Radioactivity is the spontaneous decay or disintegration of the nucleus of the atom of an
element during which it emits α, β or ϒ rays or a combination of any or all the three and energy (or heat). Radioactive elements spontaneously emit radiation from their nucleus such as radium, thorium, radon, uranium, polonium, etc.
If a small sample of radium is placed at the bottom of a small hole drilled in a block of lead. The radiation emitted from this radium emerged from the hole in a narrow beam if the rays were subjected 10 a strong magnetic field placed at the side of a beam. A photographic plate situated at appropriate sides to receive the rays showed that the paths of some rays were bent to the right, some to the left and some went straight on; unbent.
Electrically charged plates placed at the side of the beam gave the same effect. The radiations that was bent towards the negative electric plate or the south pole of the magnetic field are called the Alpha particles (α-particles)
The radiations deflected towards the positive electric plate or the North pole of the magnetic fields called Beta particles (β-particles).
The radiation that was neither affected by the electric nor magnetic feld is called Gamma rays (ϒ). They are actually electromagnetic radiations.