How close did the Nazis come to building an atomic bomb?

Nearly eight decades ago, when Allied forces and Nazi Germany were locked in mortal combat in World War II, American and German scientists were engaged in another desperate fight: a race to be the first to develop the atomic bomb.

We know how it turned out. But just how close did the Nazis come to developing a bomb? Clues to the answer, scientists believe, may lie in a handful of metallic cubes that have found their way into museums and private collections in the US and Europe.

During the war, two teams of Nazi scientists refined tonnes of uranium ore into 1,200 uranium cubes, each about 5cm on a side and weighing 2.2kg to 2.5kg. They then placed the cubes in complex arrays in reactors from which they hoped to generate plutonium for making a bomb, says Jon Schwantes, a nuclear radiochemist at America’s Pacific Northwest National Laboratory in Richland, Washington.

One team was led by a nuclear physicist named Kurt Diebner. The other was led by Nobel laureate Werner Heisenberg. “That’s the Heisenberg,” says Schwantes’ graduate student Brittany Robertson.“The Heisenberg of the Heisenberg uncertainty principle.”

Late in the war, a group of American and British scientists, intelligence officers and special forces soldiers combined in the Alsos Mission, in which they located and captured about 600 of the Heisenberg cubes. (They also raided Diebner’s facility, but his cubes have been lost to history.) Most of the captured cubes, Schwantes believes, were “folded into” the American uranium stockpile, but a few survived. “We think there’s about a dozen worldwide,” Schwantes says.

For the most part, though, nobody is sure if the few cubes that remain are actually from Heisenberg’s program. “Each has a different story for how it arrived at its current location, though most of the stories are incomplete at best,” wrote Timothy Koeth and Miriam Hiebert of the University of Maryland in Physics Today. Even a cube residing in the Smithsonian Museum in Washington, DC, is of uncertain provenance, Schwantes says.

To figure out if they were authentic, Schwantes’ team turned to radiochronometry, the science geologists and geophysicists use to date rock strata, and even the age of the Solar System. It works by comparing the amount of uranium to the amount of its decay products. From that, it is possible to determine how long it has been since the object was formed.

In the case of rocks, this involves working with very slow radioactive decay chains that play out over the course of geologic time. But for the Nazi cubes, what was needed were other, faster-moving radioactive decay chains. “The difference is that our time frame of interest is zero to 100 years, while theirs may be billions of years,” Schwantes says.

In order to conduct their study, Schwantes and Robertson obtained tiny samples of three cubes – one held by the Pacific Northwest Laboratory and the other two held by Koeth – and measured the amounts of key isotopes in them, including a rare isotope of thorium with a half-life of 75,000 years and an even rarer element known as protactinium (half-life 32,000 years).

Not that these half-lives are all that short by the standards of historians. But they are rapid enough, Schwantes says, that modern analytical instruments can determine the age of the uranium cubes with remarkable precision.

The researchers announced their preliminary results this week: the cubes’ ages are “consistent” with Nazi-era research. But with further refinement of the method, Schwantes says, it should be possible to determine their ages to within a few months – good enough not just to confirm the pedigree of these cubes, but determine which of the two labs they came from, because Diebner’s and Heisenberg’s teams were producing cubes in different years.

Other analyses, he says, can also determine the source of the uranium ore from which the cubes came – useful in helping to understand how the Nazi supply chains worked at the time the ore was mined. Also interesting, he says, is to test the surface of the cubes for traces of organic coatings.

Uranium is a metal that readily oxidises if exposed to air. In order to prevent this, Schwantes says, each research group coated its cubes with a protective seal. Heisenberg’s used a cyanide-based coating; Diebner’s used styrene.

So far, Robertson says, the team has only analysed the coating on one cube. “To be honest,” she says, “I thought this was a long shot. I hadn’t thought an organic compound would survive this long sitting next to uranium metal.”

But it did, and what she found was a surprise. Their “Heisenberg” cube had been coated in styrene. Does this mean that this is actually a Diebner cube? Or did Diebner share some of his cubes with Heisenberg?

The latter would be a significant find because a major feature of the Nazi bomb program was that it pitted research groups against each other – not in cooperation, but competition. That, in fact, may have been one of the reasons the Nazis failed to beat the US to the bomb even though they had a two-year head start: as Koeth and Hiebert wrote, that approach resulted in “fierce competition over finite resources, bitter interpersonal rivalries, and ineffectual scientific management”.

“One of the questions that has not been answered,” Schwantes adds, “was if they came together with all their resources, could they have devised a reactor design that would have gone critical [meaning one that would have produced a sustained nuclear reaction that could have produced plutonium]?”

What is known is that they didn’t get all that close to making a bomb. In 2015, a team of German scientists led by Klaus Mayer and Maria Wallenius examined one of the cubes still in Europe and found that its plutonium levels were not measurably different from the trace amounts found in natural uranium – meaning that Hitler’s bomb-development program had not proceeded far enough to come close to developing the enriched fuel needed for a bomb.

All of which is great information for World War II historians. But, as Schwantes and Robertson say, it is also a useful test of forensic methods of much greater importance.

“It’s our intent to use this as a fun project to really hone some of the new methodologies for nuclear forensic science in the hopes that those techniques can be applied to other investigations,” Schwantes says. Such investigations, he adds, could include anything from tracing the source of illicit nuclear materials to identifying the source of any other nuclear materials whose origin is not well established.

Meanwhile, Robertson says, working with these materials is sobering.

First off, she says, it’s a bit surreal to think that she is handling materials that might have been touched by one of the most famous scientists in history, Werner Heisenberg. “That’s kind of neat,” she says.

But it’s also a bit weird, she says, because it’s hard not to be aware of what these cubes might have led to, had the Nazi program been more successful. “The world would be a very different place,” she says. “I try not to lose sight of that reality.”