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. Silicon 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 4×4 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.