Impact on timing and light extraction of a photonic crystal as measured on a half patterned LYSO crystal: implications for time of flight PET imaging

The improvement of the energy and of the timing resolution is always a challenge in scintillation-based detectors. A large fraction of the photons produced by scintillation remains trapped inside the crystal. Photonic crystals have been suggested as a solution to improve light extraction. Here we will present results obtained with a nanostructured TiO₂ coating on a 50×50 mm² LYSO crystal. The objective of the present paper is to characterize the performance of this coating in both light extraction and timing performance as both parameters are critical to spatial resolution of PET systems. To avoid tricky calibration problems, especially for timing, we have manufactured a monolithic crystal with one half patterned with a photonic crystal and with one half bare. We are rotating the crystal π/2 relative to the photo-detector between each measure. We have chosen a digital Si-PM Philips DPC 3200 as photo-detector due to its excellent timing precision and stability. The impact on light extraction of the photonic crystal is very strong as 30% of the light only escaped through the naked face vs 70% through the textured face for each position. The timing effect is much more subtle and quite at odd with previous results. By averaging the measurements on four positions, we are detecting a time lag effect of photon extraction with a probability of 98%. The average lag is only of 17 ps on the detection in the textured part of the crystal. This effect, although without practical consequences for PET imaging, is nevertheless perplexing as we were foreseeing a faster exit of photons on the textured face. We propose an explanation for the effect observed.