The Faber Book of Science (42 page)

From the numerous experiments they had performed so far, they had an idea of what the pile should be, but they had not worked out the details, there were no drawings nor blueprints and no time to spare to make them. They planned their pile even as they built it. They were to give it the shape of a sphere of about 26
feet in diameter, supported by a square frame, hence the square balloon.

The pile supports consisted of blocks of wood. As a block was put in place inside the balloon, the size and shape of the next were figured. Between the Squash Court and the near-by carpenter’s shop there was a steady flow of boys, who fetched finished blocks and brought specifications for more on bits of paper.

When the physicists started handling graphite bricks, everything became black. The walls of the Squash Court were black to start with. Now a huge black wall of graphite was going up fast. Graphite powder covered the floor and made it black and as slippery as a dance floor. Black figures skidded on it, figures in overalls and goggles under a layer of graphite dust. There was one woman among them, Leona Woods; she could not be distinguished from the men, and she got her share of cussing from the bosses.

The carpenters and the machinists who executed orders with no knowledge of their purpose and the high-school boys who helped lay bricks for the pile must have wondered at the black scene. Had they been aware that the ultimate result would be an atomic bomb, they might have renamed the court Pluto’s Workshop or Hell’s Kitchen.

To solve difficulties as one meets them is much faster than to try to foresee them all in detail. As the pile grew, measurements were taken and further construction adapted to results.

The pile never reached the ceiling. It was planned as a sphere 26 feet in diameter, but the last layers were never put into place. The sphere remained flattened at the top. To make a vacuum proved unnecessary, and the balloon was never sealed. The critical size of the pile was attained sooner than was anticipated.

Only six weeks had passed from the laying of the first graphite
brick, and it was the morning of December 2 [1942].

Herbert Anderson was sleepy and grouchy. He had been up until two in the morning to give the pile its finishing touches. Had he pulled a control rod during the night, he could have operated the pile and have been the first man to achieve a chain reaction, at least in a material, mechanical sense. He had a moral duty not to pull that rod, despite the strong temptation. It would not be fair to Fermi. Fermi was the leader. He had directed research and worked out theories. His were the basic ideas. His were the privilege and the responsibility of conducting the final experiment and controlling the chain reaction.

‘So the show was all Enrico’s, and he had gone to bed early the night before,’ Herbert told me years later, and a bit of regret still lingered in his voice.

Walter Zinn also could have produced a chain reaction during the night. He, too, had been up and at work. But he did not care whether he operated the pile or not; he did not care in the least. It was not his job.

His task had been to smooth out difficulties during the pile construction. He had been some sort of general contractor: he had placed orders for material and made sure that they were delivered in time; he had supervised the machine shops where graphite was milled; he had spurred others to work faster, longer, more efficiently. He had become angry, had shouted, and had reached his goal. In six weeks the pile was assembled, and now he viewed it with relaxed nerves and with that vague feeling of emptiness, of slight disorientation, which never fails to follow completion of a purposeful task.

There is no record of what were the feelings of the three young men who crouched on top of the pile, under the ceiling of the square balloon. They were called the ‘suicide squad.’ It was a joke, but perhaps they were asking themselves whether the joke held some truth. They were like firemen alerted to the possibility of a fire, ready to extinguish it. If something unexpected were to happen, if the pile should get out of control, they would ‘extinguish’ it by flooding it with a cadmium solution. Cadmium absorbs neutrons and prevents a chain reaction.

A sense of apprehension was in the air. Everyone felt it but outwardly, at least, they were all calm and composed.

Among the persons who gathered in the Squash Court on that morning, one was not connected with the Met. Lab. – Mr Crawford H. Greenewalt of E. I duPont de Nemours, who later became the
president of the company. Arthur Compton had led him there out of a nearby room where, on that day, he and other men from his company happened to be holding talks with top Army officers.

Mr Greenewalt and the duPont people were in a difficult position, and they did not know how to reach a decision. The Army had taken over the Uranium Project on the previous August and renamed it Manhattan District. In September General Leslie R. Groves was placed in charge of it. General Groves must have been of a trusting nature: before a chain reaction was achieved, he was already urging the duPont de Nemours Company to build and operate piles on a production scale.

