Which statement about the sievert (Sv) is true?

• A. The dose rate in rem per hour
• B. Unit of activity
• C. Energy deposited per kilogram
• D. SI unit of dose equivalent; equals Gy × w_R for the tissue and radiation used

Answer: (D)

Dose equivalent, not absorbed dose, is what the sievert represents. The absorbed dose in gray tells you how much energy is deposited per kilogram, but different types of radiation cause different biological effects. To capture that, we multiply the absorbed dose by a radiation weighting factor w_R that depends on the radiation type. For a single tissue and a single radiation, the dose equivalent H equals D × w_R. Since the sievert is defined so that Gy × w_R gives Sv, the unit of dose equivalent is the sievert. For example, photons have w_R ≈ 1, so 1 Gy corresponds to about 1 Sv; alpha particles have w_R = 20, so 1 Gy of alpha radiation corresponds to about 20 Sv. The other options don’t fit: energy deposited per kilogram is measured in gray, and activity is measured in becquerels; rem per hour uses an older non-SI unit.