In addition to desirable physical properties including a density of 7.4 g/cm3 , an effective atomic number of 66, and no hygroscopicity, Lu2SiO5:Ce has well-known scintillation properties of ~30 900 photons/MeV, an emission peak near 420 nm, and a decay time of ~43 ns. These scintillation properties are achieved with Ce doping concentrations roughly in the range of 0.05 to 0.5 atomic percent relative to Lu. These properties make Lu2SiO5:Ce a widely used scintillator in positron emission tomography, in particular. We have found that both the light output and decay time may be improved by a combination of optimized crystal growth atmosphere and co-doping with divalent cations such as Ca. Scintillation light output of ~38 800 photons/MeV has been achieved as well as scintillation decay time as short as 31 ns with no long components. The relationship between growth conditions, dopant concentration, decay time, and light output is well defined, thus allowing one to reliably "tune" the crystal to the desired combination of light output and decay time. Possible explanations of the underlying mechanism are being explored and include compensation of oxygen vacancies, alteration of the relative occupancies of the cerium lattice sites, and suppression of trapping centers. In addition to higher count-rate capability and better coincidence timing, the improved decay time is expected to be particularly advantageous for time-of-flight positron emission tomography. Also, phoswich detectors comprising "standard" LSO (~43 ns decay time) and "fast" LSO (~31 ns decay time) become an attractive alternative to typical phoswich designs that often suffer from problems of mismatched light outputs and indices of refraction or the absorption of one scintillator's light by the other.

Effects of $\hbox {Ca}^{2+}$ Co-Doping on the Scintillation Properties of LSO:Ce

This paper presents a comparative study of optical and scintillation properties for various inorganic crystal scintillators, which are used, or actively pursued, by the high energy physics community for experiments. Transmittance, excitation and photo-luminescence spectra were measured for samples with a dimension of 1.5 radiation length. The transmittance data are compared to the theoretical limit calculated by using refractive index, assuming no internal absorption. Refractive index of lutetium oxyorthosilicate and lutetium-yttrium oxyorthosilicate was measured by using a V-prism. Light output was measured for these samples with Tyvek paper wrapping, and the result is presented with the quantum efficiency of the readout devices taken out. Temperature coefficient of the light output was also measured. The result of these measurements will be used in the summary table of the inorganic scintillator section for the 2008 edition of the particle data book.

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Optical and Scintillation Properties of Inorganic Scintillators in High Energy Physics

The National Electrical Manufacturers Association (NEMA) NU 2-2001 performance measurements were conducted on the Discovery RX, a whole-body PET/CT system under development by GE Healthcare. The PET scanner uses 4.2 × 6.3 × 30 mm lutetium yttrium orthosilicate (LYSO) crystals grouped in 9 × 6 blocks. There are 24 rings with 630 crystals per ring and the ring diameter is 88.6 cm. The transaxial and axial fields of view are 70.0 and 15.7 cm, respectively. The scanner has retractable septa and can operate in both 2-dimensional (2D) and 3-dimensional (3D) modes. 2D acquisitions use ring differences of ±4 for direct and ±5 for cross slices; 3D acquisitions use a ring difference of 23. The coincident window width is 6.5 ns and the energy window is 425–650 keV. Other than the detectors, the system uses the same hardware and software as a Discovery ST. The CT scanner is a 16-slice LightSpeed; the performance characteristics of the CT component are not included herein. Methods: Performance measurements of sensitivity, spatial resolution, image quality, scatter fraction and counting rate performance, and image quality were obtained using NEMA methodology. Results: The system sensitivity in 2D and 3D was measured as 1.7 cps/kBq and 7.3 cps/kBq, respectively. The transaxial resolution for 2D (3D) was 5.1 mm full width at half maximum (FWHM) (5.0 mm) at 1 cm from gantry center and the radial and tangential resolutions were 5.9 mm (5.9 mm) and 5.1 mm (5.2 mm) at 10 cm, respectively. The axial resolution for 2D (3D) was 4.8 mm FWHM (5.8 mm) and 6.3 mm (6.5 mm) at 1 cm and 10 cm from gantry center, respectively. The scatter fraction was 13.1% and 31.8% in 2D and 3D. The peak noise equivalent count rate (NECR) was 155 kcps at 92.1 kBq/mL in 2D and 117.7 kcps at 21.7 kBq/mL in 3D for a noise-free estimation of randoms. The contrast of the 22, 17, 13, and 10 mm hot spheres in the image quality phantom in 2D (3D) were 74.6% (72.4%), 56.7% (59.5%), 46.2% (44.6%), and 17.9% (18.0%), respectively. Conclusion: The Discovery RX is a scanner that possesses high NECR, low scatter fraction, and good spatial resolution characteristics.

https://jnm.snmjournals.org/content/jnumed/47/12/1960.full.pdf

NEMA NU 2-2001 Performance Measurements of an LYSO-based PET/CT System in 2D and 3D Acquisition Modes

