What material do FPDF units use to convert x-rays directly into electrical charges?

• A. Silicon photodiode
• B. Amorphous selenium (a-Se)
• C. Gallium arsenide
• D. Cadmium telluride

Answer: (B)

Amorphous selenium (a-Se) is the correct choice because it is a semiconductor material that has the unique ability to directly convert x-ray photons into electrical charges. This property is vital for the function of Flat Panel Detector (FPD) systems in radiography. When x-rays hit the amorphous selenium layer, they create electron-hole pairs, which can then be collected and processed to form digital images. This direct conversion process is what enables high-quality imaging with reduced exposure to radiation.

Silicon photodiodes, while used in some applications, do not have the same direct conversion capability for x-rays as amorphous selenium. Gallium arsenide and cadmium telluride are also semiconductors that are used in other types of imaging systems but are not the primary materials associated with FPD units for direct conversion of x-rays. Understanding the specific role of amorphous selenium helps clarify why it is the material of choice in this context.