Which charge does the storage capacitor of a detector element (DEL) store?

• A. A positive charge
• B. A negative charge
• C. Photons
• D. Electrons

Answer: (A)

The storage capacitor in a detector element holds the electric charge produced by exposure, and that charge represents the signal from the image. As X-ray exposure generates electron-hole pairs in the photodiode and the device is reverse-biased, electrons move away while holes can remain or be collected, leaving behind a net positive charge on the storage node. This accumulated charge on the capacitor creates a voltage that the readout circuitry converts into the pixel value. Photons themselves aren’t stored, and the signal isn’t stored as free electrons—the capacitor preserves the net positive charge that encodes the detected signal.