The floor in the bunker felt slick from the fine black dust that clung to everything. While the graphite bricks had been cut to proper size, the workers used modified woodworking tools to cut notches for the uranium cubes. Each of the lattice holes had to be customized, because Esau was using all the uranium and uranium oxide he had cobbled together from the scattered experiments of the other research teams.
Uranium metal cubes would be scattered at six-inch intervals in the graphite material, but Carl-Friedrich von Weizsacker’s calculations had shown that this would not be enough to make the reaction self-sustaining. Within the growing pile, they would add a circular array of uranium oxide in long tubes. In the center of this they would drop the neutron source that should trigger the whole reaction.
Von Weizsacker climbed out of the pit, saw Esau standing there and walked over to him. “I now recall the papers I discussed with you earlier, Professor Esau, the last open reports about the American nuclear research.” He wiped his blackened hands on his blackened coveralls.
“They were published in Physical Review, in English. I remember reading them on the underground railway in Berlin. I believe I received a few suspicious glances when people saw me poring over an American periodical. This was in June 1940, I believe. Some scientists had reported using Lawrence’s cyclotron at Berkeley to create element 93 by bombarding uranium. But element 93 is unstable and undergoes beta decay, turning into element 94. That is the one we want. Element 94 is fissionable, and stable, and chemically different from uranium. We can make your weapon. If we can get this pile working.”
Esau could sense von Weizsacker dancing around the real issue. He felt impatient. By focusing on the optimistic good news, von Weizsacker implied that he also had something bad to say. “I know all that. So what is the problem?”
“Well, it will be very difficult for us to make sense out of our measurements from this pile we are building,” he said with no other preamble. Specks of graphite blackened von Weizsacker’s teeth, but the smudges could not disguise his boyish features, his statue-perfect Aryan appearance.
“We are mixing the sizes of the uranium cubes, adding the uranium oxide to the metal, changing the spacing. Too many variables in everything. Normally, we would build successive piles, each one simple and straightforward, with conditions we could understand and attempt to predict. With successive attempts we can add new twists and see how that affects the readings. We will never be able to understand this reaction. It is too complicated.”
Esau met the younger man’s eyes. He sensed that von Weizsacker had been chosen as a delegate from the other scientists. He noticed that the work had stopped. He kept his voice firm.
“I am not interested in understanding it at this moment. I am interested in demonstrating that it will work! Nuclear physics fascinates me as well, but have you not been listening to the radio broadcasts? The constant bombing of Frankfurt. The American General Eisenhower announcing the unconditional surrender of Italians, and then Italy declaring war on Germany!
“I must deal with the Reichminister of Armaments, who in turn must deal with the Führer. Everyone wants a useful weapon now. Understanding can come later.” Esau turned to go back to his office, but stopped. “When will the pile be ready?”
Von Weizsacker shrugged. “They are still hooking up the counters, and we will need to take measurements at successive stages of the assembly to see how far we must go to achieve criticality.”
Esau kept staring at him, waiting for an answer.
The younger man stopped, thought a minute, then nodded. “Late this evening, I would guess.”
“Good. I will be in my office.”
The man who had discovered nuclear fission, Dr. Otto Hahn, had been chosen the de facto leader of the Kaiser-Wilhelm group. Esau fostered this impression, since he respected Hahn. And Hahn seemed more interested in his physics than in using his authority, which was fine with Esau.
The great physicist, though, treated Abraham Esau like a schoolboy to be lectured. Esau’s own grasp of nuclear physics, though considerable, did not compare with the researchers working under him. He forced himself not to act too impatient when Hahn began to teach him about the pile being constructed in the bunker. Hahn ignored the fact that Esau himself had passed along the key bit of information about graphite.
Otto Hahn himself insisted on “clarifying” it to Esau, making sure that the Plenipotentiary understood the enormity of the event about to take place. Hahn stood in Heisenberg’s old office as Esau dutifully watched the great man pace. Hahn began to talk in his quiet voice.
“We know nothing about what the Americans have done, but we can conjecture how to repeat their experiment. In principle at least.” Esau noticed the stubble on Harm’s cheeks. His moustache stood out, and his eyes looked big and sad, bloodshot from too little sleep. “This goes far beyond the tiny laboratory exercises that I did with Herr Strassman and Dr. Lise Meitner—”
“You need not credit a Jew for your discovery, Dr. Hahn,” Esau interrupted, straightening in his seat.
Hahn halted his pacing, raised his bushy eyebrows and turned to Esau. “Lise did much of the work. She had the idea first. She understood long before I did—” He stopped himself, but Esau already knew what Hahn thought. Rumors even said that he had helped Lise Meitner escape to Sweden, but Hahn had never said this aloud.
Esau didn’t want to push him. He needed Hahn’s mind, his ideas, to make a self-sustaining chain reaction. “No matter. We are worried about physical principles now, not political ones.”
Hahn nodded curtly. “So we are. We know that the uranium nucleus can fission, and that it is the scarce 235 isotope that fissions due to slow neutrons. Niels Bohr pointed that out.”
Esau let his eyes fall closed for just a moment. Bohr, the half-Jew. It seemed they permeated nuclear physics.
“But now we cannot be satisfied with causing merely a fission or two just to prove that it can be done. We must make one fission cause another, and another, and another, so that the reaction continues of its own. Then perhaps it can be useful, such as making a uranium burner to produce power. That was one of Heisenberg’s ideas.”
“We wish to make a weapon, Dr. Hahn. Not a furnace.”
“Both work on the same principle. Listen.” He held up one finger, a thick finger, with blackened pores and nails from handling the carbon blocks. It would take weeks to wash everything off. Even after a thorough shower, the pores of the skin exuded graphite dust within another hour.
“In your mind, Herr Esau, picture a mousetrap with a marble balanced just above the spring.” He stepped back and gestured to the empty floor. “Now picture this floor covered with such mousetraps, each one loaded with a marble, each one ready to snap the instant an appropriate signal is received.”
Involuntarily, Esau leaned over and looked at the bare wooden planks of the floor. Hahn glanced around as if suddenly remembering where he was, then he lowered his eyes and fixed his face into a scowl. “Professor Heisenberg was very good at these thought experiments too.”
Esau said nothing. He tapped his fingertips together and waited for Hahn to continue.
“Now, I will stand outside this room full of mousetraps…” Hahn stepped back, holding one hand up and keeping a gap between his fingers as if holding something. “I have a marble in my grasp. I toss it into the room.” He mimed the gesture.
“The marble in my hand is like a neutron that I send into our reactor. Each of our mousetraps, cocked and holding their marbles, is like a uranium nucleus waiting to fission.