The amplitude of certain thermoluminescence light emission peaks is closely related to the number of crystallographic peaks is closely related to the number of crystallographic dislocations in crystals. On an empirical basis, this has been repeatedly demonstrated for a variety of rock types in and near faults, fold structures and meteorite craters as well as from material from the vicinity of underground nuclear explosions and artificially deformed samples. Since what are being measured are the relative concentrations of crystallographic defects, it can be anticipated that the ratio of thermoluminescence to strain is not a straight line relationship. However, various studies in Canada, the United States and France, during the past ten years have indicated that the rock itself can be used as a natural strain gauge, for at least semiquantitative measurements.
Analogous to solid-state studies in metallurgy, the principal variables which will change the thermoluminescence emission peak heights are: the type and composition of the crystal; impurity elements; grain growth; heat treatment; and amount and rate of deformation. Work done in the writer's laboratory since 1964 permit some generalizations on these variables. Rock types containing quartz, feldspar and/or carbonates have been found to be the most satisfactory to work with, but measureable differences in the amount of strain have been obtained from very unsatisfactory materials, such as serpentine, using sensitive equipment. Variations in TL due to impurity or "trace" elements are negligible compared to other variables. Grain growth tends to eliminate TL by the elimination of crystal imperfections, while high temperatures such as would occur near igneous contacts tend to increase TL by the creation of various types of imperfections. TL due to strain tends to increase in rough proportion to the amount of "work hardening" and cyclical loading may raise the emission to very high levels. Subsequent plastic deformation or failure tend to reduce the emission. Analysis of the emission curves often serves to distinguish which variable has been changed.
In its simplest form, the instrumentation for measuring thermoluminescence consists of a photomultiplier tube and a method of controlled heating of the sample to temperatures near 400 degrees C. More elaborate instrumentation may include facilities for artificial irradiation, measurement at very low temperatures or measurement of the wave lengths of emission.
