Which unit is used to measure the ionization in air used in radiographic exposure?

• A. Coulomb per kilogram
• B. Gray
• C. Sievert
• D. Roentgen

Answer: (A)

Exposure in radiography is the ionization produced in air by X- or gamma rays. The unit that directly captures this ionization is the roentgen, defined as the amount of ionization produced in 1 cubic centimeter of dry air under standard conditions. Gray and Sievert measure energy deposited in tissue and biological effect, not ionization in air, so they don’t quantify exposure in air. The Coulomb per kilogram is the SI unit for exposure, but the roentgen remains the traditional unit used in radiographic exposure because it directly describes ionization in air (1 R ≈ 2.58×10^-4 C/kg). So, for measuring ionization in air in radiographic exposure, the roentgen is the best fit.