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Where in the world are Nazi Germany’s uranium cubes?

1 May 2019

Hundreds of the hand-size blocks were created for nuclear reactor research by German scientists during World War II. Physics Today tracked down several of them, but most are still AWOL.

Chandelier of cubes at Atomkeller museum
In the 1940s Werner Heisenberg and his German colleagues built an experimental nuclear reactor that incorporated chains of uranium cubes, similar to these replicas displayed in the Atomkeller Museum in Haigerloch, Germany. Though hundreds of the cubes were produced, we don’t know where most of them are today. Credit: Felix König, CC BY 3.0

In April 1945, Manhattan Project director Leslie Groves coordinated a daring exercise behind enemy lines in Germany. The objective of the mission, called Alsos, was to gather intelligence about the German nuclear research program, which involved the likes of Kurt Diebner, Carl Friedrich von Weizsäcker, Karl Wirtz, and leader Werner Heisenberg. The Alsos team, which included physicists such as Samuel Goudsmit, found more than just information; they discovered an experimental laboratory in the southwest German town of Haigerloch, where they recovered 659 uranium cubes that had been produced for an ultimately unsuccessful nuclear reactor.

Further reading from the Physics Today archives:

  1. The German uranium project by Hans Bethe (2000)
  2. Revisiting The Los Alamos Primer by Cameron Reed (2017)
  3. The scientific exodus from Nazi Germany by Andrew Grant (2018)

In the May issue of Physics Today, Timothy Koeth and Miriam Hiebert from the University of Maryland describe the history of one of those cubes. But the fate of the vast majority of the other cubes, including another 400 or so that weren’t recovered by the Alsos team, remains unknown. Some may have been destroyed for use in the US weapons enrichment program at Oak Ridge National Laboratory in Tennessee. Others may have ended up in the hands of less respectable actors: Documents at the US National Archives at College Park, Maryland, describe risky efforts by profiteers to make a quick buck from cubes sold on the black market.

Koeth and Hiebert have set up an email account——for tips regarding the whereabouts of any of these historical artifacts. If you happen to have useful information, please alert them—and us. We’d love to hear about it.

Meanwhile, Physics Today made some calls and tracked down several cubes. Here’s where we found them:

Atomkeller museum, Haigerloch, Germany

Haigerloch, Germany
The entrance to the laboratory of Werner Heisenberg’s reactor experiment (left) was underneath a medieval church and castle in Haigerloch, Germany. Today the site is home to the Atomkeller Museum (right), which showcases artifacts from the failed experimental effort. Credits: AIP Emilio Segrè Visual Archives, Goudsmit Collection (left); LepoRello (Wikipedia), CC BY-SA 3.0 (right)

Where it’s stored: The cube (which is not fully intact) is on display behind Plexiglas 15 millimeters thick at the nuclear physics museum. The site is located beneath a church in what was once an underground laboratory where German physicists built an experimental reactor. The device consisted of hundreds of uranium cubes strung together by airplane cable and suspended in a pit of heavy water to regulate the rate of fission. A replica of the uranium chandelier reactor is also on display.

How it got there: The cube likely came from a Haigerloch colleague of Heisenberg’s, says Egidius Fechter, the museum’s curator. Eventually the cube was passed to Germany’s Landesanstalt für Umweltschutz (State Institute for Environmental Protection), whose president presented the cube to the museum in 2000. (See also the letter by Fechter and Michael Thorwart, Physics Today, April 2001, page 93.)

Mineralogical Museum, University of Bonn, Germany

Where it’s stored: The cube is on display at the two-century-old museum, which is housed inside the Baroque Poppelsdorf Palace.

How it got there: The museum received the cube in 1954 from the Scientific Research Division of the Military Security Board, which was created by the UK, US, and France to oversee demilitarization in West Germany. The cube is likely one of the 659 recovered by the Alsos team from Haigerloch.

Federal Office for Radiation Protection, Germany

Where it’s stored: At the organization’s Berlin site storage facility.

