When thermoluminescent materials are exposed to ionising radiation some of the electrons which are freed by the ionisation are given sufficient energy to move through the crystal. Many of these electrons are trapped at points of imperfection in the crystal lattice and will remain there if the material is kept at room temperature. If the material is heated at some later time the electrons are released from these traps and give up their surplus energy by emitting visible light.
The amount of light emitted during the heating is measured with a
photomultiplier tube and is proportional to the
radiation dose absorbed by the
material. The TLD is raised to about
C for readout. Further heating to about
C, depending on the material, anneals the TLD so enabling
them to be re-used.