the low-temperature thermoluminescence can never be observed when the meas- 

 urements are started at room temperature because all the electrons in low-energy 

 traps are quickly driven out at room temperature. If the irradiation takes place 

 at room temperature, the light corresponding to low-temperature peaks is emitted 

 as fluorescence while the radiation is being carried out. Rocks and minerals, ex- 

 cited by traces of uranium, will have had all the low-temperature peaks of natural 

 thermoluminescence drained out at earth temperature. 



By increasing the speed of heating, one can obtain still greater sensitivity 

 for thermoluminescence because the intensity of light is determined by the num- 

 ber of electrons released and photons emitted per second. If all the electrons are 

 released in a few seconds, the light will be much brighter for a short time than 

 if it released slowly over a 10-minute period. A new apparatus has been de- 

 veloped in this laboratory in which the heating unit is simply a thin strip of 

 nichrome sheet, which is heated very quickly by applying a low-voltage current 

 at high amperage. The recorder, Model 127, which is manufactured by the San- 

 born Company, makes a line very quickly with a heated point close to, but not 

 touching, a specially prepared paper. A 10-inch recording is made in 10 seconds 

 with this apparatus. The temperature is not recorded, but rough calibrations are 

 made with materials which change colors at definite temperatures. A thermo- 

 luminescence curve obtained with this apparatus is shown in Figure 10-4. 



For some geological work, the natural thermoluminescence of the material 

 is desired. By exposing the material to intense radiation in the laboratory, it is 

 possible to obtain much greater intensity of the thermoluminescence peaks and 

 to introduce additional peaks, particularly the peaks near room temperature 

 which have drained out at earth temperatures. This irradiation can be accom- 

 plished with X-rays or with gamma rays. A convenient and inexpensive source 

 of gamma radiation may be made from cobalt metal which is then exposed to 

 neutrons in the nuclear reactor at Oak Ridge or in other nuclear reactors of the 

 Atomic Energy Commission. A small hollow cylinder of cobalt 60 kept in alum- 

 inum cylinders can give a uniform, high gamma radiation of several thousand 

 roentgens inside (Saunders, and others, 1953). The source is kept in a box of 

 lead and concrete set into a basement floor, and the samples to be irradiated are 

 packed in a small aluminum frame and lowered into the inside of the cobalt cyl- 

 inder with a pole and string and a mirror inclined to the floor at an angle of 45 

 degrees. The aluminum frame contains several samples of crystal plates or rock 

 sections, or it contains several gelatin capsules containing samples of powder. 



INORGANIC CRYSTALS Most of the alkali halides give excellent 



thermoluminescence glow curves. Schematic 

 curves have been determined for the Li, Na, K, Rb, and Cs fluorides, chlorides, 

 bromides, and iodides (Hecklesberg and Daniels, 1957). The larger anions give 



186 



