Nuclear physicist Edwin Mattison McMillan and chemist Glenn Theodore Seaborg discovered the first elements in the periodic table heavier than uranium—the so-called transuranic elements, of which plutonium is the most important. Plutonium, which is highly explosive, was the key element in the first nuclear bombs.
Plutonium’s story has fission
The news of the discovery of fission excited Edwin Mattison McMillan tremendously. He had worked with Ernest Orlando Lawrence
The fissioning of the uranium nucleus created fragments, and McMillan decided to measure how far these fragments would penetrate into various kinds of matter. He studied their range first in a stack of thin aluminum foils and then in a stack of cigarette papers. In the latter case, he smeared a layer of uranium oxide onto the top piece of paper and then exposed the stack to a beam of neutrons from the cyclotron. As expected, some of the fission fragments bored into the pile of thin papers and stopped at various depths. By taking the papers apart and measuring their radioactivity with a Geiger counter, he was able to determine the range of particular fragments. He also found that the top paper, containing the uranium, had radioactivities with different properties from the radioactivities in the other pieces of paper. In particular, he found a radioactive product that had a twenty-three-minute half-life (the time in which half the nuclei of an isotope undergo radioactive decay) and another with a half-life of a little more than two days.
In 1934, Enrico Fermi had tried to create elements heavier than uranium by adding neutrons to uranium; Otto Hahn and Fritz Strassmann had begun experiments of their own with the same intent. McMillan knew that the twenty-three-minute half-life belonged to uranium 239, an isotope
In the spring of 1940, McMillan concluded that the substance with a two-day half-life in his experiments resulted from the transformation of uranium 239 into a new element by means of electron (or beta) emission. When electron emission
Glenn Theodore Seaborg.
McMillan was assisted by Philip Abelson, a chemist at the Carnegie Institution in Washington, D.C. Using “carrier” techniques, they were able to separate the new element from uranium and characterize it chemically (a carrier is a substance, available in bulk amounts, chemically analogous to the radioelement obtainable only in minuscule amounts). At the start, McMillan suspected that element 93 would be chemically like rhenium, since element 93 would appear directly below rhenium in the periodic table; but, much to their amazement, they found that element 93’s properties made it a close relative of uranium. In their paper “Radioactive Element 93,” published in Physical Review in 1940, they suggested that this element might be part of a second “rare earth”
During the summer and fall of 1940, McMillan began looking for the product of neptunium 239’s radioactive decay. Since neptunium emitted electrons as it decayed, which would result in yet another neutron-proton conversion, McMillan reasoned that the product of its decay must be a new element with atomic number 94 and atomic weight 239. To make adequate amounts of this element, he bombarded uranium with deuterons in the 152-centimeter Berkeley cyclotron
Following academic protocol, Glenn Theodore Seaborg, who had received his doctorate at the University of California, Berkeley, in 1937, asked McMillan if he could continue his studies of element 94. McMillan agreed. Seaborg, whose postdoctoral work with Gilbert N. Lewis was on the effects of isotopic variations on the chemistry of elements, was well prepared to work out the chemistry of the new element. In the fall of 1940, he assigned his graduate student Arthur C. Wahl the thesis problem of investigating the chemical properties of element 93. Joseph W. Kennedy
On December 14, Seaborg and his colleagues bombarded uranium oxide with deuterons from the 152-centimeter cyclotron. They believed that this bombardment formed neptunium 238, an isotope with a short half-life and therefore a high radioactivity. After losing an electron, this isotope turned into element 94 with mass number 238 (and a very long half-life). During the weeks following this experiment, they were able to separate the longer-lived isotope from its short-lived precursor. The chemically separated fraction containing the new element, which exhibited the alpha radioactivity that McMillan had predicted, presented the researchers with the problem of isolating the new element from its nearest neighbors, in particular, uranium and neptunium.
The chemical key to the isolation of element 94 was that it had two oxidation states; that is, it could form two different compounds with oxygen. To put it in its higher-oxidation state required a stronger oxidizing agent than was required for neptunium. Since different chemical properties indicate a different element, the discovery of element 94’s particular oxidative properties constituted proof of its individuality. Seaborg’s team first successfully oxidized element 94 on February 23-24, 1941. Their paper describing the oxidation experiment was sent to Washington on March 7, but it was not published until 1946, because the new element’s potential military importance had become obvious.
