ATOMIC & NUCLEAR PHYSICS

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irradiation

1. Exposure of a sample or object to radiation, usually for a definite purpose. Biological and pharmaceutical materials may have their properties altered by exposure to ultraviolet radiation; X-rays are widely used in medicine and industry. Materials may be irradiated directly with radiation of a given type and energy by placing them in a particle accelerator or nuclear reactor, but it is often more practical to use the radiation from manufactured radioactive isotopes to change their physical and chemical properties as required. Neutrons and gamma rays are used to sterilize foodstuffs and control the reproduction of insect pests.
   
   The outer layers of a supernova are irradiated by neutrons produced as the core collapses to form a neutron star; these neutrons react with atomic nuclei to form heavier elements.
   
   See also ionizing radiation.
   
      • ATOMIC AND NUCLEAR PHYSICS



2. An optical effect of contrast that makes bright objects viewed against a dark background appear to be larger than they really are.