Charged particles, from the spontaneous fission of ²³⁸U in minerals and in natural and synthetic glasses, leave a trail of damage (fission tracks, radiation tracks) as they travel through a solid medium. This is the result of the transfer of energy from the particles to the atoms of the medium. These tracks, suitably enlarged by etching, can be seen in some minerals by using a petrological microscope (see polarizing microscope). The number of tracks per unit area is a function of the age of the specimen and its uranium concentration, provided that it cooled rapidly on formation and has not been reheated at a later date. The uranium concentration can be measured by counting tracks produced by fission of ²³⁵U caused by irradiation of the specimen with thermal neutrons in a nuclear reactor. Fission-track dates can, by this method, be obtained for minerals such as micas, apatite, sphene, epidote, and zircon. The dates obtained are ‘cooling ages’ and indicate the time elapsed since the temperature dropped below the 50% track retention value; the tracks are known to fade by annealing of solids at elevated temperatures. The method can also be used to date tektites, volcanic glass, and some archaeological objects.