Observation of fast scintillation of cryogenic PbI2 with VLPCs

At cryogenic temperatures (∼10 K), undoped lead iodide (PbI₂) has material and scintillation properties that are very attractive for positron emission tomography (PET). However, the PbI₂ emissions are quenched at temperatures >40 K, so close optical coupling between the scintillator and photodetector requires a photodetector that also operates at cryogenic temperatures. This suggests the visible light photon counter (VLPC), which operates at similar temperatures and combines high gain and high quantum efficiency. We proximity couple (0.001 in air gap) PbI₂ crystals with 1.0-2.5 mm dimensions to a 1 mm diameter VLPC and cool the system to 8.5 K. Signals with short (<10 ns) duration are observed. When excited with 511 keV photons, a coincidence timing resolution of 1.3 ns full-width at half-maximum is measured. While a clear photopeak is observed for 122 keV excitation, no clear photopeak is seen under 511 keV excitation (possibly due to the poor optical quality of the PbI₂ crystals). While the present configuration must be scaled-up considerably, a cryogenic PbI₂/VLPC combination may eventually become the basis for a practical time-of-flight PET camera.