Optimization of digital time pickoff methods for LaBr₃-SiPM TOF-PET detectors

The relatively new inorganic scintillator LaBr3:Ce is well suited for time-of-flight positron emission tomography (TOF-PET), since it has short scintillation decay time, high light yield and very good energy resolution. Silicion photomultipliers (SiPMs) show low noise, high gain and small transit-time jitter, and are thus well suited for fast timing applications. The work presented here focuses on the timing performance of bare LaBr3:Ce(5%) crystals coupled to commercially available SiPMs. First, relatively small crystals coupled to single SiPMs were used to study the intrinsic timing resolution of such detectors. Special attention was paid to the optimization of digital signal processing (DSP) time-pickoff methods. A coincidence timing resolution of 101 ps FWHM was achieved. Next, a monolithic LaBr3:Ce crystal was coupled to a 4x4 SiPM array and an intial timing performance characterization was performed. A single detector timing resolution of 225 ps FWHM was achieved. The timing delay induced by the scintillation photon transport was virtually constant over the depth-of-interaction (DOI) range of the detector.