Which material is commonly used as a photoconductor in direct-conversion DR detectors?

• A. CsI
• B. Amorphous Selenium
• C. Gadolinium Oxysulfide
• D. Sodium Iodide

Answer: (B)

Direct-conversion detectors rely on a photoconductive layer that turns X-ray energy directly into an electric charge, which is then collected to form the image. Amorphous selenium fits this role because it acts as a solid-state photoconductor: when X-rays are absorbed, it generates charge carriers that can be collected with an applied electric field, producing a digital signal without an intermediate light step. This direct conversion also helps preserve spatial resolution since there’s no light spread that could blur the image.

The other materials are scintillators used in indirect-conversion systems. They first convert X-rays into visible light, which is then detected by a separate photodiode or detector, adding an extra step and potential light spread. CsI, gadolinium oxysulfide, and sodium iodide are common scintillators that enable indirect conversion rather than direct photoconversion.