What traditional unit of measure is valid for energies up to 3 MeV?

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

Answer: (C)

The correct answer is Roentgen, which is the traditional unit of measure for the exposure to ionizing radiation in air. It is defined in such a way that it is specifically applicable to X-rays and gamma rays, making it suitable for measuring energies up to 3 MeV.

The Roentgen quantifies the amount of ionization produced in air by X-ray or gamma radiation. For X-rays and gamma rays up to this energy range, Roentgen is particularly relevant as it reflects the biological effect caused by exposure to these radiations.

Other units, while related to radiation measurement, serve different purposes. The rem (which measures biological effects of ionizing radiation), rad (which measures absorbed dose), and gray (the SI unit for absorbed dose) do not specifically denote exposure in air nor are they limited to the same energy considerations as the Roentgen. Hence, for measuring X-ray and gamma ray exposure up to 3 MeV, Roentgen is the most appropriate unit.