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How would neutrinos affect fusion?

  1. Mar 10, 2015 #1
    How would fusion processes be affected, by a background bath / sea of neutrinos ?

    Would the constant interactions between fusion products, and neutrinos, constantly break apart the fusion products, and so tend to "undo" the fusion?
     
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  3. Mar 10, 2015 #2

    Drakkith

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    The intensity of neutrinos would have to be near supernova levels to have any noticeable effect. For comparison, the stalled shockwave of a supernova absorbs something around 1044 joules worth of neutrinos in about 10 seconds. We're talking an absolutely staggering neutrino intensity. I'd guess you need something within an order of magnitude or so to have any effect on fusion, and even then I doubt it would have much of an effect other than heating the material up. (Someone who's knowledgeable in this area please correct me if I'm wrong)
     
  4. Mar 10, 2015 #3
    The stalled shockwave in a SNII absorbs neutrinos... And fissions as a result? Or, becomes radioactively unstable and fissions soon afterwards? Neutrinos would rip out charges from nuclei, transmuting elements to other elements?
     
  5. Mar 10, 2015 #4

    mfb

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    The effect of neutrinos depends on their energy. At very low energy, you can get elastic scattering (leading to some heat, that process is always possible) and inverse beta decays of unstable nuclei (leading to transmutation and some gamma rays). Once you reach the MeV range, you can induce more inverse beta decays and make new unstable isotopes. Significantly above an MeV you get direct nuclear reactions, mainly kicking out nucleons out of nuclei. In the GeV+ range finally you can produce new particles of all sorts, and the interactions can lead to showers in the material.

    Randall Munroe wrote an article about the effect of supernova neutrinos on a human. If we could neglect all other effects of a supernova, it would be lethal at a distance of about 2 AU. Humans are very sensitive to ionizing radiation, however - electronics would easily survive this, and if you want to get a relevant power output you would need at least something like kW/kg, or 3 orders of magnitude more neutrino flux (ignoring details of the chemical composition of humans and fusion power plants).
    Which means we found an exception to the "supernovae are bigger" rule: even the neutrino flux from a supernova, outside the star, is not sufficient to give relevant power from nuclear processes in an absorber. Not that it would matter, as the energy emitted as light is completely sufficient to vaporize your setup...
     
  6. Mar 10, 2015 #5
    How would neutrinos have affected primordial nucleosynthesis ? Neutrinos would have kicked nucleons out of nuclei ? For a given primordial matter density, at the time and scale factor and temperature of BBN, less net fusion would have occurred ( without being undone by neutrinos ) ?
     
  7. Mar 10, 2015 #6

    Drakkith

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    As far as I know neutrinos had no significant effect on big bang nucleosynthesis, as the intensity and energy were too low. Keep in mind that fusion happened at temperatures comparable to stellar cores, which aren't affected by their own neutrinos either.
     
  8. Mar 10, 2015 #7

    e.bar.goum

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    Not quite true. Neutrinos (the abundance, number of pairs and degeneracy) absolutely have an effect on BBN, but perhaps not in the way the OP envisages. The number of neutrino pairs, and the neutrino density changes the n/p ratio, and so relative abundances of 4He to 2H, 3He, 7Li - weak interactions are important.

    See:
    http://www.hindawi.com/journals/ahep/2012/268321/
    and
    http://www.sciencedirect.com/science/article/pii/S0370269303008001

    For example. There are so many papers on this topic.
     
  9. Mar 10, 2015 #8
    Yes, yet that is because stellar cores are of finite limited size. So the neutrinos can escape before interacting.

    BBN occurred across the cosmos. The whole universe was one fusing furnace. Every neutrino would have eventually been likely ( likelier ) to interact.

    Yes? Fusion processes are reversible. If the neutrino comes out as a product, it could go back in as a reactant, and undo its own kind of fusion reaction. Yes?
     
  10. Mar 10, 2015 #9
    Does that imply that "spontaneous" fission events are actually stimulated by neutrinos, from the background sea of primordial CNB cosmic neutrino background? Uranium in the earth's core fissions due to interaction with the CNB ?
     
