Long intense exposure to radioactivity can produce new imperfections which 

 provide new electron traps of different energies giving rise to new peaks in the 

 thermoluminescence glow curves. Figure 10-10 illustrates the alterations that 

 can be introduced into glow curves. Lithium fluoride gives a single peak when 

 exposed to 500 roentgens of cobalt 60 . An exposure of 10,000 roentgens increases 

 the intensity of thermoluminescent light and starts a new peak corresponding to 

 a new electron trap. Exposure to 25,000 roentgens creates more of the high- 

 temperature traps, and 40,000 roentgens creates still more. When the radiation 

 damage has become quite extensive, the thermoluminescence at the original peak 

 decreases with further exposure. 



STRATIGRAPHY Although the thermoluminescence glow curves 



vary greatly with impurities, conditions of 

 crystallization and recrystallization, pressure, radioactivity, and age, they are 

 reasonably characteristic of a particular sample of rock. If two rocks exhibit 

 the same glow curves, they are likely to have had similar histories, including the 

 deposition from water of about the same chemical composition. If the same se- 

 quence of thermoluminescence glow-curve patterns is found in two different lo- 

 cations, the geological history of the layers is probably the same. 



Saunders (1953) was the first to apply thermoluminescence measurements 

 to stratigraphy. His data are shown in Figures 10-11 and 10-12, where glow 

 curves were obtained from a large quarry of Niagara limestone near Waukesha, 

 Wisconsin. Figure 10-11 shows that in a single exposed thin bed extending for 

 nearly half a mile, the glow curves taken at frequent lateral intervals are all the 

 same. However, as shown in Figure 10-12, the glow curves vary greatly in the 

 different layers as one takes samples of different layers over a vertical height of 

 25 feet. The conditions of chemical environment at the time of deposition are 

 all the same on the lateral bed, but they differ considerably with the time of depo- 

 sition, as indicated by the vertical sampling of the different layers. Long-distance 

 correlation is much more uncertain, but Saunders found thermoluminescence 

 evidence for the correlation of limestones from three different areas — the Pa- 

 hasapa limestone of the Black Hills, South Dakota ; the Madison limestone of the 

 Wind River Mountains, Wyoming; and the Redwall limestone of the Grand 

 Canyon, Arizona. All these limestones gave 3 definite peaks in the glow 

 curves, but the peak heights varied to give 4 different types characterized by the 

 relative heights of the peaks. The sequence of these 4 types in successive ver- 

 tical layers was found to be the same in the 3 different areas' The probable 

 correlation of the limestones in these areas had already been noted from geo- 

 logical evidence. 



A summary of glow curves at different depths at a location in Iowa is shown 

 in Figure 10-13. It is clear that the thermoluminescence patterns vary greatly 



195 



