Amy Dewees MISE - Physical Basis of Chemistry Final Project April 22, 2006
Radiocarbon Dating
Introduction: Radiocarbon dating is a technique used to determine the approximate age of a sample made of organic material that is up to 50,000 years old. The amount of an isotope of carbon (C- 14) is that is present in a sample is used to determine the age of the sample. Similar techniques can be used to determine the age of other types of samples or sample that are older than 50,000 years. The process of radiocarbon dating was developed by a team lead by Willard F. Libby in the period following World War II at the University of Chicago. He and his team were the first to measure the rate of decay of C-14 and determined it’s half-life to be 5568 years. In order to confirm their hypothetical half-life for C-14, Libby and his team calculated the age of samples using the radiocarbon dating technique on objects whose age could be confirmed through historical records. These objects included a piece of the dead sea scrolls and wood from an Egyptian tomb. Libby won the Nobel Prize in Chemistry in 1960 for his work in radiocarbon dating. This technique, based in chemistry, has impacted many areas of study including geology, paleontology, archaeology, and biomedicine (“emuseum @Minnesota State University”, 2003).
The Chemistry of Radiocarbon Dating: Carbon-14 (C-14) is an isotope of carbon. An isotope is an atom of an element that has the same number of protons as other atoms of the element, but has a different number of neutrons. C-14 contains 8 neutrons instead of 6 found in carbon-12 (C-12). Most isotopes are stable and remain in their original form, but others are unstable. Unstable isotopes, like C-14, are radioactive and release particles and energy as they degrade during the process known as radioactive decay. During radioactive decay, a neutron breaks down into a proton and an electron, the nucleus gains the proton and the electron is released as a β particle or the atom gives off 2 protons and 2 neutrons, releasing an α particle. When this happens the isotope, or parent material becomes an atom of another element, known as the daughter material. In the case of C- 14, it is the parent material and nitrogen-14 (N-14) is the daughter material (Feather, 1999). 14 0 14 Radioactive decay of C-14 into N-14 can be summarized using the equation: 6 C " !1e + 7 N . Isotopes decay at a specific rate, the amount of time it takes for half the isotope atoms in a sample to breakdown from the original parent material into the daughter material is known as the half-life. The half life of carbon-14 is 5730 years. That means that in 5730 years, 50% of the C-14 atoms will break down into N-14 atoms. In another 5730 years, another 50% of the C-14 atoms will break down, leaving 25% of the C-14 atoms remaining in the sample. Scientists can use this information to determine the age of a sample (Holt, Rinehart, and Winston, 2001).
Using C-14 to Determine the Age of a Sample: C-14 is present in the atmosphere and plants take it in and incorporate it into their cells. Animals then take in the C-14 when they eat the plants or eat other animals. When an organism is alive it takes in the C-14 isotope at a constant ratio to C-12. When an organism dies, it stops taking in new carbon and the C-14 atoms it contains begin to decay, breaking down into N-14. Scientists measure the amount of C-14 remaining in the sample and use that to determine the age of the sample (“emuseum @Minnesota State University”, 2003). The measurement can be done using a Geiger counter, which measures the amount of radiation given off or by placing the sample in a mass spectrometer, which measures the relative concentrations of atoms in a sample (Nave 2005)
Problems Associated with Radiocarbon Dating: One problem with using radiocarbon dating to determine the age of a sample is that it can only be used to measure the age of a sample that is less than 50,000 years old. The reason for this is that there is not enough C-14 left in the sample to accurately measure after that amount of time. Another drawback is that it can only be used to calculate the age of organic material, not rocks. Lastly, radiocarbon dating relies on the assumption that the amount of C-14 in the atmosphere does not change. There are some scientists who question if this is accurate; it is possible that the amount of C-14 in the atmosphere varies (California Institute for Ancient Studies).
Radiocarbon and Radiometric Dating Activities: In order for students to really understand what radiocarbon dating entails, they will complete two activities. The best way for students to learn science is to experience it. The first activity, taken from SciencecNetLinks, models the decay of carbon-14 using m & m candy. The second activity, taken from Fossils! Hands-on Activities for Teachers, models the use of radiometric dating to determine the age of a material using other isotopes, including Uranium 235 and Thorium 232, which have different half-lives than C-14. In this activity students count the amount of parent and daughter material (colored beans) and use the ratio to determine the age of the sample. There are some simple calculations and the use of a graph to accomplish this, so there is a nice integration of math into the activity.
Bibliography
“California Institute for Ancient Studies: Problems with Radiometric and Carbon-14 Dating.” http://www.specialtyinterests.net/carbon14.html. Retrieved March 17, 2006.
“Emuseum@Minnesota State University at Mankato: Radio-Carbon Dating.” (2003) http://www.mnsu.edu/emuseum/archaeology/dating/radio_carbon.html. Retrieved February 10. 2006.
Feather, Ralph Jr. (1999) Earth Science. Westerville, Ohio; McGraw-Hill.
“Fossils! Hands-on Activities for Teachers: Radiometric Dating Activity.” http://www.acad.carleton.edu/curricular/BIOL/classes/bio302/Pages/Half-life.html. Retrieved spring 2004.
Higham, Thomas. (1999) “Radiocarbon Web-Info.” http://www.c14dating.com/int.html. Retrieved February 10, 2006.
Holt, Rinehart, and Winston. (2001) Holt Science and Technology Earth Science.
Nave, C.R. (2005) “HyperPhysics; Mass Spectrometer.” http://hyperphysics.phy- astr.gsu.edu/HBASE/magnetic/maspec.html. Retrieved March 17, 2006.
“Science Net Links: Radioactive Decay: A Sweet Simulation of Half-Life.” (2006) http://www.sciencenetlinks.com/lessons.cfm?DocID=178. Retrieved March 17, 2006