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Abstract
CsSrI3:3%Eu (Φ10×55mm), CsSrI3:5%Eu (Φ10×50mm) and CsSrI3:7%Eu (Φ10×45mm) single crystals have been successfully grown by the edge-defined film-fed growth method for the first time, with the growth rate reaching 10-20mm/h. We designed a crystal growth device that achieved the first growth of this binary scintillation crystal by the EFG method. The raw material purification, temperature gradient of experimental device and growth rate, which are the effect factors of crystal quality, were systematically investigated. Moreover, the effect of Eu2+ concentration on optical properties were studied. The Eu2+ 5d-4f emission band was observed at 450-455nm, and the PL decay time was determined as 1.32µs for CsSrI3:3%Eu, 1.35µs for CsSrI3:5%Eu and 0.73µs for CsSrI3:7%Eu.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Scintillation crystals are widely used as radiation sensors in many areas, such as medical imaging, homeland security and well-logging. In recent years, due to the increased demand for high-performance scintillation crystals, various types of scintillation crystals including binary halides, ternary halides, quaternary halide such as LaBr3:Ce3+[1], SrI2:Eu2+[2], CsBa2I5:Eu2+[3], KSr2I5:Eu2+[4], KCaI3:Eu2+[5] and Cs2LiYCl6:Ce3+[6] have been studied in detail.
Since the ABX3:Eu (A = Cs, B = Ca, Sr, X = Cl, Br, I) materials have promising scintillation properties, how to obtain high quality crystals and with a quickly speed attracted the attention of the reseachers [7–10]. Among them, CsSrI3:Eu2+ single crystal has been exhibited a good scintillation performance. CsSrI3 crystal was first reported by Gaby Schilling of Hannover University, and the results showed that CsSr0.92Eu0.08I3 has a light output of 65000ph / MeV, and the energy resolution is 5.9%. After that, Tennessee University grew CsSr0.93Eu0.07I3 by Bridgman method, it showed that CsSr0.93Eu0.07I3 can obtain a light output of 72000ph / MeV under the excitation of gamma ray, which can meet the special needs of homeland security and nuclear safety inspection. CsSrI3: Eu single crystal showed a good scintillation performance, but it had proven difficult to grow high quality single crystal with fast growth rate. In addition, CsSrI3 is very sensitive to both moisture and oxygen.
Previous CsSrI3: Eu single crystals were mainly grown by Bridgeman method, but we tried to use the edge-defined film-fed growth (EFG) method to obtained the crystals. The advantage of EFG method is that it can grow crystal with specific shape, the growth speed can reach 10-20 mm/h, and the growth process can be observed. At present, growing halide scintillation crystals by EFG method is lacking in the world, and even not exist in China. Therefore, how to design a crystal growth apparatus, reduce crystal defects and improve the quality have become a major problem in the current EFG method.
In this paper, CsSr1-xEuxI3(x = 0.03,0.05,0.07) single crystals with high growth rate(10∼20mm/h) were grown successfully by EFG method. We mainly studied the crystal growth device, the temperature field and the purification of raw materials. On this basis, some optical properties of the crystals were studied.
2. Experimental procedure
CsSr1-xEuxI3(x = 0.03,0.05,0.07) single crystals were grown by EFG method from the melt. The starting materials were SrI2(99.95%), CsI (99.99%) and EuI2(99.99%). SrI2, CsI and EuI2 beads were placed into a silica ampoule tube in glove box (O2 < 0.1ppm, H2O < 0.1ppm). Then the sealed ampoule was evacuated to 10−6 mbar, and heated to ∼250°C for 6h in order to remove the residual water and oxygen impurities. Then the ampoule loaded with starting materials was heated ∼660°C to obtain CsSr1-xEuxI3(x = 0.03,0.05,0.07) compounds. After sintering, the polycrystalline materials were added into the crucible equipped with a die, and the crucible held in specific ampoule, then moved ampoule into the grow furnace. At first, the furnace was heated ∼640°C, we can see that the starting materials were melting, and the die floated on the melt surface. The melt flowed upward through the center hole in the die surface by capillary action. Before growing the crystals, we put a platinum wire into the melt, and obtained the seeds by rotating and pulling the platinum wire. After that, we started to grow CsSr1-xEuxI3(x = 0.03,0.05,0.07) crystals by EFG method, during the growth process, the crystal size is controlled by adjusting the temperature and pulling speed, the pulling speed was 10-20mm/h, and the whole growth process is carried out in argon. The schematic diagram of the growth apparatus as seen in Fig. 1.
