Which statement about a photomultiplier tube is false?

• A. Converts radiant energy (light) to electrical energy (current)
• B. Amplifies the current significantly
• C. Has a very rapid response time
• D. Is composed of an iron plate and a layer of selenium

Answer: (D)

The key idea is how a photomultiplier tube responds to light. A PMT takes photons and converts them into electrons at a photocathode, then uses a chain of dynodes in a vacuum to emit many more electrons from each one, producing a large electrical current from a tiny initial signal. This built-in amplification gives a high gain, so a small light input becomes a strong electrical signal. The design also supports very fast response times because the electrons travel quickly through the vacuum with little delay.

The statement about being composed of an iron plate and a layer of selenium is not how a photomultiplier tube is built. Those materials describe a different type of light detector, a selenium-based photoelectric device, not a PMT. In a PMT, you’ll find a vacuum-sealed glass envelope with a photocathode, multiple dynodes for secondary emission, and an anode, commonly made with materials suited for electron emission and collection—not an iron plate with a selenium layer.