In a pile, Mr Greenewalt was told, a new element, plutonium, is created during uranium fission. Plutonium would probably be suited for making atomic bombs. So Greenewalt and his group had been taken to Berkeley to see the work done on plutonium, and then flown to Chicago for more negotiations with the Army.

Mr Greenewalt was hesitant. Of course his company would like to help win the war! But piles and plutonium!

With the Army’s insistent voice in his ears, Compton, who had attended the conference, decided to break the rules and take Mr Greenewalt to witness the first operation of a pile.

They all climbed onto the balcony at the north end of the Squash Court; all, except the three boys perched on top of the pile and except a young physicist, George Weil, who stood alone on the floor by a cadmium rod that he was to pull out of the pile when so instructed.

And so the show began.

There was utter silence in the audience, and only Fermi spoke. His grey eyes betrayed his intense thinking, and his hands moved along with his thoughts.

‘The pile is not performing now because inside it there are rods of cadmium which absorb neutrons. One single rod is sufficient to prevent a chain reaction. So our first step will be to pull out of the pile all control rods but the one that George Weil will man.’ As he spoke others acted. Each chore had been assigned in advance and rehearsed. So Fermi went on speaking, and his hands pointed out the things he mentioned.

‘This rod, that we have pulled out with the others, is automatically controlled. Should the intensity of the reaction become greater than a pre-set limit, this rod would go back inside the pile by itself.

‘This pen will trace a line indicating the intensity of the radiation. When the pile chain-reacts, the pen will trace a line that will go up and up and that will not tend to level off. In other words, it will be an exponential line.

‘Presently we shall begin our experiment. George will pull out his rod a little at a time. We shall take measurements and verify that the pile will keep on acting as we have calculated.

‘Weil will first set the rod at thirteen feet. This means that thirteen feet of the rod will still be inside the pile. The counters will click faster and the pen will move up to this point, and then its trace will level off. Go ahead, George!’

Eyes turned to the graph pen. Breathing was suspended. Fermi grinned with confidence. The counters stepped up their clicking; the pen went up and then stopped where Fermi had said it would. Greenewalt gasped audibly. Fermi continued to grin.

He gave more orders. Each time Weil pulled out some more, the counters increased the rate of their clicking, the pen raised to the point that Fermi predicted, then it levelled off.

The morning went by. Fermi was conscious that a new experiment of this kind, carried out in the heart of a big city, might become a potential hazard unless all precautions were taken to make sure that at all times the operation of the pile conformed closely with the results of the calculations. In his mind he was sure that if George Weil’s rod had been pulled out all at once, the pile would have started reacting at a leisurely rate and could have been stopped at will by reinserting one of the rods. He chose, however, to take his time and be certain that no unforeseen phenomenon would disturb the experiment.

It is impossible to say how great a danger this unforeseen element constituted or what consequences it might have brought about. According to the theory, an explosion was out of the question. The release of lethal amounts of radiation through an uncontrolled reaction was improbable. Yet the men in the Squash Court were working with the unknown. They could not claim to know the answers to all the questions that were in their minds. Caution was welcome. Caution was essential. It would have been reckless to dispense with caution.

So it was lunch time, and, although nobody else had given signs of being hungry, Fermi, who is a man of habits, pronounced the now historical sentence:

‘Let’s go to lunch.’

After lunch they all resumed their places, and now Mr Greenewalt was decidedly excited, almost impatient.

But again the experiment proceeded by small steps, until it was 3.20.

Once more Fermi said to Weil:

‘Pull it out another foot’; but this time he added, turning to the anxious group in the balcony: ‘This will do it. Now the pile will
chain-react
.’

The counters stepped up; the pen started its upward rise. It showed no tendency to level off. A chain reaction was taking place in the pile.

In the back of everyone’s mind was one unavoidable question.

‘When do we become scared?’

Under the ceiling of the balloon the suicide squad was alert, ready with their liquid cadmium: this was the moment. But nothing much happened. The group watched the recording instruments for 28 minutes. The pile behaved as it should, as they all had hoped it would, as they had feared it would not.

