What does the symbol R represent in radiation measurement?

The symbol R in radiation measurement stands for Roentgen, which is a unit used to measure exposure to ionizing radiation in air. The Roentgen quantifies the amount of ionization produced in air by X-rays or gamma rays, specifically focusing on the effects of such radiation on air molecules. This unit is essential in fields such as radiation protection, health physics, and radiological safety because it provides a way to assess the potential biological effect of radiation exposure based on the ionization it causes.

The Roentgen is particularly utilized in contexts where the focus is on the exposure levels of radiation, making it distinct from other units that measure different aspects of radiation. For instance, Radiation Absorbed Dose, commonly referred to as rad or gray, measures the energy deposited in a material (like human tissue) from radiation. Quality factor refers to a dimensionless factor that accounts for the type of radiation and its biological effects, while Curies is a unit of radioactivity that measures the decay of radioactive material rather than exposure or absorption. The specificity of the Roentgen to ionization in air delineates why it is the correct answer in the context of this question.