Microsoft ® Encarta ® Reference Library 2003
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| Meitner, Lise
Meitner, Lise (1878-1968), Austrian-Swedish physicist, who first identified nuclear fission. She was born in Vienna, Austria, and educated at the Universities of Vienna and Berlin. In association with German physical chemist Otto Hahn, she helped discover the element protactinium in 1918, and she was a professor of physics at the University of Berlin from 1926 to 1933. In 1938 she left Germany and went to Sweden to join the atomic research staff at the University of Stockholm. In 1939 Meitner published the first paper concerning nuclear fission (see Nuclear Energy). She is also known for her research on atomic theory and radioactivity. In her work she predicted the existence of chain reaction, which contributed to the development of the atomic bomb (see Nuclear Weapons). In 1946 she was a visiting professor at Catholic University in Washington, D.C., and in 1959 she revisited the United States to lecture at Bryn Mawr College. In 1997 the International Union of Pure and Applied Chemistry announced that the chemical element with the atomic number 109 would be given the official name meitnerium (Mt) in her honor. Microsoft ® Encarta ® Reference Library 2003. © 1993-2002 Microsoft Corporation. All rights reserved. Protactinium Protactinium, formerly protoactinium, symbol Pa, radioactive metallic element with an atomic number of 91. Protactinium is a member of the actinide series of the periodic table (see Periodic Law). It was discovered in 1918 by the Austrian-Swedish physicist Lise Meitner and the German physical chemist Otto Hahn. Protactinium is a member of the uranium-actinium radioactive-decay series and is found in uranium ores. Isotopes of protactinium ranging in mass number from 215 to 238 are known. Protactinium-233 has a half-life of 27 days. Protactinium-231, the most stable isotope, has a half-life of more than 32,000 years; by emission of an alpha particle it decays to actinium. Protactinium melts at about 1552° C (about 2826° F), boils at about 4227° C (about 7641° F), and has a specific gravity of about 15.37. See also Radioactivity. Microsoft ® Encarta ® Reference Library 2003. © 1993-2002 Microsoft Corporation. All rights reserved. Meitnerium Meitnerium, symbol Mt, chemical element with atomic number 109. It is produced artificially by nuclear fusion (in which a chemical element with larger atoms is produced by fusing together smaller atoms from other elements). Each meitnerium atom has a very large nucleus, or central mass, containing positively charged particles called protons and neutral particles called neutrons (see Atom). The large number of particles in the nucleus makes the atom unstable and causes the atom to split apart into smaller components soon after it is created. The International Union of Pure and Applied Chemistry named element 109 meitnerium (Mt), which was previously called unnilennium, to honor Austrian Swedish physicist Lise Meitner, a pioneer in the field of nuclear fission (the splitting of atomic nuclei). Meitnerium has the atomic number 109, which means that each Mt atom contains 109 protons in the nucleus. Scientists have created several isotopes of meitnerium, or forms of the element that contain different numbers of neutrons in the nucleus. For example, meitnerium-266 contains 109 protons and 157 neutrons (109 protons + 157 neutrons = atomic mass 266), and meitnerium-268 contains 109 protons and 159 neutrons. Meitnerium was first created in 1982 by researchers at the Heavy-Ion Research Laboratory in Darmstadt, Germany, by nuclear fusion of the smaller elements bismuth (Bi) and iron (Fe). Because the meitnerium nucleus contains so many particles, meitnerium is unstable and undergoes spontaneous fission, a process in which the atom breaks into smaller “daughter” components. When the atom splits, it releases energy in the form of electromagnetic waves and electrically charged bits of matter. This energy is known as radiation (see Radioactivity). German scientists at the Heavy-Ion Research Laboratory created meitnerium-266, an isotope with a lifespan of only 0.0068 seconds. The most stable isotope of element 109 is meitnerium-268, which has a lifespan of 0.14 seconds. By 1998 these two isotopes were the only confirmed isotopes of meitnerium. Meitnerium belongs to Group 9 (VIIIb) on the periodic table, which also contains the naturally occurring elements cobalt (Co), rhodium (Rh), and iridium (Ir). Cobalt, rhodium, and iridium are shiny, silvery metallic elements with melting points above 1500° C (2732° F). Because elements in the same group, or column, on the periodic table often share similar properties (a pattern known as the periodic law), scientists expect meitnerium to share properties with other Group 9 elements. However, because of the limited amount of meitnerium that can be produced and its short lifespan, scientists have been unable to determine chemical properties of this unstable element.