While experiments leading to the discovery of element 94 were occurring, Seaborg’s team, augmented by Emilio Gino Segrè, was searching for the 239 isotope of the element. They suspected that this isotope would be fissionable; that is, it would split when bombarded with slow neutrons and simultaneously produce huge amounts of energy. In the spring of 1941, Seaborg and his collaborators made this new isotope by bombarding uranium 238 with neutrons. This resulted in neptunium 239, which soon decayed into the 239 isotope of element 94. This isotope turned out to be very stable, with a half-life of twenty-four thousand years. When they bombarded isotope 239 with slow neutrons from the 94-centimeter Berkeley cyclotron, its nucleus fissioned with a release of energy greater than what scientists had obtained with uranium 235 (the fissionable isotope of uranium).
The researchers recognized immediately that the 239 isotope of element 94 had the potential to make up the highly explosive ingredient of a nuclear bomb. Therefore, as with the 238 isotope, Seaborg’s team decided to withhold public announcement of isotope 239’s fissionability until 1946. In March, 1942, when they were preparing detailed reports about their studies of the new element, they named it “plutonium.” After discussions about using “P1” or “Pu” for the element’s symbol, they chose Pu, an appropriate designation for this highly poisonous substance.
In the spring of 1942, Seaborg took a leave of absence from the University of California to join the operation to make material for an atomic bomb.
In the course of their work, Seaborg’s team developed new techniques for handling minuscule amounts of radioactive material, transforming such common apparatus as test tubes, flasks, and balances into devices that could handle adeptly pinhead-sized quantities of material. These ultramicrochemical techniques enabled the group to work out the chemistry of plutonium. In an important early experiment, they succeeded, on September 10, 1942, in weighing the first visible amount of plutonium 239 (about one ten-millionth of an ounce).
At the Metallurgical Laboratory, Seaborg and his colleagues discovered in nature minute quantities of neptunium and plutonium, the products of natural radioactive processes. In the summer of 1944, as a result of their recognition that neptunium and plutonium form part of a new series in the periodic table, Seaborg and his collaborators were able to discover two more elements, 95 and 96 (americium and curium, respectively).
The successful solutions to the problems of the chemical separation of plutonium led to the construction, in Hanford, Washington, of large plutonium-producing nuclear reactors and a massive plant designed for the chemical separation of plutonium. A ratio of about one to a billion was involved in the scale-up from the minute quantities used by Seaborg’s team to the huge amounts used in the Hanford Engineer Works
Seaborg and McMillan, the two scientists most responsible for the discovery of plutonium, believed that its use was justified in the Nagasaki bomb to bring a swift end to World War II. Afterward, they were made aware that many people associated their discovery with myriad deaths and destruction. With the development of fast breeder reactors, based on the production and recycling of plutonium, the insidious properties of plutonium became well known to the public, especially through the efforts of various groups in the environmental movement.
Seaborg, in particular, fought against the association of plutonium with the horrors of nuclear war and the poisoning of the planet. In many of his speeches and writings, he expressed the hope that plutonium would be used in peaceful ways to raise standards of living. For Seaborg, plutonium confronted humanity with the choice that several previous scientific discoveries also presented. Plutonium could, like them, be used destructively, but Seaborg believed that human beings, with deepened wisdom and understanding, could learn to use this element constructively to build a world of lasting peace and shared abundance.
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Chemistry, 1942-1962. River Edge, N.J.: World Scientific, 1999. Provides the Nobel lectures of McMillan and Seaborg from 1951. Includes brief biographies of the laureates. -
Gaddis, John Lewis, et al., eds. Cold War Statesmen Confront the Bomb: Nuclear Diplomacy Since 1945. New York: Oxford University Press, 1999. Discusses the global politics of nuclear proliferation following World War II. Includes the chapter “Longing for International Control, Banking on American Superiority: Harry S. Truman’s Approach to Nuclear Weapons.” Bibliographical references, index. -
Heilbron, J. L., and Robert W. Seidel. Lawrence and His Laboratory: A History of the Lawrence Berkeley Laboratory. Berkeley: University of California Press, 1989. Ernest O. Lawrence was the guiding spirit behind Berkeley’s Radiation Laboratory, in which bigger and better atom-smashing machines were built and used to do research. Uses material from the Lawrence papers and other sources to recount the early history of the laboratory. -
Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon & Schuster, 1986. Well researched and clearly written. Has become the principal account of how the atomic and hydrogen bombs were developed. Narrates, in graphic human, scientific, and technical detail, how the bombs evolved from basic discoveries in chemistry and physics. Illustrated with diagrams and photographs. Detailed index. -
Seaborg, Glenn T., with Eric Seaborg. Adventures in the Atomic Age: From Watts to Washington. New York: Farrar, Straus and Giroux, 2001. Seaborg’s biography, covering a range of years, from his childhood to the discovery of plutonium and the Manhattan Project to the Nobel Prize to the future of nuclear energy. Illustrations, index.
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