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Since its discovery in 1993, LGSO has been seen as a promising scintillator for positron emission tomography (PET) imaging. Recently, new high lutetium content Lu1.8Gd0.2SiO5:Ce (LGSO-90%Lu) has been produced with Ce concentration varying from 0.025 to 0.75 mol% Ce, yielding decay times in the range of 34.2 to 44.8 ns. These scintillators could potentially be used as phoswich detectors in PET for improving spatial resolution. To validate this assumption, light output, energy resolution, temperature dependence, timing resolution, and pulse-shape discrimination performance were assessed. The light output of these LGSO scintillators is 3.5 to 4.8 times higher than BGO with avalanche photodiode readout [7.5 to 8 times with photomultiplier tube (PMT)] and increases with Ce concentration. The variation of light output for temperature ranging from 10 to 50 Â°C is less than 18% with PMT readout. Energy resolution at 662 keV is about 11% (10%) with avalanche photodiode (APD) (PMT) readout. Timing resolution with APD is ~ 1.65 ns for all samples relative to a fast PMT-plastic detector. Pulse-shape discrimination of LGSO-90% Lu with 0.025 mol% Ce (Â¿ = 34.2 ns) and 0.75 mol% Ce (Â¿ = 44.8 ns) was successfully achieved in a phoswich assembly, making this new high lutetium content LGSO with adjustable decay time constants a strong candidate for a high-resolution depth-of-interaction detector intended for PET imaging.