In just the same way that water can be absorbed by a paper kitchen towel, radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air and gamma radiation travels many metres. All types of radiation become less intense the further the distance from the radioactive material, as the particles or rays become more spread out and as they become absorbed in the surrounding matter.
The smaller the amount of absorption of a source's radiation that takes place in a material, the greater the penetrating power of that radiation.
The thicker the substance, the more the radiation is absorbed. The three types of radiation penetrate materials to different degrees.
Alpha radiation is the least penetrating (i.e it has the shortest range). It can be stopped (absorbed) by just a thin sheet of paper, by the skin or by a few centimetres of air.
Beta radiation is more penetrating than alpha: it can pass through paper and the skin. It can be stopped by a thin sheet of aluminium - say 3 mm thick, or a few cm of body tissue.
Gamma radiation is the most penetrating of the three radiations. Even small levels can penetrate air, paper or thin metal. It can easily penetrate body tissue. It requires a few centimetres of lead or about 1 metre of concrete to absorb it. (Higher levels of gamma radiation can only be stopped by many centimetres of lead or many metres of concrete.)
Alpha radiation transfers more of its energy to an absorber than beta or gamma radiation - that's why it comes to a stop more quickly. Gamma radiation transfers the least energy to the abosrbing matierial.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science