First Nuclear Bomb Is Detonated

Date July 16, 1945

Physicist J. Robert Oppenheimer directed the development and design of the nuclear bomb in the Manhattan Project during World War II, culminating in the first successful nuclear explosion at Alamogordo, New Mexico. Ten days after what was called the Trinity test, the United States and Great Britain demanded Japan’s unconditional surrender. The bomb’s explosion also hailed the start of the atomic age.

Also known as Trinity test; atomic bomb; a-bomb; the bomb

Locale Alamogordo, New Mexico

Key Figures

• J. Robert Oppenheimer (1904-1967), American physicist who directed the Los Alamos Laboratory
• Leslie Richard Groves (1896-1970), American engineer and army general in charge of the atomic bomb project
• Enrico Fermi (1901-1954), Italian American nuclear physicist and 1938 Nobel laureate in physics

Summary of Event

The successful detonation of the first atomic bomb was the dramatic conclusion of nearly fifty years of scientific research and a four-year crash program of wartime development. The discovery of radioactivity by Antoine-Henri Becquerel in 1896 revealed the enormous energy locked inside the atom. Albert Einstein’s 1905 theory of relativity showed that this energy could be accounted for by a tiny loss of mass in a radioactive material.

A practical method of releasing large quantities of atomic energy was not possible, however, until after the discovery of the neutron in 1932 by James Chadwick . This discovery led Enrico Fermi in 1934 to bombard various elements with neutrons, whose lack of electric charge allowed them to penetrate the positive nucleus and transmute them to heavier radioactive atoms. Fermi showed that neutrons slowed by water were especially effective and thought he had produced transuranic elements by bombarding uranium. He failed to recognize that he had probably split the uranium nucleus into lighter nuclei.

The first evidence of uranium fission was observed by Otto Hahn and Fritz Strassmann in Berlin at the end of 1938. When they discovered radioactive barium impurities in neutron-irradiated uranium, they wrote to their colleague Lise Meitner, a refugee in Sweden from German anti-Semitic laws. She and her nephew Otto Robert Frisch saw the possibility of fission and calculated the large release of energy in the repulsion of the nuclear fragments such as barium, which matched the resulting loss of mass. This result was reported to Niels Bohr in Copenhagen and was published in the English journal Nature in February, 1939.

The energy of fission fragments was soon measured by Frisch in Copenhagen, and Frédéric Joliot and his associates in Paris, who demonstrated the release of up to three additional neutrons. Soon, several researchers in American laboratories confirmed the energy and neutron yields.

While visiting the United States from January to May of 1939, Bohr derived a theory of fission with John Archibald Wheeler of Princeton University. This theory led Bohr to predict that the common isotope uranium 238 (which accounts for 99.3 percent of the uranium found in nature) would require fast neutrons for fission, but the rarer uranium 235 would fission with neutrons of any energy. This suggested the idea of studying fission in a systematic way with a controlled chain reaction using slow neutrons.

By July of 1939, the need for government support and secrecy led Leo Szilard and Eugene Paul Wigner to approach Einstein about writing a letter to President Franklin D. Roosevelt . The president responded by forming the Advisory Committee on Uranium, which provided $6,000 to Columbia University for the purchase of fifty tons of uranium oxide and four tons of graphite to study its ability to slow neutrons.

Meanwhile, in England, Frisch and Rudolf Peierls worked out the requirements for an atomic bomb in a paper sent to the English government in the spring of 1940. They estimated that less than 1 kilogram (2.2 pounds) was required for a critical mass of pure uranium 235, in which more neutrons would cause fission than those that escaped or were absorbed. If two or more subcritical masses could be brought together rapidly enough, they would produce an explosion equivalent to several thousand tons of dynamite. They also outlined the extensive effort that would be needed to separate uranium 235 from natural uranium.