Austrian-born physicist who shared the Enrico Fermi Award (1966) with the chemists Otto Hahn and Fritz Strassmann for their joint research that led to the discovery of uranium fission. After receiving her doctorate at the University of Vienna (1906), Meitner attended Max Planck's lectures at Berlin in 1907 and joined Hahn in research on radioactivity. During three decades of association, she and Hahn were among the first to isolate the isotope protactinium-231 (which they called protactinium), studied nuclear isomerism and beta decay, and in the 1930s (along with Strassmann) investigated the products of neutron bombardment of uranium. Because she was Jewish, she left Nazi Germany in the summer of 1938 to settle in Sweden. After Hahn and Strassmann had demonstrated that barium appears in neutron-bombarded uranium, Meitner, with her nephew Otto Frisch, elucidated the physical characteristics of this division and in January 1939 proposed the term fission for the process. She retired to England in 1960.
The term fission was first used by the German physicists Lise Meitner and Otto Frisch in 1939 to describe the disintegration of a heavy nucleus into two lighter nuclei of approximately equal size. The conclusion that such an unusual nuclear reaction can in fact occur was the culmination of a truly dramatic episode in the history of science, and it set in motion an extremely intense and productive period of investigation. The story of the discovery of nuclear fission actually began with the discovery of the neutron in 1932 by James Chadwick in England (see above). Shortly thereafter, Enrico Fermi and his associates in Italy undertook an extensive investigation of the nuclear reactions produced by the bombardment of various elements with this uncharged particle. In particular, these workers observed (1934) that at least four different radioactive species resulted from the bombardment of uranium with slow neutrons. These newly discovered species emitted beta particles and were thought to be isotopes of unstable "transuranium elements" of atomic numbers 93, 94, and perhaps higher. There was, of course, intense interest in examining the properties of these elements, and many radiochemists participated in the studies. The results of these investigations, however, were extremely perplexing, and confusion persisted until 1939 when Otto Hahn and Fritz Strassmann in Germany, following a clue provided by Irène Joliot-Curie and Pavle Savic in France (1938), proved definitely that the so-called transuranic elements were in fact radioisotopes of barium, lanthanum, and other elements in the middle of the periodic table. That lighter elements could be formed by bombarding heavy nuclei with neutrons had been suggested earlier (notably by the German chemist Walter Noddack in 1934), but the idea was not given serious consideration because it entailed such a broad departure from the accepted views of nuclear physics and was unsupported by clear chemical evidence. Armed with the unequivocal results of Hahn and Strassmann, however, Meitner and Frisch invoked the recently formulated liquid-drop model of the nucleus (see above) to give a qualitative theoretical interpretation of the fission process and called attention to the large energy release that should accompany it. There was almost immediate confirmation of this reaction in dozens of laboratories throughout the world, and within a year more than 100 papers describing most of the important features of the process were published. These experiments confirmed the formation of extremely energetic heavy particles and extended the chemical identification of the products. The chemical evidence that was so vital in leading Hahn and Strassmann to the discovery of nuclear fission was obtained by the application of carrier and tracer techniques. Since invisible amounts of the radioactive species were formed, their chemical identity had to be deduced from the manner in which they followed known carrier elements, present in macroscopic quantity, through various chemical operations. Known radioactive species were also added as tracers and their behaviour was compared with that of the unknown species to aid in the identification of the latter. Over the years, these radiochemical techniques have been used to isolate and identify some 34 elements from zinc (atomic number 30) to gadolinium (atomic number 64) that are formed as fission products. The wide range of radioactivities produced in fission makes this reaction a rich source of tracers for chemical, biologic, and industrial use. Although the early experiments involved the fission of ordinary uranium with slow neutrons, it was rapidly established that the rare isotope uranium-235 was responsible for this phenomenon. The more abundant isotope uranium-238 could be made to undergo fission only by fast neutrons with energy exceeding 1 MeV. The nuclei of other heavy elements, such as thorium and protactinium, also were shown to be fissionable with fast neutrons; and other particles, such as fast protons, deuterons, and alphas, along with gamma rays, proved to be effective in inducing the reaction. In 1939, Frédéric Joliot-Curie, Hans von Halban, and Lew Kowarski found that several neutrons were emitted in the fission of uranium-235, and this discovery led to the possibility of a self-sustaining chain reaction. Fermi and his coworkers recognized the enormous potential of such a reaction if it could be controlled. On Dec. 2, 1942, they succeeded in doing so, operating the world's first nuclear reactor. Known as a "pile," this device consisted of an array of uranium and graphite blocks and was built on the campus of the University of Chicago. The secret Manhattan Project, established not long after the United States entered World War II, developed the atomic bomb. Once the war had ended, efforts were made to develop new reactor types for large-scale power generation, giving birth to the nuclear power industry. |