Which unit is used to express the absorbed dose of ionizing radiation?

• A. Gray (Gy)
• B. Sievert (Sv)
• C. Röntgen (R)
• D. Coulomb per kilogram (C/kg)

Answer: (A)

Absorbed dose is the energy deposited by ionizing radiation per unit mass of material. The unit for this physical quantity is the Gray, defined as one joule of energy absorbed per kilogram (1 Gy = 1 J/kg). This makes Gray the best answer because it directly quantifies how much energy the tissue has absorbed.

The other units describe different concepts. The Sievert accounts for the biological effect by applying weighting factors to convert absorbed dose into a measure of potential harm (equivalent or effective dose), useful for comparing risk across tissues and radiation types. The Röntgen (R) and coulomb per kilogram (C/kg) relate to exposure—the amount of ionization in air—not to how much energy is deposited in tissue, so they do not express absorbed dose.