All obtained crystals were cut and polished at a size of ∼10×8×2mm3 in the glove box using synthesis oil for characterization.
A small fraction was taken from the CsSr1-xEuxI3(x = 0.03,0.05,0.07) crystals and ground into powder for XRD test. The powder XRD measurements were obtained by an X-ray diffractometer (D8 ADVANCE).
Dynamic Vapor Sorption technique (DVS Intrinsic by Particulate Systems) was used to study the hygroscopicity. Grind the sample into powder and put it into the ultra-sensitive balance to record the weight change as a function of time with fixed humidity. The initial sample weight was 20 ± 0.5%. The weight change was record with 0.0001mg precision.
Photoluminescence spectra were measured at room temperature with a FLS 920 spectrophotometer (Edinburgh Instruments Co. Ltd., England). A Hamamatsu R928 PMT was used to record the emission intensity as a function of wavelength. For decay time measurement, the decay signal was recorded using a photomultiplier tube that was attached to an oscilloscope.
3. Results and discussion
3.1. Effect factors of crystal quality
The size of the grown crystal are Φ10×55mm (CsSrI3:3%Eu), Φ10×50mm (CsSrI3:5%Eu) and Φ10×45mm (CsSrI3:7%Eu), respectively. The as-grown crystals were shown in Fig. 2(a)–2(c). The EFG method achieves a faster growth rate, but there are many problems in the growth process, such as raw material purification, seeds selection, temperature field adjustment and growth rate control, etc. By studying these problems, we have obtained the ideal growth parameters to improve the quality of crystals.
Fig. 2. (a) CsSrI3:3%Eu single crystal grown by EFG method. (b) CsSrI3:5%Eu single crystal grown by EFG method. (c) CsSrI3:7%Eu single crystal grown by EFG method.
3.1.1. Raw material purification
The purification of raw materials is directly related to the quality of the grown crystal, including consistency, transparency, growth defects, etc. As shown in Fig. 3(a), synthesis of the compounds with unfiltered raw material, the surface of the compounds block contains a large number of black impurities, which left by the manufacturer when purifying the raw materials. The presence of these impurities greatly affected the phase purity of the synthesized compound, ultimately resulting in a large amount of impurities and defects produced in the grown crystals. We designed a special device to purify the raw materials, our purification is divided into two steps: the first step is to put the raw materials in the crucible with sand core, then put it in the sealed ampoule, pass the high purity argon, and heat to the melting point of the raw material, let the melt pass the sand core slowly flows down to the purpose of removing suspended solids and insoluble impurities in the melt. The second step is to grind the obtained raw materials, and put it into a clean quartz tube, and after continuously evacuating for 6 hours, quartz tube was sealed at the pressure of 10−4 Pa. During the evacuation process, the temperature of the material kept at 300°C to remove water and free ions in the raw materials. In Fig. 3(b), there are no black impurities on the surface of the compounds synthesized by re-filtered raw materials, which makes the synthesized compound have a higher purity. The crystals grown from compounds without filtering were cloudiness (as seen in Fig. 3(c)), but the crystals grown from compounds with filtering have a good quality and transparency (as shown in Fig. 3(d)).
Fig. 3. (a) Appearance of the surface of the synthetic compound before purification. (b) Appearance of the surface of the synthetic compound after purification. (c) The grown crystal before purification. (d)The grown crystal after purification.
3.1.2. Seed crystal
The choice of seed crystal plays a key role in the quality of the grown crystal, so how to obtain high quality seed crystal is the first problem we need to resolve. We put a platinum wire into the melt, and obtained the seed crystal by rotating and pulling the platinum wire, as seen in Fig. 4(a). The high quality parts of the polycrystal were cut, polished and used as seed crystal. The shape of the seed crystal also has a great influence on the crystal growth. For example, the diameter of the seed crystal must be smaller than the diameter of the die, Otherwise, when growing to the step of expanding the shoulder, the crystal is difficult to control and rapidly expands beyond the die edge, which makes crystal impossible to continue to grow by the die size. The size of the seed crystal in our experiment is shown in Fig. 4(b).
Fig. 4. (a) The figure of obtaining seed crystal by platinum wire. (b) The size of the seed crystal in our experiment.
