Alpha particles are also known as alpha rays, alpha radiation or alpha particles energy. They are effectively helium nuclei, containing two protons and two neutrons, and are produced naturally by the decay of heavier elements such as uranium or radium. They are high-energy particles that can cause ionization in the materials they pass through. Alpha rays are stopped by a few centimeters of air, a piece of paper or clothing, which makes them less dangerous than other forms of radiation in some contexts. Other synonyms for alpha particles include alpha rays, alpha radiation, alpha decay products, and alpha emitters.