Atomic Bomb
The invention of the atomic bomb was an historical inevitability. The scientific discovery necessary for construction of such a bomb occurred just as World War II was about to begin. It seems impossible that an enormous new source of energy (nuclear fission) would not eventually be applied in the construction of a bomb.
The scientific discovery that made the bomb possible occurred in 1934. Enrico Fermi (1901-1954), an Italian physicist, was trying to produce element number 93 by bombarding uranium with neutrons. Fermi thought he had been successful, but his results were not clear-cut. Other scientists attempted to repeat his work to clarify Fermi's findings. In 1938, Otto Hahn (1879-1968) and Fritz Strassman showed that the products of Fermi's reaction were nuclei from the middle of the periodic table.
The significance of these findings was made clear by Lise Meitner (1878-1968) and Otto Frisch (1904-1979), who demonstrated that the uranium nuclei in Fermi's experiment had actually been fissioned by neutrons. The word fission means to break apart. In the case of nuclear fission, a neutron causes a large nucleus, like that of uranium, to break apart into two roughly equal pieces, the products identified by Hahn and Strassman.
Three other important discoveries followed quickly. First, Niels Bohr (1885-1962) predicted--and the American physicist John Dunning (1937-) soon confirmed--that only one isotope of uranium, uranium-235, actually undergoes fission. Second, Leo Szilard and Walter Zinn (1906-), in rediscovering the fission reaction, found that each time a uranium nucleus fissions, an average of about two neutrons are also released.
The production of these neutrons creates the possibility of a chain reaction. A chain reaction is any reaction in which the substance needed to start the reaction is also produced as a result of the reaction. In this case, the neutrons needed to initiate nuclear fission are also one of the products of fission.
A nuclear chain reaction will occur in any piece of fissionable material that is large enough to prevent the loss of neutrons to the surrounding environment. The size of the material needed is called its critical size or critical mass.
Finally, the discovery was made that enormous amounts of energy are released during fission. Each fission of a uranium-235 nucleus releases 100 million times more energy than is released in a chemical reaction.
Many scientists recognized the significance of this series of discoveries. The first rumblings of World War II were just being heard in Europe. The possibility of an explosive weapon that used nuclear fission was obvious to at least some scientists. In a fortunate twist of history, most of the German and Italian scientists working on nuclear fission were forced to flee, or chose to flee, their native lands, thus denying Adolph Hitler (1889-1945) the possible use of such a weapon.
By 1939, a number of nuclear scientists were convinced that the United States should begin construction of a fission weapon, an atomic bomb. They persuaded Albert Einstein (1879-1955) to present a letter (actually written by Szilard) to President Franklin D. Roosevelt (1882-1945), describing the bomb and encouraging a national effort for its development.
The U.S. government moved slowly on Einstein's letter. As late as 1943, less than $300,000 had been spent on the development of a fission bomb. It was not until mid-1942 that a program began in earnest to build a bomb. At that point, President Roosevelt authorized the creation of the Manhattan Project. The Manhattan Project was the name given the new Manhattan Engineering District created within the Army Corps of Engineers. Brigadier General Leslie R. Groves (1896-1970) of the Corps was put in charge of the Project.
One of the major tasks facing the project was the production of fissionable material for a bomb. Uranium-235, the only naturally occurring fissionable isotope of the elements, makes up only 0.7 percent of the element. In order to achieve a self-sustaining chain reaction, a much larger concentration of uranium-235 was needed. Since uranium-235 and the far more common uranium-238 are chemically similar, separation of the two isotopes presents a difficult challenge.
The project explored three methods of separating uranium-235 from uranium-238, gaseous diffusion, centrifuging, and electromagnetic separation. Most of the research on separation techniques took place at a new laboratory, built specially for the purpose at Oak Ridge, Tennessee. Directing research on the three separation techniques were Harold Urey (1893-1981) (diffusion), Eger Murphree (1898-1962) (centrifuging), and E. O. Lawrence (electromagnetic).
In addition, research was begun on the production of plutonium-239, a synthetic transuranium element that is also fissionable. This research was conducted in Hanford, Washington, under the direction of Eugene P. Wigner (1901-1995), an emigre from Hungary.
Two other major research centers were established at the University of Chicago and at Los Alamos, New Mexico. Fermi directed research on the first atomic pile and the first chain reaction at Chicago, while J. Robert Oppenheimer (1904-1967) was in charge of basic research and final assembly at Los Alamos.
The first atomic bombs were assembled and tested in mid-1945. Their design was simple. Two pieces of fissionable material, each less than critical size, were placed at opposite ends of the bomb. When the bomb was dropped, one piece was propelled into the second piece by means of a conventional explosive. A mass larger than critical size was produced, and a self-sustaining chain reaction was initiated.
The first test of a nuclear weapon was conducted on July 16, 1945 near Alamogordo, New Mexico. The weapon was a success, releasing an amount of energy equivalent to 20,000 tons of TNT. Within a month, two bombs were dropped on Japan, a uranium-235 weapon over Hiroshima, and a plutonium weapon over Nagasaki. An estimated 110,000-150,000 people were killed by the two bombs and another 200,000 or more were injured by them. Research on the atomic bomb continued after the war, soon leading to the invention of an immensely more powerful weapon, the nuclear fusion, or hydrogen, bomb.
This is the complete article, containing 969 words
(approx. 3 pages at 300 words per page).
