Thermoluminescence 377 



In present terminology, the storage of energy in solid phosphors 

 is expressed in terms of electron traps, F-centers, phonons, positive 

 holes, polarons, etc. The history of these terms is beyond the scope 

 of this book, but they have been coined largely from a consideration 

 of thermoluminescence resulting from the action of streams of elec- 

 trons or radiation other than light. 



Some of the most beautiful luminescent effects can be obtained 

 from artificial phosphors, from minerals and other inorganic ma- 

 terials exposed to light, cathode rays, X-rays, or gamma rays at liquid 

 air temperatures and then warmed. Dewar (1894) appears to have 

 been the first to try this experiment, using calcium sulphide and 

 light. With certain substances, as the temperature rises, a succession 

 of thermoluminescences of various colors appears, giving a spectrum 

 in time rather than in space. 



This display might seem to have no practical value, but in recent 

 years thermoluminescence, even without the color display, is now 

 used ^° in a number of ways— to identify minerals, to detect dif- 

 ferences in surface structure of catalysts, to assess radiation damage, 

 to estimate geological age, possibly in the future to determine the 

 dates at which rocks and ceramic material were heated to high 

 temperatures. 



Candoluminescence 



No history of luminescence would be complete without mention 

 of certain phenomena which occur as a result of heating, which 

 appear different from incandescence, and have been given the name 

 of candoluminescence. It has been well known that lime and certain 

 other oxides become exceedingly bright when heated to a high tem- 

 perature. The same is true with such minerals as gadolinite, con- 

 taining rare earths, studied by T. Scheerer (1840) and H. Rose 



(1858) . On heating, at a certain temperature these earths rather 

 suddenly emit an intense light connected with a molecular change, 

 far brighter than would correspond to their temperature. Accord- 

 ing to E. L. Nichols and B. W. Snow (1892) , zinc oxide, as well as 

 lime (Nichols and M. L. Crehore, 1894) " possesses the remarkable 

 property, long since known to exist in case of other oxides, of lumi- 

 nescence by heat." The phenomenon was later called the " blue 

 glow," or candoluminescence, and regarded as fundamentally due 

 to fluorescence excited by the incandescent radiation. C. Swinton 



(1899) has shown that certain rare earths (thoria) become brilliant 

 on bombardment with cathode rays, while others (ceria) do not, 

 although both earths are equally brilliant when heated to the same 

 temperature. Candoluminescence is now regarded as selective ther- 

 mal radiation rather than luminescence. 



