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The Manhattan Project Heritage Preservation Association, Inc. "Preserving, Exhibiting, Interpreting and Teaching the History of the Manhattan Project" |
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Manhattan Project History
The Early Years (1900 - 1939)
"The URANIUM" Confusion"
| "Marie (Curie) saw our research work and I will never forget the expression of intense joy which came over her when Irene and I showed her the first artificially radioactive element in a little glass tube. I can still see her taking in her fingers (which were already burnt and swollen from radium) this little tube containing the radioactive compound - as yet one in which the activity was very weak. To verify what we had told her she held it near a Geiger-Muller counter and she could here the rate meter giving off a great many "clicks". This was doubtless the last great satisfaction of her life" - Frederick and Irene Joliot-Curie |
| "If the thickness is larger than the critical value....I can produce an explosion" - Leo Szilard; early patent amendment; July 1934. |
| "Since the neutron carries no charge, there is no strong electrical repulsion to prevent its entry into the nucleus. In fact, the forces of attraction which hold nuclei together may actually pull the neutron into the nucleus. When a neutron enters a nucleus, the effects are about as catastrophic as if the moon struck the earth. The nucleus is violently shaken up by the blow, especially if the collision results in the capture of the neutron. A large increase in energy occurs and must be dissipated, and this may happen in a variety of ways, all of them interesting." - I. I. Rabi; January 1934. |
Once James Chadwick discovered the neutron and Niels Bohr began to explain the "nucleus" of the atom, intense experimentation was the rule of the day across the many laboratories, both in Europe and the United States. The experiments carried out by the Joliot-Curies in France gave the first chemical proof of artificial transmutation. "One of the most important discoveries of the century", according to Emilio Segre in 1934. Also at this time, Leo Szilard was entertaining ideas of "chain-reactions" and even "nuclear explosions". He envisioned neutrons being much more productive than alpha particles, such as the Joliot-Curies had used in their experiments. He intuitively felt that some elements should emit two or more neutrons for each neutron captured. The question was, "which one?" As so often happens, and seemed to happen more frequently with Szilard, he lacked the necessary resources to develop his dreams into reality. Enter Enrico Fermi and his team in Rome. When Fermi began his neutron bombardment experiments he was 33 years old. In the beginning he worked alone. He intended to irradiate most of the elements in the periodic table and he started methodically with the lightest. He had little success until he reached fluorine, where he was able to induce minimal radiation. From that point forward, he and his team (E. Segre, E. Amaldi and F. Rasetti) were able to induce radioactivity in several elements, including iron, silicon, titanium, zinc, bromine, and lanthanum. They came, finally to uranium, element 92. One interesting thing they had found: Light elements generally transmuted to even lighter elements by ejecting either a proton or alpha particle. Heavier elements, however, due to a stronger electrical barrier around the nucleus, got heavier by capturing the bombarding neutron and throwing off its binding energy by emitting gamma radiation. These elements tended to become heavier isotopes of themselves. Fermi realized that bombarding uranium with neutrons was producing first a heavier isotope, uranium 239, and then a new, man-made transuranic element, atomic number 93, something never seen on earth before. However, this initial success soon collided with prior theory, and several "mysterious" results began to take shape. It was very confusing! On October 1934, Fermi made a startling discovery. In his own words: "I will tell you how I came to make the discovery which I suppose is the most important one I have made. We were working very hard on the neutron induced radioactivity and the results we were obtaining made no sense. One day, as I came into the laboratory, it occurred to me that I should examine the effect of placing a piece of lead before the incident neutrons. Instead of my usual custom, I took great pains to have the piece of lead precisely machined. I was clearly dissatisfied with something. I tried every excuse to postpone putting the lead in place. When finally, with some reluctance, I was going to put in its place, I all at once grabbed an odd piece of paraffin and placed it where the lead was to have been." The extraordinary result of substituting paraffin for a heavy element like lead was a dramatic increase in the intensity of the radioactivity. What was happening was that the paraffin was "slowing down" the speed of the neutrons, permitting them to be in the vicinity of the nucleus for a longer period of time, thus giving them more time to be "captured." Everyone from Bohr's lab in Copenhagen followed Fermi's work with fascination. From Cornell, Hans Bethe published a paper calculating the slim odds of neutron capture. All of the resulting consternation resulted in Bohr visualizing his now familiar explanation of the nucleus: rather than a single particle, the nucleus must be made up of neutrons and protons closely packed together - a result that Bohr felt did not bode well for harnessing nuclear energy. Thus by the mid-1930's the three most original living physicists had spoken to the question of creating "chain-reactions" to produce nuclear energy. Rutherford had called it "moonshine." Einstein compared it to "shooting in the dark at scarce birds." Bohr thought it "remote" at best! The first "enlightenment" can be found in a curious paper that was published in late 1934 by Ida Noddack, a respected German chemist. This paper was titled simply "On Element 93" and severely criticized Fermi's work. She said that based on Joliot-Curies' prior discoveries, instead of assuming that the newly created radioelement was heavier than uranium, he ought to have compared it to all of the known elements. She went on to say that "although in the past, elements have transmuted only into their near neighbors, it is conceivable, when heavy nuclei are bombarded by neutrons, they could break into several large fragments." If Noddack's physics was avant garde, her chemistry was sound. By 1938 her article was gathering dust on back shelves, but Bohr had promulgated the liquid-drop model of the nucleus and the confused chemistry of uranium became the "topic of the day" at laboratories everywhere.
Even though Leo Szilard was often ahead of his time, there is nothing documented to indicate that he was yet thinking of Uranium.
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