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From the 1946 book, Dawn Over Zero.
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AN important event in 1941 intensified the search for the multiplication factor greater than one. A group of workers at the Radiation Laboratory of the University of California bombarded uranium 238 with neutrons of intermediate speeds and discovered that these neutrons transformed the U.238 into an entirely new element, not known to exist in nature, which they named plutonium.
How this remarkable achievement was accomplished is a story in itself to be told hereinafter, but one outstanding fact about plutonium has a vital bearing on what is being related here: tests on the new uranium off-spring gave definite proof of an earlier conjecture that plutonium has the same fissionable properties as U.235, so that it could be used as a substitute of equal, if not even greater, efficiency.
It is impossible to overestimate the significance of this discovery, made by a group of young physicists and chemists (most of them still in their early twenties), as it opened the way for the first time to fissionable material in large quantities.
In the separation of one element from another, advantage is taken of the difference in chemical properties between the two elements, which makes possible the employment of chemical means for large-scale separation. Since U.238 and U.235 have the same chemical properties, they could therefore not be separated by chemical means, whereas methods taking advantage of the slight difference in their atomic weight would yield, by the methods then known, only insignificant amounts.
Plutonium, on the other hand, was an entirely different element from uranium, hence possessing entirely different chemical properties, which made it possible to employ chemical methods for its separation from the U.238 parent in large quantities. In a word, it meant that the material for an atomic bomb could be made available in time for use in the war.
The first submicroscopic bits of plutonium were produced by neutrons knocked out from beryllium by means of a cyclotron. By this method only insignificant infinitesimal amounts could be produced. But the fact that a neutron entering U.238 converts it automatically in a series of steps into plutonium proved beyond the shadow of a doubt that that is exactly what happens when the cannibal in the U.238 swallows a neutron. This had been suspected all along, but now there was conclusive experimental proof that it was an absolute fact.
What that meant was that, if you could manage to build a chain-reacting pile, it might be possible to arrange it so that one out of every two neutrons emitted in the fission process from the U.235 would go into splitting another U.235 atom and thus keep the chain reaction going, while the second neutron would be captured by the cannibal and convert the U.238 into plutonium, which would be better than the U.235 since it could be separated by chemical means in quantities large enough for use in atomic bombs.
Moreover, an even more far-reaching possibility suggested itself concerning the use of a controlled chain reaction for the development of atomic power. The natural supply of U.235, as already stated, is very limited, each ton of natural uranium containing only fourteen pounds (seven tenths of one per cent).
But the discovery of plutonium promised to make up more than a hundredfold for the niggardliness of nature. For, again on the assumption that half of the neutrons emitted by the fission of the U.235 in the pile would go into the making of plutonium, this would mean that for every atom of U.235 split, an atom of plutonium would be created.
In other words, after the fourteen pounds of the U.235 in each ton of natural uranium had been completely used up through fission, fourteen pounds of plutonium would have taken their place. You would eat your cake and have it too.
Since the experiments with plutonium at California indicated that, like U.235, it would also undergo fission with slow neutrons, this meant that after the U.235 in the pile was all used up, the plutonium would take its place in perpetuating the chain reaction exactly as before, so that when the fourteen pounds of plutonium had been split up, they would have been replaced by an equal amount of the substance. In this manner the chain reaction could be kept going in the pile as long as there was any uranium left.
In other words, all the 1,986 pounds per ton of U.238 in the pile would be eventually converted into plutonium, thus increasing the original amount of fissionable material by 140 times. This, of ‘course, would mean a corresponding increase in the amount of atomic energy extracted from the pile, provided the plutonium had not in the meantime been removed for atomic bombs.
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|Ernest Lawrence accepts the 1939 Nobel Prize for Physics in February 1940. His invention of the cyclotron enabled the discovery of Plutonium.|
All these possibilities were reported on July 11, 1941 by Professor Lawrence, in a memorandum to the Committee of the National Academy of Sciences, which was then studying the uranium problem. This report played a major role in expediting Government support of the Atomic Bomb Project on an all-out basis, and therefore must rank as one of our great historic documents. Following are pertinent excerpts:
"An extremely important new possibility has been opened for the exploitation of the chain reaction with unseparated isotopes of uranium.