By December, 1941, when the United States entered World War II, contracts had been arranged with about twelve American universities for uranium research. Wartime research was supervised by Vannevar Bush , with James Bryant Conant as his representative for uranium research. Three major projects included the study of uranium separation methods at Columbia, cyclotron studies at the University of California, and chain-reaction studies at the University of Chicago. Uranium bombardment in the Berkeley cyclotron led to the discovery of plutonium in 1940 and the fact that plutonium 239 was fissionable. Since uranium 238 was found to absorb neutrons without fission at some high energies, it could not be used for a bomb but offered the possibility of breeding plutonium, which could then be separated from uranium by chemical methods.

In 1941 at Columbia University, Fermi began research on fission rates in a subcritical lattice of uranium oxide and graphite and showed that a chain reaction would be sustained if impurities are reduced to less than 1 percent. His group moved to Chicago in 1942, where they constructed a reactor with 40 tons of natural uranium and 385 tons of graphite, large enough for a sustained chain reaction. When neutron-absorbing cadmium control rods were partially removed on December 2, a sharp increase in neutrons indicated success. Much larger reactors would be needed to breed sufficient plutonium for a bomb.

During 1942, the Manhattan District of the Corps of Engineers was formed under General Leslie Richard Groves, who contracted with the Du Pont Corporation to construct three secret atomic cities at a total cost of $2 billion. At Oak Ridge, Tennessee, twenty-five thousand workers built a one-thousand-kilowatt reactor as a pilot plant and spent two years building a five-thousand-stage gaseous diffusion plant for uranium 235 separation. Huge electromagnets were built also for further uranium enrichment at a cost of $500 million. A second city of sixty thousand inhabitants was built at Hanford, Washington, where three huge reactors and remotely controlled plutonium-extraction plants were completed in early 1945.

Study of fast-neutron reactions for a nuclear bomb were brought together in June of 1942 at Chicago under the leadership of J. Robert Oppenheimer. He soon became a personal adviser to Groves, who built Oppenheimer a laboratory for the design and construction of the bomb at Los Alamos, New Mexico. In 1943, Oppenheimer gathered two hundred of the best scientists of the Manhattan Project to live and work at this third secret city. Equipment was assembled, and research was begun on such problems as neutron absorption, uranium and plutonium fabrication and purification, and explosive methods for forming a critical mass.

Finally, two bomb designs were developed. A gun-type bomb called Little Boy used 15 kilograms (33 pounds) of uranium 235 in a 4,500-kilogram (5-ton) cylinder about 2 meters (2 yards) long and 0.5 meters (0.5 yards) in diameter, in which a uranium bullet could be fired into three uranium target rings to form a critical mass. An implosion-type bomb called Fat Man had a 5-kilogram (11-pound) spherical core of plutonium about the size of an orange, which could be squeezed inside a 2,300-kilogram (2.5-ton) sphere about 1.5 meters (1.5 yards) in diameter by properly shaped explosives to make the mass critical in the shorter time required for the faster plutonium fission.

By early 1945, the first usable amounts of plutonium and uranium arrived from Hanford and Oak Ridge. Enough plutonium was available by the end of May to begin critical-mass studies by Frisch’s group. A test date was set for July, when one uranium and two plutonium bombs would be finished. A flat scrub region 200 kilometers (125 miles) southeast of Alamogordo was chosen for the test site named Trinity, and observer bunkers were built about 10 kilometers (6.25 miles) from a 30-meter (33-yard) steel tower.

On Friday, July 13, one of the plutonium bombs was assembled at the site; the next morning, it was raised to the top of the tower. On July 16, after a short thunderstorm delay, the bomb was detonated at 5:30 a.m. The resulting implosion initiated a chain reaction of nearly 60 fission generations in about a microsecond. It produced an intense flash of light, followed by a fireball expanding to a diameter of about 600 meters (656 yards) in two seconds, and then it rose to a height of more than 12 kilometers (7.5 miles), forming its ominous mushroom shape. Forty seconds later, an air blast hit the observer bunkers, followed by a sustained and awesome roar. Measurements confirmed an explosive power of 18.6 kilotons of TNT (trinitrotoluene), nearly four times the predicted value.

