First experiments with LaBr3:Ce crystals coupled directly to silicon photomultipliers for PET applications

LaBr3:Ce³⁺ has high potential for time-of-flight (TOF) positron emission tomography (PET) because of its fast response (∽16 ns decay time) and high light yield (∼70.000 photons/MeV). To benefit optimally from the advantages of LaBr3:Ce³⁺, it should be read out with a photosensor having fast response, high photodetection efficiency, and low dark current. Excellent timing resolutions (≪ 100 ps) have been demonstrated with small LaBr3:5%Ce³⁺ crystals on fast PMTs. Silicon photomultipliers (SiPMs) are now becoming a promising alternative. However, present SiPMs are still relatively small (≪ 25 mm²), have limited photodetection efficiency (PDE), are often packaged in a way that complicates efficient light collection, and exhibit relatively large rise times in combination with scintillators. We are performing experiments in a dry box with small, bare LaBr3:5%Ce³⁺ crystals coupled directly to SiPMs, so as to study and optimize the intrinsic performance of SiPM-based LaBr3:Ce³⁺ scintillation detectors. Preliminary results obtained with 3 mm × 3 mm × 5 mm LaBr3:5%Ce³⁺ crystals mounted on 3 mm × 3 mm SiPMs show that coincidence timing resolutions as good as 237 ps FWHM and energy resolutions of ∼6.5% FWHM at 511 keV can already be achieved. Next-generation SiPMs are expected to further improve the detector performance.