"Experiments in the Radiation Laboratory of the University of California have indicated:
(a) that element 94 [i.e., plutonium] is formed as a result of the capture of a neutron by uranium 238 followed by two successive emissions of electrons (beta transformations), and furthermore
(b) that this trans-uranic element undergoes slow neutron fission and therefore presumably behaves like uranium 235.
"If this is so, the following three outstanding important possibilities are opened:
1. Uranium 238 would be available for energy production, thus increasing about one hundredfold the total atomic energy obtainable from a given quantity of uranium.
2. Using element 94 one may envisage preparation of chain reaction units for power purposes weighing perhaps a hundred pounds instead of a hundred tons, as probably would be necessary for units using natural uranium.
3. If large amounts of element 94 were available it is likely that a chain reaction with fast neutrons could be produced. In such a reaction the energy would be released at an explosive rate which might be described as a ’super bomb.’"
At about the same time there came news from England that French scientists working at the Cavendish Laboratory, Cambridge, had (as the Official British Report stated later) “produced strong evidence, by December, 1940, that, in a system composed of uranium oxide, or uranium metal, with ‘heavy water’ as the slowing- down medium (i.e., moderator), a divergent slow neutron fission chain reaction would be realized if the system were of sufficient size.” At that time, the British report adds, “it seemed likely that, if uranium metal were used, this critical size would involve not more than a few tons of ‘heavy water.’”
This news from Britain, coupled with the news from the University of California, galvanized our scientists into action. For it was realized at once that the Nazis, through their rape of Norway, had gained possession of the world’s largest plant for the manufacture of heavy water, and it was certain that they were using it as a moderator for a uranium chain-reaction pile.
And since it was now almost certain that such a pile would produce plutonium in large amounts, it became once again likely that the Nazis might after all be able to produce atomic bombs in time for use against England, and also against Russia, which they had invaded in the summer of 1941.
It looked as though the cannibal in uranium was not friendly after all. In fact, the latest developments revealed him to be no cannibal at all. He was at the same time both a blessing and a curse, a Dr. Jekyll and a Mr. Hyde.
There was good reason to fear that Mr. Hyde was likely to gain dominance over Dr. Jekyll, for until November 1941, nearly three years after the discovery of fission, we had approved a total of only $300,000 on projects for uranium, a mere trifle compared with the magnitude and importance of the subject, and it therefore seemed likely that the Nazis had got a head start on us in a race in which their opponents were trotting while they were galloping.
At that time the results of another study by Professors Wigner and Smyth of Princeton gave further reason for anxiety. The fragments into which the U.235 splits are highly radioactive, and since they differ chemically from uranium, it was realized that they could be extracted and used as a “particularly vicious form of poison gas.”
In their report Drs. Wigner and Smyth stated that “the fission products produced in one day’s run of a 100,000-kilowatt chain-reacting pile might be sufficient to make a large area uninhabitable.”
As Dr. Smyth hastens to point out, neither he nor Dr. Wigner recommended the use of radioactive poisons, nor has such use been seriously proposed since then by the responsible authorities. But, he adds, “serious consideration was given to the possibility that the Germans might make surprise use of radioactive poisons, and accordingly defensive measures were planned.”
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|Scientists meet at the University of California Radiation Laboratory. (left to right) Ernest Lawrence, Arthur Compton, Vannevar Bush, James Conant, Karl Compton and Alfred Loomis. In mid-1940, after the German invasion of France, the United States changed its research organization, increased funding and appointed Vannevar Bush as Chairman of the National Defense Research Committee.|
With all these dark clouds gathering on the horizon, our top scientists called a council of war. It was December 6, 1941, the day before “the day that will live in infamy.” President Conant, who officially represented Dr. Bush, announced two important decisions:
• One was that “the possibility of obtaining atomic bombs for use in the present war was great enough to justify an ‘all out’ effort.”
