Radiocarbon Dating: What is It? And How Does It Work?

By Joan Whetzel

Prior to the discovery of radiocarbon dating, scientists had some difficulty reliably dating bones and other organic remains that occurred before about 3000 BC, around the time of Egypt’s 1st Dynasty. The process of radiocarbon dating was developed by a University of Chicago team of scientists, under the leadership of Willard F. Libby. This discovery won Libby a Nobel Prize in Chemistry in 1960.

What is Radiocarbon Dating?

Radiocarbon dating measures the amount of the radioactive element know as 14C (Carbon-14) that is present in a sample of organic remains. Comparing the process to clocks and telling time, the 14C isotopes’ radioactive decay is to the clock as the radiocarbon dating is to reading the time on the clock.

Once scientists determine the amount of carbon contained in the sample of organic remains, they can determine its age, as long as the sample is younger than 60,000 years old. Organic remains older than 60,000 years old don’t contain enough carbon to date them with any accuracy.

Rate of Radioactive Decay for Carbon-14

Carbon 14 (14C) has a half-life of 5,730 years. That means that, because of radioactive decay, the amount of carbon-14 in a sample of organic remains decreases by 1/2  every 5,730 years. In addition, the quantity of atmospheric carbon-14  varies from year to year, so scientists need to re-calibrate for the yearly variations and alter their calculations accordingly in order to account for these variations.

How Radiocarbons Appear in Organic Remains

Carbon-14 is produced by cosmic rays hitting nitrogen atoms in the atmosphere. Rain and lighting brings this carbon down to the earth, where it is absorbed by the plants through their root systems. Humans and animals eat the plant life, thereby absorbing the 14C into their bones and tissues. One the plants, animals, and humans die, they no longer absorb 14C, and so the 14C slowly degrades over time.

Methods for Detecting 14C

There are 3 main methods of radiocarbon dating.

1)      Gas Proportional Counting: This method counts the beta particles give off by the organic remains sample.

2)      Liquid Scintillation: This method, popular in the 1960s, generates flashes of light every time the scintillator connects with beta particles in the organic remains sample  in liquid form.

3)      Accelerator Mass Spectrometer: Believed to be a more accurate method for radiocarbon measurement, the carbon-14 atoms are measured relative to carbon-12 and carbon-13 atoms in the sample.

The Limitations of Carbon 14 Dating

While radiocarbon dating is fairly accurate, there are a few limitations for this type of testing.

1)      Large samples of organic remains are preferable since purification and distillation – required for running the tests – strips off some of the material from the sample. Extremely small samples can be run through the mass spectrometer, but its quite expensive.

2)      Samples that are improperly collected and packaged can become contaminated with more recent carbon, thereby throwing off the test results.

3)      Radiocarbon dating has crucial upper and lower time boundaries, since the rate of decay is logarithmic, making it difficult or impossible to accurately date very recent or very old samples.

4)      the ratio of carbon-14 to carbon-13 has fluctuated considerably throughout history, which means scientists have had to compensate for the variations with a series of calibration tables an corrections to their equations. Errors can occur if calibrations are omitted or performed incorrectly.

5)      Although widely used in archaeology, radiocarbon dating is not foolproof. Single testing of one sample is not considered to produce an accurate, reliable result. It is preferable to take multiple samples and run multiple tests to verify the accuracy of the results.

Check out this website to watch a movie on how 14C tests are processed.

Chronological Methods 8 – Radiocarbon Dating.  “How is a C-14 Sample Processed?” http://archserve.id.ucsb.edu/courses/anth/fagan/anth3/Courseware/Chronology/08_Radiocarbon_Dating.html 

Bibliography

1. Beta Analytic. Radiocarbon Dating: An Introduction. Downloaded1/4/2012. http://www.radiocarbon.com/about-carbon-dating.htm 

2. Pickover, Clifford A. The Physics Book. New York: Sterling Publishing, 2011. Page 406.

3. Radiocarbon Web-Info. C14 Dating. Downloaded 1/4/2012. http://c14dating.com/ 

4. University of California, Santa Barbara. Chronological Methods 8 – Radiocarbon Dating. Downloaded 1/4/2012. http://archserve.id.ucsb.edu/courses/anth/fagan/anth3/Courseware/Chronology/08_Radiocarbon_Dating.html