3.1.3. Temperature field
We used a resistance heating furnace to grow the crystals and studied temperature field, as seen in Fig. 5. When the thermocouple heating, the furnace will produce a temperature gradient, so how to adjust the temperature field to achieve stable growth of the crystals is particularly important. At the solid-liquid interface, the local supercooling caused by temperature gradient is the driving force of crystal growth. The larger the temperature gradient, the faster the crystal grows. However, excessive supercooling will increase the thermal stress of crystals, resulting in dislocations, defects and even fracture. Initially, we grew the crystal in a lager temperature gradient as shown in Fig. 5(a), the resulting crystal is of poor quality. After that, we improved the temperature field, as shown in Fig. 5(b), and installed a post-heater on the growth device, which makes the crystal growth more stable, and the size of the grown crystal is controlled better.
3.1.4. Growth rate
We all know that the temperature gradient must be coordinated with the pulling rate. In a suitable temperature field, we initially chose to grow crystals at a pulling speed of 20mm/h, we can see Fig. 6, the upper part of the crystal was magnified, and some bubbles were observed. These bubbles may form in the melt (materials decomposition), and mainly exist in the surface of the melt. If the pulling speed is too high, these bubbles will enter into the crystals, and mainly present at the edge of the crystal. After that, we obtained a colorless, high transparency crystal by decreasing the pulling speed to 10mm/h, as seen in the lower parts of the crystal.
3.2. XRD analysis
The measured powder diffraction pattern is depicted in Fig. 7. By comparing the XRD patterns of these samples, it is obvious that the XRD powder diffraction patterns of CsSr0.97Eu0.03I3, CsSr0.95Eu0.05I3 and CsSr0.93Eu0.07I3 crystals were in agreement with pure CsSrI3[11]. Due to the CsSrI3 crystal was very sensitivity to moisture and oxygen, some other reflections reveal an increase in intensity on the samples. These reflections can be assigned to the formation of hydrates of the respective iodides are hygroscopic in the air.
3.3. DVS analysis
Hygroscopic materials absorb moisture and cause decomposition, which result in optical and structural degradation. The hygroscopicity of the grown crystals was tested at room temperature, Fig. 8 shows the changes of the powder mass over time at constant humidity. From the results we can see that the sample powder is very sensitive to humidity, and the weight of the sample powder gradually increases over time. It can be seen that the absorption of water in the sample is very serious, weight gain by absorbing water is more than 10% in just 2 hours, so it is the crystal material which easy to deliquesce. Therefore, in the crystal growth process, as well as the post-preservation and usage, we must take measures to prevent crystal from deliquesce.
3.4. PL analysis
The fluorescence spectrum upon excitation at 370nm was measured in the range of 350–600nm by a TRIAX550 spectrophotometer at room temperature, and it was reported in Fig. 9. The strongest emission located at 451-454nm is associated with the 4f→5d transition of Eu2+ ions. When the Eu2+ concentration is less than 5%, the luminescence intensity of the sample increases as the concentration of Eu ions increases. When the Eu ion concentration is 7%, since the distance between the Eu2+ ions becomes small, re-absorption between ions occurs, and thus the luminescence intensity of the sample decreases, that is, concentration quenching occurs.
Photoluminescence decay profiles of Eu2+ emission from 3%, 5% and 7%Eu doped crystals were recorded under 370nm pulsed laser excitation. Photoluminescence decay curves were fit with a single exponential function. A fast decay constant about 1.32µs, 1.35µs and 0.73µs from Eu2+ emission is observed, as seen in Fig. 10. We can see that the fluorescence decay time increases first and then decreases with the Eu2+ ion concentration rising. The reduction of decay time may be due to the increasing probability of non-radiative transitions, and this result is consistent with concentration quenching.
4. Conclusions
By designing the experimental apparatus and adjusting the temperature field, we successfully achieved the growth of CsSrI3:3%Eu (Φ10×55mm), CsSrI3:5%Eu (Φ10×50mm) and CsSrI3:7%Eu (Φ10×45mm) crystals using the EFG method. This is also the first time that the EFG method has been used to grow binary scintillation crystal materials in the world, and we achieved growth of the crystals with a growth rate of 10-20 mm/h. The Eu2+ 5d-4f emission band was observed at 451-454nm, and when the Eu2+ ion concentration is 7%, concentration quenching was observed. PL decay time was determined as 1.32µs for CsSrI3:3%Eu, 1.35µs for CsSrI3:5%Eu and 0.73µs for CsSrI3:7%Eu.
Funding
National Natural Science Foundation of China (51272130, 51772171).
Disclosures
The authors declare no conflicts of interest.
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