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| Discovery of Radioactivity |
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| The discovery of the phenomenon of radioactivity was purely accidental. In 1896, Henry Becquerel, a French scientist accidentally found that in presence of salt of uranium, photographic plates got heavily fogged even though they were wrapped in opaque paper. He concluded that the uranium salt must be giving off penetrating radiations similar to X-rays discovered one year earlier. Two years later, Madam Curie named the phenomenon radioactivity. |
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| Radioactivity is the spontaneous random emission of particles from within the nucleus of the atom. Spontaneous random
emission means that the particles are emitted in bursts at irregular intervals with no set pattern and are emitted in any direction. The number emitted per second varies between very wide limits. This process is unique in that the particles are emitted without any energy having been given to the atom. Thus, energy is obtained without energy being introduced, and so the atom itself is a source of energy. The
atom is not the same after the emission of the particles. It has changed into an atom of another substance which may be unstable (will undergo further disintegration) or stable (will not emit any more particles), it is not possible to control this change. Neither is it possible to reverse it. Changing the temperature, changes the rate at which a chemical reaction takes place. It does not change the rate at which a radioactive substance decays. Radioactive decay is independent of temperature, pressure or chemical combination. |
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| As mentioned earlier, different radioactive emissions have different penetration powers. The more massive a particle the greater the chance that it collides with other particles and the less likelihood that it will travel far. |
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| The phenomenon of spontaneous disintegration of an unstable nucleus of naturally occurring isotope accompanied by emission of active radiations line
a, b and
g radiations is known as Natural Radioactivity. |
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| These radiations come from the breakup of the central core or nuclei of heavier elements like thorium, uranium etc. Elements with atomic number above 83 have very heavy and unstable nuclei. Being unstable, these nuclei spontaneously break up to form stable elements with lower atomic numbers. In the process of this breakup, the three types of radiations are given out. |
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| Experiments by Becquerel, Rutherford and Marie Sklodowska Curie and her husband Pierre Curie had shown that uranium, radium and certain other radioactive substances gave off three types of radiations. They performed many experiments to discover the properties of these radiations. |
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| Deflection of a, b and g
rays |
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| Experiment: |
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 The radioactive material was kept in a
small cavity in a block of lead. |
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| The whole apparatus was enclosed in an
evacuated vessel as shown here. |
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| An electric field was applied. |
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| It was observed that the beam is split up
into three components. |
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| The radiations, which emerged from the
radioactive material, were made to strike a photographic plate. These
radiations produced three different spots on the photographic plate. |
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| Conclusions: |
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| The rays that deflected towards the
negative plate consist of positively charged particles and these were called
alpha rays. |
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| The rays, which deflected towards the
positive plate, consist of negatively charged particles and these rays were
known as the beta rays. |
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| The rays, which did not deflect, do not
contain any charged particles and described these rays as gamma rays. |
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| Note: |
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| Beta particles deviate more than alpha
particles showing that beta particles are lighter than alpha particles or
vice versa. |
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| Gamma rays do not deviate as they are
electromagnetic waves and hence have no charge. |
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