Figure 2.

a) Representation of the photoelectric process in a direct-conversion material, showing the excitation of electron-hole pairs that drift in opposite directions under the influence of an externally applied electric field. The moving charge induces electrical signals on the electrodes that can be read out. b) A photoelectric interaction in a scintillator also creates electron-hole pairs, but in the absence of the applied field they stay together as loosely bound pseudoparticles known as excitons. The excitons diffuse to luminescent centers where they recombinne, emitting secondary scintillation photons in the process that can be read out using an appropriate light sensor.