• The second was that the project was of such a magnitude that it required an entirely different type of organization from the one in existence at that time.
And so it was that on December 6 the decisions were taken that led to the end of the war that began a few hours later on the morning of December 7.
On November 7, 1942 a small group of men began assembling a huge collection of lumps of uranium and bricks of graphite into a spherical structure designed to force the Great God K to reveal himself.
With the bricks and lumps of uranium and graphite, purer and more plentiful than ever before, they were building a gigantic latticework in which, they hoped, a cosmic fire would be lighted by a cosmic ray from interstellar space, and be kept burning by an endless relay of neutrons that would be liberated from split atoms of uranium 235.
It was to be, as they referred to it in their matter-of-fact way, the first self-sustaining chain-reaction pile ever made.
The scene of their labors was a gloomy squash court underneath the West Stands of Stagg Field on the University of Chicago campus.
No one passing the staid, ivy-covered neo-Gothic building on Ellis Avenue, between Fifty-sixth and Fifty-seventh streets, could have had the slightest inkling of what was going on inside. In fact, so great was the secrecy in which the work was shrouded that not even the president and trustees of the university knew what a hazardous venture was being made on their premises. And it was just as well, for their unawareness no doubt spared them many a sleepless night.
Much had happened since that historic day before Pearl Harbor as a result of the decision to go all out on atomic energy for military purposes. The sneak attack the day after lent even greater urgency to the undertaking. One of the first steps was to set up three great research centers, at Columbia University, the University of Chicago, and the University of California, to be directed, respectively, by Professors Harold C. Urey, Arthur H. Compton, and Ernest O. Lawrence, all Nobel prize winners.
Those at Columbia and California were to be devoted to research and development of large-scale methods for the separation of U.235.
The one at Chicago, which was named the Metallurgical Project, was to develop methods for the production and separation of plutonium. This, of course, meant the development of self-sustaining chain-reacting piles.
The Columbia group, including Drs. Szilard, Fermi, Anderson, Zinn, and Weil, as well as the Princeton group, including Drs. Wheeler and Wigner, who had been working on the pile problems, and scores of top-ranking nuclear physicists from our university laboratories were therefore transferred to Chicago early in 1942 and went to work at once on plans for building a pile.
Since they were still hampered by the lack of pure uranium and graphite, they continued their investigations along the lines started at Columbia and Princeton, and in this way gained much additional knowledge on how to ferret out the Great God K.
But the problem of pure uranium metal and pure graphite had to be solved at the earliest possible date. So, on learning that the Westinghouse company had been producing small amounts of metallic uranium for research purposes, Dr. Compton called Dr. Harvey C. Rentschler, Westinghouse director of research, on the telephone.
“How soon can Westinghouse supply three tons of pure uranium?” Dr. Compton asked. Dr. Rentschler was aghast; the total output of pure uranium metal up to that time had been a few grams. But he was assured by Dr. Compton that a large amount was needed urgently for a vital, highly secret war project, so Dr. Rentschler and his assistant, Dr. John W. Marden, set up a makeshift laboratory for the production of “metal X.”
Within a few months they had increased production from eight ounces a day to more than five hundred pounds and cut the cost from a thousand dollars a pound to twenty-two dollars. The three tons Dr. Compton had asked for were delivered by November 1942.
Other companies began production, and by May 1942 deliveries of uranium oxide with less than one per cent impurities began coming in at the rate of fifteen tons a month.
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|A "pile" at the University of Chicago, 1942, containing graphite blocks and uranium oxide.|
A new and simpler process for purifying uranium ore, developed at the National Bureau of Standards by Dr. James I. Hoffman, was put in operation by the Mallinckrodt Chemical Works of St. Louis under the direction of Dr. John R. Ruhoff, who, as Colonel Ruhoff, was later in charge of procurement of metal for the Manhattan Engineer District, as the Atomic Bomb Project was known. As a result an additional thirty tons of oxide a month became available by July 1942.
