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1、Willard Frank Libby-Archaeological Clock 威拉德威拉德 弗兰克弗兰克 利比利比-考古学时钟考古学时钟 Hello everyone,do you like travel historic places of interest?Are you interested in archaeology?And do you know how to determine the age of fossils and relics?Let me introduce you a Nobel Prize Winner in Chemistry today,to see ho
2、w he did it.Willard Frank Libby won the Nobel Prize in Chemistry in 1960.He worked for the University of California,in USA.He won the Nobel prize because of his method to use carbon-14 for age determination in archaeology,geology,geophysics,and other branches of science.PART ONE Biography He is a Ph
3、ysical Chemist,and specialist in radiochemistry,particularly hot atom chemistry.He is good at tracer techniques,and isotope tracer work.He became well-known at University of Chicago for his work on natural carbon-14(as the radiocarbon).C-14 is used in dating archaeological artifacts,and its use in h
4、ydrology and geophysics.First of all,what is C-14?Carbon is a fundamental component in all living material.In nature,there are three isotopes:carbon-12,which is stable form,carbon-13,and carbon-14.Carbon-14 is radioactive.It forms in the atmosphere when acted upon by cosmic radiation and then deteri
5、orates.How to measure the age of archaeological artifacts?When an organism dies and the supply of carbon from the atmosphere stops,the content of carbon-14 declines through radioactive decay at a fixed rate.In 1949,Willard Libby developed a method for applying this to determine the age of fossils an
6、d archeological relics.It had been known since 1939,that cosmic rays create showers of neutrons on striking atoms in the atmosphere.And the atmosphere contains about 78 percent nitrogen,which absorbs neutrons to decay into the radioactive isotope carbon-14,Libby concluded that traces of carbon-14 sh
7、ould always exist in atmospheric carbon dioxide.Also,because carbon dioxide is continuously absorbed by plants and becomes part of their tissues,plants should contain traces of carbon-14.Since animals consume plants,animals should likewise contain traces of carbon-14.After a plant or other organism
8、dies,no additional carbon-14 should be incorporated into its tissues,while that which is already present should decay at a constant rate.The half-life of carbon-14 was determined by its codiscoverer,chemist Martin D.Kamen,to be 5,730 years,which,compared with the age of the Earth,is a short time but
9、 one long enough for the production and decay of carbon-14 to reach equilibrium.In his Nobel presentation speech,Swedish chemist Arne Westgren summarized Libbys method:“Because the activity of the carbon atoms decreases at a known rate,it should be possible to determine the time passed since death,b
10、y measuring the remaining activity,if this occurred during the period between approximately 500 and 30,000 years ago.”Libby verified the accuracy of his method by applying it to samples of fir and redwood trees whose ages had already been found by counting their annual rings.They also measured the a
11、ge of artifacts,such as wood from the funerary boat of Pharaoh Sesostris III,whose ages were already known.By measuring the radioactivity of plant and animal material obtained globally from the North Pole to the South Pole,he showed that the carbon-14 produced by cosmic ray bombardment varied little
12、 with latitude.This is the table of the measurement results of radioactivity intensity of carbon 14 at different latitudes On March 4,1947,Libby and his students obtained the first age determination using the carbon-14 dating technique.He also dated linen wrappings from the Dead Sea Scrolls,bread fr
13、om Pompeii buried in the eruption of Vesuvius(AD 79),charcoal from a Stonehenge campsite,and corncobs from a New Mexico cave.He showed that the last North American ice age ended about 10,000 years ago,not 25,000 years ago as previously believed by geologists.In nominating Libby for the Nobel Prize,o
14、ne scientist stated,“Seldom has a single discovery in chemistry had such an impact on the thinking in so many fields of human endeavour.Seldom has a single discovery generated such wide public interest.”After showing the essential uniformity of carbon-14 in living material,Libby looked for the answe
15、r to the second question by measuring the radiocarbon level in organic samples dated historicallymaterials as old as 5,000 years from sources such as Egyptian tombs.In 1950,Libby used this method to determine the construction age of the pyramids.It is incredibly coincided with the historical records
16、.Since the carbon-14 dating method is based on changes in the nucleus and is not affected by the physical and chemical conditions of the surrounding environment,the half-life of carbon-14(5730 years)is suitable for the determination of samples from thousands to tens of thousands of years ago.PART TH
17、REE Application In addition,some carbon-containing materials,such as wood,grass,shells and other animal and plant remains,are everywhere in ancient sites,and it is convenient to obtain specimens.Therefore,the carbon-14th dating method is widely used in the study of Prehistoric Archaeology and late Q
18、uaternary geology.However,the carbon-14 dating method can only be used for organisms that have died for tens of thousands of years.For some organisms that have died for millions or even hundreds of millions of years,the carbon-14 content in the remains of these organisms has been too low to be measu
19、red due to too many half-lives.Therefore,this method can not be used to measure the age of these organisms.With the help of the American Anthropological Association and the American Geological Association,Libby and his colleagues measured a large number of cultural relics and prehistoric relics of k
20、nown ages to verify whether the carbon 14 dating method was actually available.Here is the graph for Carbon 14 dating of known historical relics After learn this class,I hope you not only know about Prof.Libby,but also have some ideas about how to determine the age of fossils and relics by Carbon 14
21、.Thats todays Nobel prize winner,thank you!思思 考考 利比是著名的物理化学家，放射化学专家，热原子化学、示踪技术、同位素示踪技术专家，1947年创立了放射性碳十四测定年代的方法，在考古学中得到极其重要的应用，还应用放射性同位素氚于地球物理和水文学研究。1960年，鉴于利比发现碳-14年代测定法，为人类创造出独特的计时工具“考古学时钟”，他被授予诺贝尔化学奖。由于碳-14年代测定法依据的是原子核的变化，不受周围环境的物理、化学条件影响，而碳-14的半衰期（5730年）正适用于对几千年到几万年前的标本进行测定，另外，一些含碳的物质，如木、草、贝壳等动植物
22、遗骸在古代遗址中普遍存在，获取标本很方便。因此，碳-14年代测定法被广泛应用于史前考古学和第四纪晚期地质学的研究。同学们，咱们在物理化学这门课中是不是学过半衰期及其相关计算？对的，埋藏地下深层的沉积地层的样品，只要按C14的放射性衰变公式进行计算，便可推出待测物品的存在年代。其中，C14平均寿命是一个常数，由实验测出N0,NA即可计算出标本年代。Besides the Nobel Prize in Chemistry for 1960,he received other distinctions,including the Research Corporation Award for 1951
23、 for the radiocarbon dating technique;the Chandler Medal of Columbia University for outstanding achievement in the field of chemistry(1954);the American Chemical Society Award for Nuclear Applications in Chemistry(1956);the Elliott Cresson Medal of the Franklin Institute(1957);the American Chemical Societys Willard Gibbs Medal Award(1958);the Albert Einstein Medal Award(1959);the Day Medal of the Geological Society of America(1961).