by John Carey
To slow down neutrons was an old trick for Enrico, from the time when he and his friends in Rome had recognized the extraordinary action of paraffin and water on neutrons. So the group at Columbia – Szilard, Zinn, Anderson, and Enrico – undertook the investigation of fission of uranium under water. Water, in the physicists’ language, was being used as a moderator.
After many months of research they came to the conclusion that neither water nor any other hydrogenated substance is a suitable moderator. Hydrogen absorbs too many neutrons and makes a chain reaction impossible.
Leo Szilard and Fermi suggested trying carbon for a moderator. They thought that carbon would slow down neutrons sufficiently and absorb fewer of them than water, provided it was of a high degree of purity. Impurities have an astounding capacity for swallowing neutrons.
Szilard and Fermi conceived a contrivance that they thought might produce a chain reaction. It would be made of uranium and very pure graphite disposed in layers: layers exclusively of graphite would alternate with layers in which uranium chunks would be embedded in graphite. In other words, it would be a ‘pile’.
An atomic pile is, of necessity, a bulky object. If it were too small, neutrons would escape into the surrounding air before they had a chance to hit a uranium atom, and they would be lost to fission and chain reaction. How large the pile ought to be, nobody knew.
Did it matter whether the scientists did not know the size of the pile? All they had to do, one might think, was to put blocks of graphite over blocks of graphite, alternating them with lumps of uranium, and keep on at it until they had reached the critical size, at which a chain reaction would occur. They could also give the pile different shapes – cubical, pyramidal, oval, spherical – and determine which worked best.
It was not so simple. Only a few grams of metallic uranium were available in the United States, and no commercial graphite came close to the requirements of purity.
The 1951 edition of Webster’s New Collegiate Dictionary states that graphite is ‘soft, black, native carbon of metallic lustre; often called plumbago or black lead. It is used for lead pencils, crucibles, lubricants, etc …’ The atomic pile built in 1942, clearly included in the ‘etc.,’ was to use as much graphite as would go into making a pencil for each inhabitant of the earth, man, woman, and child. Moreover, graphite for a pile must be of a state of purity absolutely inconceivable for any other purpose. Scientists would have to be patient.
Procurement became a big and important task, one for which Fermi was not suited and which he would rather leave to others. Luckily for him, Leo Szilard did not share his aversion to interrupting research and shopping around.
Szilard was a man with an astounding number of ideas, several of which turned out to be good. He had no fewer acquaintances than ideas, a not negligible percentage of whom were important persons in high positions. These two sets of circumstances made of Szilard a powerful and useful spokesman for the small group of researchers, one who could confront the difficulties of politics with sufficient impetus to overcome them successfully. Willingly and with determination he undertook the not easy task of turning grams into tons, both of metallic uranium and of highly pure graphite.
The first question one asks when undertaking a task of that kind is: ‘Who is going to finance my enterprise and give me the cash that is needed?’ Szilard hoped he knew the answer. During the summer of 1939, with Wigner, Teller, Einstein and Sachs, he had succeeded in arousing President Roosevelt’s interest in uranium work. Now, at the very beginning of 1940, he scored his second victory and obtained the first tangible, if small, proof of that professed interest, when Columbia University received the first grant of $6,000 from the Army and Navy to purchase materials.
Thus by early spring 1940 a few tons of pure graphite started to arrive at the physics building of Columbia University. Fermi and Anderson turned into bricklayers and began to stack graphite bricks in one of their laboratories.
They were well aware that for many months, perhaps for years, there would not be uranium and graphite of good enough quality and in sufficient quantity to attempt a pile. That did not matter for the time being: they knew so very little about the properties of the substances they were to work with – of metallic uranium not even the melting point had been determined – that much study of these properties ought to be pursued and completed before they could in good conscience recommend that the Uranium Committee undertake the tremendous effort and expense that would go in the project.
So they stacked graphite bricks into a stocky column, placed a neutron source under it, observed what happened to the neutrons in the graphite and began to collect data.
This work, dull as it sounds, was considered very important; and when the Advisory Committee on Uranium met on April 28, 1940, it decided to wait for more results at Columbia University before making formal recommendations for the project. The committee made this decision despite the report that the Nazis had set aside a large section of the Kaiser Wilhelm Institute in Berlin for research on uranium.
After the study on graphite, came that on uranium: How does it absorb and re-emit neutrons? Under what conditions will it undergo fission? How many neutrons will be produced altogether?
The experiments proceeded slowly for lack of materials and Fermi would have liked to speed up his work. Besides, he was convinced that from the behaviour of a small pile he would obtain much more information pertinent to building a larger pile. Fermi and his group were able to start work on the ‘small pile’ by the spring of 1941. They demolished their column of graphite bricks and laid them down again, placing lumps of uranium among them. Slowly, as more graphite arrived at Columbia, a black wall grew up. The black wall reached the ceiling; but it was still far from being a chain-reacting pile: too many neutrons escaped from it or were absorbed inside it, and too few remained to produce fission.
