LAPPD Waveform Response to Annihilation Gammas Incident on BGO Crystals

We have measured the waveform digitized response of a Large Area Picosecond Photon Detector (LAPPD) optically coupled to a 3.0 x 5.4 x 30.0 mm3 BGO scintillator crystal (6-side polished, 5-side wrapped) placed between two LAPPD readout strip regions. We connected four LAPPD signals (two ends each for two anode strips) to a 4GHz Tektronix MS064 Real Time Oscilloscope running at 6.25GS/s and self-triggering. We captured 100000 triggers each with 6250 samples (1 microsecond interval) of waveform per dataset; datasets were obtained with optical grease coupling between the BGO crystal and the LAPPD. We processed these waveforms to produce single-photoelectron pulse lists, with each pulse having a 2D measured position of photoconversion in the LAPPD photocathode. While the LAPPD has sub-millimeter spatial resolution (especially between strips) for normally incident light, the optics of scintillation photon collection with its range of incident angles results in considerable spatial spreading of photons when being transmitted through a 5mm-thick LAPPD fused silica entrance window. Because many independent photon positions are detected, however, the event spatial resolution reduces in quadrature with collected pulses/event to provide a photopeak spatial resolution of <15mm) FWHM each photoelectron.