emit light on heating even without exposure to gamma radiation. Practically all 

 granites, and particularly the feldspars, exhibit natural bluish thermolumines- 

 cence. Most granites contain more than 1 part per million of uranium and about 

 3 times as much thorium. This is sufficient to dislodge electrons, and they ac- 

 cumulate in traps and imperfections in the crystal lattice. Shales and lignite and 

 many other rocks have radioactivity, but the crystal structure is not simple 

 enough, hard enough, and transparent enough to show thermoluminescence. 



RADIOACTIVITY OF ROCKS An extensive survey of the alpha radioactivity 



of many rocks and deposits has been made 

 (Ockermann and Daniels, 1952) by using a specially designed scintillometer 

 sensitive only to alpha particles. Eighty-two limestones gave an average radio- 

 activity of 0.4 alpha particle (ranging from 0.05 to 1.7 alpha particle) emitted 

 from a thick layer of powder per square centimeter per hour. Forty different 

 granites gave an average of 3.2, ranging from 0.2 to 9.6 alpha counts per square 

 centimeter per hour. Bentonites, residues from Wisconsin well waters and ash 

 from plants gave still higher alpha particle counts. Such materials have the po- 

 tential ability to produce thermoluminescence if the crystal lattices are suitable. 

 Certain old highly radioactive minerals containing rare earths have been 

 subjected to such intense radiation damage that the crystal lattice has been so 

 distorted that they show no diffraction patterns when a beam of X-rays is passed 

 through them. Such minerals are called metamict crystals, and they may store 

 up to 100 calories of energy per gram (Morehead and Daniels, 1952; Kurath, 

 1957), which is released as heat when the temperature is raised. 



AMOUNT OF RfiDlffTION '-* 



TEMPERATURE 



FiGURE 10-10. Effect of intense gamma irradiation on thermoluminescence 

 of lithium fluoride. 



dow curves 



194 



