Study on the inhomogeneity of LYSO crystal boules grown by the CZ method for PET applications
Introduction
Lu2SiO5:Ce/(Lu,Y)2SiO5:Ce (LSO/LYSO) was first discovered in 1990 [1], [2] and 2000 [3], [4]. It has been proven to be an excellent scintillator for PET systems in recent decades. LSO/LYSO with a density higher than 7.1 g/cm3, a decay time less than 40 ns and a light output higher than 30000 ph/MeV can be successfully grown by the Czochralski (CZ) [5], [6], floating zone [7] and micro-pulling-down methods [8]. The scintillation mechanism is based on the 5d-4f emission from two Ce luminescence centers designated Ce1 and Ce2 [9], [10]. Typically, the luminescence from the Ce1 center, with peaks at 393 nm and 425 nm, is dominant, and the luminescence from the Ce2 center is regarded as a negative factor [11]. Based on the scintillation mechanism, many studies have addressed how to improve the scintillation performance of LSO/LYSO. Air annealing was first found to be effective [12], and can significantly enhance the scintillation light output by reducing the number of oxygen vacancy (Vo) defects [13], [14], [7]. Ca codoping is also effective, and can both enhance the light output and shorten the decay time by producing stable and {+Vo} defects [15], [16]. In Ca-codoped LSO/LYSO, is involved in a more efficient luminescence mechanism [17], [18], while the {+Vo} defects can dissociate spatially correlated Vo and Ce to promote the migration of electrons to luminescence centers [19]. Some researchers also found that Ca codoping can reduce the ratio of Ce2 to Ce1, which suppresses the luminescence from Ce2 and contributes to fast scintillation decay [11]. Other codoping in LSO/LYSO crystals, such as Li [20], [21], [22], La [23], [24], Mg [18], Yb [25], [26] and Cu [27], has also been carried out to optimize the scintillation performance.
Large LSO/LYSO boules, which are used in commercial PET systems, are typically grown by the CZ method [28], [29], [30]. As the CZ growth process lasts for weeks, the growth conditions, including convection intensity, temperature gradient and melt components, will change. The as-grown LSO/LYSO boules usually show large inhomogeneities in components, defects, optics and scintillation properties along the growth direction. First, the concentrations of doping ions such as Ce and Ca change substantially along the crystal. As the segregation coefficients of Ce and Ca are approximately 0.22 [31] and 0.16 [16] in LSO/LYSO, their concentrations in the crystals (head/foot) were reported to be 200/400 ppmw and 32/43 ppmw [32]. Then, defects are more likely to appear at the bottom of the crystal. Oxygen vacancies are the most common defects and increases along the growing axis, with the melt gradually losing oxygen in a neutral growth environment [33]. Another defect, such as the macroscopic scattering center, also appears at the bottom of the crystal and is caused by constitutional supercooling at the end of the growth stage[29]. Moreover, the optical spectra(transmittance, emission, excitation and absorption) of LSO/LYSO are also inhomogeneous and are relevant to their intrinsic characteristic, such as Ce concentrations [6], Vo defects [14], / ratio [18] and Ce1/Ce2 ratio [19]. Last, the scintillation inhomogeneity of LSO/LYSO boules is greatly influenced by the characteristics mentioned above. The Ce and Ca concentrations for optimum light output are reported to be 150 to 400 ppmw [32] and 0.1%(in melt) [34], which may be reached in a certain part of the crystal. Vo defects and scattering centers also degrade the light output and energy resolution, especially in the bottom part of the crystals. Sometimes double peaks appear in the energy spectrum of the samples from the bottom of the crystal, which makes them unusable [35]. Over the years, much work has been done to improve the homogeneous scintillation. Investigations [36], [37] on the scintillation characteristics of 2186 LSO samples cut from over 400 crystal boules show that the uniformity of light output has been improved from 20% to 5%.
In the PET system, the rings of detectors consist of a large number of scintillator pixels [38]. The uniformity of the pixels is critical and can affect the overall energy resolution and time resolution of the PET system [39], [40]. As one of the most promising scintillators for PET systems, LSO/LYSO with high light output and short decay is now widely used. The growth of large LSO/LYSO is a complex physical and chemical process whose internal and external conditions continuously change over a long period of time. Therefore, property inhomogeneities along the growth direction are unavoidable. However, studies in this area are rarely reported. This paper focuses on the inhomogeneities of two types of large LSO/LYSO crystals (LYSO:Ce and Ca-codoped LYSO:Ce). We started with the growth of good-quality crystals by the CZ method. By sampling the crystals along the growth direction and air annealing, the inhomogeneities of components, defects, optical and scintillation properties are studied. This research also explores the leading factors that affect scintillation inhomogeneity, and it can guide the growth of high-quality and large LSO/LYSO crystals.
