Interferometric Measurement of Ionizing Radiation-induced Transient Changes in Complex Refractive Index

A 10-ps or less coincidence time resolution (CTR) for time-of-flight positron emission tomography (ToF-PET) can bring benefits such as improved image quality, better lesion detectability and quantification, lower dose, reduced scan time, or a combination of these. One possible approach to achieve this order-of-magnitude enhancement in the CTR compared to state-of-the-art ToF-PET systems is detection via modulation of optical properties induced by ionizing radiation (instead of scintillation), as this mechanism directly probes ionization charges created, the very first stage of the detection process that can’t be probed by scintillation. Here, we conducted an experimental study of the optical modulation induced by ionizing radiation using an interferometric setup that is highly sensitive to complex refractive index modulation. We used synchronized ultrafast electrons at the SLAC national accelerator laboratory, where a pair of polarizers and a pair of birefringent plates were installed for interferometric detection. High bandgap materials of CdS and ZnTe were selected to be probed across the visible wavelength to investigate the probe wavelength dependence of the modulation signal. First, we determined a stable operating angle of the birefringent plates in the setup, which corresponded to the angles that yielded 25% of the maximum transmission, for example, 180.45 degrees in CdS at 600nm. We further studied the ZnTe sample (bandgap at 560nm) for the probe wavelength range of 560-690nm. The detected modulation was 1.7% at 560nm and increased continuously with wavelength, reaching the maximum of 12.7% at 630 nm. But the trend reversed for probe wavelengths ¿ 630nm and modulation change decreased to 8.7% at 690nm. This result suggests that the induced optical modulation is sensitive to the probe wavelength, which should be just below the bandgap energy of the materials studied. In conclusion, this finding will provide insights toward further development of the detection system for optical modulation-based detection with the goal to achieve 10 ps CTR ToF-PET.