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Gamma rays (denoted as ?) are a form of electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions, such as electron-positron annihilation or radioactive decay. Gamma rays are generally characterized as electromagnetic radiation having the highest frequency and energy, and also the shortest wavelength (below about 10 picometers), within the electromagnetic spectrum. Gamma rays consist of high energy photons with energies above about 100 keV. Gamma rays were discovered by Paul Villard, a French chemist and physicist, in 1900, while studying uranium. Hard X-rays overlap the range of "long"-wavelength (lower energy) gamma rays. The distinction between the two terms, however, depends on the source of the radiation, not its wavelength; X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei. Due to their high energy content, gamma rays can cause serious damage when absorbed by living cells. Shielding gamma rays requires large amounts of mass. They are better absorbed by materials with high atomic numbers and high density. The higher the energy of the gamma rays, the thicker the shielding required. Materials for shielding gamma rays are typically measured by the thickness required to reduce the intensity of the gamma rays by one half (the half value layer or HVL). For example, gamma rays that require 1 cm (0.4 inches) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 6&_160;cm (2½&_160;inches) of concrete or 9&_160;cm (3½&_160;inches) of packed soil.
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