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In 1896, Henri Becquerel was performing experiments which uranium compounds. He accidently placed a crystal of potassium uranyl sulphate [K2UO2(S04)2.2H20] over photographic plate wrapped with usual black paper. On developing the photographic plate, he found the shadow of the crystal on the photographic plate. He repeated this phenomenon a number of times and came to a conclusion that the crystal of potassium uranyl sulphate had emitted some mysterious rays which could penetrate the black paper and had affected the photographic plate. Later on, it was found that thorium compounds also emitted similar rays.

This phenomenon of spontaneous emission of radiations by an element or its compound was given a name radioactivity and the substances which exhibit this phenomenon were called radioactive substances. Once discovered, radioactivity became a popular subject for scientific investigations.

Two very popular names associated with the study of radioactivity are Madam Curie and her husband Pierre Curie who discovered radioactive elements, polonium, thorium, radium and actinium. Some of these elements possessed radioactive power much larger than uranium·. In the later years very large number of radioactive elements have been discovered.



Radioactivity can be broadly classified into two categories: natural radioactivity and artificial or induced radioactivity.

(i) If a substance emits radiations by itself, it is said to possess natural radioactivity.

(ii) If a substance does not possess radioactivity but starts emitting radiations on exposure to rays from a natural radioactive substance, it is said to possess induced or artificial radioactivity.



Some points of difference between natural and artificial radioactivity are as follows:

Natural Radioactivity                                                             Artificial Radioactivity

1. It involves spontaneous                                          1. Stable nuclei are bombarded

disintegration of unstable                                           with high energy particles

nuclei with emission of a                                            to produce radioactive

or 13 particles or y-radiations                                     nuclides.

giving rise to new nuclide.

2. It cannot be controlled.                                          2. It can be controlled by controlling                                                                             the speed of the                                                                                         bombarding projectiles.

3. It is shown by heavy                                              3. It can be induced even in the

elements i.e., elements                                               lighter elements.

with high atomic number

and mass· number.



The SI unit of radioactivity is, Bacqueral (Bq) which is defined as one disintegration per second ( dps). Earlier, radioactivity was expressed in terms of curies (C;). One curie refers to the activity of one gram of radium, and is equal to 3.7 x 1010 disintegrations per second.

1Ci = 3.7 x 1010 dps = 3.7 x 1010 Bq.

1 millicurie ( m Ci) = 3.7 x 107 dps and 1 microcurie ( µ Ci) = 3.7 x 104 dps

A more recent unit of-radioactivity is Rutherford (Rd).

1 Rd = 106 dps.