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Hi, I'm new and have a couple questions.

  1. Sep 10, 2007 #1
    Don't think me silly, but the first question is, where do excess neutrons go? If I do a fission reaction and get protons they are just ionized hydrogen atoms, a free electron is just a negative charge to find a free proton somewhere. But since a neutron can't get back in a nucleus, where does it go? The only place I can think of would be the nearest gravity well. Is their a tiny ball of neutrons at the earth's core?

    The second is does antimatter have mass? If it does and then if you annihilate it do you get energy equal to the total mass or just the matter's mass?

    Thanks, Fred
  2. jcsd
  3. Sep 10, 2007 #2
    For the neutrons: I cannot tell you details about fission, but the ball of neutrons at earth's core has a simple upper bound: Free neutrons (unlike neutrons bound in a nucleus) are unstable and decay with a lifetime of something like 10 minutes.

    For the antimatter: Yes, it has mass. You get energy equal to the mass of the matter plus the mass of the anti-matter plus the kinetic energy they initially had.
  4. Sep 10, 2007 #3


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    Hi and welcome! Hope you will enjoy your stay here.

    Some of the neutrons may induce nuclear reactions with either the moderator or the reactor tanks wall. But most of them will be stucked in the control rods between the fule rods. (http://en.wikipedia.org/wiki/Control_rod)
    And as Timo pointed out, if the neutron is not bounded to a nucleus, it will decay with lifetime approx 10min, to a proton and an electron.

    The anitmatter have oppostite energy compared to their particle counterpart, but has indeed mass.
    Last edited: Sep 10, 2007
  5. Sep 10, 2007 #4
    Thanks for answering, but if free neutrons have a 10 min. halflife, how do they form neutron starts? Is not 20min enough time for at least some of them to fall to earth's center? Seems they would fall pretty quickly. If gravity stabilizes neutrons, as in a neutron star, is there a threshold size?
  6. Sep 10, 2007 #5


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    See my updated answer.

    Iam not an expert on neutron stars altough.

    Also lifetime is defined as this:

    So it is not so that they live 10mins then decay, lifetime is a measure for how long the amount 1/e of a sample has decayed (compare with half-life time)
  7. Sep 10, 2007 #6
    LOL, just a bunch of math jibberish. Why do people do that when they can't answer the question in laymens terms? I know what half life is, its a bell curve of decay and 10 min would mean half went poof. But a few might survive hours or days, and we are talking about a lot of tiny bits here with lots of empty space for them to fall through between atoms in the earth. What about large stars?
  8. Sep 10, 2007 #7


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    hehe yeah but the language of physics is math :)

    Now Iam not an expert about nuclear reactions in stars but I do know quite a bit on how reactors work.

    Now I must ask what you are meaning by "in large stars". In stars there is fusion, not fission. So you basically start with protons that fuses togheter and build up heavier elements. But in some steps there are neutrons produced, and there is also neutron capture by a bit heavier elements (there are the so called s- and r-processes).
    I belive that the star is so dense that if the neutron does not is doing n-capture, it will decay before a large amout can gather in the center of the star and disturbing its ordinary "life". But I am not an expert on Nuceal Astrophyics (at least not now hehe)
  9. Sep 10, 2007 #8
    The star reference is not really about fusion, I understand fission v fusion and iron and all, I was referring to the gravity it takes to keep neutrons from decaying, there is surely a threshold gravity that may or may not exist, and correct me if I'm wrong, but larger starts are, as a rule, less dense at the core. I just was offering where the neutrons may come from on earth, decay in the earth itself and us setting off bomb and running reactors. I have also heard that 4 neutrons may form a stable something or other and defy decay. So, do they go to the core?

