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 very large distances.
All types of radiation become less intense as they travel further away from the radioactive material this is because the particles or rays become more spread out.
The thicker the substance, the more the radiation is absorbed. The three types of radiation penetrate materials in different ways.
Alpha radiation is the least penetrating. It can be stopped (or absorbed) by a human hand.
Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.
Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead or many metres of concrete.