What is it about the density of a substance that makes it more resistant to allowing radioactivity to pass through it? Since most of the atom is empty space?
There are also good illustrations and explanations on http://www.awe.co.uk/main_site/scientific_and_technical/Factsheets/URR/ [Broken]Pat Rowe said:Radioactive elements decay to other elements because their nuclei are not stable, and in the process give off particles or energy. Alpha particles are nuclei of helium atoms, and because they are relatively large, they do not easily go through light materials such as clothing or paper. They easily pick up electrons to become helium. Beta particles are electrons that come from the nucleus (this is a little complicated, since you probably know that only protons and neutrons should be in the nucleus--if you think of a neutron as a proton plus and electron, then the electron comes from there). Beta particles penetrate light materials, but not too far into denser materials like water. Gamma rays are a real problem, however. They are high-frequency electromagnetic radiation, pure energy which can penetrate most materials, including human bodies. Molecules and cells struck by gamma rays are damaged. Dense materials, such as lead shielding, stop gamma rays. People who are exposed to harmful radioactive materials may suffer radiation poisoning and death.