Which types of radiation does the measurement Roentgen primarily apply to?

Roentgen is a unit of measurement used primarily to quantify exposure to ionizing radiation, specifically gamma rays and x-rays. This unit measures the amount of ionization produced in air by these types of radiation. When x-rays and gamma rays pass through the air, they interact with air molecules, resulting in ionization—for which the Roentgen unit is designed to give an indication of the radiation's potential to cause biological damage.

Alpha particles, beta particles, and neutrons interact differently with matter and do not produce ionization in air in a manner that is relevant to the Roentgen unit. Alpha particles, for instance, have a very short range and are not typically measured in air as they cannot penetrate it effectively. Beta particles can ionize but are also not measured using the Roentgen unit, and neutrons require different units and are generally evaluated based on their biological effects or interaction with materials rather than by ionization in air.

Thus, the correct answer highlights the specific context within which the Roentgen unit is applicable, making it relevant primarily to gamma rays and x-rays.