Now, though, Esau was bringing them sufficient inspiration, thanks to Graham Fox.
He allowed himself to smile as they drove into the muddy courtyard outside the wooden barracks. Dark stains showed where rain had soaked through the plank walls. All around, the city of Berlin pressed close, too close perhaps for such dangerous research as this, but it did make a perfect hiding place.
A dim shadow behind one of the windows watched the staff car pull up, then ducked back. Esau hoped the observer would inform the rest of the physicists exactly who had arrived for a visit.
They had come to accuse Professor Werner Heisenberg of treason.
A week before, when the Reich post had delivered a letter with Belgian postmark, Professor Esau took notice. He held up the stained envelope and frowned; it looked like cheap stationery inside. He did not recognize the bold handwriting on the address with its excessive loops and flourishes. With a letter opener he slashed open the edge.
He recognized the handwriting on the letter inside immediately. Graham Fox! It was impossible, but he could not stop himself from a smile such as he had not worn since his student days at Cambridge. He wondered how Fox had managed to get a letter through the postal blockades to Germany. But none of Esau’s initial astonishment compared to what he felt upon reading the terse but profound message.
… So, my dear friend, Fermi has achieved a self-sustaining neutron reaction moderated by graphite blocks. By virtue of Germany’s superior physicists, Heisenberg’s group should have come to this discovery on their own—could he perhaps be leading you down the wrong path? After all, no one would question Heisenberg’s claims. I will do what I can here because we must maintain parity. All humanity is at risk. Must count on you, Abraham.
At that moment Esau’s secretary—the same one who had bungled his invitation to the physics conference, and then bungled his subsequent apology letter—appeared at the door with some inane question. Esau’s shouting fit sent her scurrying back into the hall. Her heels echoed on the tile floor like gunshots.
Esau clutched the letter with sweaty fingers. A nuclear reaction moderated by graphite! Esau was astounded. According to all their careful studies—no, he corrected himself, not careful enough—they had thought heavy water was the only substance that could appropriately moderate a reacting pile. How could they have missed something as simple and common as graphite?
A nuclear reactor could produce a different element, a new element beyond uranium on the periodic table, that could be used as a substitute for the rare isotope uranium-235 in an atomic bomb. In 1940 the American Edwin McMillan, working at the University of California at Berkeley, had artificially created “element 93” by bombarding uranium with neutrons. Since uranium had been named after the planet Uranus, McMillan had decided to call his new element “neptunium” after the planet Neptune.
But physical theory predicted that the next artificial element in the series, element 94, would be a candidate for fission, just like uranium-235. Element 94 did not exist in nature, but in all likelihood could be made in the laboratory. But only if they could keep a nuclear chain reaction going. Continuing the scheme of using planetary names, this element should be named after the newly discovered ninth planet Pluto. Plutonium?
If they could produce enough of this new plutonium, Esau would not need to worry about the incredible difficulties of separating uranium-235 from the rest of the ore. They could have a German bomb sooner than expected.
But for that they needed a working reactor to “cook” the uranium until it became plutonium… and to achieve a functioning reactor, Esau had thought he needed enormous quantities of heavy water, which was exceedingly rare and precious. Even then it remained a matter for conjecture, because they had never been able to obtain enough heavy water to test the theory.
The difficulties continued to tangle worse and worse as the war went on.
Germany’s only source of heavy water had been the Norwegian Hydro Works at Vemork—and the Allies had recently destroyed the plant, bringing all heavy water production to a halt. Allied saboteurs had even sunk the ferry carrying the last few drums of dilute heavy water rescued from the ruined factory.
Esau had seen no future for the possibility of reactor research. It had left them with nothing to try but the impossible isotope separation.
Now, though, Fox’s letter implied that perhaps graphite—simple carbon—could be used instead of heavy water. Esau could not comprehend why his own researchers had ignored the possibility. Especially with the great Heisenberg at the helm.
Feeling his cheeks flush with a growing anger, Esau dug through the files and progress reports describing aspects of his disjointed program. He loathed this clerical work—he could never find anything. Progress reports had been falsified, or not submitted on time, or written in such terse, vague language that he couldn’t understand what the physicists were talking about. He had not unpacked and organized the files completely, and he did not dare risk asking his inept secretary to help him. Some of the files were from Diebner’s tenure over the Kaiser-Wilhelm Institute; others had been culled from the Armaments Ministry itself, or even von Ardenne’s work for the post office.
Surely someone must have tested graphite.
He found the records after an hour of searching. He snapped the thin file away from the stack with a brisk gesture that betrayed his own impatience. Outside, an automobile horn blasted three times, and Esau made an annoyed comment to himself. He took the papers back to his desk and spread them out, piling everything else on top of his unopened mail.
Professor Walther Bothe had made the analysis. At Heidelberg, Bothe had used a sphere of high-quality graphite larger than a meter in diameter, submerging it in a tank of water to measure its neutron absorption cross-section. According to Bothe’s test results, graphite was indeed a poor choice, swallowing far too many of the available neutrons. For the nuclear reaction to be successful, the moderator needed to slow down the neutrons to the proper speed so they could cause fissions in the uranium—slow them down, not take them out of the reaction entirely.
But if the Americans had succeeded in creating a self sustaining chain reaction using graphite and uranium, then Bothe’s results must be wrong. Wrong!
Esau squinted and rubbed the scar on his lip. The car honked again below the window, but now he no longer heard it. What if Bothe had not used pure enough carbon? Graphite had a tendency to be contaminated with boron— and other results had plainly shown that boron acted as one of the most voracious neutron swallowers. What if carbon was indeed an efficient moderator, but Bothe’s result had been masked by boron contamination? The pile would then defeat its own reaction, not because of graphite—but because of the boron poisoning.
Carbon was trivial to obtain. Absolute purification would be somewhat difficult, but vastly simpler than manufacturing heavy water or finding some way to separate the uranium-235 from the rest of the natural uranium.
This changed everything.
Esau read Fox’s letter again. This also meant the Americans were far ahead of them. Despite a German head start at the beginning of the war, the Americans had already achieved a self-sustaining chain reaction. That, too, changed everything.
Reichminister Albert Speer did not take either Esau or his nuclear program seriously. But perhaps this news would make him pay attention. If the Americans had jumped headfirst into developing an atomic weapon, could Germany afford not to do the same?