The two most common types of TLDs are
calcium fluoride and
lithium fluoride, with one or more impurities to produce trap states for energetic electrons. The former is used to record
gamma exposure, the latter for gamma and
neutron exposure (indirectly, using the Li-6 (n,alpha)
nuclear reaction; for this reason, LiF dosimeters may be enriched in lithium-6 to enhance this effect or enriched in lithium-7 to reduce it). Other types include
beryllium oxide, and
calcium sulfate doped with
thulium. As the radiation interacts with the crystal it causes
electrons in the crystal's atoms to jump to higher energy states, where they stay trapped due to
intentionally introduced impurities (usually
manganese or
magnesium) in the crystal, until heated. Heating the crystal causes the electrons to drop back to their ground state, releasing a
photon of energy equal to the energy difference between the trap state and the ground state. ==References==