It was observed by Henri Becquerel in 1896 that some minerals like pitchblende emit radiation spontaneously, and this radiation can blacken photographic plates if photographic plate is wrapped in light proof paper. He called this phenomenon radioactivity. It was observed that radioactivity was not affected by temperature, pressure or chemical combination, it could therefore be a property that was intrinsic to the atom-more specifically to its nucleus. Detailed studies of radioactivity resulted in the discovery that radiation emerging from radioactive substances were of three types :

(i) Alpha Particles: An α-particle is a helium nucleus, i.e. a helium atom which has lost two electrons. It has a mass about four times that of a hydrogen atom and carries a charge +2e. The velocity of α-particles ranges from 5 to 7 percent of the velocity of light. These have very little penetrating power into a material medium but have a very high ionising power.
(ii) Beta Particles: β-particles are electrons moving at high speeds. These have greater (compared to α-particles) penetrating power but less ionising power. Their emission velocity is almost the velocity of light. Unlike α-particles, they have a spectrum of energy, i.e., beta particles possess energy from a certain minimum to a certain maximum value.

(iii) Gamma Rays: γ-rays are electromagnetic waves of wavelength of the order of ~ 10–12 m. These have the maximum penetrating power (even more than X–ray) and the least ionising power. These are emitted due to transition of excited nucleus from higher energy state to lower energy state.

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