Thermoluminescence

The emission of light when certain solids are warmed, generally to a temperature lower than that needed to provoke visible incandescence. Two characteristics of thermoluminescence distinguish it from incandescence. First, the intensity of thermoluminescent emission does not remain constant at constant temperature, but decreases with time and eventually ceases altogether. Second, the spectrum of the thermoluminescence is highly dependent on the composition of the material and is only slightly affected by the temperature of heating. If a thermoluminescent material emits both thermoluminescence and incandescent light at some temperature of observation, the transient light emission is the thermoluminescence and the remaining steady-state emission is the incandescence. The transient nature of the thermoluminescent emission suggests that heating merely triggers the release of stored energy previously imparted to the material. Supporting this interpretation is the fact that after the thermoluminescence has been reduced to zero by heating, the sample can be made thermoluminescent again by exposure to one of a number of energy sources: x-rays and gamma rays, electron beams, nuclear particles, ultraviolet light, and, in some cases, even short-wave visible light (violet and blue). A thermoluminescent material, therefore, has a memory of its earlier exposure to an energizing source, and this memory is utilized in a number of applications. Many natural minerals are thermoluminescent, but the most efficient materials of this type are specially formulated synthetic solids (phosphors). See Luminescence

In addition to special sites capable of emitting light (luminescent centers), thermoluminescent phosphors have centers that can trap electrons or holes when these are produced in the solid by ionizing radiation. The luminescent center itself is often the hole trap, and the electron is trapped at another center, although the reverse situation can also occur. In the former case, if the temperature is low and the energy required to release an electron from a trap (the trap depth) is large, electrons will remain trapped and no luminescence will occur. If, however, the temperature of the phosphor is progressively raised, electrons will receive increasing amounts of thermal energy and will have an increased probability of escape from the traps. Freed electrons may then go over to luminescent centers and recombine with holes trapped at or near these centers. The energy liberated by the recombination can excite the luminescent centers, causing them to emit light. See Hole states in solids, Traps in solids

Radiation dosimeters based on thermoluminescence are widely used for monitoring integrated radiation exposure in nuclear power plants, hospitals, and other installations where high-energy radiations are likely to be encountered. The key elements of the dosimeters, thermoluminescent phosphors with deep traps, can store some of the energy absorbed from these radiations for very long periods of time at normal temperatures and release it as luminescence on demand when appropriately heated. The brightness (or light sum) of the luminescence is a measure of the original radiation dose.

Thermoluminescence

luminescence that arises upon the heating of a substance that was previously excited by light or by hard radiation. Thermoluminescence can be observed in many crystal phosphors and minerals and in several glasses and organic phosphors.

The mechanism of thermoluminescence involves recombination. Upon heating, trapped electrons are liberated, and they re-combine with luminescent centers that were ionized upon excitation, producing radiation. Thermoluminescence is used in mineralogy and in the study of the energy spectrum of electron traps in solids. The luminescent centers of minerals are various structural flaws, which may be caused by the conditions under which the minerals were formed or by ionizing radiation or other external effects. The thermoluminescence spectrum of minerals and the nature of their de-excitation provide information on the nature and energy parameters and the age and radiation and thermal history of rocks.

The most intense and complex thermoluminescence is found in minerals containing admixtures of rare earths, such as fluorite, apatite and anhydrite, and in many silicates (feldspar, quartz, and sodalite), carbonates, and sulfates.

REFERENCES

Marfunin, A. S. Spektroskopiia, liuminestsentsiia i radiatsionnye tsentry v mineralakh. Moscow, 1975.
Thermoluminescence of Geological Minerals. London-New York, 1968.

A. N. TARASHCHAN