Alpha particles are the heaviest and most highly charged of the nuclear radiations. Without additional energy input, these characteristics make alpha particles less penetrating than beta particles and gamma rays. Alpha particles cannot travel more than four to seven inches (10 to 18 cm) in air and are completely stopped by an ordinary sheet of paper.

Any material between the radiation source and the receptor is called shielding because it absorbs some of the radiation before it can penetrate the receptor. Different types of shielding are effective against different types of radiation. As this lesson further describes alpha, beta, and gamma radiation, it also notes what kinds of shielding is suitable for each.