Ionizing photon interactions in a material induce modulation of its optical properties at a femtosecond time scale: A new direction to improve timing for ToF-PET

In this paper, we take insight from the nascent field of ultrafast X-ray science at free electron lasers (xFELs) and prove the ultrafast nature of ionizing radiation photon induced refractive index modulation process, which can potentially be used for annihilation photon detection with significantly improved coincidence time resolution (CTR) for time-of-flight positron emission tomography (ToF-PET) systems. At the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory, the arrival time of X-ray pulses with photon energies between 0.5-10keV are detected with 10fs time resolution using an optical readout field. In this work, we used similar interferometric method with spectrum encoded timing information to observe the ionization charge induced refractive index modulation process in yttrium aluminum garnet (YAG), lutetium-yttrium oxyorthosilicate (LYSO) and bismuth silicon oxide (BSO) crystal samples. By using the ultrafast X-ray pulses with 50fs duration from LCLS, we have proved that the refractive index modulation induced by ionizing radiation photons indeed occurs within 50fs time scale for all crystal samples. This shows the potential of adopting the interferometric detection method and using the mechanism of refractive index modulation for annihilation photon detection, in order to dramatically improve PET CTR. Since our proposed method does not depend on optical scintillation as the readout mechanism, we are free to explore non-scintillation and nontraditional detection materials.