The Harshaw Chemical Company in Cleveland and the du Pont plant at Penns Grove, New Jersey, prepared, by a new simpler process, the raw materials for final purification by the Westinghouse company, which had been doing this work by a rather slow and costly method.
Similar steps were taken to obtain graphite of the highest possible purity. Following suggestions made by the National Bureau of Standards, the National Carbon Company and the Speer Carbon Company began producing, by the middle of 1942, highly purified graphite that absorbed twenty per cent fewer neutrons than the best standard commercial material previously available.
By July enough purified uranium oxide from Mallinckrodt had become available to build an improved exponential pile, the ninth in a series of what were known as intermediate piles The pile was not the right kind for a chain reaction, but the results obtained caused elation among the workers.
Earlier tests in May had already revealed that the purified materials increased the neutron multiplication factor K from 0.87 to 0.98. Now for the first time calculations based on the data showed that in a theoretical “infinitely large” pile, from which no neutrons leaked away from the sides, the K factor would be 1.007, a value greater than one.
Even before this experiment, Dr. Compton had predicted that a neutron multiplication factor “somewhere between 1.04 and 1.05 could be obtained in a pile of highly purified uranium oxide and graphite, provided that the air was removed from the pile to avoid neutron absorption by nitrogen.”
In the fall of 1942 a new and much more satisfactory method for producing pure uranium, developed independently by Professor Frank H. Spedding and his associates at Iowa State College, Ames, Iowa, and by Clement J. Rodden at the National Bureau of Standards, was introduced, and by the end of November more than one ton of metal had been produced by this method at a plant set up at Ames.
Lumps of this product, the purest so far made, became known as “Spedding’s eggs.”
By November 7 a total of 12,400 pounds of pure uranium metal had been collected at the West Stands squash court. In addition there were many more tons of uranium oxide, and tons of graphite, both of a higher purity than ever before.
Calculations on critical size—that is, the size at which the number of free neutrons produced by fission is just equal to the total lost by non-fission capture and by escape through the surface—assured them that they at last had enough uranium and graphite of sufficient purity to make a chain-reacting pile possible.
They approached their task with mixed emotions. They were naturally eager to succeed, but as human beings realizing the implications of success in this particular adventure, they were hoping, Dr. Compton told me, that they would fail.
Though all their calculations seemed to point to success, they could by no means be certain. The calculations involved many corrections, approximations, and interpretations, and hence there was the likelihood of error somewhere along the line. Some miscalculation, some wrong interpretation, might vitally affect the result.
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|Starting construction of a pile; base of a pile made from granite.|
Keeping in mind Dr. Compton’s prediction that they would meet with success “provided that the air was removed from the pile to avoid neutron absorption by nitrogen,” they began building their pile inside a huge square balloon, from which the air could be pumped out later.
Every other detail had been carefully attended to. They had figured out the dimensions that would be required to obtain the critical size—that is, the size at which enough neutrons would be liberated inside to make up for all possible losses.
They had devised all sorts of controls, for normal use in operating the pile as well as for emergencies if something unforeseen happened. The squash court was full of all sorts of sensitive devices for counting neutrons.
These neutron-counters were, in fact, the instruments that were to reveal the presence of the Great God K as soon as he arrived, if he ever did; for by counting the neutrons from one generation to the next they would be able to tell whether the generations of fission-producing neutrons kept multiplying themselves by a constant factor greater than one or whether their death rate was greater than their birth rate.
They knew only too well that without some system of birth control for neutrons an uncontrolled chain reaction might develop in which the neutrons would multiply in geometric progression so rapidly as to cause disaster for all concerned.
Not that they were likely to produce an explosion even approximating that of an atomic bomb, as that could not happen except with fast neutrons liberated in a critical mass of either pure U.235 or plutonium, under special pre arrange conditions.
