Which statement best defines exposure in radiological terms?

• A. Ionization produced in air by X- or gamma-rays
• B. Absorbed dose in tissue
• C. Activity of a radioactive source
• D. Energy deposited in shielding

Answer: (A)

Exposure is a measure of the ionization that radiation produces in air. For X- or gamma-rays traveling through air, photons interact with air molecules and create ion pairs, and exposure quantifies this ionization. Historically it was expressed in roentgens; in SI units it’s coulombs per kilogram. This quantity reflects how readily a radiation field ionizes air and is not the same as how much energy is deposited in tissue (absorbed dose) or the activity of a source (disintegrations per second), or the energy deposited in shielding. So the statement that ionization produced in air by X- or gamma-rays best defines exposure.