In an indirect capture detector, which layer converts x-ray energy into visible light?

• A. Photodiode
• B. TFT array
• C. Scintillation layer
• D. Amorphous Selenium detector

Answer: (C)

The main idea is that indirect capture detectors first turn x-ray energy into light, then convert that light into an electrical signal. The layer that does the initial conversion from x-rays to visible light is the scintillation layer. It absorbs incoming x-ray photons and emits visible photons in response. Materials like cesium iodide (CsI) or gadolinium oxysulfide are common for this purpose. The emitted light is then detected by a photodiode or a TFT-enabled light sensor, which converts the light into an electrical signal for imaging. The photodiode’s job is to translate light into electricity, not to produce light from x-rays; the TFT array mainly handles readout. Amorphous selenium represents a direct-conversion detector, where x-rays are converted straight into charge without a light intermediary, which is a different approach.