Hundreds of cubes of Nazi uranium produced as part of the German nuclear research programme went missing after the allied victory in Europe, but now scientists have developed a new way to identify them when they show up.
The race to develop nuclear technology was a critical venture during the Second World War, though nuclear weapons never entered into the conflict in Europe, and more than a tonne of the radioactive metal was hidden in secret laboratories throughout Nazi territory.
Now, new techniques to identify these uranium cubes have been presented at a meeting of the American Chemical Society, and could help investigations into the illegal trafficking of nuclear material.
Although the Nazi nuclear weapons programme had a two-year head start on the US effort, it never escaped the laboratory, in large part due to the regime’s politicisation of academia which caused many researchers to flee the country or defect to either the US or Soviet Russia.
During the early 1940s, there were several German scientists who were competing to exploit nuclear fission to contribute towards the war effort.
These included Werner Heisenberg, a Nobel Prize winner who had been attacked by the regime as a “White Jew” for his work on theoretical physics – something opposed by the Aryan Physics movement which was based on an opposition to Einstein’s Relativity.
Heisenberg was initially based in Berlin but was moved to a secret laboratory beneath a medieval church in the town of Haigerloch in the Swabian Alps to try to avoid the Allied troops.
Another researcher, Kurt Diebner, was based at a different experimental laboratory in Gottow, and the uranium cubes were produced at these sites to fuel nuclear reactors.
The cubes, measuring about two inches on each side, were hung on aircraft cables to make a kind of nuclear chandelier that was submerged in heavy water – water made with the hydrogen isotope deuterium – in the hope that the uranium decay would provoke a self-sustaining nuclear chain reaction.
Ultimately, the design failed.
When US and British forces reached the Haigerloch laboratory in 1945 more than 600 of the uranium cubes were shipped to the US, after being dug up from a field near the town.
Some of these may have been used in the American nuclear weapon efforts while others today belong to collectors, including research institutions.
But hundreds of the cubes from the Diebner laboratory disappeared.
One cube is held at the Pacific Northwest National Laboratory (PNLL) in the US, but nobody really knows how it got there, according to Dr Jon Schwantes, the principal investigatory behind the new research. The team also worked with Dr Timony Koeth at the University of Maryland which also has access to a few other cubes.
The PNLL cube is used to train international border guards and nuclear forensics researchers to detect nuclear material being trafficked.
Although it’s labelled a Heisenberg cube, doctoral student at the PNLL, Brittany Robertson, says that assertion is anecdotal.
Robertson turned to a technique called radiochronometry, the nuclear version of the technique used by geologists to identify the age of carbon samples based on radioactive isotope content.
As explained by the American Chemical Society: “When the cubes were first cast, they contained fairly pure uranium metal. As time passed, radioactive decay transformed some of the uranium into thorium and protactinium.”
Robertson has adapted a radiochronometry procedure to separate and quantify these elements in PNNL’s cube, developing a method that shows how their relative concentrations reveal how long ago the cube was made.
If refined, the method could also allow researchers to analyse rare-earth element impurities in the cube, revealing where the original uranium was mined which would indicate whether it was produced for the Heisenberg or Diebner group.
Robertson and Dr Schwantes are working with Dr Carlos Fraga at PNLL to examine the coatings of the cubes too, something which different laboratories applied to prevent the uranium from oxidising.
Curiously the PNNL team recently discovered that the University of Maryland’s cube, which is labelled as a Heisenberg cube, is coated in styrene – although Heisenberg’s group used a cyanide-based coating.
The team has now learned that some of the cubes from Diebner’s group were sent from Gottow to Heisenberg’s secret laboratory in Haigerloch, as Heisenberg attempted to amass more fuel for his reactor.
“We’re curious if this particular cube was one of the ones associated with both research programmes,” Dr Schwantes said. “Also, this is an opportunity for us to test our science before we apply it in an actual nuclear forensic investigation.”
The researchers said that working with material from the dawn of the nuclear age was intriguing, but recognised the objects were linked to an especially horrific period in history.
“I’m glad the Nazi programme wasn’t as advanced as they wanted it to be by the end of the war,” Robertson said, “because otherwise, the world would be a very different place.”