Section snippets
Crystal growth and sample preparation
The CZ method is widely used in the growth of large LYSO crystals. In this paper, high-quality LYSO crystals were grown by regulating the rotation rate of the CZ process to maintain the solid–liquid(SL) interface as a slightly convex(toward the liquid) shape. This kind of SL interface shape has been proven to be beneficial in the growth of large scintillation crystals [41]. First, we pregrew a 72 mm LYSO crystal from a 120 mm 120 mm Ir crucible and studied the relationship between the SL
Components
The component analysis of LYSO:Ce and Ca-codoped LYSO:Ce are shown in Table 1, Table 2. The Ce concentration in the LYSO:Ce crystal gradually increases from top to bottom, ranging from 340 ppmw to 520 ppmw. The segregation coefficient of Ce changes along the growth direction over a range from 0.218 to 0.262. Its fluctuation is relatively small (less than 5.0%), which is a result of the stable temperature gradient in the growth interface controlled by regulating the rotation rate. Impurity
Conclusions
In this paper, the inhomogeneities, including the components and optical and scintillation properties, of large LYSO:Ce and Ca-codoped LYSO:Ce are studied. In PET detectors, the scintillation consistency of the crystal pixels is very important. To grow high-quality LYSO boules with uniform light output and energy resolution, some factors that will affect the scintillation properties are concluded by using the experimental data above. First, proper doping concentrations of Ce and Ca in the
CRediT authorship contribution statement
Rui Zheng: Conceptualization, Methodology, Software, Writing - original draft. Ji Chen: Conceptualization, Validation. YueFeng Deng: Conceptualization, Validation. Yongqing Chang: Investigation, Visualization. Yu Liu: Investigation, Visualization. Ran Cheng: Software, Investigation. Qingguo Xie: Supervision. Peng Xiao: Writing - review & editing, Project administration.
Declaration of Competing Interest
None.
Acknowledgments
This work was jointly supported by the National Key Research and Development Program of China #2016YFF0101500, the National Natural Science Foundation of China Grant #61671215, and the project of the Science and Technology Department of Hubei Province #2016CFA005.
References (44)
- et al.
Crystal growth and scintillation properties of LSO and LYSO crystals
J. Cryst. Growth
(2013) - et al.
Shaped crystal growth of Ce3+ doped Lu2(1–x)Y2xSiO5 oxyorthosilicate for scintillator applications by pulling-down technique
J. Cryst. Growth
(2006) - et al.
Luminescence and energy transfer processes in Lu2SiO5: Ce3+ scintillator
J. Luminesc.
(2006) - et al.
The effect of co-doping on the growth stability and scintillation properties of lutetium oxyorthosilicate
J. Cryst. Growth
(2008) - et al.
Revealing the role of calcium codoping on optical and scintillation homogeneity in Lu2SiO5:Ce single crystals
J. Cryst. Growth
(2018) - et al.
Growth and characteristics of LYSO scintillation crystals
J. Crystal Growth
(2005) - et al.
Czochralski growth and characterisation of large Ce3+: Lu2SiO5 single crystals co-doped with Mg2+ or Ca2+ or Tb3+ for scintillators
J. Cryst. Growth
(2005) - et al.
Characterization of three LYSO crystal batches
Nucl. Instrum. Methods Phys. Res., Sect. A
(2015) - et al.
Czochralski growth of rare earth oxyorthosilicate single crystals
J. Crystal Growth
(1993) - et al.
The interface inversion process during the czochralski growth of high melting point oxides
J. Cryst. Growth
(2004)
Spiral formation during czochralski growth of rare-earth scandates
J. Cryst. growth
Cerium-doped lutetium oxyorthosilicate: a fast, efficient new scintillator
Nucl. Sci. IEEE Trans.
Crystal growth and optical characterization of cerium-doped LYSO
J. Appl. Phys.
Defects identification and effects of annealing on LYSO single crystals for scintillation application
Materials
The role of cerium sites in the scintillation mechanism of LSO
Annealing effects on czochralski grown Lu2Si2O7:Ce3+ crystals under different atmospheres
J. Appl. Phys.
Air atmosphere annealing effects on LSO: Ce crystal
IEEE Trans. Nucl. Sci.
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