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Neutron Matter

  1. Jul 13, 2008 #1
    I seek some links for pointers to be able to answer the question ..

    "Given that neutrons can approach each other easier than protons, and that the Strong Force will hold neutrons to neutrons when they get within the (short!) range where the Strong Force can be effective, why do we not have pure neutron matter in abundance - without the need for it to be part of a collapsing star, or other extreme physical conditions" ?
    Last edited: Jul 13, 2008
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  3. Jul 13, 2008 #2


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    Free neutrons have a half life of about 15 (?) minutes. There is no way to build neutron matter up except by intensive gravitational force. Also neutron-neutron collisions tend to be elastic. It is very had to get them to stick together.
  4. Nov 15, 2010 #3


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    I've been thinking about this problem too. And I understand the 15 minute thing is a limiting factor. But I still have another question, relating to the neutrons generated in the sun. btw its been 14 years since I did my degree so forgive me if my question is a bit naive.
    If a neutron is produced in the sun, what is the mean free path of it once it has been thermalised. My intuition tells me that it would be quite far and I also read that slow neutrons have been shown to follow parabolas (due to the influence of gravity), I assumed because the mean free path is so great that it has time to dip before it hits the next particle.

    So I'm curious how far a neutron would fall in a star before it decays.
    I would also be curious to know to what extent this would happen in a protostar that is engaging in deuterium burning or purely contracting gravitationally. In a diffuse protostar how far would it fall? Is this a significant process of mass transport and does it increase the ability of a star to shine by the release of gravitational potential energy.

    This also got me wondering if there is ever enough neutrons falling into a star to create a central very dense neutron gas, and whether the gravitational binding energy of such a theoretical gas would ever be large enough to equal the mass defect between a neutron and the sum of a proton and an electron, and whether consequently this negative potential would stabilize the free neutron and stop it from decaying.

    One last related question: is a neutron star a solid state of neutronium and are there any other liquid or gaseus states of neutronium. This is almost the same question as above, i.e. do neutron gasses exist and are they stable?
    Last edited: Nov 15, 2010
  5. Nov 16, 2010 #4


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    Free neutrons will decay, so neutron gasses couldn't exist. The interior of a star is quite hot, so neutrons would tend to bounce around, rather than fall, so accumulation any place wouldn't happen.
  6. Nov 16, 2010 #5


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    Isn't something else going on as well? Supposed you made your lump of neutronium somehow. Wouldn't it decay fairly quickly until a stable balance of neutrons and protons were reached? I dont' know why, but neutrons seem to be stabilized only in within certain ratio with protons. So if you got 100 neutrons to stick together, they would quickly decay until they reached the stability window.
  7. Nov 17, 2010 #6


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    The decay you described could happen, or the whole thing could come apart. There is no mechanism I am aware of (except the interior of a neutron star) that could get 100 neutrons to stick together in the first place.
  8. Nov 17, 2010 #7
    ~ 10 min half life is plenty of time to test these ideas.

    It has been shown experimentally that while a neutron and a proton interact and can form a bound state (the nucleus of deuterium), a neutron and a neutron will not form a bound state.

    So the implied assumptions in the original question are the problem here. The strong force is more complicated that presumed in that question.
  9. Nov 17, 2010 #8
    I've heard it's because identical Fermions don't bind in three dimensions, which is the same reason He-2 doesn't exist. But I don't know how to show it.
  10. Nov 17, 2010 #9


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    The answer to why neutron matter doesn't exist depends on how many neutrons you're talking about and what you mean by "exist," i.e., over what time-scale you want it to be stable.

    For very large values of the neutron number N, neutron matter is stable. That's what a neutron star is.

    For N=1, neutron matter has a lifetime of 15 min.

    For N=2, neutron matter is believed to have a lifetime of a tiny fraction of a femtosecond. The reason is that the strong nuclear force only has a range of about 1 fm, but if you confine a neutron to a 1 fm region, the Heisenberg uncertainty principle requires it to have an energy of about 10 MeV. Experiments show that the strong nuclear force isn't strong enough to bind two neutrons that have this much energy.

    The reason that the N=3, 4, ... systems aren't bound is basically the same as for N=2. This continues until you get to a neutron star.

    It is not completely certain experimentally that the dineutron and tetraneutron are not bound, but searches have given negative results.

    The reason the deuteron is bound but the dineutron apparently isn't is that the attraction between a neutron and proton coupled to spin 1 is fairly strong, but you can't put two neutrons in a spin-1 state due to the exclusion principle, and the attraction between two neutrons in a spin-0 state is somewhat weaker.
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