Fast growth of cerium-doped lutetium yttrium orthosilicate single crystals and their scintillation properties☆
Graphical abstract
According to the chemical bonding theory of single crystal growth, Ce:LYSO bulk single crystals with diameter of 64 mm and length of 220 mm and mass of 4.2 kg were grown via the Czochralski method with the maximum pulling rate of 3.5 mm/h for nuclear irradiation (22Na) detector device, light yield of 26193 ph/MeV and decay time of 38 ns are obtained.
Introduction
Scintillators can convert ionizing radiations, such as γ-rays, X-rays, α or β particles, into visible-ultraviolet light pulses.1 The scintillator materials often determine the performance of the scintillation detector. Excellent scintillator materials should possess the properties of high light yield, short decay time, and high density.2,3 A heavy inorganic scintillator with high light yield (∼40000 photons/MeV)4 is needed for various applications, for example, particle detection, and medical imaging systems.5 Among various inorganic scintillator materials, cerium-doped lutetium yttrium orthosilicate (Ce:LYSO) crystal is prominent owing to its high density (7.1 g/cm3), short decay time (45 ns), and excellent energy resolution (7.1% at 662 keV).6,7 The main obstacles of using Ce:LYSO crystals in various physical devices are two-fold: the availability of high-quality crystals in sufficiently large size and the high cost associated with their high melting point.8
LYSO single crystal is considered as a solid solution of two different silicates, Lu2SiO5 and Y2SiO5.9, 10, 11 The advantages for LYSO compared with LSO are: lower melting temperature (2100 °C instead of 2150 °C), decrease in the amount of lutetium oxide in the starting melt, lower melt viscosity that reduces the rare-earth oxide inclusions and causes an apparent reduction in concentration of optical scattering centers in LYSO.10 LYSO is usually available as bulk single crystals grown by Czochralski (Cz) methods at high temperature of 2100 °C for weeks. To reduce cost, the development of fast growth technology is necessary, which can reduce power consumption and iridium volatility during growth. However, rare earth orthosilicate single crystals were often grown at low pulling rates.12, 13, 14, 15, 16, 17, 18, 19 For example, Ce:LYSO with size of ϕ75 mm × 155 mm at rate of <1.0 mm/h,12 ϕ30 mm × 60 mm Ce:LYSO crystal at the rate of 2.0 mm/h,13 and Ce:LYSO with size of ϕ80 mm × 200 mm and rate of 1–3 mm/h were reported.15 Li et al. reported a Nd:LSO single crystal grown at 1–2 mm/h rates.19 The pulling rates of Ce:LYSO and other rare earth orthosilicate single crystal by the Cz method were ≤3 mm/h.
Recently, we have reported the fast growth (pulling rates > 3 mm/h) of high-quality rare earth single crystals by controlling crystallization thermodynamics and kinetics in different size zones.20, 21, 22 Among the reported as-grown LYSO single crystals, few works focused on the anisotropic growth of LYSO single crystal. Based on the chemical bonding theory of single crystal growth, LYSO crystal with exposing {100}, {001}, {110}, and {111} surfaces are thermodynamically stable. Thus, its preferential pulling direction for fast growth is [010] direction.23
The novelty of the present work relies on the fast growth of large-sized LYSO and Ce:LYSO single crystals (diameter: 64 mm; length: 220 mm, weight: 4.2 kg) via the Cz method with pulling rate of 2.2–3.5 mm/h along [010] direction. In addition, the quality of Ce:LYSO single crystals can also be guaranteed. This work shows a very mature method for production of Ce:LYSO single crystals.
Section snippets
Experimental
Low-doped and high-doped Ce:LYSO single crystals were grown by the Cz method.24,25 For example, raw materials of CeO2, Lu2O3, Y2O3 and SiO2 with purity of 99.995% were appropriately predried and weighed according to Eq. (1) before mixing.(1−x−y)Lu2O3 + yY2O3 + SiO2 + 2xCeO2 = Lu2(1−x−y)Y2yCe2xSiO5 + 12xO2
Ce:LYSO samples with different Ce doping ratios were obtained by changing the ratio of Lu2O3:Y2O3:SiO2:CeO2. For example, y = 0.16 was used in this work. The sinters were then charged into an
Results and discussion
Fig. 1 shows the phase diagram of Lu2O3–SiO2 system according to calculated and experimental data.26, 27, 28 In Lu2O3–SiO2 binary system, intermediate compounds are known to be Lu2SiO5, Lu4Si3O12, and Lu2Si2O7 compositions at high temperature. For Y2O3–SiO2 system, the intermediate phases of Y2SiO5, Y2Si2O7 and Y4Si3O12 were reported.29 Actually, it is very difficult to achieve the real phase equilibria in Lu2O3–Y2O3–SiO2 system due to the slow diffusion rate in solid phases at low temperatures
Conclusions
In conclusion, high-quality Ce:LYSO single crystals were grown by fast Cz pulling technique with the control of crystallization thermodynamics and kinetics. The pulling rate can reach 3.5 mm/h which is larger than the reported LYSO crystals with the use of Cz growth technology. The preferred fastest growth direction along [010] is a key factor for fast pulling rate. Also, high quality and large size of Ce:LYSO bulk single crystals with diameter of 64 mm and length of 220 mm and mass of 4.2 kg
Acknowledgements
K.C. also acknowledges Qilu Young Scholars Program of Shandong University. We would like to thank Dr. Congting Sun and Dr. Yulei Chang for their help when preparing this work.
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Foundation item: Project supported by National Natural Science Foundation of China (51832007), Research and Development Project of Scientific Instruments of the Chinese Academy of Sciences (YJKYYQ20170073) and Natural Science Foundation of Shandong Province (ZR2020ZD35).