Sub-surface Laser Engraving in Thick Hexagonal Scintillation Crystals

Positron Emission Tomography detectors are commonly dense pixelated inorganic scintillators. These are embedded in a reflector whose thickness hinders the sensitivity. Pixel arrays manufacturing is laborious and expensive. The extensively known laser processing can be employed as an alternative. A grid of laser induced microcracks at the focal spot creates a semitransparent optical barrier to guide scintillation light towards the photosensor. In this work, a cost-effective nanosecond laser has been used to validate this approach. Its design flexibility has been exploited to develop hexagonal patterns of 1.25 mm of edge to serve a hexagonal photodetector, building block of an innovative quasi-spherical scanner. Field flood images of individual readout in LYSO blocks of 5 mm, 7 mm and 10 mm of thickness are presented alongside the pixel resolvability. Depth of interaction has been developed with two shifted layers of 5 mm in a 10 mm crystal. The crystal resolvability and energy resolution confirm the validity of the engraving solution in a complete system.