A system to date the age of rocks.
This is a dating technique used in materials which contain small amounts of radioactive substances.
The radioactivity causes ionization of free electrons, and some of the electrons produced become trapped within holes and crystal defects.
The sample to be tested is heated to around 500 degrees Celsius to release these trapped electrons in the form of visible light. The amount of this thermoluminescence per rad of ionizing radiation provides an estimate of the latest time the sample was heated to 500 C.
For the most part, this technique is used to date stalagtites, stalagmites and ancient pottery.