TL/OSL (Thermoluminescence/Optically Stimulated Luminescence)

TL/OSL Details
1. Instrument NameRiso TL/OSL Reader with facility for
A) Optically Stimulated Luminescence (OSL) Dating
a) Blue Light Stimulated Luminescence (BLSL) Dating using Blue light emitting diodes
b) Infra-red Stimulated Luminescence (IRSL) Dating using infrared diodes
B) Thermoluminescence Dating (TL)
2. Model/MakeTL-DA-20 Riso National Laboratory, Denmark
3. Principle

Luminescence dating is based on the principle that certain naturally occurring minerals (e.g. quartz, feldspar), which are previously exposed to ionizing radiation, will emit light (luminescence) when they are thermally or optically stimulated. Depending on the type of stimulation source, the phenomenon is termed thermally stimulated luminescence (TL) or optically stimulated luminescence (OSL). The OSL is further divided, based on the colour (wavelength) of the excitation light source, into Blue Light Stimulated Luminescence (BLSL) and Infrared Stimulated Luminescence (IRSL)

4. Description

Luminescence dating requires a proper resetting of the previously acquired (pre depositional) luminescence in the natural minerals into a very low level (natural zeroing event), either by exposure to sun light during pre-depositional transportation (by wind, water etc.) or by a thermal event (pottery making, baking by lava, fusion crest of meteorites), before deposition. Following the natural zeroing event and subsequent burial, the natural minerals begin luminescence acquisition afresh from the ionizing radiation (alpha, beta and gamma) constantly provided by the decay of radioactive elements (U238, Th232, K40, Rb) present in the sediments and also from the cosmic rays.

For calculating the age, the radiation energy stored in the mineral (known as Paleodose) and the annual radioactivity rate (annual dose) from the surrounding sediments has to be calculated.

The time elapsed since sedimentation, i.e.,

Age = Paleodose
        Annual Dose

In practice, the paleodose is calculated by measuring the luminescence intensity in the natural mineral and regenerate the equivalent luminescence intensity by artificial radiation dose from a calibrated beta source attached to the TL/OSL instrument. For calculating the annual dose rate, the elemental concentration of U, Th and K (by XRF or ICP-MS) in the sediment and water content in the sediment are required.

5. Applications

Luminescence dating provides absolute ages and has very important application in Quaternary geology and archaeology.

The following events can be dated by luminescence technique

i) The most recent sun bleaching (application in sediment dating)

ii) The most recent thermal event (application in archaeology, baking by lava flows, fusion crust of meteorites)

iii) Crystallization event (application in travertine’s in caves)

6. Material needed for analysis

Sediment Dating :

Sediment sample should be collected in steel pipe to avoid any exposure to sun light. About 500 gm sample is required. In the lab (under sub-dued red light condition), chemical treatment, sieving and mineral extraction will be carried out. Both coarse- and fine-grained material can be used for dating. a)  Coarse grain technique: Extracted quartz or feldspar mineral in the size range 90-200m

b) Fine-grain technique: polyminerals in the size range 4-11m

Pottery Dating :

Few pieces of pottery and associated sediment/soil