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In 1896, the phenomenon of radioactivity was accidentally discovered by French scientist Henri Becquerel. He was performing experiments with uranium compounds. He accidentally placed a crystal of potassium uranyl sulphate over photographic plate wrapped with usual black paper.
On developing the plate, the found the shadow of the crystal on it. He repeated this phenomenon a number of times and come to the conclusion that the crystal of potassium uranyl sulphate had emitted some mysterious rays which could penetrate the black paper and had affected the photographic plates. Later, it was found that thorium compounds also emitted similar rays.
Investigations of Pierre and Marie Curie in later years led to the discovery that atoms of certain other elements like radium and polonium undergo spontaneous decay and emit similar radiations. This phenomenon of spontaneous emissions of radiations by an element or its compound is called radioactivity and substances which exhibit this phenomenon are called radioactive substances.
Three types of radiation from recognized radioactive elements are a-particles, b-particles and g-radiation. It was realized that radioactivity of an element is independent of its physical state, its chemical environment or temperature, suggesting that it is a property of the nucleus i.e., a nuclear phenomenon.
Rutherford studied the penetrating power of these radiations and their behavior in electric and magnetic field. His conclusions have been summarized in the table below:
| Property | α - rays | β - rays | γ - rays |
|---|---|---|---|
| Mass | 6.67x10-27 kg or 4 a.m.u | 9.11 x10-31 kg | Negligible |
| Identity | Helium nuclei He2+ | Electrons | High energy radiations |
| Charge | + 2 units | - 1 unit | 0 |
| Velocity | Nearly 1/10 th that of light | Nearly same as that of light (0.03-5)x10-13J | Same as that of light |
| Energy | (6 – 16)10-13J | (0.03 - 5)x10-13 J | Same range as β particles (1 - 5)x10-13J |
| Effect of electric and magnetic field | Deflected towards negative pole | Deflected towards positive pole | Not deflected |
| Penetrating power | Small - a few cms of air | Large, 100 times that of α - particle about 1-2m in air | Very large 10000 times that of α -rays stopped only by thickness of about 15- 20cm of lead |
| Effect an photographic plate and zinc sulphide | Affected more strongly then by β and γ rays | Effect is less then α -rays | Least effect |