Analysis of the last years theoretical studies and track simulations to conclusion that primary stages (electron scattering and e-h thermalization) play the key role in the following scintillator efficiency. The long thermalization length comparing to Onsager radius is the main reason for geminate pair concentration decrease and later luminescence losses. The easiest way for thermalization length decrease is the scintillation crystal doping or even transfer to the mixed crystals (solid solution). The simple model of modification of electrons scattering and e-h pairs thermalization for the mixed crystals is proposed. It is shown that solid solutions have higher light output independently on the crystal type. Analysis of experimental data confirmed this conclusion. This phenomenon is found for halide, oxide and sulfates scintillators. The similar behavior is typical for mixed anion and/or cation systems. The key role of initial track formation stages is illustrated by the same trend for activated scintillators and pure crystal with intrinsic luminescence. These estimations and experimental data lead to the conclusion that the scintillation efficiency improvement by mixed crystal use can play an important role in the search and development of new scintillators.

Scintillation Efficiency Improvement by Mixed Crystal Use

Following our previous studies, a further investigation on three long (2.5 times 2.5 times 20 cm3) LSO and LYSO crystal samples was carried out, and was compared to a long BGO sample of the same size. The optical and scintillation properties, including transmittance, photo-luminescence and excitation spectra, were measured. The result of light output and light response uniformity, measured by using both PMT and APD, are reported. Their applications in future high energy physics experiments are discussed.

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Large Size LSO and LYSO Crystals for Future High Energy Physics Experiments

Because of their high stopping power and fast bright scintillation, cerium doped silicate based heavy crystal scintillators, such as GSO, LSO, and LYSO, have been developed for medical instruments. Their applications in high energy and nuclear physics, however, are limited by lacking high quality crystals in sufficiently large size. The optical and scintillation properties, including the transmittance, emission and excitation spectra and the light output, decay kinetics and light response uniformity, as well as their degradation under gamma-ray irradiation were measured for two long (2.5times2.5times20 cm) LYSO samples from CPI and Saint-Gobain, and were compared to a BGO sample of the same size from SIC. Possible applications for crystal calorimetry in future high energy and nuclear physics experiments are discussed

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Large size LYSO crystals for future high energy physics experiments

The high energy and nuclear physics community is interested in fast bright heavy crystal scintillators, such as cerium-doped LSO and LYSO. An investigation is being carried out to explore the potential use of the LSO and LYSO crystals in future physics experiments. Optical and scintillation properties, including longitudinal transmittance, emission and excitation spectra, light output, decay kinetics and light response uniformity, were measured for three long (2.5×2.5×20 cm) LSO and LYSO samples from different vendors, and were compared to a long BGO sample of the same size. The degradation of optical and scintillation properties under γ-ray irradiations and the radiation-induced phosphorescence were also measured for two long LYSO samples. Possible applications for a crystal calorimeter in future high energy and nuclear physics experiments are discussed.

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Large size LSO and LYSO crystal scintillators for future high-energy physics and nuclear physics experiments

This paper presents a study of the gamma-ray induced radiation damage effect in large size (2.5 times 2.5 times 20 cm3) LSO and LYSO crystal samples. Optical and scintillation properties, including longitudinal transmittance and photo-luminescence spectra, light output and light response uniformity with PMT and APD readout, are measured before and after gamma-ray irradiations with an integrated dose up to 106 rad for three LSO and LYSO samples from different vendors. It was found that 300degC thermal annealing removes all radiation induced absorption. The photo-luminescence spectra measured before and after the irradiations were found to be consistent, indicating that the scintillation mechanism is not damaged. The radiation damage recovers very slow under the room temperature, indicating that the radiation damage level in LSO and LYSO crystals is not dose rate dependent. It was also found that the overall radiation damage in LSO and LYSO crystals is small as compared to other crystal scintillators commonly used in high energy and nuclear physics experiments.

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Gamma-Ray Induced Radiation Damage in Large Size LSO and LYSO Crystal Samples

This paper presents a study of correlations between the initial optical and scintillation properties and their radiation damage for mass produced lead tungstate crystals. A correlation was observed between crystal's initial light outputs and the values of its initial longitudinal transmittance at 360 nm. A strong correlation was found between the emission weighted radiation induced absorption coefficients and the relative losses of the longitudinal transmittance at 440 nm. Correlations were also observed between the relative losses of crystal's light output and the relative losses of its longitudinal transmittance at 440 nm, or the emission weighted radiation induced absorption coefficients. No correlations were observed between crystal's radiation hardness and its initial longitudinal transmittance or the slope of the initial longitudinal transmittance across the band edge

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Jianming Chen

Papers

Large Size LSO and LYSO Crystals for Future High Energy Physics Experiments

Large size LYSO crystals for future high energy physics experiments

Large size LSO and LYSO crystal scintillators for future high-energy physics and nuclear physics experiments

Gamma-Ray Induced Radiation Damage in Large Size LSO and LYSO Crystal Samples

A Study on Correlations Between the Initial Optical and Scintillation Properties and Their Radiation Damage for Lead Tungstate Crystals