What unit is used to measure radioactivity or decay rate?

The Curie is the unit used to measure radioactivity, specifically the decay rate of radioactive materials. One Curie is defined as the amount of radioactivity in a sample that produces 3.7 x 10^10 disintegrations per second, which reflects the activity level of the material. This unit is named after Marie Curie, who conducted pioneering research on radioactivity.

In contrast, the other units provided serve different purposes in the field of radiation measurement. The Joule is a unit of energy, used in many physical contexts but not directly related to the decay of radioactive materials. The Sievert is a unit used to measure the biological effect of ionizing radiation on human tissue, taking into account the type and energy of radiation. The Gray, on the other hand, is a unit used to measure the absorbed dose of radiation, indicating the amount of energy deposited in a material per unit mass.

Understanding these distinctions is essential in radiological science, as each unit has specific applications that are crucial for safety assessments, treatment planning, and research within the field. Thus, the Curie is the appropriate choice for measuring radioactivity.