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

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

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

4.  The half-life of 14 C 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 14 C remains to be measured.

5. Two methods for making radiocarbon measurements: 1. Decay count or conventional methods. Beta particles emitted from a sample are counted. Two techniques: a.Gas proportional counting. Carbon converted to a gas (e.g. carbon dioxide). b.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). 2. Accelerator Mass Spectrometry (AMS). The number of carbon atoms and proportions of isotopes are directly counted. (The natural abundance of 14 C is about one 14 C atom per trillion atoms of 12 C.) Short time to count (<30 min) and requires small amount of carbon (1 mg).

6. n + N 14 7 C + p 14 6 neutronproton

7. IntCal09 Calibration Curve Tree RingsMarine Data

8. IntCal09 Calibration Curve

11. 0.1680.832 5329-5377 5450-5586

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

13. Radiocarbon dates from Kettle Lake, North Dakota