The roentgen measures ionization in air caused by exposure to which of the following?

The roentgen is a unit of measurement that specifically quantifies the amount of ionization produced in air by X-rays or gamma rays. It was originally defined based on the amount of X-ray or gamma radiation that would create a certain number of ion pairs in air under standardized conditions.

X-rays and gamma rays are both forms of electromagnetic radiation, which possess the ability to ionize atoms in air, hence leading to measurable ionization. In contrast, while alpha and beta particles can also cause ionization, they interact differently with matter and are typically measured in different units. Alpha particles, for instance, have a high mass and charge, causing significant ionization but over a very short range, while beta particles, being electrons or positrons, have different ionization characteristics.

Thus, the roentgen is specifically associated with measuring exposure from X-rays and gamma rays rather than all types of ionizing radiation, which is why the correct answer focuses on X-rays and gamma rays alone.