OPTIMIZATION OF SCINTILLATION DETECTOR TIMING SYSTEMS USING MONTE-CARLO ANALYSIS

Monte Carlo analysis is used to model statistical noise associated wit h scintillation-detector photoelectron emissions and photomultiplier t ube operation. Additionally, the impulse response of a photomultiplier tube, front-end amplifier, and constant-fraction discriminator (CFD) is modeled so the effects of front-end bandwidth and constant-fraction delay and fraction can be evaluated for timing-system optimizations. Such timing-system analysis is useful for detectors having low photoel ectron-emission rates, including Bismuth Germanate (BGO) scintillation detectors used in Positron Emission Tomography (PET) systems. Monte C arlo timing resolution for a BGO/photomultiplier scintillation detecto r, CFD timing system is presented as a function of constant-fraction d elay for 511-keV coincident gamma rays in the presence of Compton scat ter. Monte Carlo results are in good agreement with measured results w hen a tri-exponential BGO scintillation model is used. Monte Carlo sim ulation is extended to include CFD energy-discrimination performance. Monte Carlo energy-discrimination performance is experimentally verifi ed along with timing performance (Monte Carlo timing resolution of 3.2 2 ns FWHM versus measured resolution of 3.30 ns FWHM) for a front-end rise time of 10 ns (10-90%), CFD delay of 8 ns, and CFD fraction of 20 %.