The Bastard Brigade

by Sam Kean

  Despite his grief, Weizsäcker attended the second meeting of the Uranium Club in late September. In truth, he found topics like cosmic rays more stimulating than nuclear fission. Always calculating, however, he realized the political importance of the topic, and he argued sotto voce among his colleagues that conducting bomb research would give German scientists prestige within the military and access to more funding. If they played things right, he added, they might even be able manipulate the Nazis for their own ends—a highly dangerous game.

  Weizsäcker was also instrumental in persuading Otto Hahn—who had moral qualms about fission research—to join the club. He was perhaps less sure about his old friend Heisenberg: the two men had barely seen each other during the previous few years. They nevertheless had a warm reunion when Heisenberg reported for duty, and both would become key members of this new and potentially lethal clique.

  In one of its first acts, the Uranium Club assigned each scientist to one of two projects. The first involved enriching uranium.

  Two main types of uranium atoms exist in nature, uranium-235 and uranium-238. Each type—called an isotope—has the same number of protons (ninety-two). They differ in the number of neutrons, with uranium-238 having three extra. In most situations this difference makes no difference at all, since uranium-235 and -238 behave the same way in chemical reactions. But those extra neutrons make a huge difference in chain reactions. When a neutron strikes uranium-235, it undergoes fission and releases more neutrons, kicking chain reactions into high gear. In contrast, when a neutron strikes uranium-238, the atom simply absorbs it. The nucleus doesn’t fission and no extra neutrons get released. So if you want to make a fission bomb, you need as much uranium-235 and as little uranium-238 as possible. Otherwise, the 238 will absorb too many neutrons and the chain reaction will peter out.

  The problem is, the fission-happy 235 isotope is rare in nature: for every 140 uranium atoms out there, just one is uranium-235. The obvious solution, then, is to separate the 235 and concentrate it, producing what’s called enriched or “supernatural” uranium.

  But enriching uranium is easier said than done. Most of the tricks chemists use to separate substances rely on chemical differences between them: you bathe them in acid or something, and one substance dissolves away while the other remains behind. But those tricks fail with uranium-235 and -238 because, again, they’re the same element and behave the same way in chemical reactions. Scientists therefore had to invent new ways to separate uranium, all of which were cumbersome and required ridiculous amounts of energy; in some cases, they were separating isotopes literally atom by atom. No less a scientist than Niels Bohr declared atomic weapons impossible for this very reason. Indeed, the enrichment of uranium seemed the single biggest obstacle to making nuclear weapons, which is why the Uranium Club dedicated a whole team to the problem.

  The second Uranium Club project involved building a nuclear reactor, which the Germans called a Uranium Machine. Reactors are basically small-scale chain reactions in the lab. Because the uranium in them isn’t enriched, reactors won’t explode in a mushroom cloud the way bombs do. But you can use them to study how chain reactions work, which is vital knowledge for making bombs.

  Heisenberg joined the Uranium Machine team, and he absolutely threw himself into the task. Still stinging over the “Jew physics” debacle, he wanted to prove to German officials that he wasn’t a mere theoretical wizard; he could do solid, practical work and build useful things, too. Heisenberg was also ruthlessly competitive. He loved to crush people at Ping-Pong and other games, and saw physics as no less of a bloodsport. Producing the world’s first self-sustaining chain reaction would be quite an honor for Germany.

  By early December 1939 Heisenberg had produced two secret reports on chain reactions, one focused on energy production and one on bombs. To begin experimental work, he opened a small lab in a wooden shed attached to a scientific institute in Berlin, just off a lane with cherry trees. Inside the shed was a six-foot-deep pit lined with brick—the world’s first test chamber for nuclear reactors. Because they were working with radioactive materials, scientists entering the shed had to wear goggles, overalls, and face masks, an ominous sight. And if this wasn’t enough to deter folks from snooping around, they code-named the place the Virus House.

