Study on the inhomogeneity of LYSO crystal boules grown by the CZ method for PET applications

Highlights

• Grow large LYSO:Ce,Ca crystals by stabilizing solid–liquid interface of CZ method.

• Study the inhomogeneities of components, spectra and scintillation of LYSO:Ce,Ca.

• Both Ca codoping and air annealing improve the scintillation uniformity.

• Correlate Ce and Ca concentrations with scintillation uniformity of LYSO crystals.

Abstract

In this paper, $\mathrm{\Phi }$ 72 $\times$ 200 mm LYSO:Ce and Ca-codoped LYSO:Ce crystal boules were grown by the CZ method. The growth conditions were stabilized by controlling the rotation rate to maintain a slightly convex solid–liquid interface shape. Samples 3.9 $\times$ 3.9 $\times$ 20 mm in size were cut from the crystal boules along the growth direction. The inhomogeneities, including the components and the optical and the scintillation properties, of the as-grown and air-annealed samples were studied. The component analysis showed an increasing trend of Ce and Ca concentrations along the growth direction of the crystals. By combining the defect study of the samples, appropriate ranges of doping concentrations on high-quality LYSO crystal growth were determined. The optical properties, including the emission, excitation and absorption spectra, of the samples were investigated. The spectra indicated the differences of Vo defect, Ce1/Ce2 ratio and Ce4+ content in the samples, which can be related to the scintillation performances. Scintillation studies showed that light output and energy resolution vary greatly from top to bottom of the as-grown LYSO:Ce. These properties can be improved to achieve better uniformity by air annealing and Ca codoping. The light output can reach 106.6% (relative to a standard sample) with a uniformity of $\pm$2.6%, and the energy resolution can reach 9.4% with a uniformity of $\pm$0.2% for the air-annealed Ca-codoped LYSO:Ce samples. In conclusion, this paper describes some factors that will affect the scintillation inhomogeneity and can guide the growth of high-quality LSO/LYSO crystals.

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 ${\text{Ce}}^{4+}$ and {${\text{Ca}}_{\mathit{Lu}}$+Vo} defects [15], [16]. In Ca-codoped LSO/LYSO, ${\text{Ce}}^{4+}$ is involved in a more efficient luminescence mechanism [17], [18], while the {${\text{Ca}}_{\mathit{Lu}}$+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], ${\text{Ce}}^{4+}$/${\text{Ce}}^{3+}$ 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 $\pm$20% to $\pm$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.