Radioactivity
Radioactivity is the spontaneous emission of radiation in the form of particles or high energy photons resulting from a nuclear reaction.
Radioactive decay, also known as nuclear decay or radioactivity, is the process by which the nucleus of an unstable atom loses energy by emitting radiation, including alpha particles, beta particles, gamma rays and conversion electrons.
A material that spontaneously emits such radiation is considered radioactive.
Units of Radioactivity
The International System of Units (SI) unit of radioactive activity is the becquerel (Bq), named in honour of the scientist Henri Becquerel. One Bq is defined as one transformation (or decay or disintegration) per second.
An older unit of radioactivity is the curie, Ci, which was originally defined as "the quantity or mass of radium emanation in equilibrium with one gram of radium (element)".
Today, the curie is defined as 3.7×1010 disintegrations per second, so that 1 curie (Ci) = 3.7×1010 Bq.
For radiological protection purposes, although the United States Nuclear Regulatory Commission permits the use of the unit curie alongside SI units