But an uncontrolled chain reaction even with slow neutrons in a large mass of uranium would liberate great quantities of atomic energy in the form of heat, and this heat, if allowed to become great enough, might vaporize the uranium and graphite in the of the pile, and the vapor produce such tremendous pressure that it would explode the pile and the West Stands as well into a fiery cloud of uranium-graphite dust.
And that would be only part of the story. The extremely hot metal vapor might cause a Chicago fire even more disastrous than the one started by Mrs. O’Leary’s Cow; for the tremendous radioactivity of the flaming vapor would prevent firefighters from getting anywhere near the flames.
Not only that, but the radioactive poisons that would be scattered over a wide area by the explosions would make the Chicago University campus and a large part of Chicago’s South Side uninhabitable for some time.
But the builders of the pile were not taking any such risks. In fact, it would have been impossible even to attempt to build a pile were it not for the existence of effective birth-control methods for neutrons.
Two elements in particular, boron and cadmium, are voracious eaters of neutrons. So, as the pile was built up layer by layer, strips of cadmium and rods of boron steel were inserted at regular intervals in such a way as to make certain, to the best of their knowledge, that the number of neutrons would never rise beyond a desired level.
Since up to then atomic energy was being liberated in large amounts only in the sun and the stars, the pile they were building, which was to light an atomic fire for the first time on earth, was, in a sense, a miniature model of the sun or a star.
As a cradle for the star about to be born they laid a timber framework resting on the squash-court floor. The structure was to be, properly enough, a sphere, as that was calculated to yield the best results.
The uranium metal and uranium-oxide lumps were to be spaced in a cubic lattice, embedded in graphite. The graphite was cut in bricks and built up in layers.
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|A pile under construction. On the board is written Layer 7 - Graphite, Layer6 - cylinders Black Powder.|
At the corners of the square graphite bricks in each alternate layer they placed the uranium lumps. The lumps of the six tons of the pure metal were placed in the center of the structure. Surrounding it was a latticework of graphite bricks, at the corners of which were placed lumps of purified uranium oxide.
Ten slots passed completely through the pile. Three of these near the center were to hold boron-steel rods for purposes of safety and control. The others were to hold neutron-absorbing cadmium strips for additional safety and for experimentation.
Actually, any one of the cadmium strips alone could dispose of enough unwanted neutrons to keep the expected chain reaction within bounds. All that would be necessary to bring the neutron birth rate down to a desired level, at which it could not get out of hand, would be to push the cadmium strip to a measured distance within the pile, each centimeter of strip absorbing a definite number of neutrons.
Conversely, to bring the birth rate up to the level at which the multiplication factor greater than one would appear, it would only be necessary to pull the strip out a definite measured distance.
One of the three safety rods was to be operated automatically by two electric motors, which pushed the rod in when the intensity of the reaction increased above the desired level, and pulled it out when it was decreased below that level. Both within the pile and near it were placed instruments for measuring the intensity of neutrons at any given moment.
A remote-control room was set up for use if it was found necessary to shut off the main room (where the pile was) because of lethal radiations.
From the very beginning the cadmium strips and boron-steel rods were placed in “retard” position to make certain that the desired multiplication factor did not appear by surprise. This later turned out to be very fortunate indeed, for it actually arrived much earlier than was expected.
The work proceeded from early morning until late at night for twenty-four days. With each layer the total number of the neutrons born in the first generation was found to be greater than that born at the previous layer, but the rate of increase was not great enough to maintain a chain reaction, as the multiplication factor K was in each case less than one. For each one hundred neutrons that produced fission, less than one hundred new fission-producing neutrons were being born.
Things were still going at about the same pace on December 1, when the eleventh layer was completed. The sphere was then nearly three-quarters complete and still there was no sign of a multiplication factor greater than one.
Late that evening Dr. Fermi, who had been given the name Dr. Farmer for security reasons, had gone to bed. Dr. Zinn and the others stayed on working into the night.
Somehow as they worked on, piling up the bricks of the twelfth layer, they thought they heard a marked change in the tempo of the clicks from the neutron-counters. With each brick the tempo seemed to increase. The neutrons were definitely coming out at a faster rate.
