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Radio Carbon Dating

Radio Carbon 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, 1990) This decay process itself allows the remaining radiocarbon in the sample to be detected and estimated, since the intensity of beta radiation produced is dependent on the amount of carbon-14 present. The more atoms of Carbon-14 there are, the more disintegrations will occur in a given time, giving off the electrons whose emission is detected in the laboratory (pg. 258 Renfrew, 1973). A feature of the radioactive decay process is that each isotope of each element decays at a specific rate. The radioactive decay of C-14 follows the principal of half-life. The half-life (T1/2) is the mount of time necessary for 1/2 of the radioactive material to decay. (Bowman, 1990) During the beginning years of radiocarbon dating the T1/2 was determined to have been 5560 +/- 30 years this was known as Libby’s Half-life. (Libby, 1955) This T1/2 was change to a more accurate number of a half-life of 5730 years +/- 30 years. (Renfrew, 1973) Figure 2, shows the T1/2 of C-14. The short half-life of C-14 makes this dating technique available for samples of 70,000 years or younger. The amount of C-14 at that moment is too insignificant to be measured. (Renfrew, 1973) There is good evidence, however, that the production of carbons 14 and thus the ration of C-14 to C-12 has varied somewhat over the past several thousand years. This creates a problem in C-14 dating because C-14 dating rest on the assumption that the concentration of C-14 in time is a constant. (Bowman, 1990) The use of dendrochronology offered a way to mitigate this inaccuracy. It helped verity the dates acquired by C-14 dating. Dendrochronology is the method of using tree rings for dating. This method entails the counting of the rings that are produced annually by trees. When patterns emerge among trees of the same region, one can start counting in one tree and finish in an older tree. This method is very accurate since each ring coincides with a calendar year. A graph was created which is used to convert C-14 dates to tree years. (Renfrew, 1973). Figure 3. {"T o Calibrate a radiocarbon date, its position on the horizontal axis (x-axis) is found. The point on the curve which lies vertically above it is then located. The corrected, calendar date is the date on the vertical axis (y axis), reading horizontally along to the left from this point on the curve." (Pg. 71 Renfrew, 1973)} The concentration of C-14 became more stable after there was the 1950’s testing of A-bombs. (Renfrew, 1973). When it was introduced, there were some reported problems of radiocarbon dating. Most of the problems reported dealt with the selection and collection of samples to be dated. All the samples are delicate and can be contaminated in different ways. For example, the contamination of a sample, by mixing it with samples of other dates could occur; therefore, the dates rendered could not have been accurate. A sample could have been contaminated due to a fungus growth on the sample; thus again, the dates obtained would have been altered due to the fungus. This problem can be alleviated by burning the sample in a current of pure oxygen. All these problems have nothing to do with radiocarbon dating, but with the origin and handling of the samples. (Libby, 1955). A shortcoming of radiocarbon dating is that it is limited. Dates older than 70,000 years cannot be dated because the amount of C-14 is insignificant. (Renfrew, 1973). Further research in the field of dating will perhaps one day bring forth a new method of dating that will allow scientist to date earlier years. From the time of its introduction, C-14 dating has been in a constant process of improvement. Today, C-14 dating technique involves the of small gas counters and accelerator-mass-spectrometry (AMS) techniques that were developed in the 1970’s. These new techniques require only a small amount of a sample for analysis. This new technique broadens the reach of the effects of the discovery of radiocarbon dating. (Bowman, 1990). These allowed C-14 dating to be involved in dating one of the most famous artifacts, the Shroud of Turin. ("Radiocarbon dating of the Shroud of Turin", 1989) The Shroud of Turin is the supposed burial cloth of Jesus. The shroud itself appears to show a person who was crucified. The Shroud of Turin became an object of some veneration because of its supposed association with Christ. Its history dates back at least as far as the mid. 14th century AD. The fist photograph of the shroud showed the man as a negative image, a three-dimensional picture. This, along with other discoveries, such as the supposed presence of pollen spores from Israel on the cloth has suggested the shroud might be an important and genuine relic. ("Radiocarbon dating of the Shroud of Turin", 1989) In the 1980s, the archbishop of Turin, the Pontifical Custodian of the shroud, gave permission to a group of scientist to date small pieces of fabric sampled from the schroud. Under the inspection of the British Museum, radiocarbon labs at Tucson (US), Oxford (England), and Zurich (Switzerland) dated the samples, along with three control samples of varying ages. FIGURE 4 shows the results of the three labs. The results gave a certainty of 95%. The results of the three labs were consistent. Figure 5 shows the calibration of the age of the shroud using the mean of the three labs to determine it. The results demonstrate that the shroud belongs to 1260-1390 AD (rounded to the nearest +/-10 yr.); this is the time when the shroud first appeared in historical records. ("Radiocarbon dating of the Shroud of Turin", 1989) The results shows that the shroud was and is only a medieval artifact not the actual burial cloth of Jesus Christ. Today, there are well over 130 radiocarbon dating laboratories around the world. They all charge different amounts of money to give a date of a specific sample; it may cost approximately $250. (Bowman, 1990) Many advances are taking place with radiocarbon dating, and many more will come. As one can see, radiocarbon dating has many uses and implications because it can date anything that was once living (with limitations). Dr. Libby said it best,"[Radiocarbon dating] has become a valuable research tool, improving our knowledge of chronology". (p. 141,1955) The uses of radiocarbon dating are many, this allows its use to affect many different fields of science including hydrology, atmospheric science, oceanography, geology, paleoclimatology, archaeology, biomedicine, and many much more. Radiocarbon dating proved to be one of the most important discoveries of the 20th century. This is all due to Libby and his associates; in thanks for this achievement, Libby was awarded the Nobel Prize in Science in 1960. (Bowman, 1990) Bibliography: Bibliography Bowman, S. (1990) Radiocarbon Dating. London: British Museum Pub. Libby, W. F. (1955) Radiocarbon Dating. Chicago: University of Chicago Press. "Radiocarbon Dating of the Shroud of Turin". (1989, February 16). Nature, 337, (6208). 611-615. Renfrew, C. (1973). Before Civilization: the Radiocarbon Revolution and Prehistoric Europe. New York: Alfred A. Knopf, Inc.

Word Count: 1837