  Two years before the start of the Manhattan Project, then, the Uranium Club had scientists working on two key aspects of nuclear weapons: enriching uranium and producing a self-sustaining chain reaction. The German atomic bomb project was off to a rip-roaring start.

  Carl Weizsäcker, the diplomat’s son, soon provided a third key ingredient. In the early days of the war, he often read journals like Physical Review while riding the subways in Berlin. This earned him dirty looks from other passengers, since the articles were written in English, the language of the enemy. (Later in the war, one German scientist was sentenced to death for listening to the BBC.) But Weizsäcker cared little what hoi polloi thought. He did some of his best thinking on the subway, in fact, never more so than one fateful morning in July 1940.

  As noted, uranium-235 undergoes fission when a neutron strikes it, while uranium-238 does not. So it makes sense that most nuclear scientists focused on studying 235. But in doing so, they overlooked something. When 238 absorbs a neutron, it becomes uranium-239, a slightly fatter version of itself. It then undergoes what’s called beta decay. During this process a neutron in the nucleus transforms into a proton and an electron; the atom spits the electron out, while the proton stays behind.

  Intriguing stuff. But while most scientists stopped at this point, Weizsäcker saw further. After the atom of uranium-239 underwent beta decay and gained a proton, it would become a new element, element 93. And Weizsäcker realized that this new element 93 would also be unstable and would also undergo explosive fission. Moreover, you could make this element cheaply in a reactor, since you wouldn’t need to enrich anything beforehand; you could just bombard the common 238 isotope with neutrons. Best of all, because 93 was a different element than uranium, chemists could easily dissolve it out and isolate it. It had all the advantages of uranium-235 and none of the drawbacks.

  Neutrons of thought were soon pinging everywhere in Weizsäcker’s brain—a full mental chain reaction—and by the time he reached his train stop, he had it all worked out. In truth, Weizsäcker bungled some details here. Element 93, now called neptunium, also undergoes beta decay and transmutes one more time into element 94, plutonium, which is the really dangerous stuff. Weizsäcker nevertheless got the most important thing right—that the Germans could use 238 in a Uranium Machine to mass-produce a fissile element, then quickly separate that element out. The son of one of the most important officials in the Third Reich, in other words, had just dreamed up a cheap and much more practical means of making fuel for nuclear bombs.

  Aside from the striving Diebner, the diplomat’s son Weizsäcker, and the brilliant but obtuse Heisenberg, one other member of the Uranium Club played a crucial role in its history. Unfortunately for Walther Bothe, his contribution involved no great insight or breakthrough, but a monumental boner.

  Bothe occupied a strange position in German science—half legend and half loser. He’d made himself a legend for his bravery and sangfroid during World War I. After earning his Ph.D. in 1914, he joined a German machine gun unit and was taken prisoner in Russia; he spent the next five years in a desolate camp in Siberia. Unbowed, he continued to do independent research in mathematics and theoretical physics there, even constructing his own table of logarithms to aid his calculations. He also found time to learn the Russian language and to court and marry a Russian woman, with whom he returned to Germany in 1920 as a war hero—a stirring example of dignity in trying circumstances.

  But the 1930s were rough on Bothe. Exactly like the Joliot-Curies—in fact, before the Joliot-Curies—he’d turned up strong evidence for the existence of the neutron in several experiments, only to misinterpret his results. Unlike the Joliot-Curies, he had no othe
r major discoveries to offset that embarrassment. To compound his troubles, the luster of his marriage had tarnished and he was growing increasingly restless with domestic life. Worst of all, as a mild opponent of the Nazis, he found himself marginalized within German science, pushed aside for colleagues who had more ardor for the Aryan agenda. This triple weight of professional, personal, and political pressure finally proved too much: his health collapsed and he suffered a nervous breakdown, eventually checking himself into a sanatorium.