  11. Mar 10, 2015 #10

    Drakkith

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    Not in the way I envisaged it either. Thanks for the links!
     
  12. Mar 10, 2015 #11

    e.bar.goum

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    No worries. BBN is peripherally part of my research, and the physics there is really quite interesting!
     
  13. Mar 11, 2015 #12

    mfb

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    A few decays are neutrino-induced, but the rate is completely negligible compared to "normal" radioactivity.
    There is a proposed dedicated experiment to measure the cosmic neutrino background and neutrino masses, PTOLEMY. Out of 1.6*1024 tritium decays per year, about 10 to 1000 (depending on the neutrino masses) would be due to the cosmic neutrino background.
     
  14. Mar 11, 2015 #13
    Um? What is the net number of unpaired neutrinos in universe?

    Also: on long term average after oscillations, are the net lepton charges of various flavours conserved?
     
  15. Mar 11, 2015 #14

    e.bar.goum

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    As in number of neutrino families/flavours (electron, tau, muon). For some reason, "pairs" is often used in this context.
     
  16. Mar 12, 2015 #15
    How could you tell the difference, between stimulated decays, and spontaneous ones? Why couldn't there simply be many times more CNB neutrinos? Maybe the stimulated decays of various unstable isotopes could sketch out the energy profile of the neutrinos? Uranium has a half life of 4.5gyr... Does that mean the energy barrier is high, say 100eV for sake of argument, so we know the CNB has very few neutrinos of 100eV... Whereas other isotopes with lower energy barriers, say 1-10eV , decay much more rapidly, implying that the CNB has many more neutrinos at the lower energies? Perhaps the isotopes would sketch out a Maxwellian distribution for the neutrinos??
     
  17. Mar 12, 2015 #16

    mfb

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    You can measure the energy. If you absorb a thermal neutrino, the electron (or positron) will always have the same energy, while for a regular decay you get a continuous energy spectrum below this energy. If you absorb a neutrino with a higher energy, you'll get an even higher energy for the electron.
    That would require a completely different universe, otherwise there is nothing where those neutrinos could come from.

    The half-life of uranium depends on the isotope, the long-living ones all have alpha decay as main channel - no neutrinos involved. The energy barrier for beta decay is always the W mass, at 80 GeV, which requires a virtual W (apart from decays of the top quark).
    You cannot just invent unmotivated arbitrary energy barriers, one for every isotope, just to match one experimental value per isotope. That is not how science works. But the model does not work at all anyway.
     
  18. Mar 12, 2015 #17
    It depends on the density.

    Imagine a cubic centimeter of space, and neutrino flies through it. Let's say that the plasma inside is so opaque to neutrinos that chance of interaction is 0.5 (50%).

    When a volume of space expands, say, x10 in every dimension, the density falls thousand times.

    When neutrino crosses our "former centimeter^3" which is now a cubic decimeter, neutrino only crosses ten, not 1000, of its constitutient centimeters. Now _the same matter_ is only 0.005 opaque to neutrino.

    If space expands fast enough, soon after BB the integral chances of interaction for neutrinos may end up being substantially less than 1 even if they'd fly thru expanding Universe for the rest of eternity.
     
  19. Mar 12, 2015 #18
    How would a Fermi sea cool on expansion?
     
  20. Mar 13, 2015 #19
    The energy barrier for weak interaction is always that 90 GeV ...
    Yet the chances of interaction at lower energies are non zero and at higher energies are still less than one

    Radioactive decays involving the Weak Force interaction are always possible apparently... You could claim that the decays have a small unknown activation energy, similar to chemistry, and that neutrinos kick the metastable system up over the local energy barrier, so that the system can then decay exothermally with a range of energies for all of the products (?)

    http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/neutrino3.html
     
  21. Mar 13, 2015 #20

    mfb

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    That is a result of quantum mechanics, right.
    You can claim a lot of things, that does not make them correct.
    There is absolutely no indication or theory of any process like that. And keep in mind that we do not allow personal theories here.
     
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