Radioactivity also known as radioactive decay, nuclear transformation, and nuclear disintegration is a spontaneous process by which an unstable parent nucleus emits a particle or electromagnetic radiation and transforms into a more stable daughter nucleus that may or may not be stable. An unstable daughter nucleus will decay further in a decay series until a stable nuclear configuration is reached. The radioactive decay is governed by the formalism based on the definition of activity and the radioactive decay constant. Henri Becquerel discovered the process of natural radioactivity in 1896 and soon thereafter in 1898 Pierre Curie and Marie Curie discovered radium and polonium and coined the term “radioactivity” to describe the emission of “emanations” from unstable natural elements. Frédéric Joliot and Irène Joliot-Curie discovered artificial radioactivity in 1934.