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Fission & the nucleus

  1. Sep 23, 2012 #1
    1.Uranium-235,after taking a neutron turns into Xe and Sr and 2 neutrons or it turns into Ba and Kr and 3 neutrons or many other things. How it is decided that Ur nucleus will change into what?
    2.Why aren't we still using fusion to produce energy?
    3. Isomers-what's the reason behind the nucleus having same things(p,n,e) being in different energy states?
  2. jcsd
  3. Sep 23, 2012 #2


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    I think it's purely random.

    We have never used nuclear fusion to produce energy. We don't know how to yet. But there are many people around the world working on a way to use fusion to provide power. The ITER project is thought to be able to reach breakeven, but it is still several years away from construction.

    It's simply a possible state for the nuclei to be in, similar to how an electron can be excited into a higher energy state.
  4. Sep 23, 2012 #3


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    Each fission event is random because the state of the U-235 atom and the absorption of the neutron (and formation of the composite nucleus) involve randomness (stochasticism).


    The absorption of a neutron by U-235 produces an excited U-236 nucleus. About 16% of the events result in a γ-decay without fission. Then U-236 may decay, or it may absorb a neutron and form U-237, which may decay to Np-237.

    When U-236 fissions, it splits into two nuclei of Z1 and Z2, where Z2 = 92-Z1. The atomic masses of the two nuclei are A1 and A2, where A1+A2+2,3 n = 236. Two or three neutrons may be released in the reaction, but there are 'delayed' neutrons, which are released from some of the neutron-rich fission products. Delayed neutrons represent less than 0.7% of neutrons of each generation, and the are critical to the control of a nuclear reactor.

    The most probably fission reaction (based on yield) is one producing Te-134 (Z=52), with the complementary nuclide being, Zr-100 (Z=40), and two neutrons. Other probable results are:

    Xe-140, 139, 138 with Sr 94, 95 with 2 or 3 neutrons (as appropriate, i.e., A1+A2+#n = 236),
    Ba-144, 143 with Kr-89, 90 with two or three neutrons (as appropriate).

    In a fission reactor, the neutron field is far from monoenergetic, and in fact, there is a spectrum of neutron energies from 0.01 eV up to several MeV or about 8 orders of magnitude of energy in units of eV. Some fission occur in U-238 due to fast neutrons - about 8 to 10% of fissions. Most fissions occur in U-235 until sufficient Pu-239 has been produced to compete for neutrons with U-235. The accumulation of Pu-239 and other fissile transuranics is continuous, and the proportion of fissions in transuranics is continually increasing until discharge.

    As Drakkith indicated, fusion has not been used for commercial energy production. The physics is straightforward, the engineering is a challenge.

    Nuclei have different energy levels due to internal arrangement of nucleons. Some may exist in a metastable energy state, i.e., they have a relatively long half-life, e.g., Kr-85m or Xe-135m, and those nuclei decay via isomeric transition (gamma decay). Z and A do not change. Those nuclei could decay by beta decay, which is a competing process.
  5. Sep 24, 2012 #4
    Thank you very much for your answer guys... Drakkith and specially to Astronuc for ur elaborate explanation...

    I knew its random... But what is the reason behind this randomness? There must be something... Aren't there any theories about this randomness?

    And about isomers... Its still not that clear to me... I can view electrons being in different energy states and radiating or absorbing energy... Can you please help me picture this about the nucleus... I mean how can I view the protons and neutrons being in different energy states?
  6. Sep 24, 2012 #5


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    If there would be something, it would not be random.
    In fact, you can show that quantum mechanics cannot be deterministic within its "textbook interpretation" (collapse interpretation).

    There are deterministic interpretations of quantum mechanics, but they do not allow us (within our universe) to predict the event either.

    The same can happen for nucleons, and even for the whole nucleus. There is no fundamental difference between those systems.
  7. Sep 24, 2012 #6
    Well that's not true is it? You are still happy to call die rolls "random", or lottery number draws "random". These things are purely deterministic though. We think them random only because of our lack of knowledge about the initial conditions of the system.

    But yes quantum mechanics seems to be different to this, although my brain still does not like to accept it...
  8. Sep 24, 2012 #7
    Actually this isn't true. It's impossible even by classical physics without quantum uncertainty to predict the outcome of a draw from a rolling barrel filled with numbered balls as long as they have been turned enough times. There are too many individual and chained sequences of collisions with unlimited sensitivity to initial conditions.

    Classical mechanics can't predict which way a perfectly inverted pencil will fall and even a vanishing force can tip it.

    So no, true randomness in physical systems like the lottery balls does exist.
  9. Sep 24, 2012 #8
    "Unlimited" is a strong word. In a classical system you can arbitrarily increase the precision of your measurements of the system without limit also, so unless your randomisation procedure introduces infinite destruction of information or your precision hits the quantum limit (and hence the randomisation is quantum mechanical in origin) then you can in principle predict it. The equations of motion are deterministic.

    In practice, yes, but in principle, no. Quantum mechanics is different at the level of principle. Or if you want to say it in a glorified way, God himself cannot predict the outcome of quantum events, but he does know what lotto numbers are coming up next :p. Assuming lotto numbers are not so chaotic as to be sensitive to quantum events, which I am pretty sure is the case.
    Last edited: Sep 24, 2012
  10. Sep 24, 2012 #9

  11. Sep 24, 2012 #10
    That system (with those initial conditions) is clearly sensitive to quantum effects (and so outside the domain where we consider Newtonian physics to be predictive) so is not very interesting for the purposes of our discussion. I had not seen it before though; it is a cute trick. But if these kind of systems are the only ones whose evolution cannot be determined from their initial conditions alone then I stand by my statement, since I do not see how you can claim the lottery ball system is like that one. Not to mention that those initial conditions occupy zero phase space, so that the probability of finding the system in that initial state is zero, unless God prepares it for you. You do not have to do crazy things like this to kill determinism in quantum mechanics.
    Last edited: Sep 25, 2012
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