  Needing a break from Germany, the balding, mustached Bothe sailed to the United States in the summer of 1939 for a speaking tour. He planned on spending the trip alone, focused on science, but on the voyage over he met Ingeborg Moerschner, a vivacious and flirty blonde thirteen years his junior. To his astonishment, Moerschner reciprocated his affection, and long-dormant feelings of love (and lust) soon quickened inside him. He spent every waking hour on the ship mooning over her—and many nonwaking hours as well, as the middle-aged physicist and young Nazi civil servant became lovers.

  In a delightful coincidence, both were headed to New York first, so Bothe accompanied her to the World’s Fair in Queens, where they strolled the grounds arm in arm. By the sheerest luck, their schedules overlapped again a few weeks later in the Bay Area, where she was taking a job at the German consulate in San Francisco and Bothe planned to study cyclotron design at the University of California at Berkeley. So they rendezvoused there and went sightseeing. Bothe still had a wife and two children back home, but he pushed all that out of his mind and happily succumbed to folly.

  The overlap in their schedules was no coincidence, of course. She’d been spying on him from the moment they’d met onboard, reporting his comings and goings to Berlin. (Bothe’s foreign wife and eagerness to travel to America no doubt put him under suspicion.) Bothe remained oblivious to this, however, and when he returned to Germany late that summer, he was a changed man. He even showed some zeal for Moerschner’s Nazi Party. During the first meeting of the Uranium Club, when other members hemmed and hawed about developing a nuclear bomb, Bothe rallied them and urged their support. “Gentlemen,” he said, “it must be done.”

  The club assigned him a crucial task within the reactor project. As Enrico Fermi had proved a few years earlier, the speed of neutrons plays an important role in fission. In particular, scientists knew that uranium-238 prefers fast neutrons, while the fission-happy uranium-235 can work with slow ones. The trick to building a reactor, then, is reducing the speed of neutrons, to decrease the chances of capture by 238 and boost the chances of capture by 235. For technical reasons, scientists referred to reducing the speed of neutrons as “moderating” them.

  Fermi had slowed neutrons down with pond water and paraffin, but scientists suspected that other substances might work equally well as moderators, especially graphite. (Most people today think of graphite as the “lead” in pencils, but this is actually pure carbon.) Bothe set out to answer two questions about graphite. First, how well did it slow neutrons down? As a small atom, carbon seemed promising in this regard, but he had to verify that experimentally. Second, did graphite absorb neutrons? Some elements have a bad habit of gobbling up stray neutrons and snuffing out chain reactions. Bothe needed to determine whether carbon was one of them.

  He worked with massive hunks of graphite, spheres three feet wide. These had a “chimney” bored into them, and the experiments started when he dropped a neutron source down the chimney into the center of the sphere. These neutrons would fire outward, colliding with carbon atoms as they pinballed their way toward the surface. Eventually they’d either get absorbed on the inside or emerge from the sphere and ping a detector. Based on the number and the speed of the neutrons that emerged, Bothe could determine how effectively the graphite was slowing them down and how many neutrons it absorbed. Pretty straightforward.

  Unfortunately, Bothe couldn’t keep his mind on the work. He’d never quite broken off the affair with Moerschner, who had moved to Lisbon after the U.S. government shut down the Nazi consulate in San Francisco. (It was riddled with spies.) And Bothe’s wife Barbara soon found out about the dalliance, reportedly after Moerschner called his house one night and Barbara picked up. Despite Barbara’s wrath—or perhaps because of it—Bothe clung to the affair, and by his own account he found himself constantly distracted. On the first anniversary of their meeting, he wrote to Moerschner, “I have been speaking of physics the entire day, while thinking only of you.” He also referred to himself as a “drunken teenager,” and recalled playing Beethoven’s “Moonlight Sonata” over and over while pining for her.

  Given his mindset, it’s no wonder his experiments didn’t go well. According to theory, the average neutron pinballing its way through a graphite sphere should travel twenty-eight inches before being absorbed; Bothe’s neutrons averaged just twenty-three. He’d tried to buy the purest graphite he could, but he suspected that contaminants—such as boron and cadmium, both of which gobble up neutrons by the bushel—might still be lingering inside. So he tried to develop purer sources of carbon. His top idea involved burning simple syrup and other sugars normally used in sweets, scorching them into a black, smoldering mass. At one point, in other words, a key ingredient in the Nazi nuclear bomb was candy.

