Which component of a semiconductor detector creates the initial electrical signal?

• A. Photocathode
• B. Photoconductor crystal
• C. Amplifier
• D. Metal housing

Answer: (B)

In a semiconductor detector, the initial electrical signal comes from the detector material itself. When radiation such as X-rays or gamma rays interacts with the semiconductor, it deposits energy that excites electrons from the valence band into the conduction band, creating electron–hole pairs. These charge carriers are produced right at the point of interaction and, under the applied electric field, drift toward the electrodes to generate an electrical current—the first signal that the readout system measures. The active detecting material in this case is the photoconductor crystal, which is why that choice is the correct one. A photocathode would only emit electrons in response to light in vacuum-tube devices, not from the energy deposited by X-rays inside a solid-state detector; an amplifier merely boosts an existing signal, and the metal housing surrounds the detector without creating a signal.