Click, click, click. “We knew then,” Dr. Zinn told me later, “that if we pulled out the control rods, the thing would pop. But we did not want to wake Dr. Fermi.” It was unthinkable to open the show in his absence.
They were there early as usual the next morning, one of the coldest in Chicago. The squash court was badly heated, but the atomic bricklayers carried on in total oblivion of the cold and the gloom. Dr. Zinn was master of ceremonies that cold December 2. Present were Drs. Fermi, Szilard, Anderson, Well, Compton, Wigner, Samuel K. Allison, N. Hilberry, Volney C. Wilson, John Marshall. There was one young woman in the group, Leona Woods, who later became Mrs. Marshall.
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|Picture taken in 1946 of scientists involved in the first sustained chain reaction at the University of Chicago. Bottom-left is Enrico Fermi.|
Everyone present gradually became aware that one of the great moments in history was near. The neutrons were being born at a rate faster than had been anticipated for the twelfth layer of the structure. The calculations had apparently been on the conservative side.
By noon they were agreed that the goal might be near. Another hour or two and the first self-sustaining chain-reacting pile in the history of man, generating atomic energy, might be a reality.
And then Fermi, the imperturbable, said: “Let’s go to lunch!” The room became empty and silent. The pile looked grotesque in its balloon, which had been found not to be necessary after all.
Meantime, three blocks away, a momentous conference had been in progress since early that morning in Room 209, Eckhert Hall, on the University of Chicago campus: Among the group were Dr. Crawford H. Greenewalt, chemical engineer, a member of the board of directors of the E. I. du Pont de Nemours and Company, Inc.; Roger Williams, assistant general manager of the du Pont explosives department; Warren K. Lewis, professor of chemical engineering at the Massachusetts Institute of Technology; and T. C. Gary, also of the du staff.
This group had been selected for their engineering background as a reviewing committee to appraise the entire Chicago Metallurgical Project. None of them had the slightest inkling of the goings-on in the West Stands squash court.
In particular, they were discussing what was no doubt the strangest proposal ever made to the heads of a large industrial plant. The du Pont company had been asked by General Groves, who less than three months earlier had been placed at the head of the newly created Manhattan Engineer District, to undertake the construction and operation of large-scale plants, to cost hundreds of millions of dollars, for the production of plutonium and its chemical separation from its uranium parent.
No such problem had ever before confronted a group of practical engineers and industrialists. They had been asked to construct a type of plant that nobody had ever built, to manufacture a product that had been made only in submicroscopic amounts, which nobody was sure could ever be made in quantity.
Worse still, the only way to produce plutonium on a large scale was through a self-sustaining chain reaction, and yet there had been no definite experimental proof that such a chain reaction was possible.
And even if they were to succeed in building these fantastic plants, involving tremendous engineering problems of a revolutionary nature, and managed to operate them successfully in the production of plutonium, it might still take years to work out a method for its concentration.
How was one going to design plants for chemical procedures that still remained to be worked out?
And yet, as the discussion in Eckhert Hall proceeded that morning and continued in the afternoon, all these difficulties faded into insignificance in the face of one ominous possibility. We were at war. Our very existence was at stake.
The Nazis had had a head start on us. All indications were that a chain reaction was definitely possible, and we could not afford to lose any more time. In a war for survival one must take calculated risks.
The design and construction of operating plants must go on simultaneously with the laboratory experiments. That was a revolutionary concept never heard of before in industry.
And there was the further handicap that because of the strict secrecy ordered by President Roosevelt, the industrialists could not be told what was going on underneath the West Stands of Stagg Field.
When Dr. Fermi and his team came back from luncheon and it appeared that the work was about to reach a climax, Dr. Compton obtained permission to invite one, and only one, of the group at Eckhert Hall to witness the proceedings.
Dr. Compton called Dr. Greenewalt on the telephone. “Could you come over to the squash court below the West Stands of Stagg Field without delay?” Dr. Compton asked. “Don’t ask any questions and tell no one where you are going.”
