PET Detector Block with DOI Capabilities Based on a Large Monolithic BGOCrystal

In this work we report on the performance of a 50×50×15 mm
³
monolithic BGO block with black-painted lateral walls enabling depth of interaction (DOI) capabilities. A directional optical barrier, called retro-reflectors, is coupled to the entrance scintillator face bouncing back the scintillation light in the direction from which the light came and, therefore, preserving the narrow light distribution while adding to the overall signal. Two SiPM array configurations were tested with total dimensions of
_~
5×5 cm
²
, namely 12×12 and 16×16 SiPM (all SiPMs were 3×3 mm
²
). In addition, a nanopattern at the exit face of the scintillator (coupled to the SiPM array) was eventually used, increasing the scintillation light extraction. We have studied the detector spatial resolution using collimated 511 keV photon beams from
²²
Na sources at normal incidence. The DOI resolution was evaluated using collimated beams with lateral incidence at 90 deg. An average FWHM for the DOI resolution of 5.3±0.3 mm has allowed us to separate the crystal thickness into 3 DOI layers. Here, we have studied the detector spatial resolution as a function of the DOI layer obtaining 2.5±0.4 mm at the entrance layer (15-10 mm), 2.2±0.4 mm (10-5 mm) and 1.6±0.4 mm (5-0 mm, near the photosensor). An average energy resolution as good as 20% FWHM was obtained for the whole crystal volume.