Era Of Atomic Age Begin Part-2/2
To keeps such a chain reaction under control it was necessary to keep the reproductive ratio of neutrons at optimal levels. Fermi had carried out certain experiments in July 1941, to obtain measurements of the reproduction factor of neutrons, called “k”, which was the key to the problem of a chain reaction. If this factor could be made sufficiently greater than 1, a controlled chain reaction could be made to take place in a mass of material of practical dimensions. If it were less than 1, no chain reaction could ensue. One of the first things that had to be determined was how best to place the uranium in the reactor.
Fermi and Leo Szilard suggested placing the uranium in a matrix of the moderating material, thus forming a cubical lattice of uranium. This placement appeared to offer the best opportunity for a neutron to encounter a uranium atom. Of all the materials which possessed the proper moderating qualities, graphite was the only one which could be obtained in sufficient quantity of the desired degree of purity. At Chicago, the work on sub-critical size piles was continued. By July 1942, the measurements obtained from these experimental piles had gone far enough to permit a choice of design for a test pile of critical size. At that time, the dies for the pressing of the uranium oxides were designed by Zinn and then tailor-made to fit the criteria. It was a fateful step since the entire construction of the pile depended upon the shape and size of the uranium pieces.
Construction of the main pile at Chicago started in November. The project gained momentum, with machining of the graphite blocks, pressing of the uranium oxide pellets, and the design of instruments. Fermi’s two ‘construction’ crews, one under Zinn and the other under Herbert L. Anderson, worked almost around the clock, Day after day the pile grew toward its final shape. And as the size of the pile increased, so did the nervous tension of the men working on it. Logically and scientifically they knew this pile would become self-sustaining. It had to. All the measurements indicated that it would. But still, the demonstration had to be made. As the eagerly awaited moment drew nearer, the scientists gave greater and greater attention to details, the accuracy of measurements, and exactness of their construction work.
During the early afternoon of December 1, tests indicated that critical size was rapidly being approached. At 4 p.m., Zinn’s group was relieved by the men working under Anderson. Shortly afterward the last layer of graphite and uranium bricks was placed on the pile. Zinn, who remained, and Anderson made several measurements of the activity within the pile to determine whether it would become self-sustaining. Both had agreed, however, that should measurements indicate the reaction would become self-sustaining they should suspend further activity until Fermi and the rest of the group could be present.
Consequently, the control rods were locked and further work was postponed until the following day.That night the word was passed to the men who had worked on the pile that the trial run was due the next morning. About 8.30 on the morning of Wednesday, December 2, the group began to assemble in the squash court. At the north end of the squash court was a balcony about ten feet above the floor of the court. Fermi, Zinn, Anderson and Compton were grouped around instruments at the east end of the balcony. On the floor of the squash court, just beneath the balcony, stood George Weil, whose duty was to handle the final control rod. In the pile were three sets of control rods. One set was automatic and could be controlled from the balcony. Another was an emergency safety rod. Attached to one end of this rod was a rope running through the pile and weighted heavily on the opposite end. The rod was withdrawn from the pile and tied by another rope to the balcony. Hilberry was ready to cut this rope with an axe should something unexpected happen.
At 9.45 a.m., Fermi ordered the electrically operated control rods to be withdrawn. The man at the controls threw the switch to withdraw one set of control rods. A small motor whined. All eyes were glued to the lights which indicated the rods’ positions. A quick calculation on the data supplied by the instruments made it clear that this was not enough. The reaction was not sustained. Cautiously, Fermi went on ordering a withdrawal of more control rods and to increasing extents. The whole team went on conducting the cautious experiment without any success till well after midday.
Everyone was tired and hungry. Fermi ordered a break and everyone adjourned for lunch. They reassembled at 2 O’clock and resumed work immediately. At 2.50 p.m. the control rod came out another foot. The counters nearly jammed, the pen headed off the graph paper. But this was not it. Counting ratios and the scale of the graph had to be changed. “Move it six inches,” said Fermi at 3.20 p.m. Again the change-but again the levelling off. Five minutes Fermi called: Pull it out another foot.” Weil with”the rod. Fermi computed the rate of rising of the neutron counts over a minute period. He silently, grim-faced, through some calculations on his slide rule. In about a minute again computed the rate of rising. If the rate was constant and remained so, he would know the reaction was self-sustaining. He repeated the procedure a few more times. Finally, he announced, “The reaction is self-sustaining.” “O.K., Zip in,” called Fermi to Zinn, who controlled that rod. The time was 3.53 p.m. Abruptly, the counters slowed down, the pen slid down across the paper. It was all over. A man had initiated a self-sustaining nuclear reaction and then stopped it. He had released the energy of the atom’s nucleus and controlled that energy. Arthur Compton went to convey the good news by long distance telephone to James Conant who was waiting at Harvard.
“The Italian navigator has landed in the New World,” said Compton.
“How were the natives?” asked Conant. “Very friendly.”With those words, mankind entered the atomic age.
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