant, l the kinetic order, s the frequency factor, and β the heating rate (1 Ks−1 in this measurement). Equation 5 can be modified into eq 6 adopted in the fitting process,39 ⎛ E ⎞ I(T ) = sn0 exp⎜ − t ⎟ ⎝ κBT ⎠ ⎧ ⎛ E ⎞ ⎪ (l − 1)s ×⎨ × T × exp⎜ − t ⎟ ⎪ β ⎝ κBT ⎠ ⎩ ⎫l /(1 − l) ⎡⎛ ⎞ ⎛ κBT ⎞2 ⎛ κBT ⎞3⎤ ⎪ κ T B × ⎢⎜ ⎟ − 2⎜ ⎟ + 6⎜ ⎟ ⎥ + 1⎬ ⎪ ⎢⎣⎝ Et ⎠ ⎝ Et ⎠ ⎝ Et ⎠ ⎥⎦ ⎭ (6)

The fitted data agrees well with the experimental data (see Figure 13 right inset). All the TL curves could be fitted into five TL peaks for all samples. The specific TL parameters are listed in Table 3. As for the main TL peak at about 400 K, its trap depth is about 1.12−1.13 eV. This result is consistent with the calculated result of 1.17 eV for the F− center (two-electron captured by iodine vacancy) in SrI2 by using PBE0,40 which likely functions as a deep electron trap in SrI2, similar to its role in the other halides.41 We recall that even the matrix composition shows the iodine excess; the iodine-deficiency, e.g. iodine vacancies, at localized regions are still likely to be formed. Therefore, this assignment of the F− center is reasonable. As for the other two main TL peaks at about 58 K (Etrap ≈ 0.1 eV) and one within 500 and 550 K (Etrap ≈ 1.5 eV), it is difficult to speculate about their specific nature with

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CONCLUSIONS Three SrI2:Eu2+ single crystals each treated with different melt aging time were grown by the Bridgman technique under identical growth conditions. The matrix composition in the finished crystal can approach stoichiometry when using meltaged raw materials. The formation of second phase inclusions, e.g., Sr-rich inclusions, in the overtreated melt-aged SrI2:Eu2+ H



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REFERENCES

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Figure 14. Afterglow profiles of SrI2:Eu2+ single crystals with different melt aging treatments after continuous X-ray irradiation. The signal level with X-rays being on is normalized to 100%.

single crystal was caused by local constitutional supercooling of the melt during directional solidification. Simultaneous improvement in the light yield, energy resolution, scintillation decay-time, and afterglow was achieved in melt-aged SrI2:Eu2+. The SrI2:Eu2+ single crystal with 24 h melt aging treatment showed a notable enhancement of light yield up to 87,000 ± 4350 photons/MeV, a better energy resolution of 3.4% at 662 keV and a reduction of nearly 1 order of magnitude in afterglow compared to the nonmelt-aged SrI2:Eu2+, which possesses 71,600 ± 3580 photons/MeV and 4.6% at 662 keV. The light yield optimization could be explained by the suppression of deep traps located at about 1.55 eV below the conduction band. The energy resolution improvement was ascribed to the optimization of inhomogeneity and light yield. A continuous shortening of scintillation decay-time was observed with the increase of melt aging treatment, which was associated with the near complete removal of the shallow traps (Etrap ≈ 0.1 eV). The afterglow suppression effect is reported for the first time. Moreover, an unwanted increase of the afterglow level in the time scale of 104 s in overtreated melt-aged SrI2:Eu2+ could originate from the formation of the F− center (two-electron iodide vacancy center) acting as deep electron traps at 1.1 eV below the conduction band. More importantly, this new approach should be applied to other high figure-of-merit metal halides because of the potential beneficial effect on the crystal quality and scintillation performances.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the U.S. Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded Grant #2012-DN-077-ARI067-04. Research at ORNL for one author (L.A.B.) was supported in part by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UTBattelle for the U.S. Department of Energy and in part by the Nuclear Nonproliferation Program (NA-22) of the National Nuclear Security Administration, U.S. Department of Energy. This support does not constitute an express or implied endorsement on the part of the Government. I



Article

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Defect Engineering in SrI2:Eu2+ Single Crystal Scintillators - Crystal

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