It is a spontaneous emission of radiation from a substance usually an unstable .This process is also known as nuclear or radioactive decay). In the year 1896, Henri Beckerel a French scientist working with phosphorescent materials discovered that a uranium compound placed on a photographic plate which was covered with light black paper causing the plate to be exposed by blackening the pate itself. This implied that some sort of radiation was coming from the uranium which could pass through. The material was radioactive given-off radiation. Other scientists that researched on radioactivity were;v This radioactivity was spontaneous with the radiation being omitted without anything causing it apparently.
Types of radioactivity
From several experiments that have been carried out, it has shown that there are three different types of radioactive emission called;
Alpha particles ; whose emission is easily stopped by paper or thick card
Beta particles ; whose emission is easily stopped by aluminum sheet of different thickness and are negatively charged.
Properties of alpha particles
1) Alpha rays are deflected by magnetic field.
2) The direction of deflection of alpha rays shows that they are positively charged
3) They are relatively massive and so have more momentum and tend to continue in their direction of travel.
4) Alpha particles are absorbed by paper and air and are easily stopped aluminum thin sheet.
5) They have little penetrating power, traveling no more than 5cm. Being large, it is soon stopped by air molecules.
6) They are capable of stripping electrons from their atoms because alpha particles have comparatively large mass and momentum.
Properties of beta particles
1) There are also deflected by magnetic field but in the opposite direction to alpha particles.
2) The direction of deflection of beta rays shows that they are negatively charged with –e .
3) They are relatively light and as a result easily deflected.
4) They are deflected by different amount showing that they have varying speed..
5) They have good penetrating power of about 1 metre.
6) They can penetrate a few millimeter of aluminium.
7) They produce less ionization in air than alpha particles.
Properties of gamma rays
1) They are electromagnetic waves similar to light wave but of very short wavelength.
2) They are electrically uncharged since they are not deflected by a magnetic field.
3) They are highly penetrative compared to the other two types and can only be stopped by thick lead blocks.
4) They have high penetrating power compared to the other two types.
5) They have much less ionization than alpha rays.