Which of the following is a man-made source of radiation?

• A. Medical X-rays
• B. Radon
• C. Cosmic Rays
• D. Terrestrial sources

Answer: (A)

Distinguishing man-made from natural radiation sources is what this item tests. Medical X-rays are produced by X-ray machines that generate ionizing photons specifically for imaging, so the radiation comes from human-made equipment. In contrast, radon is a natural radioactive gas formed from uranium decay in rocks and soil, cosmic rays are high-energy particles originating in space, and terrestrial sources refer to natural radioactive materials in the Earth’s crust. Because these are all natural phenomena that exist without human creation, they are natural background sources. Therefore, the medical X-rays stand out as the man-made source of radiation.