The effect of high excitation density in promoting nonlinear quenching that is 2nd or 3rd order in electron-hole density is generally understood to be a root cause of nonproportionality in scintillators. We report and discuss quantitative data on just how fast these nonlinear channels are in specific cases. Kinetic rate constants for the creation of excitons from electrons and holes and for their quenching by dipole-dipole transfer have been measured in CsI and NaI. We show in addition that the strong radial concentration gradient in an electron track gives rise to fast (~ picoseconds) diffusion phenomena that act both as a competitor in reducing excitation density during the relevant time of nonlinear quenching, and as a determiner of branching between independent carriers and pairs (excitons), where the branching ratio changes along the primary electron track. We use the experimentally measured nonlinear quenching rate constants and values of electron and hole carrier mobilities to carry out quantitative modeling of diffusion, drift, and nonlinear quenching evaluated spatially and temporally within an electron track which is assumed cylindrical in this version of the model. Magnitude and inequality of electron and hole mobilities has consequences for quenching and kinetic order that vary with dE/dx along the path of an electron and therefore affect nonproportionality. It will be demonstrated that in a material with high mobilities like high-purity germanium, Auger recombination is effectively turned off by diffusive carrier dilution within < 1 fs in all parts of the track. In alkali halide scintillators like CsI and CsI:Tl, electron confinement and high-order quenching are accentuated toward the end of a particle track because of hole self-trapping, while separation of geminate carriers is accentuated toward the beginning of the track, leading to 2nd order radiative recombination and opening additional opportunities for linear trapping.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.