Radiation is energy that travels in the form of waves (electromagnetic radiation) or high-speed particles (particulate radiation). Particulate radiation happens when an unstable (or radioactive) atom disintegrates. Electromagnetic (EM) radiation, on the other hand, has no mass and travels in waves. EM radiation can range from very low energy to very high energy, and we call this span the electromagnetic spectrum. Within the EM spectrum, there are two types of radiation -- ionizing and non-ionizing. Radiation is often categorized as either ionizing or non-ionizing depending on the energy of the radiated particles. Ionizing radiation carries more than 10 eV, which is enough to ionize atoms and molecules and break chemical bonds. This is an important distinction due to the large difference in harmfulness to living organisms. A common source of ionizing radiation is radioactive materials that emit α, β, or γ radiation, consisting of helium nuclei, electrons or positrons, and photons, respectively. Other sources include X-rays from medical radiography examinations and muons, mesons, positrons, neutrons and other particles that constitute the secondary cosmic rays that are produced after primary cosmic rays interact with Earth's atmosphere. Gamma rays, X-rays and the higher energy range of ultraviolet light constitute the ionizing part of the electromagnetic spectrum.
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