Radiation is energy that is transmitted in the form of waves or particles. As a radioactive element decays, it emits alpha or beta radiation, alongside gamma radiation. Alpha and beta radiation consist of energy released in the form of particles that can be stopped by light-weight shielding such as a piece of paper. Gamma radiation is more penetrating and is emitted as waves of energy rather than particles. It can only be stopped by 40 cm of lead, 4 metres of water, or 2 metres of concrete, for example.