A team of researchers is using state-of-the-art forensic techniques to solve the riddle of the origin of uranium cubes that were used as part of the Nazi effort to develop nuclear weapons during the Second World War.
The story of the Allied Manhattan Project, which was established in 1942 to beat Germany in the race for "the Bomb" is well-known, but what is much less familiar to the public was how the German effort compared.
In the popular imagination, and in the fears of Allied leaders of the day, it was easy to imagine that the two projects were mirror images of one another. In fact, they couldn't have been more different. The Allied effort was the pooling of resources by the United States, Britain, and Canada, with the best scientists and engineers sent to specially made, secret facilities operating under military discipline and with the full backing of Allied governments and a budget that wouldn't see anything comparable until the US Moon landings in the 1960s.
Because Germany had begun work on its bomb in April 1939, shortly after atomic fission had been demonstrated in the laboratory, the Allies thought they were three years behind and worked frantically to create their own weapon before a German version was dropped on London or Washington.
The truth was that, far from being on track to loading atomic bombs onto V2 rockets, the German effort was remarkably half-hearted. Part of the reason was that the Nazi's anti-Jewish pogroms had caused the world's leading physicists to flee the German sphere of influence for the West, while those who were left were either conscripted into the Army or sent to work on projects like the first ballistic missiles.
Worse, the armaments minister Albert Speer and heads of the Uranium Club, as the project was nicknamed, deliberately played down the idea of making a bomb to make sure that Hitler never became enthusiastic about it. This was partly because the project scientists didn't think they could build a bomb before the end of the war, but mainly because nobody wanted to face the Fuhrer one with news of a failure, which tended to reduce one's life expectancy.
As a result, the German project was unfocused. Broken up into several groups, the project worked largely on a theoretical bomb while preferring to focus on developing an atomic motor, and was bounced from one department to another. They made so little progress that when key scientist Werner Heisenberg was informed of the bomb being dropped on Japan after his capture, he refused to believe it.
When Germany fell in 1945, the British and the Americans had special teams rushing across the country to secure enemy research facilities before the invading Red Army could capture them. These included a German experimental reactor called Leipzig-IV at Haigerloch, which consisted of about 650 cubes of uranium about two inches (5 cm) wide that were spaced on strings of aircraft wire and then suspended in a vat of heavy water, where the light hydrogen atoms were substituted with heavy ones of deuterium isotopes.
These cubes were supposed to produce a fission reaction with the water acting as a moderator, slowing down the neutrons enough to increase the chances of them interacting with a uranium atom. This didn't work because later calculations showed that at least another thousand cubes would be needed and the reactor was damaged when oxygen accumulated inside, leading to an explosion and the world's first nuclear accident.
After the war, many of these cubes were sent to the United States and Britain, but in the process many were lost track of and over the last 75 years some have cropped up in the most unlikely places from time to time, without any provenience to show where they came from or where they'd been since.
One of these cubes ended up at the Pacific Northwest National Laboratory (PNNL) in Richland, Washington. How it got there is a mystery – it wasn't even certain it was actually one of the missing German reactor cubes or if it was from some other project. In hopes of finding the answer, a team under Jon Schwantes applied modern forensic techniques to compare surviving cubes and figure out which research groups they were associated with.
The PNNL cube is believed to be a Heisenberg Cube from Haigerloch, but the evidence is largely anecdotal. To put things on a more scientific basis, team member Brittany Robertson used a technique called radiochronometry to gather some hard facts.
Radiochronometry is essentially the same as carbon dating, which is used to help date archaeological finds. When an organism is alive, it absorbs carbon from the surrounding environment. Some of this carbon is the radioactive isotope carbon-14, which is created by cosmic rays striking the atmosphere and has remained at a more or less constant ratio before the introduction of industrial pollution and atmospheric atomic weapons tests.
When the organism dies, the carbon-14 decays at a known rate. By measuring the ratio of carbon-14 to normal carbon, it's possible to calculate the date of, for example, an Egyptian mummy with surprising accuracy.
When applied to the reactor cubes, similar insights can be found. Since the cubes were originally made of almost pure uranium, it's a relatively simple matter to determine their age. In addition, the trace elements and their isotopes can also tell a lot about the cubes and their origins, including where the original ore was mined. The latter is important because this can help determine if the PNNL cube is from the Heisenberg group, which started in Berlin but moved to Haigerloch, or the Kurt Diebner research group operating at Gottow.
Another part of the detective game is looking at the coating on the cube that was applied in the 1940s to prevent oxidation. Heisenberg's group used a cyanide-based coating, but the PNNL cube is coated with styrene, which is the same as some of the Diebner cubes. This means to cube might have been from the Diebner group, or it could be one of those that were later transferred from the Diebner group to the Heisenberg group.
According to the team, this investigation is of more than historic value. The same techniques used on the reactor cubes also have applications in the tracing of illegal nuclear materials trafficking, which can include illegally obtained nuclear reactor fuel, materials that could be used in the manufacture of weapons, terrorist activities, and simple smuggling of radioactive isotopes used in medicine and industry.
“We’re curious if this particular cube was one of the ones associated with both research programs,” says Schwantes. “Also, this is an opportunity for us to test our science before we apply it in an actual nuclear forensic investigation.”
The findings of the research were presented at the ACS Fall 2021 meeting.
Source: American Chemical Society
