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TL/OSL (Thermoluminescence/Optically Stimulated Luminescence)


Instrument Details
Riso TL/OSL Reader with facility for
  • Optically Stimulated Luminescence (OSL) Dating
  • Blue Light Stimulated Luminescence (BLSL) Dating using Blue light emitting diodes
  • Infra-red Stimulated Luminescence (IRSL) Dating using infrared diodes
  • Thermo Luminescence Dating (TL)

Model/Make:TL-DA-20 Riso National Laboratory, Denmark
 
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)

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.

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

  • The most recent sun bleaching (application in sediment dating).
  • The most recent thermal event (application in archaeology, baking by lava flows, fusion crust of meteorites).
  • Crystallization event (application in travertine's in caves).

 

Material needed for analyses
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.

  • Coarse grain technique: Extracted quartz or feldspar mineral in the size range 90-200m.
  • Fine-grain technique: polyminerals in the size range 4-11m.

 

Pottery Dating

Few pieces of pottery and associated sediment/soil

Type of Analysis

We perform optically Stimulated Luminescence Dating of sand samples using Quartz from Glacial, lacustrine, fluvial, aeolian environments.

Sample Preparation

We prepare sample using established protocols and detailed in An Introduction to Optical Dating - M. J. Aitken - Oxford University Press

A Short Guide regarding how to prepare and how to Send Samples

For each sample should be collected in 8-10 inches long and ~1.5 inch diameter iron light protected cylinder. The sample should be quartz rich sand (90-250 μm). Please note that a few seconds exposure of sediment to sunlight may destroy a sample and therefore care should be taken while collecting and packing the sample. Samples should sent wrapped clearly labelled with their sample code and package should be clearly marked "geological samples with no live material has no commercial value and Do not expose to light". Sample package should contain the latitude longitude, elevation (amsl) and depth of the sample. Please send samples only after contacting the Laboratory and knowing the cost and turn-around time.

 
 
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