How it got there: The cube is probably not from Haigerloch but rather from Gottow or Stadtilm, where teams led by Diebner also had been pursuing reactor experiments. According to a 2013 article in the German Münchner Merkur newspaper, some boys in the town of Garmisch-Partenkirchen stole a bunch of cubes in April 1945 from a military truck that was retreating from the advancing Allied forces. The children didn’t find the black blocks very interesting—until discovering that when thrown the cubes would spark upon impact. The boys tossed dozens of the cubes into the Loisach River. Some years later another group of kids found a small black cube on the bank of the Loisach, says Klaus Mayer, deputy head of the nuclear safeguards and forensics unit of the European Commission’s Joint Research Centre. They too discovered that their toy produced sparks. A concerned parent showed the cube to a doctor, who discovered the block was radioactive. Eventually a private collector took possession of the cube and, in the mid 1990s, passed it on to the Federal Office for Radiation Protection. Mayer entered the story more recently, when he and his colleagues performed a nuclear forensic investigation of both that cube and the one at the Atomkeller museum. Measurements confirmed the cubes were likely made between 1940 and 1943, with the Atomkeller’s mined from material in what was then Czechoslovakia. The analysis found no indication of significant neutron bombardment, which supports the historical evidence that the Germans’ reactor failed to achieve criticality.

Pacific Northwest National Laboratory, Richland, Washington

Heisenberg cube at PNNL
Nuclear forensics scientist Jon Schwantes holds up the uranium cube that’s stored at the Pacific Northwest National Laboratory. Credit: Andrea Starr/PNNL

Where it’s stored: Since 2012, the cube has been used in a two-week nuclear nonproliferation training course offered to immigration and border security students from 40 member states of the International Atomic Energy Agency. “It’s a prop used . . . to exemplify what materials might look like in a developing nuclear program,” says Jon Schwantes, a senior nuclear forensics scientist at PNNL.

How it got there: Richard Libby, a former scientist for PNNL who now works at the US State Department, suspects the cube originally came to Department of Energy headquarters in Washington, DC, from the Alsos mission. In the mid 1990s DOE shipped the cube and other materials to PNNL. “When it arrived at the lab, I heard it might be one of the cubes from the Heisenberg experiments,” Libby recalls. In 2002 PNNL scientists conducted nondestructive analyses on the cube to estimate its age, but the results were inconclusive.

National Museum of American History, Washington, DC

Where it’s stored: The Smithsonian has had a cube in its collection since 1983, though it is not exhibited.

How it got there: The donor of the cube, Merril Eisenbud, was an environmental scientist who became the Atomic Energy Commission’s first health and safety officer in 1947. The AEC was gathering uranium for one of its programs, and it’s probable that many of the cubes recovered by the Alsos mission went to the AEC operations office in New York. When Eisenbud was directing the transfer of uranium materials in 1954 between two facilities, he discovered a cube in a pile of scrap and kept it for “historical interest.” When he retired in 1959, the AEC gifted him the uranium cube he saved from the garbage. In his 1983 donation letter to the Smithsonian, Eisenbud wrote, “There should be no question about the authenticity.”

Harvard University, Cambridge, Massachusetts

Harvard cube
Harvard University’s uranium cube is sometimes used for demonstrations. Photo courtesy of Harvard Natural Sciences Lecture Demonstrations

Where it’s stored: In the school’s Science Center. Owen Gingerich, professor emeritus of astronomy and history of science, once used it for demonstrations, including at a 2005 lecture at Harvard’s famed Memorial Church. He’d remove the cube from its lead-lined container and display the surge in signal of a Geiger counter. Harvard lecture demonstration technician Allen Crockett says the cube is still used occasionally as a “show-and-tell demo.”

How it got there: The deputy science director of the Alsos mission, Edwin Kemble, also was Harvard’s physics department chair, and he brought a cube back to Cambridge in 1945. Gingerich says he and other faculty initially thought the cube was the only one left from the German effort. Then he showed it to von Weizsäcker. “He had a broad smile,” Gingerich says. “He said he recognized it but that there had been hundreds of them, not just one.”

Rochester Institute of Technology, Rochester, New York (previously)

Where it’s stored: Unknown. Michael Kotlarchyk, head of the School of Physics and Astrophysics, says he believes the university once had a cube, which was stored in a cylindrical container lined with shielding. But in the early 1990s, due to changes in a law regarding the storage of radioactive materials, the university decided to dispose of the cube. “I guess in hindsight, we might have made the wrong decision,” Kotlarchyk says.

How it got there: “Unfortunately, we don’t seem to have any information about how we obtained the cube,” Kotlarchyk says.

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