The Faber Book of Science (41 page)

The operation of the atomic pile was the result of almost four years of sustained work, which started when discovery of uranium fission became known, arousing enormous interest among physicists.

Experiments at Columbia University and at other universities in the United States had confirmed Enrico’s hypothesis that neutrons would be emitted in the process of fission. Consequently, a chain reaction appeared possible in theory. To achieve it in practice seemed a vague and distant possibility. The odds against it were so great that only the small group of stubborn physicists at Columbia pursued work in that direction. At once they were faced with two sets of difficulties.

The first lay in the fact that neutrons emitted in the process of uranium fission were too fast to be effective atomic bullets and to cause fission in uranium. The second difficulty was due to loss of
neutrons: under normal circumstances most of the neutrons produced in fission escaped into the air or were absorbed by matter before they had a chance of acting as uranium splitters. Too few produced fission to cause a chain reaction.

Neutrons would have to be slowed down and their losses reduced by a large factory if a chain reaction was to be achieved. Was this feasible?

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 working. 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.