The rest of the story is well known. Eugene Wigner, the
Hungarian-born
physicist who in 1939 with Szilard and Einstein had alerted President Roosevelt to the importance of uranium fission, presented Fermi with a bottle of Chianti. According to an improbable legend, Wigner had concealed the bottle behind his back during the entire experiment.

All those present drank. From paper cups, in silence, with no toast. Then all signed the straw cover on the bottle of Chianti. It is the only record of the persons in the Squash Court on that day.

I was very surprised to discover that her hair smelled exactly the same. Of course, after I stopped and thought about it, there was no reason why her hair should smell different in such a short time. But to me it
was a kind of shock, because in my mind, something enormous had just happened – and yet nothing had happened.

After we’d made the calculations, the next thing that happened, of course, was the test. I was actually at home on a short vacation at that time, after my wife died, and so I got a message that said, ‘The baby is expected on such and such a day.’

I flew back, and I arrived
just
when the buses were leaving, so I went straight out to the site and we waited out there, twenty miles away. We had a radio, and they were supposed to tell us when the thing was going to go off and so forth, but the radio wouldn’t work, so we never knew what was happening. But just a few minutes before it was supposed to go off the radio started to work, and they told us there was twenty seconds or something to go, for people who were far away like we were. Others were closer, six miles away.

They gave out dark glasses that you could watch it with. Dark glasses! Twenty miles away, you couldn’t see a damn thing through dark glasses. So I figured the only thing that could really hurt your eyes (bright light can never hurt your eyes) is ultraviolet light. I got behind a truck windshield, because the ultraviolet can’t go through glass, so that would be safe, and so I could
see
the damn thing.

Time comes, and this
tremendous
flash out there is so bright that I duck, and I see this purple splotch on the floor of the truck. I said, ‘That’s not it. That’s an after-image.’ So I look back up, and I see this white light changing into yellow and then into orange. Clouds form and disappear again – from the compression and expansion of the shock wave.

Finally, a big ball of orange, the center that was so bright, becomes a ball of orange that starts to rise and billow a little bit and get a little black around the edges, and then you see it’s a big ball of smoke with flashes on the inside of the fire going out, the heat.

All this took about one minute. It was a series from bright to dark, and I had
seen
it. I am about the only guy who actually looked at the damn thing – the first Trinity test. Everybody else had dark glasses, and the people at six miles couldn’t see it because they were all told to
lie on the floor. I’m probably the only guy who saw it with the human eye.

Finally, after about a minute and a half, there’s suddenly a tremendous noise – BANG, and then a rumble, like thunder – and that’s what convinced me. Nobody had said a word during this whole thing. We were all just watching quietly. But this sound released everybody – released me particularly because the solidity of the sound at that distance meant that it had really worked.

The man standing next to me said, ‘What’s that?’

I said, ‘That was the Bomb.’

The man was William Laurence [author of
Dawn
Over
Zero
].
He was there to write an article describing the whole situation. I had been the one who was supposed to have taken him around. Then it was found that it was too technical for him, and so later H. D. Smyth came and I showed him around. One thing we did, we went into a room and there on the end of a narrow pedestal was a small silver-plated ball. You could put your hand on it. It was warm. It was radioactive. It was plutonium. And we stood at the door of this room, talking about it. This was a new element that was made by man, that had never existed on the earth before, except for a very short period possibly at the very beginning. And here it was all isolated and radioactive and had these properties. And we had made it. And so it was
tremendously
valuable.

Meanwhile, you know how people do when they talk – you kind of jiggle around and so forth. He was kicking the doorstop, you see, and I said, ‘Yes, the doorstop certainly is appropriate for this door.’ The doorstop was a ten-inch hemisphere of yellowish metal – gold, as a matter of fact.

What had happened was that we needed to do an experiment to see how many neutrons were reflected by different materials, in order to save the neutrons so we didn’t use so much material. We had tested many different materials. We had tested platinum, we had tested zinc, we had tested brass, we had tested gold. So, in making the tests with the gold, we had these pieces of gold and somebody had the clever idea of using that great ball of gold for a doorstop for the door of the room that contained the plutonium.