Fission

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spontaneous

neutron induced 1n + 235 U 92 -->


Heavy nuclei split into two fragments of roughly equal mass. Energy is released in the process. Fission powers nuclear reactors and "small" nuclear weapons. [magnify, animate]

Chain reaction notes

In 1933, Szilard fled to London to escape Nazi persecution. While in London, he read an article written by Ernest Rutherford in the London Times, after which he conceived the idea of a nuclear chain reaction. In the following year, he filed a patent on the nuclear chain reaction. He first attempted to create a chain reaction using Beryllium and Indium, but neither yielded the reaction he deliberated. In 1936, he assigned the chain-reaction patent to the British Admiralty to ensure secrecy of the patent. In 1938, he moved to New York. After learning about fission in 1939, he concluded that uranium would be the element capable of the chain reaction.

In 1939, Szilard worried that Germany would discover the atomic bomb. He urged other scientists to keep fission private in order to postpone Germany manufacturing the bomb. In a letter prepared for Albert Einstein, Szilard wrote to President Roosevelt warning about the possibility of creating an atomic weapon and urged the US government to develop this weapon before Germany. This letter was a catalyst in involving the US government in atomic research, which led to establishing the Manhattan Project. On December 2, 1942, Szilard and Enrico Fermi were successful in creating the first controlled nuclear chain reaction. http://www.nuclearfiles.org/rebios/szilardsleo.html

Leo Szilard recalls the day

variant 1
As the light changed to green and I crossed the street, it … suddenly occurred to me that if we could find an element which is split by neutrons and which would emit two neutrons when it absorbs one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction …. I didn't see at the moment just how one would go about finding such an element, or what experiments would be needed, but the idea never left me. In certain circumstances it might be possible to set up a nuclear chain reaction, liberate energy on an industrial scale, and construct atomic bombs.

variant 2
I found myself in London about the time of the British Association meeting in [12] September 1933. I read in the newspapers a speech by Lord Rutherford, who was quoted as saying that he who talks about the liberation of atomic energy on an industrial basis is talking moonshine. This set me pondering as I was walking the streets of London, and I remember that I stopped for a red light at the intersection of Southampton Row [at Russell Square]. As the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons and emit two neutrons when it absorbed one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction. I didn't see at the moment just how one would go about finding such an element, or what experiments would be needed, but the idea never left me. Soon thereafter, when the discovery of artificial radioactivity by Joliot and Mme. Joliot was announced, I suddenly saw that tools were at hand to explore the possibility of such a chain reaction. I talked to a number of people about this …. [I]in the spring of 1934 I had applied for a patent which described the laws governing such a chain reaction. It was the first time, I think, that the concept of critical mass was developed and that a chain reaction was seriously discussed. Knowing what this would mean - and I knew it because I had read H.G. Wells - I did not want this patent to become public. The only way to keep it from becoming public was to assign it to the government. So I assigned this patent to the British Admiralty.

variant3
On Tuesday, September 12, 1933, while waiting at the lights to cross the road to the British Museum in Bloomsbury, Leo Szilard, a Hungarian theoretical physicist, had the flash of insight which was to result in the Little Boy and Fat Man bombs being dropped on Hiroshima and Nagasaki less than 12 years later. "As the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons, and which would emit two neutrons when it absorbs one, such an element could sustain a nuclear chain reaction."

Just take excerpts from this letter by Leo Szilard

I feel that I ought to let you know of a very sensational new development in nuclear physics. In a paper in the Naturwissenschaften Hahn reports that he finds when bombarding uranium with neutrons the uranium breaking up into two halves giving elements of about half the atomic weight of uranium. This is entirely unexpected and exciting news for the average physicist. The Department of Physics at Princeton, where I spent the last few days, was like a stirred-up ant heap.

Apart from the purely scientific interest there may be another aspect of this discovery, which so far does not seem to have caught the attention of those to whom I spoke. First of all it is obvious that the energy released in this new reaction must be very much higher than in all previously known cases. It may be 200 million (electron-) volts instead of the usual 3-10 mil-lion volts. This in itself might make it possible to produce power by means of nuclear energy, but I do not think that this possibility is very exciting, for if the energy output is only two or three times the energy input, the cost of investment would probably be too high to make the process worthwhile.

Unfortunately, most of the energy is released in the form of heat and not in the form of radioactivity.

I see, however, in connection with this new discovery potential possibilities in another direction. These might lead to a large-scale production of energy and radioactive elements, unfortunately also perhaps to atomic bombs. This new discovery revives all the hopes and fears in this respect which I had in 1934 and 1935, and which I have as good as abandoned in the course of the last two years. At present I am running a high temperature and am therefore confined to my four walls, but perhaps I can tell you more about these new developments some other time. Meanwhile you may look out for a paper in "Nature" by Frisch and Meitner which will soon appear and which might give you some information about this new discovery. http://www.nuclearfiles.org/redocuments/1939/390125-szilard-strauss.html

Summary

Problems

practice

  1. INCOMPLETE
    A nucleus is a ball of positively charged protons glued together with the help of uncharged neutrons. Splitting a nucleus into two roughly equal halves increases the separation between the protons as a group and reduces their electrostatic potential energy. Determine the following quanities for an idealized fission reaction …
    1. the relative amount of the original electrostatic potential energy released
    2. the absolute amount of the original electrostatic potential energy released by uranium 235 given that its diameter is 12.3 fm and its total charge is +92 e.
    Solutions …
    1. Dividing a ball of nuclear matter in half results in two spheres with half the charge and half the volume of the original. Volume is proportional to the cube of radius, so the new spheres will have a diameter that's smaller by the cube root of a half. Apply these changes to the equation for electrostatic potential energy.
       
       
      Thus, each daughter nucleus has roughly 40% of the electrostatic energy of the parent nucleus. But …
       
      40% + 40% ≠ 100%
       
      The remaining 20% is liberated to do work -- like boiling water to make steam to generate electricity or, in the case of nuclear weapons, laying waste to a city. INCOMPLETE
    2. INCOMPLETE
  2. Write something.
    • Answer it.
  3. Write something.
    • Answer it.
  4. Write something.
    • Answer it.

numerical

  1. problems

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