“Radioactivity is a process in which nuclei of certain elements undergo spontaneous disintegration without excitation by any external means.’’ and the elements whose atoms disintegrate and emit radiations are called radioactive elements.
Henry Becquerel (1891)observed the spontaneous emission of invisible, penetrating rays from potassium uranyl sulphate , which influenced photographic plate in dark and were able to produce luminosity in substances like ZnS.
Later on, M.M. Curie and her husband P. Curie named this phenomenon of spontaneous emission of penetrating rays as, Radioactivity.
Curies also discovered a new radioactive element Radium from pitchblende (an ore of U i.e. ) which is about 3 million times more radioactive than uranium. Now a days about 42 radioactive elements are known.
The radioactivity may be broadly classified into two types,
(1) If a substance emits radiations by itself, it is said to possess natural
(2) If a substance starts emitting radiations on exposure to rays from some natural radioactive substance, the phenomenon is called induced or artificial radioactivity.
Radioactivity can be detected and measured by a number of devices like ionisation chamber, Geiger Muller counter, proportional counter, flow counter, end window counter, scintillation counter, Wilson cloud chamber, electroscope, etc.