    As to the second question, a lot of magazines and books over the years have read, at least to me, that antimatter and matter reactions would produce energy at E=mc2, but they never tell you if antimatter has mass. Last time I remember reading a book on the subject that mentioned it, the book implied that no one knew and that antimatter may actually have anti-mass in that it would fall away from a gravity well produced by matter and that's why we don't see big explosions in space as an antimatter and matter galaxies collide. I find it confusing and intriguing.
  10. Sep 10, 2007 #9


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    Well the only massless particles are the photons. I know wiki is not the best source, but I dont have time right now to write of my books in elementary particle physics


    "Thinking further, Dirac found that a "hole" in the sea would have a positive charge. At first he thought that this was the proton, but Hermann Weyl pointed out that the hole should have the same mass as the electron. The existence of this particle, the positron, was confirmed experimentally in 1932 by Carl D. Anderson. During this period, antimatter was sometimes also known as "contraterrene matter"."............

    And if antimatter would not have mass, then positrons would not bend the same way as electrons do in a magnetic field (as we know they do)

    The heavier star the more dense core, but the more massive star, the shorter lifetime.

    I would like to know the name of that book of yours that you read..

    And I have no idea if gravity is related to neutron-lifetime, I have never herd of it, do you have a reference?

    neutrons on earth comes from fission reactors, atomic bombs, natural radioactive decays and a very little bit is produced in the atmosphere from cosmic rays (Not so sure about the last one though, its very late here in sweden now hehe..)

    I belive that the amount of neutrons that reaches the core of the eath is a tiny tiny bit of the bit that reacts with matter on the way.. so maybe it exists a ball of neutron, but the matter getting denser and higher Atomic number, and higher prob that induce n-capture. What is the significance of your question? just curiousity or? =)
  11. Sep 10, 2007 #10


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    Neutrons do in fact readily enter other nuclei increasing the mass number by 1 without changing the charge, Z. Most often, an absorbed neutron will produce a radionuclide which will subsequently decay, mostly likely by beta-decay, or alpha-decay if the original nucleus is heavy enough. For certain, nuclei, e.g. U-233, U-235 and Pu-239, 241, fission would likely occur.

    Free protons eventually find an electron. As far as we know, there is net charge neutrality locally around the earth, so + ions or free nuclei will eventually find electrons and free electrons will evenually recombine with positive ions.

    As far was we know, there is no ball of neutrons at the center of the earth, which is thought to be liquid Fe. Neutrons would be absorbed by nearby nuclei, and would be absorbed very near their origin, if they did not decay.

    Neutron stars are entirely different forms of matter than we find on earth, or even in the sun. The matter is so dense that it's thought to be one big mass of nucleons rather than atoms. Nucleons interact with one another. If a neutron decays, it would do so into a proton, electron and antineutrino, but with mass so dense, a proton will interact with an electron again forming a neutron and neutrino. Gravity is only one force within a star, and it is significant because of the huge mass (and density) involved.

    The concept of antimatter has been adequately explained. Positrons will annihilate with electrons to produce two gamma rays. Anti-baryons will annhilate with corresponding baryons in pi-meson shows, and the mass-energy is converted to mass and kinetic energy of smaller particles.
  12. Oct 4, 2007 #11
    Neutrons could never reach the center of the earth even if they lived for a very long time. They scatter with the nuclei they meet. So they bounce in all directions, never getting far in 1 particular direction. The collisions with nuclei cause them to loose energy (=speed) untill they are almost at speed = 0 (although they never completely stand still).

    In fact nobody knows what neutron stars are really made of. Maybe in a neutron star there are no neutrons at all...
    Last edited: Oct 4, 2007
  13. Oct 4, 2007 #12


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    What is your soruce for this?
  14. Oct 4, 2007 #13
    Wait a minute :confused: Was this a mistake, or have I understood the antiparticles wrong? Don't the antiparticles have precisely the same energies as the particles themselves?
  15. Oct 4, 2007 #14


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    well, it depends on how you see it. The electron has negative charge, and positron has positive charge; so they get opposite energies in E-fields and so on. I should perhaps have been more careful, sorry
  16. Oct 4, 2007 #15
    Okey the electric potential goes like that. But the kinetic energies are positive for both.
  17. Oct 4, 2007 #16


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    Kinetic energy is ALWAYS positive, and so is rest mass(energy) too
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