It became evident that the experiment could not be pursued to find success in that same laboratory. A larger room, with higher ceiling, was needed. No such room was available at Columbia, and somebody would have to look for one elsewhere. Fermi was absorbed in his research. His work was too important to be interrupted. So Herbert Anderson took off his overalls, put on a suit, coat and a hat, and went scouting in New York City and its suburbs in search of a loft that could house a pile. He spotted several possibilities and began some bargaining aimed at the best deal.
Before Herbert could make a final choice, Enrico learned that he, his group, his equipment, and the materials he had gathered would have to move to Chicago. It was the very end of 1941…
The best place Compton [Professor Arthur H. Compton of the University of Chicago, who had been appointed head of research into chain reaction] had been able to find for work on the pile was a squash court under the West Stands of Stagg Field, the University of Chicago stadium. President Hutchins had banned football from the Chicago campus, and Stagg Field was used for odd purposes. To the west, on Ellis Avenue, the stadium is closed by a tall grey stone structure in the guise of a medieval castle. Through a heavy portal is the entrance to the space beneath the West Stands. The Squash Court was part of this space. It was 30 feet wide, twice as long, and over 26 feet high.
The physicists would have liked more space, but places better suited for the pile, which Professor Compton had hoped he could have, had been requisitioned by the expanding armed forces stationed in Chicago. The physicists were to be contented with the Squash Court, and there Herbert Anderson had started assembling piles. They were still ‘small piles,’ because material flowed to the West Stands at a very slow, if steady, pace. As each new shipment of crates arrived, Herbert’s spirits rose. He loved working and was of impatient temperament. His slender‚ almost delicate, body had unsuspected resilience and endurance. He could work at all hours and drive his associates to work along with his same intensity and enthusiasm.
A shipment of crates arrived at the West Stands on a Saturday afternoon, when the hired men who would normally unpack them were not wo
rking. A university professor, older by several years than Herbert, gave a look at the crates and said lightly: ‘Those fellows will unpack them Monday morning.’
‘Those fellows, Hell! We’ll do them now,’ flared up Herbert, who had never felt inhibited in the presence of older men, higher up in the academic hierarchy. The professor took off his coat and the two of them started wrenching at the crates.
Profanity was freely used at the Met. Lab. It relieved the tension built up by having to work against time. Would Germany get atomic weapons before the United States developed them? Would these weapons come in time to help win the war? These unanswered questions constantly present in the minds of the leaders in the project pressed them to work faster and faster, to be tense, and to swear.
Success was assured by the spring. A small pile assembled in the Squash Court showed that all conditions – purity of materials, distribution of uranium in the graphite lattice – were such that a pile of critical size would chain-react….
While waiting for more materials, Herbert Anderson went to the Goodyear Tyre and Rubber Company to place an order for a square balloon. The Goodyear people had never heard of square balloons, they did not think they could fly. At first they threw suspicious glances at Herbert. The young man, however, seemed to be in full possession of his wits. He talked earnestly, had figured out precise specifications, and knew exactly what he wanted. The Goodyear people promised to make a square balloon of rubberized cloth. They delivered it a couple of months later to the Squash Court. It came neatly folded but, once unfolded, it was a huge thing that reached from floor to ceiling.
The Squash Court ceiling could not be pushed up as the physicists would have liked. They had calculated that their final pile ought to chain-react somewhat before it reached the ceiling. But not much margin was left, and calculations are never to be trusted entirely. Some impurities might go unnoticed, some unforeseen factor might upset theory. The critical size of the pile might not be reached at the ceiling. Since the physicists were compelled to stay within that very concrete limit, they thought of improving the performance of the pile by means other than size.
The experiment at Columbia with a canned pile had indicated that such an aim might be attained by removing the air from the pores of the graphite. To can as large a pile as they were to build now would be impracticable, but they could assemble it inside a square balloon and pump the air from it if necessary.
The Squash Court was not large. When the scientists opened the balloon and tried to haul it into place, they could not see its top from the floor. There was a movable elevator in the room, some sort of scaffolding on wheels that could raise a platform. Fermi climbed onto it, let himself be hoisted to a height that gave him a good view of the entire balloon, and from there he gave orders:
‘All hands stand by!’
‘Now haul the rope and heave her!’
‘More to the right!’
‘Brace the tackles to the left!’
To the people below he seemed an admiral on his bridge, and ‘Admiral’ they called him for a while.
When the balloon was secured on five sides, with the flap that formed the sixth left down, the group began to assemble the pile inside it. Not all the material had arrived, but they trusted that it would come in time.
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.