Abstract: An optical ionizing radiation detector system (10) for determining the three-dimensional spatial distribution of all of the secondary electrons produced by the passage of ionizing radiation through a selected gas is provided. The detector system (10) can be used over a wide range of particle energies and gas pressures. The ionizing radiation passing through the gas produces prompt fluorescence and secondary electrons. A coincidence detector (30) recognizes coincident events of prompt fluorescence in the gas contained within that chamber to initialize the system cycle. At that time, an AC electric field is generated by an R.F. pulse generator (40) to localize the secondary electrons proximate their respective positions of production. Cameras (42) and 42'), preferably of a digital type, produce an image of the localized electrons to determine their three-dimensional spatial positions.