Significance

The successful development of the atomic bomb had an immediate impact in ending World War II and a longer-term influence on postwar weapons development and nuclear technologies. On March 9, 1945, 325 American B-29 bombers dropped two thousand tons of incendiary bombs on Tokyo, resulting in 100,000 deaths from the firestorms that swept the city. Still, the Japanese military refused to surrender, and American military plans called for an invasion of Japan, with estimates of up to a half million American casualties, plus as many as 2 million Japanese casualties.

On August 6, after authorization by President Harry S. Truman, the B-29 named Enola Gay dropped the uranium bomb Little Boy on Hiroshima at 8:15 a.m. On August 9, the plutonium bomb Fat Man was dropped on Nagasaki. Approximately 100,000 people died at Hiroshima out of a population of 400,000, and about 50,000 more died at Nagasaki. Japan offered to surrender on August 10, and after a brief attempt by some army officers to rebel, an official announcement by Emperor Hirohito was broadcast on August 15.

Bibliography

Bernardini, Carlo, and Luisa Bonolis, eds. Enrico Fermi: His Work and Legacy. New York: Springer, 2004. A laudatory history of Fermi, his work, and his place in the development of nuclear physics. Includes the chapter “The Birth of Nuclear Energy: Fermi’s Pile,” which discusses Fermi’s work on nuclear chain reactions.

Bird, Kai, and Martin J. Sherwin. American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer. New York: Knopf, 2005. A biography of the physicist who directed the Los Alamos Laboratory and the detonation of the first nuclear bomb. Richly detailed, this work also explores Oppenheimer being branded a traitor and accused of treason during the McCarthy era of the 1950’s.

Junck, Robert. Brighter than a Thousand Suns. Translated by James Cleugh. New York: Harcourt Brace Jovanovich, 1958. A very readable historical account of the personal lives and political struggles of the atomic scientists who discovered nuclear fission and developed the bomb, with a particular focus on Oppenheimer.

Norris, Robert S. Racing for the Bomb: General Leslie R. Groves, the Manhattan Project’s Indispensable Man. South Royalton, Vt.: Steerforth Press, 2002. A biography of Groves, who was in charge of the Manhattan Project. Also discusses how he later shaped policies regarding nuclear weapons at the national as well as international level.

Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon & Schuster, 1986. The most comprehensive history of the atomic bomb available for general readers. Rich in human, political, and scientific detail, it provides the complete story of how the bomb was developed, from the discovery of radioactivity to the development of the hydrogen bomb. Nearly eight hundred pages of text are supplemented by about seventy pages of documentation. Includes a good index.

Schroeer, Dietrich. Physics and Its Fifth Dimension: Society. Reading, Mass.: Addison-Wesley, 1972. A good textbook on the interaction of science and society, with chapters on building the bomb, the decision to drop it, the hydrogen bomb, and the fallout problem. Illustrations and references.

Smyth, Henry De Wolf. Atomic Energy for Military Purposes. Princeton, N.J.: Princeton University Press, 1945. This volume is the official report of the scientific and technical work that was done from 1940 to 1945 to develop the bomb. It covers many details of both civilian and government involvement in the project. Includes several technical appendixes and indexes.

Trinity Remembered, the Trinity Test: Historic Documents, Photos, and Videos. A Web site devoted to the history of the detonation of the first nuclear bomb. Includes a wealth of resources and links to primary sources. http://www.trinityremembered.com/.

Williams, Robert C., and Philip L. Cantelon, eds. The American Atom: A Documentary History of Nuclear Policies from the Discovery of Fission to the Present, 1939-1984. Philadelphia: University of Pennsylvania Press, 1984. This volume includes the Szilard-Einstein letter, the Frisch-Peierls memorandum, the letters of Oppenheimer, and many documents on the Manhattan Project and postwar developments related to nuclear weapons.