Figure 10-1. Typical glow curve (Escabrosa limestone) . 



luminescence. The light emitted is increased greatly, and additional peaks are 

 produced in the glow curves by further exposure to X-rays. Tests on small com- 

 mercial mineral collections indicated that over one half of the minerals exhibit 

 natural thermoluminescence. Figure 10-2 indicates the extent of natural thermo- 

 luminescence found after examining hundreds of samples. The intensity ranges 

 are arbitrary and qualitative, number 1 being barely detectable and number 4 

 giving sufficient light, temporarily, by which to read a newspaper, if gram quan- 

 tities are used. 



Many minerals and rocks, and several crystals, particularly the alkali 

 halides, grown in the laboratory or produced commercially, show thermolum- 

 inescence after exposure to X-rays or gamma rays. 



Thermoluminescence is an old phenomenon, and there are early records of 

 the emission of light on heating certain minerals. The emission of light by 

 glasses and minerals, previously exposed to X-rays or radioactivity, was observed 

 soon after these radiations had been discovered. The research now going on in 

 phosphors for fluorescent lighting and similar developments has supplied a back- 

 ground of knowledge which is helpful in studying thermoluminescence. The 

 presence of minute traces of impurities is important both in phosphorescence and 

 in thermoluminescence. 



THEORY To exhibit thermoluminescence, a substance 



must have an orderly structure such as exists 

 in crystals or a semi-orderly structure as in glass. Furthermore, it must be a 



180 



