Performance comparison of small GYGAG (Ce) and CsI(Tl) scintillators with PIN detectors

Background: Thallium doped CsI(Tl) scintillators read out with PIN photodiodes have been successfully used for high performance pixilated solid-state gamma cameras for cardiac SPECT and general nuclear systems. These cameras allow for both emission and transmission scans because of their good energy resolution and count-rate performance. We have reported previously on the performance of detector modules employing small CsI(Tl) scintillators (2.8×2.8×6mm) where the dominant limiting factor for both energy resolution and count-rate performance is CsI(Tl) itself. Methods: We compared identical shaped CsI(Tl) and newly introduced Ce-doped (Gd,Y)3(Al, Ga)5O12 (GYGAG) transparent ceramics (with fast decay time), read out with our PIN photodiode based detector modules, characterizing the light yield, energy resolution and count-rate performance obtainable with GYGAG at 140keV. Results: We measured luminosities of 42,000 and 64,000 Ph./MeV for GYGAG and CsI(Tl) respectively. GYGAG had a decay time that allowed use of a 6µs shaping time instead of the 12µs required for optimal energy resolution with CsI(Tl), enabling >2x improvement in count-rate performance. Though GYGAG was more proportional than CsI(Tl), the energy resolution was not improved over CsI(Tl) at 140keV due to its lower luminosity. However, we measured energy resolution of 4.0% at 662keV for GYGAG on our 3mm photodiode detector modules, better than the 5.3% estimated for CsI(Tl). After exposure to ambient light, GYGAG's residual fluorescence was ∼800 times that from CsI(Tl) and had 3 decay components. The longest component contributed at 6% of the total, and had a decay time troublesome for PMT measurements, though it did not significantly affect the energy resolution measured with photodiodes.