Optimization of scintillation-detector timing systems using Monte Carlo analysis

Monte Carlo analysis is used to model statistical noise associated with scintillation-detector photoelectron emissions and photomultiplier tube 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 photoelectron-emission rates, including bismuth germanate (BGO) scintillation detectors used in positron emission tomography (PET) systems. Monte Carlo timing resolution for a BGO/photomultiplier scintillation detector, CFD timing system is presented as a function of constant-fraction delay for 511-keV coincident gamma rays in the presence of Compton scatter. Monte Carlo results are in good agreement with measured results when a tri-exponential BGO scintillation model is used. Monte Carlo simulation is extended to include CFD energy-discrimination performance. Monte Carlo energy-discrimination performance is experimentally verified along with timing performance (Monte Carlo timing resolution of 3.22 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%