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  • 8.0 The First Nuclear Weapons

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    Over the last 40 years, there has been a discernible increase in the number of scholars who have focused their research on early industrial organizations, a field of study that has come to be known as Archaeotechnology.

    Archaeologists have conducted fieldwork geared to the study of ancient technologies in a cultural context and have drawn on the laboratory analyses developed by materials scientists as one portion of their interpretive program.

    Corroded iron from the Java Sea Wreck.

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    Chinese Warring States arrowhead dating to about ā€” B. A wrought-iron Roman cleaver. Large spear from Burkino Faso, Africa. Paperweight made by reworking iron from the Himeji Castle in Japan. In this article, an overview is presented of the status of the radiocarbon dating of iron-based materials. Recent advances include simplification in sample preparation and reduction in sample size for accelerator mass spectrometry measurements, and the potential use of rust as a viable source of material for radiocarbon dating.

    Additionally, a summary is presented of all 63 previously published results for iron-based materials and 29 new results that have not been published previously. These materials range from low-carbon wrought irons to medium to very high-carbon steels and cast irons.

    Artifact dates range from several hundred years ago to several thousand years ago. Brief descriptions are given of some of these examined samples to illustrate issues and complexities that can arise in determining the age of iron-based carbon materials using radiocarbon dating. A recent summary has been published 1 of techniques for dating that range from astronomical methods to cover time scales from the age of the universe e. One well-known method for dating is based on the use of isotopic techniques.

    Included are reactions such as the uranium-to-lead transformation utilized for dates that range from 1 billion years to 4. Perhaps the best-known isotopic technique, however, is that of radiocarbon [e. The present paper deals with an issue of great interest to materials scientists and archeologistsā€”the dating of iron-based materials that contain carbon. In addition, however, the corrosion products or rust from these materials is included since they can also be used for dating in some cases.

    For the case of iron-based materials, the time span of interest is from the start of the Iron Age in the regions of interest about B. The most appropriate method for this time span and group of materials is 14C dating. It is key to point out that the usefulness of the method of dating carbon in iron-based materials relies on the source of the carbon found in the materials see sidebar.

    For the case of iron-based materials, van der Merwe and Stuiver 2 first demonstrated that it was feasible to extract the carbon from different iron-based materials and use it to establish their age using radiocarbon dating. A total of 15 samples of iron-based materials were dated by beta counting at Yale University 23 using a dependable method to extract carbon from iron utilizing flow-through combustion in oxygen with cryogenic trapping of CO2.

    These studies showed that in a wide range of cases, the carbon in iron-based materials could be extracted and reliably radiocarbon dated.

    The Yale beta counter, however, required significant amounts of carbon compared to the amounts that were usually available from artifacts without consuming or damaging them.

    The amount of carbon required was 1g, equivalent to 50 g of a 2. In the late s, radiocarbon dating by accelerator mass spectrometry AMS became common.

    This new methodology required only 1 mg instead of 1 g of carbon. Inthe present authors published 9 a new carbon-extraction method for iron based on a sealed-tube combustion with CuO in quartz. This greatly simplified the previous technique and required only materials readily available in the standard AMS graphite-preparation laboratory: Unlike the previous techniques, no exotic gas-trapping equipment is required.

    Thus, over the years, the sample-size requirement has been greatly reduced and the carbon-extraction procedure has been simplified. However, as has been mentioned, for a radiocarbon date on iron to be meaningful, the carbon extracted from the iron-based material must be from biomass contemporaneous with original manufacture. In addition to fossil fuels such as coal and coke, other carbon sources such as geological carbonates e.

    Complications arising from the recycling of artifacts must also be considered. These limitations of the dating technique have been well summarized by van der Merwe 3 and Cresswell. If rust can be dated reliably, it opens up a large number of possibilities for dating iron artifacts.

    Investigators will not need to cut into valuable artifacts for clean metal, but will be able to use surface corrosion products. This potentially opens the way for dating precious samples such as the iron plate found in the Great Pyramid at Gizeh, 1011 now at the British Museum.

