US researchers work to trace mystery uranium cubes back to Nazi nuclear program

As the Allies pushed into Nazi Germany in April 1945, a special team hunted for the Third Reich’s nuclear weapons program and famed physicist Werner Heisenberg.

In the town of Haigerloch, hidden in a cave beneath a castle, the Allied team uncovered an experimental nuclear reactor and buried in a field nearby, 659 uranium cubes. Heisenberg fled into the night, riding a bicycle with a backpack full of the radioactive blocks.

Most of the so-called “Heisenberg cubes” were lost after the war. Researchers in the US are now, for the first time, carrying out nuclear forensic analysis on three uranium cubes believed to be from Nazi laboratories, in a project that could have historical significance, as well as implications for nuclear security.

Brittany Robertson and Jon Schwantes of the Pacific Northwest National Laboratory in Washington State revealed the project on Tuesday at a meeting of the American Chemical Society.

“It’s somewhat surreal and somewhat intimidating,” Robertson said of working with an item likely handled by the famous Nazi scientist Heisenberg. “We’re dealing with these historical artifacts that are in limited quantity and we’ve got to get as much information as possible off of a very tiny amount of material.”

The artifacts’ Nazi origin “has been claimed by a number of people that have had access to these cubes, but to our knowledge, never actually confirmed experimentally,” she said. Robertson is pioneering new techniques to determine the material’s origin, as part of her doctoral thesis.

German scientist Werner Heisenberg in 1933. (German Federal Archive, unknown author, CC-BY-SA 3.0)

Her new forensic methods could “enhance the capabilities of the nuclear forensic community in important ways,” Schwantes said.

One of the cubes used in the experiment is housed in the Pacific Northwest National Laboratory. The other two cubes come from the personal collection of their collaborator, Timothy Koeth, of the University of Maryland.

The US and Nazi Germany were both racing to develop nuclear technology during the war. The Germans had a head start, with several teams competing to develop nuclear fission with the end goal of developing a weapon. Heisenberg’s group worked out of Berlin, before moving to Haigerloch in southwest Germany to evade Allied troops, while scientist Kurt Diebner headed a research group in Gottow.

Between the two facilities, the Nazis amassed between 1,000-1,200 uranium cubes. The blocks are about two inches long on each side, are charcoal-gray and weigh about five pounds. The Germans suspended hundreds of them on cables in heavy water as part of a failed attempt to produce plutonium.

A replica of the failed nuclear reactor the Nazis constructed at Haigerloch. (CC BY-SA 3.0, ArtMechanic, Wikipedia)

“The purpose of their reactors was to produce plutonium for their weapons program, so actually they weren’t interested in the uranium itself for their weapons program,” Schwantes said. “All indications suggest they were unsuccessful in that.”

Koeth came into possession of one of the cubes in 2013. An acquaintance delivered it to him with a cryptic note reading, “Taken from the reactor that Hitler tried to build.” He traced his cube’s likely origin back to Heisenberg, in research published in 2019 in the science journal Physics Today.

Samuel Goudsmit in 1928. (Public domain)

The Allied effort to track down the Nazis’ science program, called the Alsos Mission, was initiated by Leslie Groves, who headed the secret US effort to build a nuclear weapon, known as the Manhattan Project. The team included military, science and intelligence personnel, and was co-led by the Jewish, Dutch-American physicist Samuel Goudsmit, whose parents were both killed by the Nazis. Goudsmit and Heisenberg were friendly acquaintances before the war.

After starting in Italy, the Alsos team moved into southern Germany with the Allied advance and Heisenberg’s scientists fled their laboratory. They buried the uranium cubes, hid the heavy water in barrels, and concealed documents in a latrine. The Allied troops entered Haigerloch in April 1945, arrested and interrogated the scientists, and discovered the nuclear materials. Heisenberg was captured in German territory the following month, before being flown to England and held in a safe house.

Goudsmit later wrote that the German nuclear laboratories were “well equipped, but compared to what we were doing in the United States, it was still small-time stuff.”

Alsos Mission members dismantle the experimental Nazi nuclear reactor in Haigerloch. (Public domain)

The Heisenberg uranium cubes and heavy water were shipped to the US, which is where the trail gets murky. Most were likely used for the US weapons program. Koeth speculated that others ended up with Manhattan Project personnel as spoils of war or souvenirs. One cube was reportedly found in a creek in Germany, possibly tossed there by Heisenberg himself; another, in a drawer in New Jersey.

Most of the 400 or so Diebner cubes likely ended up in the Soviet Union, while others went on the black market in Europe.

There are around a dozen cubes believed to be in the US today, in both public and private collections, including the Smithsonian Institution and Harvard.

Robertson and Schwantes have only rumors to explain how their cube ended up at the Pacific Northwest National Laboratory, but they believe their forensic work can place its origin in Nazi Germany and may be able to trace it to either Heisenberg or Diebner’s lab. Definitive proof of the cubes’ origin would be hard to come by, but by analyzing the cubes’ age, composition, and known inconsistencies, they can trace the objects to the Nazi labs with a high level of confidence.

They are using a process call radiochronometry to date the cubes and line up the time frame with the Nazi program, and have confirmed that one of the cubes is natural uranium, which is consistent with the material used in the program.

Uranium cubes from the Nazi stockpile on display in a museum at Haigerloch. (Felix König, Wikipedia, CC-BY 3.0)

They are also analyzing the chemical coating of the cubes in an experimental process that could indicate which lab they came from. One of the cubes was coated with styrene, indicating it had been in Diebner’s lab, which used the coating material. Heisenberg’s team coated its uranium with cyanide. The researchers analyze the cubes by dissolving tiny amounts of uranium flakes from the surface layers.

If they are able to determine the age of the cubes down to the exact year, the age could also indicate which lab they are from, since Heisenberg and Diebner’s production efforts came one year apart.

Pure uranium cubes are rare, so if the age of the material, the coating, and contaminants are all consistent with the Nazi program, the researchers can be near certain of where they came from.

Members of the Allied Alsos Mission dig up uranium cubes hidden by German scientists in a field in Haigerloch in World War II. (Public domain)

The researchers tracking down Heisenberg’s cubes acknowledge the fun in the project, but keep its sinister origins in mind.

“Had they been successful,” Robertson said of the Nazi scientists, “the world would be a very different place, both within Europe and around the world. I try not to ever lose sight of that reality.”

Her lab’s cube could now help authorities combat nuclear threats, in a stark departure from its original, intended use.

The object is already used by the lab for educational purposes, such as training border guards and students. The new forensic analysis methods could help authorities in future nuclear investigations as part of non-proliferation or safeguard activities, by allowing investigators to determine the history of an object, corroborate its paperwork, or verify claims about its provenance, Robertson said.