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Nuclear radiation (or particles) such as α, β, and γ– rays cannot be observed directly. They are detected by secondary effects which they produce in the materials through which they pass. There are 3 such effects and a number of detecting instruments are based upon them. These effects are :
i) Ionisation ii) Photographic Action iii) Fluorescence 
i) Ionisation : A charge particle like α-particle, β-particle & γ-rays etc. on passing through matter produces ions by collision with molecules of the material. This ionisation is the basis of many instruments (detecting) such as Ionisation Chamber,  G.M. Counter, Cloud Chamber and Bubble Chamber. 
ii) Photographic Action : Nuclear particles ( α, β , γ) leaves track if they travel through photographic plates. The track produced in photographic plates becomes observable when the photographic plate is developed. 
iii) Fluorescence : Nuclear radiations especially α- particles produces flashes of light. When they fall upon a screen coated with a fluorescent material, such as Zinc sulphide. These flashes of light are called Scintillations.