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    It may also be possible to date completely rusted artifacts, commonly found in waterlogged early Iron-Age sites in Europe and in underwater shipwrecks. Previous investigators had been careful to remove rust from iron prior to dating for fear that it adds contamination.

    A key issue though, is whether any of the original carbon remains within the matrix of rust and other corrosion products. If not, rust and similar materials are clearly of no interest for radiocarbon dating and should probably be removed since, at best, they can do no good. However, if original carbon is present, the corrosion products themselves may be appropriate targets for dating, subject to solving the potential contamination problems.

    Most of the carbon in iron-based materials is in the form of the orthorhombic, crystalline iron carbide Fe3C known as cementite. Morphologically, cementite appears either as spheroidized particles or as pearlite. For compositions exceeding the eutectoid level of about 0. The thickness and sizes of all of these carbides can vary enormously, depending upon composition and heat-treatment history. For steels that have been quenched to form martensite body-centered tetragonal structurethe carbon is essentially in solid solution in the iron up to the eutectoid composition, beyond which it too will usually be in the form of carbides.

    Despite the complex range of possible amounts and morphologies of the cementite, the thermodynamic stability of iron carbide is significantly greater than that of iron.

    So, as iron rusts, the carbide phase will be more stable than the matrix and will remain behind. The question then becomes one of kinetics: How long will it take for the carbide to oxidize compared to the iron matrix? As long as the carbon remains in the rust, in whatever form, it will potentially be available for radiocarbon dating. Although little appears to have been published on this subject, Knox 12 reported the detection of iron carbide in the remaining oxide from a corroded 2,year-old Iranian steel dagger.

    The present authors and van der Merwe 14 have recently completed a study in this area. This work provides some evidence for the reliability of dating corrosion products from artifacts that have rusted in the air, in the ground, and under water, although it does not prove that all such samples can be successfully dated.

    Nonetheless, iron samples that had completely rusted produced plausible radiocarbon dates, but issues of contamination and post-depositional carbon exchange must be thoroughly tested in a variety of field settings before rust dating can be considered a validated technique. What the study does show, then, is that there is no a priori reason why the method should not work on rust. The work suggests that accurate radiocarbon dates may be obtainable with minimal material and with minimal risk to artifacts.

    Artifacts are designated as follows: Weight percent carbon vs. This table provides sample identification, radiocarbon years Before Present [B. This nomenclature is used in radiocarbon dating to avoid the variation introduced by calibration charts that convert radiocarbon years B.

    Sincea wide range of iron-based materials have been investigated Figures abcdand e. The determined ages range from relatively recent materials B. Sample sizes range from less than 0. Sample conditions range from clean metal to rusty metal to very corroded metal. The nuances of radiocarbon dating of iron-based materials will be explained by way of example in this paper.
    Fission-track dating This is a special type of dating method that makes use of a microscope rather than a mass spectrometer and capitalizes on damaged zones, or tracks, created in crystals during the spontaneous fission of uranium In this unique type of radioactive decaythe nucleus of a single parent uranium atom splits into two fragments of similar mass with such force that a trail of crystal damage is left in the mineral.

    Immersing the sample in an etching solution of strong acid or base enlarges the fission tracks into tube-shaped holes large enough to be seen under a high-powered microscope. The number of tracks present can be used to calculate the age of the sample if the uranium content is known. Fortunately, the uranium content of precisely the spot under scrutiny can be obtained by a similar process when working with a polished crystal surface.

    The sample is bombarded with slow thermal neutrons in a nuclear reactorresulting in induced fission of uranium as opposed to spontaneous fission of uranium The fission tracks produced by this process are recorded by a thin plastic film placed against the surface of the sample.

    The uranium content of the material can then be calculated so long as the neutron dose is known. The preservation of crystal damage i.

    Alternately, primary ages can be calculated if the rock was formed at the surface and cooled quickly. Under these conditions the calculated fission-track ages of two minerals with widely different annealing temperatures would be identical. The accuracy achieved depends on the number of tracks counted, so that artificial glass coloured with 10 percent uranium can be dated as soon as 30 years after manufacture. With uranium levels of a few parts per million, samples as young asyears can be dated by counting tracks for one hour.

