US scientists working to solve Heseinberg cube mystery

No-one is sure how PNNL's Heisenberg cube - which the laboratory uses to help train international border guards and nuclear forensics researchers to detect nuclear material - got there, said Jon Schwantes, the project’s principal investigator. The laboratory also has access to a few other cubes, including through a collaboration with the University of Maryland, but their provenance is also uncertain. "We don’t know for a fact that the cubes are from the German programme, so first we want to establish that,” Schwantes said. “Then we want to compare the different cubes to see if we can classify them according to the particular research group that created them.”

"The project is pretty neat in that it's predicated on the assumption that the uranium metal cubes that we currently have access to are in fact from Nazi Germany," doctoral student Brittany Robertson, who has developed the techniques, said. "We have the opportunity to apply modern nuclear forensics techniques to actually confirm this pedigree."

Radiochronometry, a process which uses the decay products produced by the decay of uranium in the sample, is used to confirm the age of the sample is consistent with the years in which it is known that Nazi Germany was producing uranium metal for use in its programme. Other techniques can then be used to analyse the surface coatings on the cubes to confirm whether the cubes were from similar batches and even from similar programmes.

In the early 1940s, two German scientists - Werner Heisenberg and Kurt Diebner - were leading parallel efforts aimed at exploiting nuclear fission, working at different locations and using different scientific methods. These reactors were to be fuelled by the uranium cubes, measuring about 5 centimetres on each side and weighing just under 2.5 kg, hung on cables submerged in heavy water, but the design failed to produce the hoped-for chain reaction.

It is thought that between 1000 and 1200 of the cubes were made, Robertson said, but the fate of many of them is unknown with most "lost to history". Some 600 cubes removed from Heisenberg's site at Haigerlof by Allied forces were shipped to the USA. Most of those were probably used in the USA's nuclear programmes, and today the whereabouts of only about 12 cubes is known, so they are extremely rare historic samples.

So far, the radiochronometry experiments have confirmed that the cubes being analysed by PNNL contain natural, unenriched uranium dated to the time period at which it is known that the German programme was producing such fuel, Robertson said. Chemical analysis of the surface coating of one of the cubes has shown the presence of styrene. It is known that Heisenberg's team used a cyanide-based coating to inhibit surface oxidation of the fuel cubes, whereas Diebner's used a styrene coating. It is also known that some cubes from Diebner's Gottow site were also used at Haigerlof. Further analysis might be able to show which team the cubes were used by, or indeed if they were used by both.

This is the first time that analysis of the surface coatings of the cubes has been attempted, Robertson said, adding: "I thought it would be a long shot - that an organic [coating material] after this many years would still be detectable." The surface coatings of other cubes are now undergoing analysis, she added.

Robertson said she found it "surreal " to work with unique objects that are linked to an horrific time in history, but in addition to corroborating historic claims the work will also be applicable in nuclear non-proliferation and safeguards work going forward.

As well as advancing nuclear forensic science, Schwantes said, the work is also providing a valuable opportunity to demonstrate these techniques before application in an actual nuclear forensic investigation.

The researchers are presenting their work, which is supported and funded by PNNL, in the Nuclear Forensics session of the American Chemical Society's Fall 2021 meeting which runs from 22-26 August.