The discovery of radioactivity was quite accidental. In 1895, Henri Becquerel kept a photographic plate wrapped in thick black paper in the same drawer in which some uranium salts were kept. After a couple of days, he found to his utter surprise that the photographic plate had become fogged or affected. The fogging of the photographic plate, even in the absence of light, led Becquerel to conclude that the uranium salts emitted some invisible rays which had penetrating power similar to that of X-rays. It was later found that these rays could cause ionization of air. These rays were called radioactive rays and the phenomenon of emission of these rays was called radioactivity.
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Later, Marie Curie discovered polonium, thorium, radium etc. These elements were also found to be radioactive. The radioactivity of radium is nearly three million times more than that of uranium. Since then many more radioactive elements have been discovered.
The constituents of radioactive rays are of three types: positively charged alpha -particles, negatively charged beta-particles and uncharged gamma-radiation. Thus the phenomenon of spontaneous emission of particles like alpha-rays, beta-rays and gamma-rays from some substances is known as radioactivity and such substances are called radioactive substances.
Types of Radioactive Rays
In 1904 Rutherford passed the radiations emitted by radioactive Demerits through a strong electric field. The radiations were found to separate into three different types.
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(i) Alpha (α) rays : The rays which are deflected towards the negative plate are called alpha (α) rays or alpha particles. These particles are actually helium nuclei (He++). They are positively charged particles.
(ii) Beta (β) rays : The rays which are deflected towards the positive plate are called beta (β) rays or beta particles. These rays consist of electrons, i.e., they consist of negatively charged particles.
(iii) Gamma (γ) rays : The rays which do not suffer any deflection at all are neutral. They are called gamma (γ) rays.
Properties of radioactive rays
Properties of a-rays
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(i) Velocity : The α-rays emanate from the radioactive substance with high velocities ranging from 1.4×109 to 1.7×109 cm/s, i.e., α-particles move with a velocity of about 10,000 miles per sec.
(ii) Ionizing power : They ionize a gas through which they pass. The ionizing power is 100 times greater than that of β-rays and 10,000 times greater than that of γ-rays.
(iii) Action on photographic plate : They affect a photographic plate feebly.
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(iv) Penetrating power : They have the least penetrating power. They can be stopped by a 0.1-mm thick aluminium foil or by a sheet of paper. They have a range of a few centimetres in air.
(v) Phosphorescence : They produce phosphorescence when allowed to fall on zinc sulphide or barium platinocyanide.
(vi) Effect of electric field : In an electric field, they are deflected towards the negative plate. This shows that a-rays or a-particles are positively charged.
Properties of β-rays
(i) Velocity : β-particles come out from the nuclei of radioactive elements with velocities which range from 1% to 99% of the velocity of light.
(ii) Ionizing power : Their ionizing power is much less than that of a-particles.
(iii) Action on photographic plate : They affect a photographic plate more strongly than a-rays.
(iv) Penetrating power : Their penetrating power is 100 times greater than that of α-rays. They can be stopped by a 2-cm thick sheet of aluminium. They have a range of a few metres in air.
(v) Phosphorescence : Their power to produce phosphorescence on zinc sulphide is much less.
(vi) Effect of electric field : In an electric field, they are deflected towards the positive plate. This shows that they are negatively charged particles.
Properties of γ-rays
(i) Nature : They are high-frequency electromagnetic waves.
(ii) Velocity : γ-rays have velocity equal to that of light.
(iii) Ionizing power : They ionize the gas through which they pass, but the ionization produced by them is comparatively small.
(iv) Phosphorescence : They are capable of producing phosphorescence in zinc sulphide etc.
(v) Effect of electric field : In an electric field, they are not deflected at all. This shows that y-rays are neutral.
(vi) Penetrating power : Their penetrating power is very high as compared to that of β-rays. They can pass easily through 30 cm thickness of iron but can be stopped by about 5 cm of lead or concrete. Their range is about 100 metres in air.
(vii) Action on photographic plate : They darken a photographic plate deeply.