Which units are used to express absorbed dose and dose equivalent, respectively?

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

Answer: (B)

Absorbed dose and dose equivalent use different units because they measure different things: energy deposited per unit mass versus the potential biological effect of that energy. Absorbed dose is expressed in gray, with 1 gray meaning 1 joule of energy deposited per kilogram of tissue. Dose equivalent adjusts that energy for how damaging it is biologically, and is expressed in sievert. The sievert equals the gray multiplied by a radiation weighting factor that reflects how different types of radiation affect tissue; for diagnostic X-rays the factor is 1, so 1 Gy equals 1 Sv. Historically, older CGS units like rad and rem were used (1 Gy = 100 rad, 1 Sv = 100 rem), but today the standard SI units are gray for absorbed dose and sievert for dose equivalent.