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Irradiation of ionic crystals by ionizing rays produce in these crystals different effect such as electronic excitation , ionization and atomic dislocation. After a short time the excited electrons will return to their ground states. Ionization will produce electrons and holes which recom— bine or be trapped.
Trapping of electrons can onby be possible when there have been imperfections in the crystal, or when imperfections have been produced in it during irradiation. These imperfections can be Schottky defects or Frenkel defects or Dislocations or Imperfections of ctystal lattice due to the impurities in crystals. According to the energie of trapped charges and crystal defects we will have a series of absorption bands. If this absorption band is in the wave - length of the visible region the irradiated crystal will be colored. These imperfections are known as «Color Centers» . One of the best known color centers is the F center : it correspond to an electron trapped in an anion vacancy, and the other important color center is M center which is produced by two adjacent F centers.
In this work the absorption spectrum of F and M centers have been investigated in the sodium and potassium chloride single crystals.