Ionizing radiation has energy sufficient to ionize an atom, generally greater than:

• A. 0.1 eV
• B. 1000 eV
• C. 10 keV
• D. >4 eV

Answer: (D)

Ionizing radiation is defined by its ability to remove electrons from atoms, which requires energy at least as large as the first ionization energy of the atoms. The smallest first ionization energies among elements are around a few electron volts, roughly about 4 eV, so any radiation with energy greater than that can ionize some atom. The room-temperature thermal energy is only about 0.025 eV, far below this threshold, so it cannot ionize. Energies like 0.1 eV are not enough to ionize, while energies in the keV range (thousands of eV) are far above the minimum needed but aren’t needed to define what makes radiation ionizing; the key point is the threshold itself. Therefore “greater than 4 eV” best captures the concept of ionizing radiation.