The three types of radiation differ in how far they can pass through materials. This is called their penetrating power. Alpha particles are the least penetrating. They travel only a few centimetres in air and are stopped by a sheet of paper or by skin.
Beta particles are more penetrating. They travel about a metre in air and pass through paper, but they are stopped by a few millimetres of aluminium. Gamma rays are the most penetrating. They pass easily through paper and thin aluminium, and are only absorbed by thick lead or by several metres of concrete.
You can show this with a Geiger-Müller tube, a source and aluminium sheets of different thicknesses. First measure the background count rate with no source. Then place the source in front of the tube and add sheets between them. If the count rate is little changed by a sheet of paper but drops to the background level with a few millimetres of aluminium, the source emits beta particles (a source that drops to background with paper alone emits alpha particles). If the count rate stays well above background even with aluminium, the source emits gamma rays.
