Disintegration of Uranium by Neutrons

Dear class,

On Wednesday we will be discussing the Nature paper (Feb 11, 1939) by Meitner and Frisch entitled “Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction”. This is a short paper (1.25 pages). When reading it you should consider the “Who, What, When, Where and How”. Likely I will call upon people in class to discuss the paper, so be prepared. Comments should be constructive and thought out.

A few things to keep in mind.

1) Where was Meitner when this paper was published? Why?

2) What is masuruium?

3) Why do the authors use the strange series of “eka-” prefixes?

4) The Curie’s just missed out on several discoveries relating to the nucleus. Was this one of them?

Please have your questions posted here in by Tuesday evening to give the presenter time to consider them. 🙂

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14 Responses to “Disintegration of Uranium by Neutrons”

  1. Hunter Burgin Says:

    To begin, I must say that it was a great misfortune to deny Lise Meitner (and O.R. Fisch) the Nobel Prize for their work with nuclear fission and Uranium during the late 1930s. After reading the article and doing my own scientific research on the web regarding Uranium and nuclear fission I was left with one question. As detailed in the 1939 paper, Fisch and Meitner propose the “liquid drop model” to describe the way in which Uranium splits when bombarded by a neutron and energy. As described in the article, when heavy elements such as Uranium split, their close packing within the nucleus result in the almost equal splitting of the element into two smaller elements (for example Krypton-36 and Barium-56). As we know this paper was published almost 75 years ago and at the time the explanation given for the exact disintegration of the uranium element was “dependent on finer structural features and perhaps partially chance”. I was wondering if this answer had been further explained by any other scientists in the field of nuclear fission. Perhaps the splitting of Uranium can be altered by the amount of energy and neutrons introduced in order to elicit a split other than Kr-36 and Ba-56?

    • profhurst Says:

      Good thought. There are a lot of factors inside the nucleus that go into deciding what daughter products are created. Even numbers are more stable than odd, and there is likely to be many subsequent beta decays as the neutron numbers settle down.

      For uranium fission there are typically many products, not just the two, and hence why nuclear fall out can have a lot of different products (which then decay further).

  2. Porter Marsh Says:

    The text said that Fermi had to rely on nuclear isomerism to explain the different radioactive periods found when Uranium is bombarded by neutrons. Later it says that “it might not be necessary to assume nuclear isomerism,” and that the difference in radioactive period could be due to different isotopes. My question is whether or not Meitner investigated the cause of the different radioactive periods. Relying on either nuclear isomerism or the existence of isotopes would explain the different periods and Meitner does explain the logic behind the isotope method. However, he only says that relying on nuclear isomerism might not be necessary without appearing to support one or the other. Did he not know?

    • profhurst Says:

      Do you mean Meitner (who is female)?

      Different nuclear isotopes (isomers) do have different consequences (including half-life) under neutron addition, compare what happens when you hit U-238 with a neutron verses U-235.

  3. Kevin Greenwood Says:

    In the second column about halfway down, Meitner challenges Fermi’s statement that the radioactive periods ascribed to elements heavier than uranium are also due to lighter elements, citing technetium in particular. In Fermi’s paper, he categorizes two products with very similar chemical properties to elements heavier than uranium. One product had a 13 minute half-life and another had a 90 minute half-life. How did she connect these products to the idea of fission products rather than ascribing these radioactive periods to the beta-decay product? She mentions in the last paragraph that the body with half-life of ~24 minutes is probably (probably) uranium-239, which does go on to become neptunium-239. Would there not be a reason for Meitner to entertain the idea that neptunium and its isotopes could be responsible for Fermi’s results, and not the fission products?

  4. Josh Ellsworth Says:

    The half lives ascribed to Masurium(112 I’m assuming) seem too long by a couple of orders of magnitude. Why did Meitner and Frisch assume that to be the case, instead of an isotope of Ru or Pd, which could also decay into Cadmium? Did they have the means to distinguish their proposed decay chain products from each other, given the observed half life, or were they simply hopeful to have observed an elusive element?

  5. Adam Settimo Says:

    After reading a very brief history of Lise Meitner and the events of the time, it is hard to not focus a little on that horrible situation, but after gaining a little background of the history and relations between some of these physicists, reading this “fantastic explanation,” of fission is quite great. The third paragraph seems really sets the stage for this ‘explanation.” The brief history I read really set the context for this paper and can be found here:

    http://www.aps.org/publications/apsnews/201502/physicshistory.cfm

    While I have many questions related to the context above I will save them for later discussion and get to the few quick questions that I have.
    What is meant by in the fourth paragraph about the formation of lighter elements being rejected due to “physical reasons?”
    What were the methods of the time used to determine similarity between these new bodies and other elements, or groups?
    When discussing the surface tension of nuclear matter being offset by the charge of the droplet, I could use some help expanding on this.

    • profhurst Says:

      I like that you identify the specific paragraphs.

      In the 4th, they mention that alpha decay is unlike, particularly the emission of a bunch of alpha particles in such a short period of time. Tunneling was known from ten years before (Condon paper from 1928), but it was not thought conceivable that something huge e.g. a set of protons and neutrons equivalent to barium, could do it.

      Instead it was the new process of nuclear “disintegration” that provided the new (to them) mechanism.

  6. Tony J Says:

    It’s curious to see the early stages of the filling and nomenclature of the unstable elements. I wonder why masuruium got renamed technetium. I recall reading about nuclear fission as a small child and wondering what elements uranium broke up into. I predicted 2 palladium ( 92/2=46) since the text book I saw didn’t specify. Is there a reason why it spits into barium and krypton?

    • profhurst Says:

      The first part of the question is a bit too simple, a quick google search turns up why. So next time state your first question, the answer, and then the question that arises from there. Why do we think they did not isolate technetium (as masurium is now know).

      Similar with the second part as well. What isotopes of palladium would you get? Is this likely?

  7. Daniel Begay Says:

    This was an interesting paper that left me with a lot of questions, but what stood out to me in the 3rd paragraph, was the investigation done by Hahn and Strassmann. When they followed up on the Curie’s finding, they discovered that the three radioactive bodies form from uranium via neutron bombardment were isotopes of barium and NOT radium. I was curious to find out what this “investigation” entialed, as they did not go into further detail in this paper.

    I came across a paper explaining that they used fractional crystallization and fractional precipitation via barium salt solutions (concentrate radium in solution)

    I’m curious to how they hypothesized that uranium would break down into radium (which is later to discovered as isotopes of barium)? The article I found states:

    “These new radioactive products are apparently due to decay of 239U by succesive emission of two alpha particles. By this process the element with the nuclear charge of 92 must decay to a nuclear charge of 88; that is, to radium.”

  8. profhurst Says:

    Lots of well thought out comments here, good job. I’ll try to reply soon.

  9. Josh Ellsworth Says:

    Second attempt at posting. My question is about the assumption by Meitner and Frisch that they observed Masurium/Technetium isotopes (112) that exhibited half lives on the order of tens of seconds whereas currently that isotope is predicted to have a half life of around 0.3s. Why didn’t the authors ascribe the observed activity to a Rhodium or Palladium isotope? Was this just wishful thinking on their part or were they able to discern five beta decays and the respective products leading to Cadmium?

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