Simulation studies to understand sensitivity and timing characteristics of an optical property modulation-based...

The concept of using the modulation mechanisms of a material's optical properties for annihilation photon detection has been proposed as a potential method to significantly improve the coincidence time resolution (CTR) of positron emission tomography detectors. However, the possibility of detecting individual 511 keV photons with largely improved CTR using the proposed detection method has not yet been demonstrated, either experimentally or theoretically. In addition, the underlying physical picture of the optical modulation effects induced by annihilation photons has not been fully understood. In this work, we perform simulation studies including generation of the annihilation photon-induced ionization energy deposition trajectory, estimation of the charge carrier cascade time and temporal variance, simulation of the distribution of ionization-induced charge carrier density, and calculation of the strength of the modulation of two optical parameters: the absorption coefficient and the refractive index, as well as evaluation of the resulting optical intensity and phase change experienced by a probe laser beam. Our simulation results show that the average absorption coefficient modulation induced by individual 511 keV photon interactions is around 0.04 cm^-1, and the average refractive index change is 3.6 ×  10^-5, leading to modulations in the probe laser intensity of around 0.1% and phase modulation of around 0.05 radians. We have also found that the ionization process induced by a single 511 keV photon interaction occurs within 2.3 ps with a temporal variance of 0.4 ps. The fundamental limit on CTR using the optical property modulation-based detection mechanism is estimated to be around 1.2 ps full width at half maximum. Our simulation results indicate that with proper experiment design, it is possible to detect the ionization produced by an individual 511 keV photon with significantly improved CTR using the optical property modulation-based detection concept.