The New encyclopaedia Britannica Volume 29 MACROPEDIA Knowledge in Depth FOUNDED 1768 15 TH EDITION Encyclopaedia Britannica. Inc. Robert P. Gwinn, Chairman, Board of Directors Peter B. Norton, President Philip W.Goetz, Editor in Chief Chicago Auckland/Geneva/London1 Madrid/ Manila/ Paris Rome/Seoul/Sydne y /Tokyo/Toronto 'Let knowledge grow from more to more and thus be human life cnnched.* The Encyclopedia Brirannica is published with the editorial advice of the faculties of the University of Chicago. Additional advice is given by committees of members drawn from the faculties of the Australian National University. the universities of British Columbia (Can.). Cambridge (Eng.), Copenhagen (Den. 1. Edinburgh (Scot. ). Florence (Italy). London ( Eng.). Marburg (W.Ger.). 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Technology of 5-5 \IT Force began production of an advanced cruise mis- example. banum and krypton~equalledthat of the ura- 511~. which would incorporate stealth technologies such as nium nucleus. Much energy was reieased in the process. radar-absorbent materials and smooth. nonreflective sur- This news set off experiments at many laboratories. Bohr face shapes. The advanced cruise missile would have a worked with John Wheeler at hnceton: they postulated range of over 1 .a00 miles. (S.0.F1 that the uranium isotope uranium-235 was the one un. dergoing fission: the other isotope. unnium-2 38. merely Nuclear weapons absorbed the neutrons. It was discovered that neutrons were produced during the tission process; on the average. Nuclear weapons derive their enormous explosive force each fissioning atom produced more than two neutrons. from either the fission or fusion of atomic nuclei. Their If the proper amount of matenal were assembled. these sqnificance may ktbe appreciated by the coining of free neutrons might create a chain reaction. Under special the words kiloton ( 1,000 tons) and megaton (one million conditions, a very fast chain reaction might produce a ions) to describe their blast effect in equivalent weights of very large release of energy; in short. a weapon of faniasnc TNT. For example, the first nuclear fission bomb. the one power might be feasible. dropped on Hiroshima. Japan. in 1945. released energy The fint atomk bomb. The possibility that such a equaling 15.000 tons ( 15 kilotons) of chemical explosive weapon might first be developed by Nazi Germany from less than 130 pounds (60 kilograms) of uranium. alarmed many scientists and was drawn to the attention Fusion bombs, on the other hand. have given yeids up of President Franklin D. Roosevelt by Albert Einstein. 10 almost 60 megatons. The first nuclear weapons were then living in the United Slates. The president appointed bombs delivered by aircraft: warheads for strategic ballistic an Advisory Commitiee on Uranium: it reported that a missiles. however, have become by far the most important chain reaction in uranium was possible. though unproved. nuclear weapons (see above Strategic missiles). There are Chain-reaction experiments with carbon and uranium also smaller tactical nuclear weapons that include artillery were starred in New York City at Columbia University. projectiles. demolition munitions (land mines), antisub- and in March 1940 it was confirmed that the isotope marine depth bombs, torpedoes, and short-range ballistic uranium-235 was responsible for low-veloaty neutron fts- and cruise missiles. The U.S.stockpile of nuclear weapons sion in uranium. The Advisory Committee on Uranium reached its peak in 1967 with more than 32.000 warheads increased its support of the Columbia expenments and of 30 different types; the Soviet stockpile reached its peak arranged for a study of possible methods for separating of about 33.000 warheads in 1988. the uranium-235 isotope from the much more abundant The basic principle of nuclear fission weapons (also uranium-238. (Normal uranium contains approximately called atomic bombs) involves the assembly of a sum- 0.7 percent uranium-235. most of the remainder being I acnt amount of fissile material (e.g..the uranium isotope uranium-238.) The centrifuge process. in which the heavi- uranium-235 or the plutonium isotope plutonium-239) er isotope is spun to the outside. as in a cream separator. to "go supcrcnticaT-that is. for neutrons (which cause at first seemed the most useful. but at Columbia a rival fission and are in turn released during fission) to be pro process was proposed. In that process. gaseous uranium dud at a much faster rate than they can escape from hexafluonde is diffused through barriers. or filters: more the assembly. There arc two ways in which a subcnucal molecules containing the lighter isotope, uranium-235. assembly of fissionable matenal can be rendered super- would pass through the filter than those containing the cntical and made to explode. The subcntical assembly heavier isotope, slightly enriching the mixture on the far mav consist of two pans. each of which is too small to side. A sequence of several thousand stages would be have a positive multiplication rate: the two pans can be needed to enrich the mixture to 90 percent uranium-235: shot together by a gun-type device. Alternatively. a sub- the toud bamer area would be many acres. critical assembly surrounded bv a chemical high explosive Dunng the summer of 1940. Edwin McMillan and Philip may be compressed into a supercntical one by detonating Abelson of the University of California at Berkeley dis- the explosive. covered element 93. named neptunium; they inferred that The basic principle of the fusion weapon (also called the this element would decay into element 94. The Bohr and Discovery thermonuclear or hydrogen bomb) is to produce ignition Wheeler fission theoy suggested that one of the isotopes. of coodttions in a thermonuclear fuel such as deuterium. an mass number 239, of this new element might also fission plutonium Isotope of hydrogen with double the weight of normal hy- under low-velocity neutron bombardment. The cyclotron fission drogen. or lithium deuteride. The Sun may be considered at the University of California at Berkeley was put to work a thermonuclear device: its main fuel is deuterium. which to make enough element 94 for experiments: by mid-1 94 1. It consumes in iu core at temperatures of 18.000,000* to element 94 had been firmly identified and named pluto- 36.000,000' F ( 10,000.000" to 20.000.000' C).To achieve nium. and its fission characteristics had been established. comparable temperatures in a weapon, a fission triggering Low-velocity neutrons did indeed cause it to undergo fis- device is used. sion. and at a rate much higher than that of uranium- 235. The Berkeley group, under Ernest Lawrence, was THE DEVELOPMENT OF FISSION WEAPONS also considering producing large quantities of uranium- Following the discovery of artificial radioactivity in the 235 by ~umingone of their cyclotrons into a super mass ^%. the Italian physicist Ennco Fermi performed a se- spectrograph. A mass spectrograph employs a magnetic ne~of experiments in which he exposed many elements to field to bend a current of uranium ions: the heavier ions low-veiocitv neutrons. When he exposed thorium and ura- furanium-238) bend at a larger radius than the lighter ions nium. chemically different radioactive products resulted. (uranium-235). allowing the two separated currents to be 'ndjcating that new elements had been formed. rather collected in separate receivers. hnmerely isotopes of the original elements. Fenni con- In May 194 1 a review committee reported that a nuclear cluded that he had produced elements beyond uranium explosive probably could not be available before 1945. (clement 92). then the last element in the penodic table: that a chain reaction in natural uranium was probabty 18 called them transuranlc elements and named two of months off. and that it would take at least an additional auscnium (element 93) and hcspenum (element 94). year to produce enough plutonium for a bomb and three ^"ng the autumn of 1938. however, when Fermi was to five yean to separate enough uranium-235. Further. it diving the Nobel Pnze for his work. Otto Hahn and was held that all of these estimates were optimistic. In late Strassmann of Germany discovered that one of the June 1941 President Roosevelt established the Office of elements was actually banum (element 56).