Which unit, historically used in the United States, is used to express radiation doses including equivalent dose?

• A. Gray
• B. Sievert
• C. Rad
• D. Rem

Answer: (D)

The key idea is distinguishing between absorbed dose and dose equivalent, and recognizing which units were used historically in the United States to express those values, including the biological effect on the body. Dose equivalent is the absorbed dose adjusted by a radiation weighting factor to reflect how damaging the specific type of radiation is to tissue. In the United States, the unit used for this dose equivalent in the past was the rem (Roentgen equivalent man). That makes rem the unit that explicitly covers “including equivalent dose” in the historical US context.

For contrast, Gray is the SI unit of absorbed dose (energy deposited per unit mass), and the rad is an older cgs unit for absorbed dose. The Sievert is the SI unit for dose equivalent, which is the modern equivalent of rem, but it is the current standard rather than the historical US unit. Since the question points to the unit historically used in the United States to express doses that include the biological effect (dose equivalent), rem is the best answer.