  These experiments failed, too, and Bothe finally approached an industrial firm to obtain one more graphite sphere for a definitive test. He asked for the purest stuff they had, a ball nearly four feet wide. Not quite trusting their assurances, he ran some purity assays of his own, charring a small chunk of the sphere and testing the ashes. It seemed sufficiently clean. You can imagine his shock, then, when the experiments failed yet again. In this ultrapure graphite, neutrons traveled only fourteen inches on average before being gobbled up, half of what theory predicted. Against all expectations, graphite seemed to be a terrible moderator, and Bothe wrote up a report to this effect.

  It was one of the most consequential blunders in science history. Another German scientist, working outside the club, read the report and realized that the lovesick, loopy Bothe had committed a mistake during the purity assay. The ashes he’d examined were no doubt pure carbon; but the process of heating and burning the graphite would have driven off all the impurities that gobble up neutrons, leaving no trace behind in the ashes. This scientist wrote a report to counter Bothe’s, but for some reason it never gained traction, perhaps because of bureaucratic rivalries between different groups. As a result, Bothe and the Uranium Club probably never knew.

  Given the high cost of graphite and Bothe’s seemingly definitive experiments, the Uranium Club ruled out carbon. Instead they focused on a different moderator, so-called heavy water. Scientifically, this represented a modest shift. Politically, it was a radical change, and would soon leave dead bodies scattered across the wilds of northern Europe.

  CHAPTER 10

  Heavy Water

  In September 1939, in a speech in Danzig, Adolf Hitler promised to unleash upon the Allies “a weapon which there was no way to defend against.” Lord knows what fantasy he was indulging, but a few Allied officials jumped to an immediate conclusion: atomic bombs. In support of this theory, they cited the fact that Germany had already banned the export of uranium from the Reich. Equally disturbing, French intelligence agents reported that the Nazis had taken a keen interest in heavy water.

  Just as uranium comes in different varieties, 235 and 238, so does hydrogen. Most hydrogen atoms are simple, with a single proton for a nucleus. One of every 6,400 hydrogens, however, has a proton and a neutron; this is called heavy hydrogen (or deuterium, D). Fuse two heavy hydrogens onto an oxygen atom and you have heavy water (D2O). Heavy water looks just like regular water, though it does have different properties. It’s denser, and if consumed in large quantities (several gallons), it can damage DNA and interfere with basic metabolism. Compared to regular water, it also intensifies nuclear chain reactions. That’s because regular water, when mixed with uranium, tends to absorb some of the neutrons flying aroun
d, which hampers chain reactions. Heavy water doesn’t absorb neutrons, merely slows them down, so it’s perfect for priming uranium-235.

  Given the scarcity of heavy water—it makes up just one of every 41 million water molecules—only one firm in the world bothered producing it in 1940. This firm, Norsk Hydro, managed a series of power stations on a bleak plateau a hundred miles west of Oslo, an area of Norway with an ugly reputation: one sixteenth-century chronicle said of the people there that their “chief delight is to kill bishops, priests, bailiffs, and superiors.” One of those power stations, called Vemork, was the biggest hydroelectric plant in the world. It sat on a forbidding ledge near a massive waterfall, and most of the electricity it produced got channeled into making fertilizers and explosives. The remainder was diverted into a nearby building for electrolysis. This process involved zapping huge tanks of water with electric currents to separate the H2 from the O. And as a by-product of electrolysis, they collected heavy water in a series of specialized fuel cells in the basement (more on these cells later). Unfortunately, the market for heavy water proved anemic. Between 1934 and 1938, Vemork sold just eighty-eight pounds total, for around $4,000 per pound. In modern terms, they made craft batches of the stuff, rarely selling more than a third of an ounce per order.