One of the Common Radiometric Dating Techniques is the Carbon-14 Radiometric Technique - Case Study Example

CARBON-14 RADIOACTIVE DATING Introduction Radiometric dating is a technique used to materials based on knowledge of the decay rates of naturally occurring isotopes, and the current abundances. One of the common radiometric dating techniques is the carbon-14 radiometric technique (Wikipedia). Carbon-14 dating is a way of determining the age of certain archeological artifacts of a biological origin. Any object produced or shaped by human craft, particularly a tool, cloth, a weapon, or an ornament of archaeological or historical interest can be dated using this technique. It is also used in dating things such as bone, wood and plant fibers (Howstuffworks, Brain). Carbon 14 and its origin Carbon-14 is a radioactive isotope of carbon, with a half-life of 5,730 years. Naturally occurring radiocarbon is formed as a secondary effect of cosmic-ray bombardment of the upper atmosphere. Carbon-14 is continuously created through collisions of neutrons generated by cosmic rays with nitrogen. The carbon-14 ends up as a trace component in atmospheric carbon dioxide (CO2). The carbon-14 atoms produced are then incorporated into carbon dioxide (CO2) molecules to produce 14CO2 molecules which mix with the most common 12CO2 molecules in the atmosphere. The 14CO2 enters plant tissue as a result of photosynthesis or absorption through the roots. Animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to intake new carbon-14 and the existing isotope decays with a characteristic half-life (5730 years). The proportion of carbon-14 left when the remains of the organism are examined provides an indication of the time lapsed since its death. The carbon-14 dating limit lies around 58,000 to 62,000 years (Wikipedia). When a negatively charged electron strikes one of the positively charged protons in a nitrogen atom, the positive and negative charges cancel out, turning the proton into a neutron. The electron is so light compared to a proton or a neutron that it doesn’t change the weight of the proton. So, this nitrogen atom that used to have seven protons and seven neutrons now has six protons and eight neutrons and still has 14 atomic mass units (6 + 8 = 14). Since it has six protons, it is no longer nitrogen. It is carbon. Specifically, it is 14C. 1         14         14         1     n   +     N   →     C   +     H 0         7         6         1   The production of 14C is the result of a random collision between an electron from space and a nitrogen atom in the atmosphere. The amount of 14C produced in the atmosphere is balanced by the continual decay of 14C to produce 14N and a beta-particle: 14         14         0     C   →     N   +     e 6         7         1-   If the amount of nitrogen in the atmosphere stays the same, and the amount of cosmic radiation stays the same, 14C will be produced at a steady rate. If the atmosphere started out with absolutely no 14C, there would be a certain amount after one year of exposure of nitrogen to cosmic radiation. After two years, there would be twice as much 14C. The “birth rate” is constant. As long as a plant is alive, it takes carbon dioxide from the air and water from the ground and converts them into sugar. Since about 0.6% of the carbon in the carbon dioxide it breathes is 14C, about 0.6% of the carbon atoms in the sugar it builds will be 14C. Animals eat plants to get the sugar they need to survive. Since 0.6% of the carbon they get from sugar is 14C, about 0.6% of the carbon in their muscles, bones, fat, etc. will be 14C (Jones, 2000). After the organism dies and becomes a fossil, carbon-14 continues to decay without being replaced.  To measure the amount of radiocarbon left in a fossil, scientists burn a small piece to convert it into carbon dioxide gas.  Radiation counters are used to detect the electrons given off by decaying C-14 as it turns into nitrogen.   The amount of C-14 is compared to the amount of C-12, the stable form of carbon, to determine how much radiocarbon has decayed, therefore, dating the fossil (NDT Course Material). Carbon-14 is also made commercially for use in medical or biological tracer research. Carbon-14 is produced in nuclear reactors by the capture of neutrons by nitrogen, carbon, or oxygen present as components of the fuel, moderator, or structural hardware. The contribution to the carbon-14 global inventory from commercial nuclear reactors and DOE facilities in the United States has been less than 600 Ci per year, or less than 1/500,000th of the natural steady-state level. Carbon-14 was produced at Hanford by neutron activation of carbon in graphite-moderated plutonium-production reactors. Carbon-14 is also present in the graphite moderator of these shutdown reactors and in certain wastes associated with previous reactor operations, as well as in wastes from ongoing decommissioning activities, including for spent graphite (Argonne National Laboratory). Discovery of Carbon 14 In February 1940, Martin Kamen, working together with a graduate student colleague named Sam Ruben at the Berkeley Radiation Laboratory, looked for and found a long-lived radioisotope of carbon. The discovery C-14 revolutionized biochemistry as it had the potential for tracing the path of carbon metabolism in living organisms. Their early experiments were with C-11, a radioisotope with a half-life of only twenty-one minutes. This was too short an existence for their experiments to be fruitful, and it was soon realized that a long-lived isotope was essential. C-14 turned out to have a half-life of about 5,600 years, was appropriate for dating past archaeological and other events (Doolittle, 500-503). According to Kamen "Carbon-14, the long-lived carbon isotope, is the most important single tool made available by tracers methodology, because carbon occupies the central position in the chemistry of biological systems." In 1985, Kamen won the Fermi award, the nations oldest prize for achievements in science and technology (Keystone paper). Uses of Carbon 14 The radiocarbon method is applied in many different scientific fields, including archeology, geology, oceanography, hydrology, atmospheric science, and paleoclimatology. Two main uses of carbon-14 are in diagnostic medical procedures and radiocarbon dating to determine the age of previously living animals and plants. In medicine, carbon-14 can be injected to study abnormalities of metabolism that underlie diabetes, gout, anemia, and acromegaly (adult “gigantism”), and to trace the metabolism of new drugs. However, its