non-conductor or a semi-conductor for electricity, and it must be exposed to 

 ionizing radiations which release electrons within the crystal. These high-en- 

 ergy ionizing radiations may be hard or soft X-rays, gamma rays, alpha rays, 

 or beta rays. The electrons move around within the crystal, and some of them 

 fall into traps from which they are driven out later when the temperature of 

 the crystal is raised high enough to supply sufficient kinetic energy. There are 

 several different kinds of electron traps: (1) imperfections and vacancies in 

 the crystal lattice produced at the time the crystal is formed, or created later by 

 mechanical pressure or thermal treatment; (2) statistical imperfections due to 

 kinetic motions which increase at higher temperatures; (3) distortions produced 

 by impurity ions of larger or smaller size than those which comprise the crystal 

 lattice; or (4) ion dislocations or holes produced by continued exposure to radio- 

 active bombardment. 



One of the commonest types of traps is the so-called "F-center" due to a 

 missing negative ion, such as a chloride ion in a sodium chloride crystal. An 

 electron trapped in such a vacancy absorbs visible light and makes the crystal 

 colored. In lithium fluoride, the ions are very small, and the frequency of the 

 light absorption lies in the ultraviolet. 



An example of the effect of foreign ions of different size is given by the ad- 

 dition of silver chloride to sodium chloride. When sodium chloride is fused and 

 mixed with 1 mole percent of silver chloride and recrystallized, the gamma-ray- 

 induced thermoluminescence is a hundred times more intense than the original 

 sodium chloride. A crystal prepared by cooling a fused mixture of sodium 

 chloride and potassium bromide and exposed to gamma rays gives thermolum- 

 inescence that is greater than that of the crystals of either pure salt alone. 



After an electron becomes trapped in one of these several types of holes, it 

 remains there until the temperature is raised sufficiently high to supply the 

 kinetic energy necessary for its release. When an electron is released and it 

 falls to another trap, or combines with an ion, at a lower energy level, it emits 

 light. The greater the number of electrons released at one time, the greater the 

 intensity of light, and the greater the difference in energy level involved, the 

 shorter is the wave length of the light emitted ; i.e., it is more blue or ultraviolet. 

 The color of the emitted light reveals information concerning the nature of the 

 traps and the impurities in the crystal, but the exact, quantitative relations have 

 not been well-worked out yet. 



EXPERIMENTAL Qualitative observations of thermolumines- 



cence can be made easily in the dark by 

 simply dropping finely granulated crystals onto a plate heated just below red 

 heat. A frying pan on an electric hot plate or kitchen stove can be used in a 

 darkened room. The crystals or minerals should be crushed to give particles 



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