Which traditional unit measures X or gamma radiation only?

• A. Rad
• B. Roentgen
• C. Gray
• D. rem

Answer: (B)

The roentgen is the traditional unit of measurement that specifically quantifies exposure to X or gamma radiation in air. This unit denotes the amount of X or gamma radiation that produces a measurable ionization in air. It is crucial for radiography as it directly correlates to the potential biological effect and exposure of living tissues to radiation.

In contrast, the rad is a unit used to measure absorbed dose, regardless of the type of radiation, and is applicable to both X rays and other forms of ionizing radiation. The gray is the SI unit that modern usage has largely adopted for measuring absorbed dose, similar to the rad but more widely used internationally. The rem is a unit that measures the biological effect of radiation on human tissue, factoring in the type of radiation but not specifically limited to X or gamma types.

Thus, the roentgen is unique among these units as it specifically addresses the exposure to X and gamma radiation, making it the correct answer to the question.