Thermoluminescence and optical absorption studies of highly pure KCl crystals

The existence of colored specimens of normally colorless minerals has been known for much longer than a century and has created excitement among mineralogists, physicists and chemists. Since early 1800, researchers have studied this coloration phenomenon through bleaching experiments, thermoluminescence, and reproduction of the coloration by electric sparks, ultraviolet light, high energy corpuscular radiation and chemical means. The discovery of ionizing radiation (X-rays and radioactivity) and the observation that this coloration could be reproduced by exposure to this radiation enhanced the interest which led to a more systematic and continuous study. Pohl1 did the most fundamental study of coloration on alkali halides and gave the name "color centers" to the "special electronic configuration in solids that gives rise to optical absorption in a normally transparent spectral region."2 The phenomenon involved in the coloration was realized to be important if one is to understand the electronic processes in solids