about 1 mm. in diameter. Large particles will heat through so slowly that the 

 thermoluminescence light is too weak to observe; i.e., the number of electrons 

 released and the number of photons emitted per second is too small to affect the 

 eye. If the material is pulverized too finely, the light comes off in a brief flash. 



In quantitative measurements, the material, in the form of a thin crystal, 

 a thin section of a mineral, or a layer of 100- to 200-mesh size powder, is placed 

 on a metal plate which is heated uniformly at a rate of about 25 to 50 degrees 

 centigrade per minute, taking about 10 minutes to go from room temperature to 

 red heat. As the temperature rises, the crystal glows for a while as electrons are 

 released from traps of one energy level, and then the crystal becomes dark when 

 all the electrons have been removed. As the temperature continues to rise, light 

 may be emitted again temporarily as the electron traps of higher energy levels 

 are emptied. Several alternating periods of light and dark may be produced be- 

 fore the temperature becomes high enough to give a continuous background of 

 red light produced by black-body radiation. At this point, the experiment is dis- 

 continued because any thermoluminescent light emitted would not be detectable 

 against the bright light emitted by the hot plate. 



Figure 10-1 gives a typical thermoluminescence glow curve for a limestone 

 (Escabrosa limestone) . The straight, slanting line is a record of the temperature 

 of the heating plate as measured with a thermocouple, whereas the glow curve 

 with two main peaks is a record of the intensity of thermoluminescence as meas- 

 ured with a photomultiplier tube. These quantities are automatically plotted 

 against time. 



The apparatus used for obtaining these glow curves is shown in Figure 10-3. 

 A silver (or iron) plate is placed over a spiral of nichrome resistance wire which 

 is placed in a box of transite, or in a metal box filled with insulating material. A 

 Variac or variable resistance in series with the heating coil is adjusted so as to 

 gradually raise the temperature at a uniform rate. This is usually done by hand 

 regulation although it is possible to have a slowly rotating, curved arm and slider 

 arranged to turn the Variac at predetermined rates. 



The sample to be measured for thermoluminescence is placed in one of the 

 compartments on the hot plate, and the thermocouple is set in the other, or at- 

 tached directly to the plate. If the sample is in the form of a powder, it is placed 

 in a shallow circular dish with sides about 1 millimeter high. It is leveled off, 

 giving a weight of about 1/3 gram. The light emitted increases with the amount 

 of material heated, but after a depth of a millimeter or less is reached (depend- 

 ing on the fineness of the powder) further additions do not increase the light 

 intensity because the light emitted by the lower material is scattered before it gets 

 out. Moreover, the time lag in heating the surface material becomes appreciable 

 if the material is too thick. In another procedure, about 50 milligrams of powder 

 is placed on a glass plate with a drop of water containing a trace of detergent. 



183 



