Which of the following is NOT listed as ionizing radiation?

• A. Cosmic
• B. Gamma
• C. X-Ray
• D. Ultraviolet

Answer: (D)

Ionizing radiation comprises photons or particles with enough energy to eject electrons from atoms, creating ions. Gamma rays and X-rays are classic examples because their energies are high enough to ionize matter readily. Cosmic radiation includes highly energetic particles and photons that also cause ionization as they pass through materials. Ultraviolet light, while energetic, generally has photon energies of only a few electron volts, well below what’s needed to ionize most atoms in typical biological contexts; its primary effects are photochemical rather than ionizing. So ultraviolet is not considered ionizing radiation in this context.