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25-Jan-2018 09:23

TL Age = Palaeodose (P) _____________ (ARD) TL samples may be collected in open ended or opaque PVC tubes approximately 12cm in length and 6cm in diameter.

Whilst it advisable to protect the sample from direct sunlight there is no need to sample at night and the orientation of the specimen is not important.

Aliquots of both the bleached and the unbleached quartz are deposited onto a series of aluminium planchettes and a number of these are incrementally irradiated using a calibrated90Sr plaque source.

Each planchette, complete with its sample aliquot, is heated to 500 deg C at a controlled rate and in an oxygen free atmosphere.

The presence of rubidium and cosmic radiation generally play a lesser but contributory roll, and the total radiation dose delivered to the TL phosphor is modified by the presence of water.

The period since deposition is therefore measured by determining the total amount of stored TL energy, the palaeodose (P), and the rate at which this energy is acquired, the annual radiation dose (ARD).

Because certain of the daughters within the uranium and thorium decay chains are gaseous it is necessary to wait a period of three weeks before introducing the cell into the counter.

This period allows the decay chains to be re-established.

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The sample collected should be taken from the centre of a homogenous sphere of 30cm radius.

This process requires that the sample be crushed to an extremely fine grain size such that all of the short range alpha particles may be detected.

The crushed sample is placed in immediate contact with a scintillation screen which is sealed in an alpha counting cell which in turn is positioned on a photo multiplier tube assembly.

Care must also be taken to ensure that the sample is taken from an undisturbed site and has not been subject to re-exposure by bioturbation.

Upon arrival in the laboratory TL samples normally consist of two parts: the sample to be dated and a modern analogue sample for the surface residual correction.

If a field gamma spectrometer is available measurements should be made in the field by placing the gamma probe into the hole from which the sample has been removed.