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Thermoluminescence dating:Thick source alpha counting

Wang Weida;Xia Junding;Research Laboratory for Conservation and Archaeology,Shanghai Museum;  
The thick source alpha counting is a important technique for the measurement of annual alpha dose in thermoluminescence dating.This project is studied by the Research Laboratory for Conservation and Archaeology,Shanghai Museum and the alpha counting facility is first made in China by Shanghai Institute of Nuclear Research,Academia Sinica.A major advantage of using alpha counting for the determination of alpha contribution to the annual dose is that,in most cases the count rate be put directly into the age equation i.e.,it is not necessary to go through calculation which 'invole U and Th concentration and dose-rate conversion factor.When alpha counting is used to assess the beta and gamma contribution from the uranium and thorium decay chains then either equal U and Th activities are assumed or seme attempt to separate the components is made using the "pairs" technique,but a long counting times are required for the measurement of "pair-rate".For the "typical" pottery composition of 41 Bq/kg and pence pair rate is only about 0.2 pairs/ks,that is,takes 10 weeks to accumulate 1000 pairs,implying a atalistical uncertainty of ±3%even then.Thus the counting of pairs is hardly a practical proposition for determining thorium content routinely though it is useful for special samples.However,it is preferable to use direct thermolumimescence dosimetry for the annual beta dose particulary as this obviates the need for determination of the potassium content by chemical analysis.
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