Heat or light can eject charges from traps T back into the conduction band.
When an electron recombines with a luminescence center L, a photon is emitted.
This phenomenon forms the basis of thermoluminescence and optical dating.
The amount of light emitted during luminescence measurement of the sediment depends upon the total radiation dose to which the crystalline material has been exposed during its burial and is called as natural signal.
This measured signal provides a measure to the palaeodose received in the intervening period of burial.
The amount of the accumulated palaeodose is proportional to both the rate of radiation absorption by the material, and the time that has elapsed between the initializing event and the luminescence read-out.
The following simple equation relates these quantities: The total radiation dose which material receives annually is called as annual radiation dose.
The rate of radiation dose can be assessed to the desired accuracy.
Ionising radiation exists in the form of alpha, beta and gamma rays, and originates from radioactive decays of the naturally occurring elements, uranium, thorium and potassium present in the sediments.Cosmic rays also contribute to the total level of radiation.Trapping: Upon exposure to nuclear radiation, some bound electrons of the atoms making up a mineral's lattice are detached from their parent nuclei and become freely mobile: they are said to enter the conduction band.Structural defects in the lattice (vacancies, interstitial atoms, and substitutional impurities) create localized charge deficits, which act as traps T for the conduction electrons.Most electrons recombine or are briefly trapped in very shallow traps, but a few are trapped at deep traps and remain there over geological time-scales (1-1000 Ma).The now charge-deficient ion that contributed the trapped charge becomes a luminescence center L Recombination: Electrons trapped in deep traps T do not readily recombine unless induced to do so by natural "clock-resetting events", or under strictly controlled laboratory conditions.