    When dealing with very old materials, high-uranium samples must be avoided because there are so many interlocking tracks that they can no longer be counted. A special feature of fission-track dating lies in its ability to map the uranium distribution within mineral grains. In a uranium map for single zircon grains, the outer zones that grew during a major melting event contained much more uranium than the grains originally present.

    The uraniumā€”lead age was highly biased toward the younger event, and the primary age could be determined only after the outer zones were removed. In practice, fission-track dates are regarded as cooling ages unless proved otherwise.

    It might also be noted that uncertainties in results may arise from an uneven distribution of uranium, statistical errors in counting, and inaccurate estimates of neutron flux dose of neutrons.

    Fission-track dating can be used on a wide variety of minerals found in most geologic materials, and it is relatively inexpensive to apply.
    Carbon dating and other cosmogenic methods The occurrence of natural radioactive carbon in the atmosphere provides a unique opportunity to date organic materials as old as roughly 60, years. Unlike most isotopic dating methods, the conventional carbon dating technique is not based on counting daughter isotopes.

    It relies instead on the progressive decay or disappearance of the radioactive parent with time. Newly created carbon atoms were presumed to react with atmospheric oxygen to form carbon dioxide CO2 molecules. Radioactive carbon thus was visualized as gaining entrance wherever atmospheric carbon dioxide entersā€”into land plants by photosynthesis, into animals that feed on the plants, into marine and fresh waters as a dissolved component, and from there into aquatic plants and animals.

    In short, all parts of the carbon cycle were seen to be invaded by the isotope carbon Invasion is probably not the proper word for a component that Libby calculated should be present only to the extent of about one atom in a trillion stable carbon atoms.

    So low is such a carbon level that no one had detected natural carbon until Libby, guided by his own predictions, set out specifically to measure it. His success initiated a series of measurements designed to answer two questions: Is the concentration of carbon uniform throughout the plant and animal kingdoms?

    After showing the essential uniformity of carbon in living material, Libby sought to answer the second question by measuring the radiocarbon level in organic samples dated historicallyā€”materials as old as 5, years from sources such as Egyptian tombs. With correction for radioactive decay during the intervening years, such old samples hopefully would show the same starting carbon level as exists today.

    His conclusion was that over the past 5, years the carbon level in living materials has remained constant within the 5 percent precision of measurement. A dating method was thus available, subject only to confirmation by actual application to specific chronologic problems. Expressed as a fraction of the contemporary level, they have been mathematically converted to ages through equation 5 above.

    Archaeology has been the chief beneficiary of radioactive-carbon dating, but late glacial and postglacial chronological studies in geology have also been aided greatly. The occasional exceptions all involve nonatmospheric contributions of carbondepleted carbon dioxide to organic synthesis.

    Specifically, volcanic carbon dioxide is known to depress the carbon level of nearby vegetation, and dissolved limestone carbonate occasionally has a similar effect on freshwater mollusks, as does upwelling of deep ocean water on marine mollusks. In every case, the living material affected gives the appearance of built-in age. In addition to spatial variations of the carbon level, the question of temporal variation has received much study.

    Of more recent date was the overcompensating effect of man-made carbon injected into the atmosphere during nuclear bomb testing. The result was a rise in the atmospheric carbon level by more than 50 percent. Fortunately, neither effect has been significant in the case of older samples submitted for carbon dating.

    The ultimate cause of carbon variations with time is generally attributed to temporal fluctuations in the cosmic rays that bombard the upper atmosphere and create terrestrial carbon Whenever the number of cosmic rays in the atmosphere is low, the rate of carbon production is correspondingly low, resulting in a decrease of the radioisotope in the carbon-exchange reservoir described above. Studies have revealed that the atmospheric radiocarbon level prior to bce deviates measurably from the contemporary level.

