Which material used in flat-panel detectors directly converts x-ray photons into an electrical signal?

• A. Amorphous Selenium
• B. Cesium iodide
• C. Amorphous Silicon
• D. Gadolinium

Answer: (A)

Direct conversion in flat-panel detectors means the material turns x-ray energy straight into electrical charges without any intermediate light. Amorphous selenium does exactly that: when x-rays are absorbed, it creates electron-hole pairs that are swept by an electric field and collected to form the image. The other materials are used in indirect conversion. Cesium iodide acts as a scintillator, turning x-rays into visible light which is then detected by a photodiode, typically made of amorphous silicon. Amorphous silicon is mainly a photodiode layer in indirect detectors, not the direct converter. Gadolinium-based materials (scintillators like GOS) likewise convert x-rays to light first. So the material that directly converts x-ray photons to an electrical signal is amorphous selenium.