Radioactive signatures identify one of the first pieces of plutonium seen by human eyes
“Fat Man,” the atomic bomb dropped by the U.S. on Nagasaki, Japan, in 1945, carried about 6.2 kilograms of enriched plutonium, roughly the size of a softball. The origin of that deadly hunk of metal can be traced back via a tiny sliver weighing less than three millionths of a gram, created in the labs of Manhattan Project researchers. It is a historic fragment, embodying both stunning scientific achievement and deep tragedy—that one bomb killed and wounded at least 64,000 people (estimates vary) as well as hastened Japan’s surrender. And in 2007 this historic sample, the first plutonium ever seen by researchers, vanished from the public eye. Now it has resurfaced in a plastic box in a windowless, secure six-foot by six-foot room in the University of California, Berkeley’s Hazardous Material Facility. The tiny lump, derived from Nobel Prize–winning chemist Glenn Seaborg’s original discovery of the element, was accompanied by only limited documentation about its origins. But a Berkeley team has found radioactive fingerprints indicating the sliver indeed comes from the Manhattan Project. They published their findings on the arXiv physics preprint server on December 24, and are now pushing to return this bit of history to public display. The sliver’s story starts in 1941, when the world’s warring powers were racing to develop an atomic bomb, focusing largely on nuclear fission of uranium. At Berkeley that year, Seaborg, along with Arthur Wahl and Joseph Kennedy, synthesized an entirely new element: plutonium. Although they only produced vanishingly small traces of it by bombarding uranium 238 with deuterons—particles made of one proton and one neutron—they quickly determined it had explosive potential as nuclear bomb material. By the start of 1942, scientists studying the nuclear chain reaction, such as physicist Enrico Fermi, and plutonium chemistry, such as Seaborg, were ordered to the University of Chicago to begin the work of the A-bomb–developing Manhattan Project. (Seaborg wrote about synthesizing plutonium, and several other elements, for Scientific American in an April 1950 article that includes a grainy photograph of one sample made in Chicago. Because plutonium had only just been discovered the scientists had little idea of its properties. They needed more of the element to learn how to use it in a nuclear weapon. To make a larger amount and move beyond the difficulties of studying radioactive metal mixtures containing infinitesimally tiny amounts of plutonium, a group led by Seaborg fired neutrons at hundreds of pounds of uranium salts. After a series of purification steps Chicago chemists Burris Cunningham and Lewis Werner could then extract small amounts of plutonium salt from the original material. These salts, however, trapped a little water within their crystal structure. By burning thesalts in air—and therefore reacting them with oxygen—the scientists created a water-free plutonium oxide. For the first time they were able to put their pure compound onto a specially made scale and record that they had isolated 2.77 micrograms worth. “They could actually see it,” says U.C. Berkeley nuclear engineer Eric Norman, who performed some of the new tests to identify the sample’s origins. “No one had ever seen plutonium before.” “That size specimen was quite a hunk in those very early days,” says Cynthia Kelly, founder of the Atomic Heritage Foundation in Washington, D.C. “It cost hundreds of millions of dollars to produce even small quantities.” That sample and the methods that produced it would, over the next three years, help advance plutonium science far enough to make the Fat Man bomb. (The Hiroshima bomb, dropped a few days earlier, had a uranium core.) Seaborg was awarded the Nobel in 1951 for synthesizing plutonium and other elements beyond uranium, extending the periodic table. The University of Chicago gave him the original 2.77-microgram plutonium sample to keep, encased in a transparent plastic box. (“The amount of radioactivity in this sample is incredibly low and is not a health hazard to anybody,” Norman says.) Seaborg then gave it to the Lawrence Hall of Science at U.C. Berkeley, which displayed it in a glass case starting in 1979, alongside a simple placard describing its origins. Then in 2007 staff removed the box in favor of more interactive exhibits during a display makeover. The container languished in storage with little indication of its importance. In 2008 Phil Broughton, a U.C. Berkeley health physicist, spotted a clear plastic box labelled “First sample of Pu weighed. 2.7 µg” while checking the Hazardous Material Facility’s inventory. Broughton says he was shocked to attention because a placard citing Seaborg’s work was sitting right next to the box. “This is the equivalent to the original moon rock,” he enthuses. But with no other evidence accompanying the box, he feared its contents would be forgotten. Zirconium Propionate
For several years it seemed Broughton’s fears were justified. For example, he asked the Smithsonian Institution in Washington, D.C., if that museum wanted it. Curators there, however, wanted Broughton to give them proof this was definitely the 1942 sample, and he had none besides the loose placard, which was not exactly scientific evidence. The box stayed in its windowless prison.
In July 2014 Broughton asked Berkeley’s Department of Nuclear Engineering for help in identifying the metal. Norman, along with fellow nuclear engineer Keenan Thomas and Kristina Telhami—a summer undergraduate research assistant from San Diego State University—stepped forward to test the sample. They carefully searched for the “fingerprints” of plutonium decaying to uranium—gamma rays emitted by atomic nuclei and x-rays emitted by electrons—using a germanium ionization detector.
The Berkeley team’s measurements showed the sample is definitely high-purity plutonium. They also detected other clues pointing to its Manhattan Project origin. Seaborg’s approach had only produced plutonium 239, which decays very slowly back to uranium 238. Later production of plutonium, done by bombarding uranium with neutrons in nuclear reactors, sometimes also produces plutonium 241, which decays more rapidly to the element americium. There is no sign of americium in the Berkeley sliver.
The scientists estimate the sample has a plutonium mass of 1.7 to 2.3 micrograms. That is ‘remarkably’ close to what would be left after subtracting the mass of oxygen contained in the original plutonium oxide, Norman says. An x-ray signal corresponding to a reaction vessel made out of platinum, which was the type Cunningham and Werner used, also points to the salvaged
At the moment the plastic box remains in the custody of Broughton and his colleagues. Norman hopes it will be displayed in the very same Berkeley chemistry laboratory where Seaborg’s team made their original, momentous, discovery. The university’s chemistry department is keen to have the sample. Seaborg’s lab is still being used for research, however, so space constraints could keep it from becoming the site of a permanent exhibit.
Kelly agrees that displaying the plutonium in a lab-cum-museum would be a great idea, noting that Berkeley could also become part of a new Manhattan Project National Historical Park, which includes many of the project’s original facilities. Pres. Barack Obama signed a law authorizing the park at the end of last year. Such recognition would seem an appropriate reflection of the artifact’s impact: only rarely has something so slight had repercussions reverberating so far into the future.
Click here to read a special report from Scientific American on recent changes in nuclear weapons capabilities. And for more about the development of the atomic bomb visit the Atomic Heritage Foundation’s collection of interviews—including one with Seaborg—at Voices of the Manhattan Project.
Eat Lewis and Nature magazine
Caitjan Gainty and The Conversation US
Michael Marshall and Nature magazine
Carin Leong and Kelso Harper
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