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Lost City Expedition: Science

How Old Is Lost City?- Carbon 14 Dating

The question “How old are you?” is often one of the first questions that comes to mind when meeting someone. The same is true in science and other fields when encountering a new sample or unknown specimen. However, a fossil, rock, or gas will not answer the question “How old are you?” and so very careful analytical methods have been developed to place an age to these samples. If the sample contains carbon, and is not older than ~40,000 years, it can be dated very accurately using a technique known as “carbon-14 dating” or “radiocarbon dating”.

The attached picture shows the accelerator (the huge brown tube) in the foreground, and the source in the background (inside the fence). The carbon ion beam moves toward the camera.
Radiocarbon dating is based on measurements of the number of 14C atoms, a rare and special isotope of carbon. While most carbon atoms are the stable isotope 12C, meaning that they have six protons and six neutrons and are stable over time, 14C atoms have 8 neutrons and 6 protons and radioactively decay over time. Carbon-14 atoms are created in the upper atmosphere by high-energy cosmic rays hitting nitrogen in air, and are incorporated in to living tissue by the process of breathing, photosynthesis, or other metabolic processes. Once an organism dies, the “fresh” 14C begins to radioactively decay at well-known and constant rate (half of the remaining carbon-14 disappears every 5700 years). Thus, if we can count the number of 14C atoms compared to the number of 12C atoms, we can use algebra to back calculate the age of the sample. Of course, the trick is actually counting the atoms; as only one out of every trillion, 1x1012, carbon atoms is a 14C atom, in some cases we must accurately count only 1000 atoms! Radiocarbon dating is only accurate for samples younger than about 40,000 years, why?

One method to accurately count carbon-14 atoms is called accelerator mass spectrometry (AMS). This method utilizes a huge spectrometer, an instrument the size of ten large classrooms that consists of a source, an accelerator and various detectors. There are not very many laboratories that make AMS measurements. For this project we have used one of the best AMS facilities, Lawrence Livermore National Laboratory’s Center for Accelerator Mass Spectrometry. Basically, what the instrument does is accelerate a beam of carbon ions to very high energies. At high energy the carbon ion beam can be manipulated using large magnets so that the various isotopes (12C and 14C) get directed towards different detectors. Furthermore, high energies are required for the detector that counts the carbon-14 atoms to work properly. Every sample requires a few hours of preparation, about 10 minutes for the instrument to measure, and $300+ in instrument costs.