What is the unit for measuring the amount of radiation exposure?

The measurement of radiation exposure is quantified using the unit called the Roentgen. This unit specifically refers to the amount of ionizing radiation that produces a certain amount of ionization in air. It's primarily used to measure X-rays and gamma rays in terms of their capacity to ionize air, which is essential for understanding potential biological impacts on living organisms.

In contrast, the other terms listed do not represent units of measurement for radiation exposure. Gamma ray refers to a type of electromagnetic radiation, while an isotope is a variant of a chemical element that has a different number of neutrons. A gamma meter, on the other hand, is a device used to detect and measure gamma radiation but does not itself serve as a unit of measurement. Understanding the specific role and definition of the Roentgen helps clarify its importance in the context of radiation exposure assessment and safety.