In radiology, which unit is commonly used to express absorbed dose in the context of the source material?

• A. rad
• B. rem
• C. sievert
• D. gray

Answer: (A)

Absorbed dose is the energy deposited per unit mass of the material being irradiated. The unit used to express this energy absorption historically is the rad, a CGS unit where 1 rad equals 0.01 gray. When the discussion centers on the dose absorbed by the actual material (the source material), rad has been the traditional way to quantify how much energy the material captures. Rem and sievert pertain to biological effects (equivalent or effective dose) rather than the pure energy deposited, and gray is the modern SI unit for absorbed dose. The question’s phrasing points to the historical, material-focused measurement, making rad the best fit.