Monte Carlo simulations of energy, time and spatial evolution of primary electrons generated by 511 keV photons in various scintillators

The intermediate phases between the initial interaction of ionizing radiation in scintillators and the production of scintillation light have been extensively studied over the years. Observable features of scintillators such as energy and time resolution have been in part explained through pre-scintillation mechanisms. Whereas the early energy transfer processes following the photoelectric (or Compton) interaction of an energetic photon with an atomic electron in scintillator materials were fully described, their spatio-temporal characteristics have had limited quantification. For 511 keV photon interaction in (TOF-)PET detectors, the production of a primary electron carrying most of the energy leads to non-localized energy deposit distribution having some temporal extent. Here, we characterize the spatial, temporal and energy evolution of the primary electron generated by 511 keV photons in different scintillators using Monte Carlo simulations. We aim to provide a general quantitative description detailing the dynamics of energy transfer, such as the electron ejection angle, track length and tortuosity, time course and remaining energy as a function of the initial interaction point. Simulation results show that the electron track is tortuous with a short extent and a rapid energy transfer within 100 μm in high-Z (e.g., BGO) or high density (e.g., LuAP:Ce) materials while the extent nearly doubles in lighter materials like BaF2 in which the track tortuosity is 10%–50% lower. The time fluctuations of the photoelectron full energy deposit remain below 1 ps FWHM, therefore adding negligible jitter to the time resolution of coincident detectors. These simulations provide a better understanding of the main carrier dynamics from simple characteristics as some detector concepts require low-range, contained energy deposit (e.g., pixelated crystal arrays) whereas others benefit from a greater extent of energy deposition (e.g., structures designed for energy sharing).