The three types of nuclear radiation differ in how far they can pass through materials. 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 travel much further in air and pass through aluminium, and they are only reduced, not completely stopped, by thick lead or several metres of concrete.
You can investigate this with a Geiger-Müller tube. Place a source a fixed distance from the tube and measure the count rate. Then put paper or aluminium plates of increasing thickness between the source and the tube and measure the count rate each time. Subtract the background count measured with no source. The thicker the absorber needed to cut the count rate to background level, the more penetrating the radiation.
