The photomultiplier tube within a scintillation detector is to convert:

• A. X-rays into electrons
• B. Electrons into more electrons
• C. Light into electrons
• D. X-ray into light

Answer: (C)

Photomultiplier tubes respond to light and convert it into an electrical signal. In a scintillation detector, the X-ray energy is first absorbed by a scintillator, which then emits photons of visible light. The PMT takes those photons and frees electrons from its photocathode when they strike it, turning light into electrons. This initial electron emission is then amplified through a chain of dynodes, producing a strong electrical pulse proportional to the amount of light detected. So the PMT’s job is light-to-electrons (with amplification), not X-rays directly to electrons or X-rays to light.