Radiation consists of high-energy particles (or waves) emitted from the nucleus of an unstable atom. There are three main types of ionizing radiation emitted by the unstable nuclei of radioactive atoms: alpha, beta and gamma radiation. Substances that contain unstable atoms are described as radioactive. The particles or rays they emit are types of radiation.
All types of radiation become less intense as they move away from the radioactive material. This is because particles or rays are more scattered. The thicker the substance, the more radiation will be absorbed.