“Radiation” in physics refers to particles and waves that travel freely through space (including the atmosphere). There are different types of radiation:  helium nuclei (historically known as alpha radiation), electrons (beta radiation), high energy photons (X-rays and gamma-rays), neutrinos, and neutrons.

• Helium nuclei (alpha radiation) are the heaviest common type of radiation.  The helium nucleus is 8000 times heavier than an electron, and has a +2 charge.  Most helium nuclei could be stopped by a piece of paper, because the strong charge causes it to interact a lot with nearby atoms and lose energy rapidly.
• Neutrons are 2000 times heavier than an electron, and they have no electric charge.  Because they are neutral (0 charge), they can penetrate quite deeply into most materials.  Some types of atoms and molecules, like water, can easily absorb neutrons.  Neutron radiation is the only type that can cause a material to become radioactive.  For this reason water is used to help control nuclear reactions that depend on neutron radiation.
• Electrons have a -1 charge, so they interact with nearby atoms, but not as strongly as helium nuclei.  Thus they penetrate deeper than helium nuclei, but not as deep as neutrons.
• High-energy gamma-ray photons have 0 charge, so they can penetrate very deep into all materials, even several feet into lead and concrete.
• Neutrino radiation very rarely interacts with any kind of material.  A single neutrino has a good chance of being able to travel through the entire Earth without being absorbed or stopped!

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