LYSO Crystal Block
Lutetium-yttrium orthosilicate scintillator. 9×6 crystal array per block, 64 blocks per ring, 24 rings = 13,824 crystals total on the Discovery 690.
Fits
Field notes
LYSO is self-luminous (natural Lu-176 background). This is normal and factored into calibration — don't diagnose "noise" from an idle ring as a fault without checking the background reference.