Picosecond Time Resolved Studies of Photon Transport Inside Scintillators

The traveling time of scintillation photons from the ionization track to the photodetector varies from photon to photon. For scintillators used in (time-of-flight) positron emission tomography (TOF-)PET systems the resulting optical transit time-spread can have a significant influence on the timing resolution. Using a femtosecond pulsed diode laser in combination with a fast imaging streak camera we measured this time-spread on the picosecond time scale for both polished and etched Ca co-doped and non co-doped LSO:Ce and LYSO:Ce crystals of 3 ×3 ×5 mm³, 3 ×3 ×10 mm³, and 3 ×3 ×20 mm³ in size. Measurements were performed with the crystals wrapped on 5 of their sides in 1 to 4 layers of 0.1 mm thick white reflecting Teflon tape (PTFE). Optical photons can migrate through Teflon tape for some time before they re-enter the crystal. This Teflon time-spread was measured as well. The roughness of the crystal surfaces were determined using a surface profiler. In addition to experiments, the optical transit time-spread was obtained by means of ray-tracing software and compared to experimental data. In addition, an analytical model about optical transit time-spreads was derived.