“My marble strikes a mousetrap, setting it off. The spring snaps up, sending my initial marble and its own marble flying into the air. Each of those two marbles strikes another mousetrap, sending two new marbles into the air, plus the same two all over again. Now we have four marbles launched in different directions, heading to different targets. And it repeats again, and again, all in a few seconds! It is like a firestorm, yes? Suddenly the air is filled with flying marbles. The sounds of clacking and springing and snapping!
“This intense reaction will continue for only a moment until all the mousetraps have sprung. All the marbles fall to the floor. Do you see how much energy I have released by simply tossing one particle?” He snapped his black-stained fingers. “That is how your bomb will work, all in an instant.”
Hahn stepped back into the office. “But that is not how our first chain reaction must work. We cannot have everything used up in an instant. We need the reaction to continue in a much slower, controlled manner, because we are using the excess flying marbles to build our new element 94. How do we do this? How can we control such an inferno?
“Imagine perhaps the room filled with cocked mousetraps again, but most of them are not loaded with marbles. Only a few of them. The rest are bare. We must get the right amount of mousetraps loaded—the right amount of uranium-235 in the mixture—and we must also space the mousetraps at the appropriate distances from each other so our result is that on average each marble that strikes a mousetrap causes exactly one more marble to fly in the air. In this way the reaction will continue at a controllable rate for as long as we require it.”
Esau smiled. “Most elegant.”
“The universe is elegant,” Hahn answered, “but secretive. It is up to us to unravel these secrets. In times of war we must unravel them faster than we might like.”
“That is the right attitude, Dr. Hahn.” Esau smiled in a way that might have been considered patronizing.
Hahn stiffened. “Professor Esau, I have already invented one terrible weapon in my life. During the Great War, Fritz Haber and I were the first to consider using poison gas against the enemy. Phosgene, chlorine gas, mustard gas. We were the first. It was our idea. Fritz Haber’s wife was a chemist herself, the first woman ever to receive a degree from Breslau University. She despised her husband’s work. She called it an abomination of science.”
Hahn lowered his eyes, letting them sink deeper behind his bushy eyebrows. “Dr. Clara Haber committed suicide when her husband refused to stop his work on our ‘super weapon.’ “
Esau decided to show compassion in his voice. “I am sorry to hear that.”
“Fritz Haber told me that a scientist belongs to the world in times of peace, but to his country in times of war. So now it is a time of war, and once again I must turn my work to the benefit of Germany. No matter what it does to the rest of the world.”
He stared at his fingers. “I like to consider myself a gentle man, but if you count all the victims of poison gas in the Great War, I already have the blood of over a million people on my hands.” He raised his eyes. “Please don’t treat me as if I am not aware of what we are doing here.”
He glared at Esau once, then left the office.
“It will go critical in the next few layers, Professor Esau.” Esau blinked, startled. He had fallen asleep with elbows sprawled on the wooden desktop. He glanced at von Weizsacker waiting by the door, then he looked at the clock. It was just past two in the morning. “I will be there shortly.” He blinked sleep away from his eyes.
A few moments later he ran along the gravel path to the bunker. Inside, naked bulbs flooded the pale walls and graphite-dusted floor, making it look like a bad black-and-white photograph. Two men continued to assemble the pile; the rest stood waiting behind the cinder-block observation wall that would shield them from stray radiation.
The pile had filled the deep pit. Graphite bricks and chunks of uranium stood in a blocky, somewhat spherical configuration. Neutron counters placed at various locations clicked from the presence of stray particles by the natural uranium decay. Otto Hahn and Paul Harteck stood beside opposite detectors, recording neutron counts as each layer was added to the pile.
Diebner climbed down from the pile. “That is the last layer, according to our calculations.” He kept his voice neutral. “If it doesn’t work now, we must begin again from scratch.”
Suspended above the pile from a chain on the ceiling, six tubes of uranium oxide hung partially inserted within the mound of black bricks. They would be the last pieces to enter the reacting pile. Von Weizsacker stood by a lever that would release a massive counterweight in case of an emergency; the weight would fall and yank out the uranium oxide rods, bringing the pile back to a subcritical state. As an added safety measure, Diebner and Harteck had mounted a drum filled with boric acid solution over the pile; in an extreme situation they could dump the solution into the pile, where the boron would swallow up all the free neutrons and smother the chain reaction. Esau winced at the drum’s precarious position. An accident could spill the boric acid into the graphite bricks, ruining the ultra-pure carbon that had been so difficult to obtain.
“Are we ready to begin?” he asked.
The others looked to Hahn, who handed his notebook to someone else. “Yes. First we must add our neutron source. Spontaneous neutrons should be sufficient, but this will make sure the reaction commences.” He raised an eyebrow. “We are tossing our first marble into the room, Professor Esau.”
Esau nodded.
“Then we will drop the remaining uranium oxide rods into the pile. This should bring us to criticality. The reaction will be self-sustaining.”
Esau folded his arms across his chest. “You may proceed.” But Hahn had already gone to the equipment piled along the walls, opening a small wooden case lined inside with lead foil. He withdrew a thin glass cylinder.
“This neutron source contains radon gas and beryllium powder. You may find it ironic that the Nazis confiscated it from the laboratories of Frederic and Irene Joliot-Curie in Paris.”
Diebner laughed. He had taken many of the Joliot-Curie notes from their laboratories, claiming the discoveries as his own. No one else said anything.
Hahn climbed the ladder to the top half of the graphite pile rising from the pit in the floor. Suspending the glass tube from a thin chain, he dangled it and let the neutron source slide down into the central hole. The clicking of the counters increased. Hahn looked over his shoulder at them. “That is as we expected. Everything is now in place.”
Esau felt nervousness chewing inside of him. “Fine. We are already behind the Americans. No use wasting time. Let’s see if the reactor works.” He listened to the counters rattling and thought of Hahn’s mousetraps.
Paul Harteck spoke up. “Would everyone please step behind the shielding wall? The leaded glass observation windows should protect you.” No one needed to be reminded twice.
“Perhaps we should proceed an inch at a time,” Hahn said, coming around behind the wall. “We will gain more information that way.”
Esau crowded up so he could see through the narrow window. “We can repeat the experiment later if you require such niceties. For now, we must see if all of us have a future here! If we do not show success with this, certain people will be very upset.” He turned to von Weizsacker. “Lower the uranium oxide.”
Von Weizsacker looked to Hahn, then Diebner, as if searching for someone to counteract Esau’s orders, but no one would speak out loud. Some of the assistants edged toward the door. Von Weizsacker released the catch on the chain, letting the six rods of uranium oxide fall into place inside the pile, bringing the pile beyond its critical limit.