Radio Carbon Dating





Radiocarbon Dating
When we think of history, we think of important people, places, cultures, events, and much more. The backbone of history rests on its chronology. It gives us the "when" of basic analysis. It gives us a frame of reference, the order of things. Before having an "absolute" way of determining dates, history was based in guesses and assumptions. Many attempts were made to organize the dates of the past. Some of these attempts were made by geologist. Geologist used the idea of "stratigraphic succession" (Renfrew, 1973) which is based on the "principle that when successive layers or strata are observed in position, the underlying ones are the earliest." (Pg. 23 Renfrew, 1973) By setting the layers in chronological order, it only gave a sequence not a real date. Another method that geologist used in order to date, was the measuring of sediment deposit. They measured the rate at which sediment forms at the bottom of lakes; nevertheless, this method was unsuccessful because it relied in the assumption that the rate of sediment deposit is a constant. It is not. (Renfrew, 1973)
The most famous ways to date has been the Three Age System, which divides prehistory in the three ages that we have come to know as the stone, bronze and iron ages. Eventhough this method is still used today, it only gives approximations, no absolute dates. (Renfrew, 1973) There were other attempts to absolute date, but they all were still based in approximation, no real dates. This made the past seem like a fog of facts and assumptions. Willard F. Libby, and a team of scientist from the University of Chicago, developed a method of dating to clear up the "fog" that made up our history. Libby’s method was Radiocarbon dating. (Bowman, 1990)
In 1949, Libby announced the fist radiocarbon dates in a conference in New York. This changed history forever. While it created controversy, due to some people’s attachments to the old ways of dating and doubts in this revolutionary method, it proved to be the closest method to have an accurate chronology of history. In 1955, to prove the accuracy or radiocarbon, Libby published a graph that showed the comparison of the results of radiocarbon dating of specimens from Egypt. These specimens had already an absolute known date. The graph proved the accuracy of Libby’s radiocarbon dating. Figure 1 (Renfrew, 1973)
Libby developed the method of radiocarbon dating though his observation of how cosmic rays create radiocarbon. From outer space cosmic rays infiltrate earth’s atmosphere. In the upper atmosphere, these rays hit nitrogen and oxygen atoms in the air. (Renfrew, 1973). When the neutrons of these high–energy particles (mostly protons) hit nitrogen atoms, Carbon 14 (C-14) is created. The nitrogen atom (atomic number 7) has an atomic mass of 14 (with 7 protons and 7 neutrons). When the nucleus of the nitrogen is hit by the cosmic ray’s neutrons, the atomic number of the atom decreases by one. The make-up of its atomic mass changes, a proton is emitted and the neutrons are increased by one; therefore, the atomic mass number stays the same. Because the atomic number has changed, a new element, carbon 14, with atomic number 6, has an atomic mass of 14 (with 6 protons and 8 neutrons). (Bowman, 1990) The reaction is 14N + n = 14c + p (where n is a neutron and p is a proton). The newly formed carbon 14 combines with the airs oxygen creating carbon dioxide; this is quickly assimilated into the carbon cycle. (Libby, 1955)
This C-14 is one of the three isotopes of carbon. The other isotopes are carbons 12 and 13. C-14 is different from the other two isotopes because it is unstable or radioactive, hence the name radiocarbon. is absorbed in a nearly constant ratio in all living organisms. Because C-14 is continually created at a constant rate in the upper atmosphere, and distributed throughout the carbon cycle, the ratio of carbon 14 to carbon 12 is remarkably constant in both the atmosphere and living organisms. (Renfrew, 1973)
Through the death of an organism, the spontaneous decay of C-14 takes place. Through Beta Radiation, a beta particle or electron, is emitted form the C-14 atom, allowing this atom to regress to its original nitrogen form. (Bowman,