Telling Time: The use of radiocarbon (14C)
in dating
Dr. Eric Grimm, Illinois State Museum
12C 13C
14C
Carbon Isotopes
6 protons6 neutrons
6 protons7 neutrons6 protons
8 neutrons
14C14N
N + e + ve147C14
6electron
(beta particle)electron
antineutrino
––
β− decay
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.
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 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).
n + N 147 C + p 14
6
neutron proton
IntCal09 Calibration Curve
Tree Rings Marine Data
IntCal09 Calibration Curve
IntCal09 Calibration Curve
0.168 0.832
5329-5377 5450-5586
Lab Nr Depth (cm) 14C age Material datedCAMS-32009 1507–1512 1960 ± 50 charcoal, Schoenoplectus seed, Chenopodium seedCAMS-57143 1688–1690 2580 ± 40 charcoalCAMS-105788 2058.5–2059.5 4200 ± 45 charcoalCAMS-116255 2538.5–2539.5 6900 ± 35 charcoalCAMS-113585 2934.5–2935.5 9530 ± 35 charcoal
Radiocarbon dates from Kettle Lake, North Dakota
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