The front cover image points to the Feature Article by Williams et al. (pp. 426–438). A gamma ray interacting with a solid launches a cascade of high energy electrons whose ionization tracks may be considered roughly gaussian cylinders of about 3 nm initial radius, depicted by the red inner cylinder (where the electron moves from left to right). Carriers with different mobilities are separated in the strong radial gradient of excitation density n, more so in the beginning than end of the track due to drift confinement. Nonlinear quenching of excitons by dipole–dipole transfer competes on a picosecond time scale with diffusion to reduce electron–hole density in the track, reducing the luminescent yield in a scintillator by different amounts depending on position along the track. Variation of local light yield along the track results in a fundamental limitation on the energy resolution of a radiation detector called nonproportionality. In this article, luminescence quenching dependence on excitation density is measured by a sub-ps band-gap laser pulse, illustrated here by the streak camera display. The measured rate constants for nonlinear quenching are used in a numerical model of competing diffusion and quenching to examine trends underlying nonproportional light yield in CsI:Tl.