What is the special unit used to measure exposure levels of radiation?

The special unit used to measure exposure levels of radiation is the Roentgen. This unit specifically quantifies exposure to ionizing radiation, particularly X-rays and gamma rays. It is defined based on the amount of ionization produced in air by the radiation. The Roentgen is used primarily in the context of radiation protection and safety because it correlates directly with the potential biological effect of radiation exposure in air, making it especially relevant for monitoring and understanding exposure levels in environments where radiation is present.

In contrast, the other units listed measure different aspects of radiation: the Sievert measures the biological effect of radiation on human tissue, the Becquerel quantifies the radioactivity of a sample (the decay of radioactive atoms), and the Gray measures the absorbed dose of radiation in matter, indicating how much energy is deposited by radiation in a given mass. Each of these units serves a unique purpose in radiation safety and does not specifically focus on the measurement of exposure levels in the same way the Roentgen does.