In the most general sense, TL (thermoluminescence), OSL (Optically stimulated luminescence), techniques measure the amount of energy stored within the electronic lattice of insulators.
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
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. 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.
Basis of the TL date measurement
Luminescence dating of sediment relies upon the fact that the geological luminescence signal of the sediment is reduced to a near-zero residual due to exposure to daylight during weathering and transport (see Aitken, 1985, 1998). 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:
Assessment of the radiation dose rate
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. The amount of uranium and thorium are measured with the help of Alpha counters using ZnS coated screens whereas potassium is measured by Gamma counters or by using chemical analytical methods.
Evaluation of the palaeodose
The palaeodose is evaluated by comparing the natural luminescence intensity of the sample with the increase of luminescence signal output induced by known amounts of additional radiation. By extrapolating the growth curve until it intersects the initial natural intensity of the sample, the dose accumulated since the initializing event can be found. Multiple aliquot (disc containing fraction of sample), single aliquot are the two major methods that are used commonly. Additive dose or regeneration dose are the two different protocol used in conjunction with multiple or single aliquot methods (see Aitken, 1998).
The main categories of materials, which may be dated by Luminescence, are archaeological pottery, riverine, aeolian, glacial and lacustrine sediments. Fault gouges, and stalagmites are other important materials that can be dated with this technique. Archaeological potteries, while they are also datable by luminescence, are not often useful in archaeological applications, since their typology is usually a more precise indicator of age. However, a very similar application is the dating of burnt daub, where the age might not be otherwise known.
Derivatives of Luminescence Dating
Luminescence is a property by virtue of which any material emits light in response to some external stimulation and depending upon the stimulation this method is broadly has two derivatives: 1. Thermoluminescence (TL) where the material is stimulated thermally. 2. Optically stimulated Luminescence (OSL) where the material is stimulated with light source. The type light source to be used depends upon the mineral used for dating. Blue or Green light is used when dealing with quartz and for feldspar infrared light is commonly used.
The samples are pretreated with 10% HCl and 30% H2O2 to remove carbonates and organics and sieved to obtain 105-150mm size. Density separation using Na-Polytungstate (r=2.58 g/cm3) is carried out to separate quartz and feldspar minerals. The quartz fraction is further etched with 40% HF for 80 min followed by 12N HCl for 30 min to remove the alpha skin. 4-11mm grain size was used in silt dominated samples are samples. In this case the samples after treatment by HCl and H2O2 are deflocculated in 0.1N sodium oxalate and washed and suspended in acetone for Stokes separation of the 4-11mm fine silt fraction. Coarse-grained sediments were mounted on stainless steel discs with help of Silkospray'. Blue-Green OSL on quartz mineral extracts using a filtered halogen lamp and IRSL studies on fine-grained sediment using infrared diodes is generally carried out. Detection optics comprised optical filters like Hoya U340 and Schott BG-39 filters for quartz and Corning 7-59 and Schott BG-39 filters for feldspar coupled to EMI 9635 QA Photomultiplier tube is used. Measurements are commonly done on Riso TL-DA-15 reader. b-Irradiation is done using 25 mCi 90Sr/90Y source. Alpha irradiation is done using Americium-241. The digital data for age computation is done using the special software like Analyst and Grun.
Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London, 359 pp.
Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press. London, 267 pp.