What term describes the number of unstable atoms in a source that decay per second?

The term that describes the number of unstable atoms in a source that decay per second is "activity." Activity is a key concept in nuclear physics and radioactivity, specifically quantifying the rate at which a radioactive source emits radiation. It is measured in decays per second, typically represented in units such as becquerels (Bq), where one becquerel corresponds to one decay event occurring per second.

Understanding activity is essential for applications such as radiation safety, medical imaging, and nuclear energy, where knowing the decay rate helps assess potential exposure levels and the behavior of radioactive materials over time. The concept is distinct from half-life, which refers to the time taken for half of the radioactive atoms in a sample to decay, and count rate, which refers to the number of decays detected by a measurement instrument, not necessarily per second. Nuclear fusion, on the other hand, is the process of combining light nuclei to form a heavier nucleus, which is unrelated to the decay of unstable atoms.