Exposure is defined as

• A. The rate of energy deposition in a given mass
• B. The number of ionizations detected in air
• C. The biological risk weighted for radiation type
• D. The energy absorbed per unit mass

Answer: (B)

Exposure is the amount of ionization produced in air by radiation, effectively reflecting how many ion pairs are created per unit mass of air (traditionally described as the charge produced per kilogram of air). Because ionizations in air correspond to the electrical charge liberated by radiation, describing exposure as the number of ionizations detected in air is a practical and accurate way to capture this concept. In more precise terms, exposure is tied to the total ionization charge per mass, not to how quickly energy is deposited (that would be dose rate) or how much energy is absorbed per mass (that’s absorbed dose in gray), or to the biological risk weighting used for dose equivalence or effective dose. The other options describe those different quantities, so the choice that references ionizations detected in air best matches the concept of exposure.