Photon pile-up (${\mu }_{\mathrm{event}}\ll 1$)

Two or more photons detected within one unresolved spatial and temporal event cell can be registered as one event, have their energies combined, or be rejected by event grading. In an X-ray CCD, low mean occupancy per event island per frame is needed for reliable individual-photon spectroscopy. Reducing frame time or spreading the image can reduce this effect.