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Telling Time: The use of radiocarbon ( 14 C) in dating Dr. Eric Grimm, Illinois State Museum

Telling Time: The use of radiocarbon ( 14 C) in dating Dr. Eric Grimm, Illinois State Museum. Carbon Isotopes. 13 C. 12 C. 6 protons 6 neutrons. 14 C. 6 protons 7 neutrons. 6 protons 8 neutrons. β − decay. 14 C. 14 N. –. –. 14 7. 14 6. C. N + e + v e. electron

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Telling Time: The use of radiocarbon ( 14 C) in dating Dr. Eric Grimm, Illinois State Museum

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  1. Telling Time: The use of radiocarbon (14C) in dating Dr. Eric Grimm, Illinois State Museum

  2. Carbon Isotopes 13C 12C 6 protons 6 neutrons 14C 6 protons 7 neutrons 6 protons 8 neutrons

  3. β−decay 14C 14N – – 14 7 14 6 C N + e + ve electron (beta particle) electron antineutrino

  4. The half-life of 14C is 5730 ± 40 years, although this value may not be very accurate. Most radiocarbon dates are reported with the “Libby” half-life 5568 ± 30 years. Calibration corrects for errors in the exact value. • After about 10 half lives, too little 14C remains to be measured.

  5. Two methods for making radiocarbon measurements: • Decay count or conventional methods. Beta particles emitted from a sample are counted. Two techniques: • Gas proportional counting. Carbon converted to a gas (e.g. carbon dioxide). • Liquid scintillation. Carbon is converted to a liquid (e.g. benzene). • Requires a relatively long time to count (24+ hours) and large amount carbon (1 g). • Accelerator Mass Spectrometry (AMS). The number of carbon atoms and proportions of isotopes are directly counted. (The natural abundance of 14C is about one 14C atom per trillion atoms of 12C.) Short time to count (<30 min) and requires small amount of carbon (1 mg).

  6. 14 7 14 6 n+ N C + p neutron proton

  7. IntCal09 Calibration Curve Tree Rings Marine Data

  8. IntCal09 Calibration Curve

  9. IntCal09 Calibration Curve

  10. 0.168 0.832 5450-5586 5329-5377

  11. http://calib.qub.ac.uk/calib/

  12. Radiocarbon dates from Kettle Lake, North Dakota

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