Improved LabPET detectors using Lu1.8Gd0.2SiO5:Ce (LGSO) scintillator blocks

The scintillator is one of the key building blocks that critically determine the physical performance of PET detectors. The quest for scintillation crystals with improved characteristics has been crucial in designing scanners with superior imaging performance. Recently, it was shown that the decay time constant of high lutetium content Lu_1.8Gd_0.2SiO₅:Ce (LGSO) scintillators can be adjusted between 30 ns and 48 ns by varying the cerium concentration from 0.025 mol% to 0.75 mol%, thus providing interesting characteristics for phoswich detectors. The large light output (90-120% NaI), the better spectral match and the high initial photoelectron rate (~200 phe^-/ns) of these scintillators with avalanche photodiode (APD) readout promise to provide superior energy and timing resolution. Moreover, their improved mechanical properties as compared to conventional LGSO (Lu_0.2Gd_1.8SiO₅:Ce) make block array manufacturing readily feasible. To verify these assumptions, new phoswich block arrays made of LGSO-90%Lu with low and high mol% Ce concentrations were fabricated and assembled into LabPET modules. Typical crystal decay time constants were 32 ns and 48 ns, respectively. We therefore report on the initial evaluation of this modified version of the LabPET detector module. Phoswich crystal identification performed using a non-optimized digital pulse shape discrimination algorithm yielded an average 10% error. At 511 keV, energy resolution of 20 ± 2% and 15 ± 1% were obtained, while coincidence timing resolution between 4.9 ± 0.3 ns and 4.1 ± 0.1 ns were achieved. The improved characteristics of this new LGSO-based phoswich detector module are expected to enhance the LabPET scanner performance, first by improving sensitivity due to the overall higher stopping power of the detector module, and second by narrowing the coincidence time window, thus minimizing the random event - - rate. Altogether these two improvements will significantly enhance the noise equivalent count rate performance of an all LGSO-based LabPET scanner.