CHAPTER IX 



THERMOLUMINESCENCE 



Introduction 



BECAUSE of its early discovery and long usage, the term " thermo- 

 luminescence " has been retained for the emission of light on 

 heating a substance to relative low temperatures— far below the point 

 at which incandescence begins. The word implies that heat energy 

 excites the luminescence, but actually heat merely liberates energy 

 in the form of luminescence, energy from absorbed light, cathode 

 rays, etc., which has previously been stored in the material. The 

 more appropriate term thermostimulation might be applied, to con- 

 form with the modern explanation of thermoluminescence as the 

 liberation by rise in temperature of trapped electrons, whose transi- 

 tions result in the emission of light. 



Thermoluminescence is strikingly exhibited by minerals, which 

 have been previously exposed to light or to cathode rays at liquid 

 air temperatures, where no luminescence is visible, but as the 

 mineral is warmed, a bright light emission occurs, often changing 

 color as the temperature rises. Historically, the term " thermal phos- 

 phor " was first applied to solids which luminesce on slight heating 

 above room temperature, without any reference to previous treat- 

 ment of the material. It is because of this historical connotation 

 that thermoluminescence is rated a chapter heading rather than a 

 section of the chapters on phosphorescence or radioluminescence. 



Seventeenth-Century Observations 



Robert Boyle undoubtedly noted ^ the thermoluminescence of 

 diamonds in October, 1663, when he held a particular one near a 

 hot but non-luminous piece of iron and saw the diamond glow. 

 The discovery of a similar property in the mineral fluorspar, some 

 varieties of which are remarkably thermoluminescent, must be 

 ascribed to Johann Sigismund Elsholtz (1623-1688) of Berlin, physi- 

 cian to the Elector, Friedrich Wilhelm. He published a book, De 

 Phosphoris Observationes, etc., in 1681 of which the first part, " De 

 Phosphoris Quatuor " appeared separately in 1676 and contained 

 the account of fluorspar. When the finely ground mineral was 

 dusted as letters and figures on a plate, slight heating would result 



1 Works of Boyle, ed. by T. Birch, 797, 2nd ed., 1772, and his book on Colours (1664) ; 

 see figure 37. 



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