The primary by-product of x-ray interactions in air is which of the following?

• A. visible light
• B. thermal energy
• C. free electrons
• D. radioactive

Answer: (D)

When X-ray photons pass through air, most interactions transfer energy to electrons and eject them from atoms. This creates free electrons and the corresponding positively charged ions—ion pairs. That ionization is the immediate, primary consequence of the interaction and is what makes the air electrically active along the path of the X-ray. The energy that goes into heating the air is a secondary effect, and X-ray interactions do not make the material radioactive—that would require nuclear changes not produced here. Visible light isn’t produced as a primary outcome in these interactions. So the best answer is the generation of free electrons (with accompanying positive ions) due to ionization.