What is the SI unit of activity?

The SI unit of activity is the Becquerel (Bq). This unit specifically measures the rate of radioactive decay, quantifying the number of disintegrations occurring in a radioactive substance per second. One Becquerel is defined as one disintegration per second, making it a fundamental measurement in the field of nuclear medicine and radiology, where understanding the activity of radioactive isotopes is essential for both safety and treatment efficacy.

The other options represent different physical quantities: Gray (Gy) is the SI unit for absorbed dose, which measures the amount of radiation energy absorbed per unit mass of tissue. Sievert (Sv) is the unit that quantifies biological effects of radiation, taking into account the type of radiation and its potential impact on human health. C/kg represents the unit of ionization in air, which is related to radiation exposure rather than activity. Therefore, the correct answer being the Becquerel emphasizes its specific role in measuring the inherent activity of radioactive materials.