    In the year bce it was about 8 percent above what it is today. In the context of carbon dating, this departure from the present-day level means that samples with a true age of 8, years would be dated by radiocarbon as 7, years old. The problems stemming from temporal variations can be overcome to a large degree by the use of calibration curves in which the carbon content of the sample being dated is plotted against that of objects of known age.

    In this way, the deviations can be compensated for and the carbon age of the sample converted to a much more precise date. Calibration curves have been constructed using dendrochronological data tree-ring measurements of bristlecone pines as old as 8, years ; periglacial varve, or annual lake sediment, data see above ; and, in archaeological research, certain materials of historically established ages.

    It is clear that carbon dates lack the accuracy that traditional historians would like to have. Until then, the inherent error from this uncertainty must be recognized. A final problem of importance in carbon dating is the matter of sample contamination.

    If a sample of buried wood is impregnated with modern rootlets or a piece of porous bone has recent calcium carbonate precipitated in its pores, failure to remove the contamination will result in a carbon age between that of the sample and that of its contaminant.

    Consequently, numerous techniques for contaminant removal have been developed. Among them are the removal of humic acids from charcoal and the isolation of cellulose from wood and collagen from bone. Today contamination as a source of error in samples younger than 25, years is relatively rare.

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    Beyond that age, however, the fraction of contaminant needed to have measurable effect is quite small, and, therefore, undetected or unremoved contamination may occasionally be of significance. A major breakthrough in carbon dating occurred with the introduction of the accelerator mass spectrometer. This instrument is highly sensitive and allows precise ages on as little as 1 milligram 0. The increased sensitivity results from the fact that all of the carbon atoms of mass 14 can be counted in a mass spectrometer.

    By contrast, if carbon is to be measured by its radioactivity, only those few atoms decaying during the measurement period are recorded. By using the accelerator mass spectrometer, possible interference from nitrogen is avoided, since it does not form negative ion beams, and interfering molecules are destroyed by stripping electrons away by operating at several million volts. The development of the accelerator mass spectrometer has provided new opportunities to explore other rare isotopes produced by the bombardment of Earth and meteorites by high-energy cosmic rays.

    Many of these isotopes have short half-lives and hence can be used to date events that happened in the past few thousand to a few million years. In one case, the time of exposure, like the removal of rock by a landslidecan be dated by the presence of the rare beryllium 10Be isotope formed in the newly exposed surface of a terrestrial object or meteoroidal fragment by cosmic-ray bombardment.

    Other applications include dating groundwater with chlorine 36Cldating marine sediments with beryllium 11Be and aluminum 26Aland dating glacial ice with krypton 81Kr. In general, the application of such techniques is limited by the enormous cost of the equipment required.

    Uranium-series disequilibrium dating The isotopic dating methods discussed so far are all based on long-lived radioactive isotopes that have survived since the elements were created or on short-lived isotopes that were recently produced by cosmic-ray bombardment. The long-lived isotopes are difficult to use on young rocks because the extremely small amounts of daughter isotopes present are difficult to measure.

    A third source of radioactive isotopes is provided by the uranium - and thorium -decay chains. Uraniumā€”thorium series radioisotopes, like the cosmogenic isotopes, have short half-lives and are thus suitable for dating geologically young materials. The decay of uranium to lead is not achieved by a single step but rather involves a whole series of different elements, each with its own unique set of chemical properties.

    In closed-system natural materials, all of these intermediate daughter elements exist in equilibrium amounts. That is to say, the amount of each such element present is constant and the number that form per unit time is identical to the number that decay per unit time.

    Accordingly, those with long half-lives are more abundant than those with short half-lives. Once a uranium-bearing mineral breaks down and dissolves, the elements present may behave differently and equilibrium is disrupted.

    For example, an isotope of thorium is normally in equilibrium with uranium but is found to be virtually absent in modern corals even though uranium is present. Over a long period of time, however, uranium decays to thoriumwhich results in a buildup of the latter in old corals and thereby provides a precise measure of time. Most of the studies using the intermediate daughter elements were for years carried out by means of radioactive counting techniques; i.

