The existence of the three distinct types of radiations, α, β and γ−rays can be easily found by the following experiment.A small amount of radium is placed at the bottom of a small hole drilled in a lead block, which is kept in an evacuated chamber
A photographic plate is placed at a short distance above the lead block. A strong magnetic field is applied at right angles to the plane of the paper and acting inwards. Three distinct traces can be seen on the photographic plate when it is developed. The trace towards left is due to positively charged particles. They are named α -particles.The trace towards the right is due to negatively charged particles.They are named β-particles.The undeviating trace is due to neutral radiations which are called γ−rays.If an electric field is applied, the α-rays are deflected towards then negative plate,β−rays towards the positive plate and γ−rays are not deflected.