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Alpha radiation are particles sent out by radioactive nuclei and each particle consists of two protons and two neutrons bound together. Alpha particles carry a positive charge and are the nuclei of helium atoms. Many radioactive nuclei that are more massive than lead will emit alpha particles during their decay. However, unlike beta and gamma radiation, alpha radiation can be blocked by the human skin. The alpha particles are emitted by radioactive nuclei such as uranium or radium in a process known as alpha decay.