    The introduction of highly sensitive mass spectrometers that allow the total number of atoms to be measured rather than the much smaller number that decay has resulted in a revolutionary change in the family of methods based on uranium and thorium disequilibrium. Thorium dating The insoluble nature of thorium provides for an additional disequilibrium situation that allows sedimentation rates in the modern oceans to be determined.

    In this case, thorium in seawater, produced principally by the decay of uranium, is deposited preferentially in the sediment without the uranium parent. This is defined as excess thorium because its abundance exceeds the equilibrium amount that should be present. With time, the excess decays away and the age of any horizon in a core sample can be estimated from the observed thoriumto-thorium ratio in the seawater-derived component of the core.

    Sedimentation rates between 1 and 20 mm 0. Lead dating The presence of radon gas as a member of the uranium-decay scheme provides a unique method for creating disequilibrium. The gas radon Rn escapes from the ground and decays rapidly in the atmosphere to lead Pbwhich falls quickly to the surface where it is incorporated in glacial ice and sedimentary materials.

    By assuming that the present deposition rate also prevailed in the past, the age of a given sample at depth can be estimated by the residual amount of lead Principal cosmogenic and uranium-thorium series radioisotopes The principal cosmogenic and uranium-thorium series radioisotopes are listed in the table.
    Born 14 May ; died 26 Apr at age Now known as the Eightfold Way after Buddha's Eightfold Path to Enlightenment and blissthe scheme grouped mesons and baryons e.

    James Hardy Born 14 May ; died 19 Feb at age James Daniel Hardy was an American surgeon who headed teams that performed the first human lung transplant in ; the first animal-to-human heart transplant in ; and a double-lung transplant that left the heart in place in Three years before Christiaan Barnard performed the first successful human-to-human heart transplant, on 23 Janyear-old Boyd Rush was admitted to the hospital.

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    No human was heart available; Hardy decided to use the heart of a chimpanzee named Bino. The newly-transplanted heart beat on its own; but it was too small to maintain independent circulation and Rush died after 90 minutes. Hardy had to endure some severe criticism. Overall, his work helped to alter perceptions about the transplantation process. Nicholas Kurti Born 14 May ; died 24 Nov at age As a young man, inhe left the Univeristy of Berlin to flee from the Nazis.

    Most of his life was then spent at Oxford, England. During WWII, he investigated how to separate the isotopes of uranium which became of interest in the Manhattan Project to build the atomic bomb. He had a side interest in food science, combining his hobby as a chef with a study of the physics and chemistry of cooking, for which he coined the term molecular gastronomy.

    With his wife, he edited the first Royal Society cook book: But the Crackling Is Superb: The components of the mixture moved at different rates, producing a series of bands. He is known in particular for his study of the chlorophylls and the carotenoids. However, in Tswett's own lifetime, chromatography remained virtually unrecognized as a scientific tool. In the s, it was rediscovered and then spread worldwide.

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    The chromatography technique he invented is now widely used to separate substances from mixtures. American mathematician who originated the idea, postumously given his name - for the Fields Medal. It became the most prestigious award for mathematicians, often referred to as the equivalent of a Nobel Prize for mathematicians. As a professor at the University of Toronto, he had worked to bring the International Congress of Mathematicians to Toronto This was approved on 24 Feb He coined their names.

    He published this essay, which dealt with animal and plant chemistry, in In the same year he also published a successful general text-book of chemistry, A Manual of Elementary Chemistry, Theoretical and Practical, which survived decades after him through twelve editions, the last in He died at the young age of Nature bears long with those who wrong her. She is patient under abuse. But when abuse has gone too far, when the time of reckoning finally comes, she is equally slow to be appeased and to turn away her wrath.

    In science it often happens that scientists say, 'You know that's a really good argument; my position is mistaken,' and then they would actually change their minds and you never hear that old view from them again.

    They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion.

    8.0 The First Nuclear Weapons

    I used to wonder how it comes about that the electron is negative. There is no reason whatever to prefer one to the other. Then why is the electron negative? It is the facts that matter, not the proofs. Physics can progress without the proofs, but we can't go on without the facts

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