main use to date has been to determine the age of fossils and other dead organic material (Argonne National Laboratory). Important discoveries using carbon 14 Much of the initial interest in carbon-14 came from archeology, for the isotope could assign dates to Egyptian mummies and the like. Carbon-14 dating excited scientists mainly as it helped to shed light on human evolution — the timing of human development as a species, and how climate changes had affected. It was especially fascinating to discover that humans arose something like 100,000 years ago, and were deeply influenced by the ice ages. A few scientists noticed that the techniques might also be helpful for the study of climate (Weart, 2004). Elliot "Ken" Volkin and Lazarus Astrachans (1956) discovered what they called "DNA-like-RNA," which François Jacob and Jacques Monod later identified as "messenger RNA." Volkin infected bacterial cells of Escherichia coli with the bacteriophage virus, added phosphorus-32, isolated nucleic acid from the preparation, and hydrolyzed it with sodium hydroxide to make alkaline products that were separated using ion-exchange chromatography. The results of experiments with phosphorus-32 were confirmed using a carbon-14 precursor that was specifically incorporated into the nucleic acid bases (Oak Ridge National Laboratory). In 1991, hikers in the Tyrolean Alps of Europe made a remarkable discovery. They found an almost perfectly preserved body of a prehistoric man, whom scientists named Ötzi. The discovery was made possible because recent warming of the atmosphere had caused glaciers in the region to retreat, exposing objects that had been buried under the ice for millennia. Ötzi’s fate was matched by a variety of well-preserved plant and animal species that were found close by. Carbon dating of samples from the site established the time of Ötzi’s demise at approximately 5,300 years ago (BRSP-15, 1-9). Problems with carbon 14 dating Carbon dating is applicable only to matter which was once living and presumed to be in equilibrium with the atmosphere. Carbon dating is controversial for a couple of reasons. There is considerable uncertainty in carbon dating and there are several variables that contribute to this uncertainty.  The proportion of C-14 in the atmosphere in historic times is unknown.  The C-14:C-12 atmospheric ratios is known to vary over time. Besides, various plants have differing abilities to exclude significant proportions of the C-14 in their intake. Besides it varies with environmental conditions also. The varying rates at which C-14 is excluded in plants also means that the apparent age of a living animal may be affected by its diet. Therefore, an animal that ingested plants with relatively low C-14 proportions would be dated older than their true age (NDT Course Material). The ratio of C-12 to C-14 in the atmosphere decreased during the industrial revolution due to the dramatic increase of CO2 produced by factories. Volcanoes discharge CO2 which could just as effectively decrease the ratio. Specimens which lived and died during a period of intense volcanism would appear older than they really are if they were dated using this technique. The ratio can further be affected by C-14 production rates in the atmosphere, which in turn is affected by the amount of cosmic rays penetrating the earths atmosphere. The amount of cosmic rays penetrating the earths atmosphere is itself affected by things like the earths magnetic field which deflects cosmic rays. Precise measurements taken over the last 140 years have shown a steady decay in the strength of the earths magnetic field. This means theres been a steady increase in radiocarbon production (which would increase the ratio). This dating scheme is controversial because the dates derived are often wildly inconsistent. For example, "One part of Dima [a famous baby mammoth discovered in 1977] was 40,000 RCY [Radiocarbon Years], another was 26,000 RCY, and wood found immediately around the carcass was 9,000-10,000 RCY" (All about Archeology). The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon-14 by a few percent; conversely, the amount of carbon-14 was increased by above-ground nuclear bomb tests that were conducted into the early 1960s. Also, an increase in the solar wind or the earths magnetic field above the current value would depress the amount of carbon-14 created in the atmosphere. Hazards of carbon 14 Carbon-14 poses a health hazard only if it is taken into the body, because it decays by emitting a weak beta particle with no gamma radiation. The beta particle emitted by carbon-14 has low energy and cannot penetrate deeply into tissue or travel far in air. Carbon-14 behaves the same as ordinary carbon, both in the environment and in the human body. Hence, a significant fraction of the carbon-14 taken in by either ingestion or inhalation is absorbed into the bloodstream, where it is transferred to all organs of the body. The health hazard of carbon-14 is associated with cell damage caused by the ionizing radiation that results from radioactive decay, with the potential for subsequent cancer induction (Argonne National Laboratory). Conclusion The C-14 technique has helped to estimate some of the most important historic events. However, due to its limitations and lack of accuracy, this technique cannot serve as a most reliable method for dating. C-14 has been a source of important discoveries such as messenger RNA, archeological events, human evolution through the ice age etc. These important discoveries have further contributed to the development in science and technology. Work Cited All about Archeology, Carbon Dating - What Is It And How Does It Work? AllAboutArchaeology.org, 25 May 2006, Argonne National Laboratory, Carbon-14. August 2005, Human Health Fact Sheet, 26 May 2006, Brain Marshall. How Carbon-14 Dating Works. Howstuffworks, Inc. 26 May 2006 BRSP-15, Carbon-14 Dating or, How Old Are Those M&Ms? 1-9, 27 May 2006, Doolittle, Russell F. Martin David Kamen: Biographical Memoirs. December 2004, Proceedings of the American Philosophical Society, Vol. 148, No. 4, 500-503. Jones Do-While, Radioactive Dating Explained. July 2000, Science Against Evolution, 25 May 2006, Keystone paper, 1940: Radioactive Dating. 27 May 2006, NDT Course Material, Carbon-14 Dating. 26 May 2006, Oak Ridge National Laboratory, ORNLs Unsung Discovery 26 May 2006, Weart Spencer, Uses of Radiocarbon Dating, July 2004, The Discovery of Global Warming, 25 May 2006, Wikipedia, Radiometric Dating. 21 May 2006 Wikimedia Foundation, Inc. 26 May 2006 Read More