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Playing with NEUTRONS. Neutrons thought experiments.

  1. Feb 2, 2013 #1
    What would happen if:

    1- Two neutrons approach each other slowly. Do they glue?
    2- A rainfall of neutrons fell to the Earth. I mean very slowly (very low energy) and a huge amount, the same mass as the oceans, i.e.

    Thank you very much!.
  2. jcsd
  3. Feb 2, 2013 #2


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    1. This question is vague. What do you mean by "slow", i.e. at what speed exactly? "Fast" neutrons that are emitted from a chain reaction in a nuclear reactor has to be "slowed" down by water to promote fission when it is absorbed by the fissile material. However, these slow neutrons are still at the thermal energy. It slow when compared to what it had when it was emitted, but this is still a considerable speed when compare to our typical driving speed. Without you defining what you mean by "slow", this question cannot be answered.

    2. You didn't pose any question here, simply a statement.

  4. Feb 2, 2013 #3
    1. i.e. 1 mm/sec speed, I mean the smothest speed you can imagine. Do the bounce or glue?

    2. Yes I posed a question, read the beginning, What would happen if: A rainfall of neutrons.... The question is what would happen with the neutrons, do they "live" mixed with the water, do they evaporate, do they sink, etc, ?
    Last edited: Feb 2, 2013
  5. Feb 2, 2013 #4
    Well since neutrons are radiation, I don't think it would be good if they just started raining down. Just think what would happen if a neutron were allowed to escape a nuclear reactor, same thing applies to this case. As for your other question there's a variety of things that can happen, I'm assuming you haven't studied any nuclear science or physics
  6. Feb 2, 2013 #5
    And yes the neutrons can survive in water, granted they would lose a considerable amount of energy (kinetic energy) like the previous poster said in a nuclear reactor after fission has occurred there are some neutrons (prompt) are released. Now these neutrons are moving extremely fast and in order to slow them down to thermal energy (.025ev) a moderator is used, one popular moderator is water. Since water is composed of hydrogen which is almost the same mass of a neutron it slows the neutron down enough so that it can go fission with another atom of a fissile material such as uranium 235. So the long and short of it is yes the neutrons would survive but they would lose a lot of their initial energy
  7. Feb 2, 2013 #6


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    The neutron is not stable except when it's bound in an atomic nucleus (or the core of a neutron star); it decays fairly quickly into a proton, an electron, a neutrino, and a fair amount of energy. So neither #1 nor #2 can happen.
  8. Feb 2, 2013 #7


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    1. No, they scatter.

    2. Even at thermal temperatures, ~ 300K = 0.026 eV, a neutron or proton moves at a speed of ~ 2200 m/s. Slow moving neutrons would either be absorbed by nuclei they encounter, or they would decay.

    Light nuclei can absorb only so many neutrons, 1 or 2 or 3, e.g., O16 + n = O17 + n => O18 + n = O19. O16 is natural and about 0.9976 of O, and O16, O17 and O18 are stable, while O19 decays quickly to F19. Hydrogen (p) would readily absorb a neutron become D (d). F19 + n = F20 => Ne20 + β + anti-neutrino.

    A mass of neutrons with a mass of the earth's ocean(s) is not a phenomenon that one is likely to encounter. As described in question 2, it would an impossible phenomenon.
    Last edited: Feb 2, 2013
  9. Feb 2, 2013 #8
    I doubt an umbrella would be very helpful here...
  10. Feb 2, 2013 #9
    @bubal I don't know how old you are or how much you know about physics , although judging from what you write I would say very little , but remember that the sea or water or any chemical and substance including you on this world and in the universe is made up of atoms which consist of elementary particles, neutron is just one of those elementary particles and neutron can't swim or sink or drown he is not a human being into water, try to visualize that when a neutron approaches let's say water he doesn't see water like we do from our human perspective he sees alot of elementary particles , and then according to his speed and other factors, he interacts with the elementary particles that water is made up of.

    to get some simple things clear use google search words like "elementary particle" "nucleus"
    Oh and by the way there is no such thing as a neutron sea, elementary particles usually make up atoms and elements they don't just live on their own in large clouds.
  11. Feb 2, 2013 #10
    @Crazymechanic. Perhaps you didnt understand the question. I stated "Neutrons thought experiments.
    ". A thought experiment, originally named in German Gedankenexperiment is an hypothesis. i.e. an train that runs at 99% of the speed of light. I UNDERSTAND that it is impossible such train, but it is one of the most typical Gedankenexperiments. And, no, am not very little, I am 46.

    That said. The answer that I was looking for is the one that said that "The neutron is not stable except when it's bound in an atomic nucleus", so this explains that if a small neutron star collided with a big planet, the neutrons wont be there forever, but they would turn into protons and electrons, so we would have hydrogen, i.e.
  12. Feb 2, 2013 #11


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    Even a very small neutron star has enormously greater mass than even the largest planet. The collision of the two would most likely result in the planet being reduced to a thin smear on the surface of a completely undamaged neutron star.

    And if you're considering a bunch of neutrons less massive than a neutron star... Well because the neutrons are unstable except when forced together by the enormous gravitational field of a neutron star, that bunch of neutrons will decay all by itself whether there's a planet nearby or not.

    BTW, if you were wondering why neutrons in a neutron star are stable... It's because the gravitational force inside a neutron star is strong enough to crush protons and electrons together to form neutrons. They'd like to decay, but they can't.
  13. Feb 2, 2013 #12
    Ok I understand you now @bubal
    , anyway like nugatory said a neutron star for neutrons is like the famous alcatraz for prisoners.
    The thing is nothing man made can come even close to that so I guess the neutron sea thing will always stay just a mind visualization.
  14. Feb 2, 2013 #13

    I asked a Dumb question. Of course they get absorbed because there is no coulombic repulsion counteracting the nuclear force.

    Carry on
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