11.16.09 - Monday 1 Limits of C14 a Conventional...

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Monday, November 16, 2009 1) Limits of C14 a) Conventional radiocarbon dates range from 150-40,000 years (6-7.5 half lives) b) Now there are two methods of obtaining C14 date i) The Conventional Method which counts beta particle emissions (need 10+ grams of material) ii) Accelerator Mass Spectrometry (1) AMS directly counts the C14 atoms (2) Typically less than one gram Is all that is needed (3) Can date items up to 60,000 years ago (4) More accurate 2) Potassium-Argon Dating a) Principle of radioactive decay b) Used to date volcanic rock c) Counts decay Ar40 to K40 d) Rock dating 4-5 billion years to 100,000 years ago e) Notably used to date early hominid sites in East Africa 3) Thermoluminescence a) Used to directly date baked prehistoric materials such as clay vessels b) Now also being applied to rocks and soil c) Pottery stores energy by trapping electrons when it is fired; this energy slowly dissipates d) Pottery is re-heated in the lab until visible light is emitted e) Often not very accurate (sometimes a 30% error rate)
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