Radioactivity

The binding-energy curve shows that nuclear stability begins to decline once the atomic number exceeds $Z=82$; nuclei beyond this point, together with certain lighter isotopes, are collectively called unstable or radioactive nuclei. Left alone, an unstable nucleus will spontaneously transform into a more stable nucleus by emitting a particle - specifically a $^{4}_{2}He$ nucleus (alpha decay), an electron or positron (beta decay), or a high-energy photon (gamma decay).

This spontaneous emission of highly penetrating alpha, beta or gamma radiation by certain nuclei is called radioactivity, and the substances that emit it are called radioactive elements. Radioactive nuclei include both naturally-occurring heavy elements ($Z>82$) and lighter or heavier radioactive isotopes (radioisotopes) of otherwise stable elements - for instance, the carbon isotope $^{14}_{6}C$ is radioactive while $^{12}_{6}C$ is not. Radioisotopes have many practical applications, including carbon dating (section 8.6.6) and cancer treatment.

Radioactivity was first discovered by Henri Becquerel in 1896, and studied in great further detail soon after by Marie Curie and her husband Pierre Curie. In India, the Saha Institute of Nuclear Physics (SINP) in Kolkata is a premier institute for ongoing research in nuclear physics. …