SI unit of radioactivity is: March 2013 (c, f)
**Core Concept**
The SI unit of radioactivity is a measure of the activity of a radioactive substance, which is defined as the number of nuclear decays per unit time. This unit is essential in nuclear medicine and radiation therapy to quantify the radiation exposure and safety of patients and medical staff.
**Why the Correct Answer is Right**
The correct answer is **B. Becquerel (Bq)**. The Becquerel is the SI unit of radioactivity, named after Henri Becquerel, who discovered radioactivity in 1896. It is defined as one nuclear decay per second. The Becquerel is a fundamental unit in nuclear physics and is used to express the activity of radioactive substances, including isotopes used in medical applications such as positron emission tomography (PET) and nuclear medicine.
**Why Each Wrong Option is Incorrect**
**Option A:** The Gray (Gy) is the SI unit of absorbed radiation dose, not radioactivity. It measures the energy deposited per unit mass of tissue.
**Option C:** The Sievert (Sv) is the SI unit of radiation dose equivalent, which takes into account the biological effect of radiation. While related to radioactivity, it is not the unit of radioactivity itself.
**Clinical Pearl / High-Yield Fact**
When working with radioactive substances, it's essential to understand the difference between activity (Bq) and dose (Gy or Sv). This distinction is crucial in ensuring patient safety and optimizing the therapeutic effect of radiation treatments.
**Correct Answer: B. Becquerel (Bq)**