As Dr. Greenewalt arrived, the last pure uranium eggs were being placed in the corners of the graphite bricks. Dr. Compton made hurried explanations in whispers. The job was nearly done.
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|Artist’s depiction of those present to witness the chain reaction at Stagg Field.|
They were still not quite out of the woods, however. Nobody was sure just how sensitive the controls were. There might still be a last-minute catastrophe. So extra-special precautions were taken.
Two young physicists in the group, Dr. Alvin C. Graves, of the University of Texas, and Harold V. Lichtenberger, of Millikan College, Decatur, Illinois, were selected to serve in what their colleagues referred to as a suicide brigade.
They stood in silence on a high platform overlooking the pile, each holding a bucket full of a potent cadmium solution, ready to quench any fire that might start if the cadmium and boron bonds were broken. For two hours they stood there, waiting for a signal to go into action, hoping the while that the nerves and muscles of mere humans would respond quickly enough.
As the pile-builders were approaching what they suspected might be critical size, they proceeded more and more cautiously. A careful check was made on the automatic controls in the balcony. After the last egg had been deposited in its graphite nest, all the cadmium strips but one were pulled out. The last one was then pulled out slowly to the proper distance The suicide brigade stood on the alert.
Faster came the neutrons and more frequent were the clicks of the counters—eight hundred, nine hundred, a thousand, eleven hundred. Intently they stood around the recorder that gave the count of the number of neutrons per minute.
In the eleventh layer the count of the first generation of neutrons had stopped at eight hundred. If it now rose to only a little more than Sixteen hundred per minute they would know that a multiplication factor greater than one had been reached.
Click, click, click—twelve hundred, fourteen hundred, sixteen hundred. And then there came a still small voice:
sixteen hundred and one. Two, three, four, five. Six, seven, eight. Nine, ten.
The atomic age had come in on tiptoe. The fission-producing neutrons were multiplying themselves by a constant factor of 1.0006; for each neutron that went into the splitting of one U.235 atom, more than one neutron was being born to carry on. A self-perpetuating chain reaction, and with it the dream of atomic power, had become a reality.
It was 3.30 in the afternoon of December 2, 1942. Along with the hour of 5.30 of the morning of July 16, 1945, this date and hour must go down in the annals of man as one of the two distinct birthdays of the Atomic Age—one marking the birth of atomic energy in a controlled reaction with slow neutrons; the other marking its birth in an uncontrolled chain reaction with fast neutrons.
As the critical size required to sustain a chain reaction had been found to be fully twenty-five per cent smaller ‘than had been expected, the scientists added just one more layer, the thirteenth, for luck, and called it a day.
In its final appearance this first man-made star was thus an incomplete sphere, flat at the top, a shape geometers call an oblate spheroid. As such it may be said to be a miniature model of the earth, except that it is flattened only at its North Pole.
Dr. Greenewalt lost no time rushing back to Room 209, Eckhert Hall, where the discussion of whether it would be wise for the du Pont company to undertake the construction of giant chain-reacting piles for the large-scale production of plutonium was still going on.
“Gentlemen,” Dr. Greenewalt said, “there is no need for further discussion.” He had been sworn to absolute secrecy, so he could not tell anything about what he had seen. But, as Dr. Compton said, “though Greeney did not say anything, all you had to do was to look at him. Greeney’s eyes popped,” Dr. Compton added.
And as the reviewing committee decided then and there to recommend to the du Pont company to proceed with plans for designing, building, and operating mammoth piles for producing plutonium (which were completed two years later, under a dollar-a-year contract, at the Hanford Engineer Works, near Pasco, Washington, at a cost of nearly $400 million), Dr. Compton held a short long-distance conversation with Dr. Conant.
“The Italian navigator has arrived in the New World and found the continent much smaller than he thought it was,” said Dr. Compton.
“I hope the natives received him kindly,” said Dr. Conant.
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