Moderation of Fast moving thermal neutron

  1. If a fast moving thermal neutron can be slowed down to increase absorption chances, is there a way it can be brought a halt?

    If yes, what will be the behavior of such neutron?(Apart from a simple fact that reaction comes to a halt)
     
  2. jcsd
  3. mfb

    Staff: Mentor

    Quantum mechanics does not allow an absolute "halt" - in theory, you can bring it to its ground state in some volume, I don't know if that has been done yet (unless you count as nucleus as volume where a neutron is in its ground state).
     
  4. mathman

    mathman 6,435
    Science Advisor
    Gold Member

    I assume you are talking about neutrons in nuclear reactors. Thermal neutrons are in thermal equilibrium with their surrounding, which means that the scattering of a neutron leaves it on the average at the same energy that it had before.
     
  5. Astronuc

    Staff: Mentor

    One can produce 'ultra-cold' neutrons. However, as mathman indicated, thermal neutrons are in thermal equilibrium with the atoms around them, and in a nuclear reactor, the thermal neutrons have an energy distribution, more or less like gas atoms have an energy distribution. When we talk of air temperature, we really refer to an average property of a gas.

    http://www.grc.nasa.gov/WWW/BGH/temptr.html
     
  6. Say you put a neutron into liquid helium 4.

    It cannot be absorbed because helium 5 is not a bound state. If it is helium 4 cleaned of any dissolved helium 3 (which could absorb neutrons) then the neutron should be stable until it undergoes free neutron beta decay (timescale 10 minutes).

    What does a free neutron do with liquid helium 4? Is it repelled by interaction with alpha particles and electrons, or does it permeate the liquid? Will it stay afloat on the surface between liquid helium and vacuum, or will it sink into the liquid under its own weight? Assume that absolutely any kinetic energy that the neutron may have at first is carried away by low energy phonons into the liquid... how high will a neutron in its ground state rise above an impermeable surface?
     
  7. Since neutrons are uncharged and only interact by nuclear forces, they cannot form a liquid. They will bounce around the helium until either they decay or scatter out of the container.
     
  8. How would they scatter out of the container?

    On examination, it turns out that helium does possess a positive optical potential for neutrons. About 10 neV.

    Meaning that neutrons below 10 neV (about 1,4 m/s) would bounce back from helium surface... and, in 1 g, fall back from height of about 10 cm.

    He wets all substances except Cs (which is anyway bad because it is a good neutron absorber).

    Would He climb the vessel walls to the height of at least 10 cm? And would the film reflect the incident neutrons before they come into contact with vessel walls (which do have nonzero absorption cross-section)?
     
  9. mfb

    Staff: Mentor

    Superfluid helium? Sure.

    A container of superfluid helium for ultracold neutrons... sounds interesting.
     
  10. What would a neutron do if it is forced into He and moderated to ultracold while under the surface?

    An electron repels the indistinguishable electrons of He. If it is overcome, by more than 1 eV of energy, then the Fermi repulsion creates a bubble, of about 1,9 nm size in shallow He, displacing about 500 He atoms.

    A neutron is mainly repelled by the alpha particles. And its repulsive optical potential off the flat surface is just 10 neV - 8 orders of magnitude smaller than what the electron has.

    If a neutron is forced into liquid helium 4 and slowed down inside, what effect does its scattering/repulsion have on the nearby liquid?
     
  11. Helium-4 has an appreciable 1/v scattering cross section for thermal and cold neutrons. You could not store neutrons indefinitely in such a container. There is nothing binding the neutrons to the helium. It would be like trying to contain sand in a vibrating pasta strainer.
     
  12. Gravity. The neutrons at 10 neV would rise just 10 cm above the surface of He. And as established before, the He film climbing the walls of the container would prevent the neutrons from even coming into contact with the material of the walls.
    Why vibrating?
     
  13. Gravity does not bind neutrons to helium, it binds them to the Earth. Which means if the effect of gravity was significant, the neutrons would fall through the helium like sand through a strainer. In reality the neutrons will diffuse out of the helium in all directions as the force of gravity is negligible compared to their thermal energy.

    This is incorrect. You can calculate the macroscopic cross section for helium, it is pretty low. The very thin layer of helium on the edge of the tank would stop only a tiny fraction of neutrons passing through. My rough calculation for the MFP comes out to be on the order of 20 cm (at 1e-5 eV, the lowest data available, σ=2 b, ρ=0.14 g/cm3).
     
  14. mfb

    Staff: Mentor

    If that effective potential of 10neV exists (and it certainly exists for several solid materials - that's how transport lines of neutrons work), I don't see how a very slow neutron could enter the helium, if thermal energy is not sufficient to give 10neV. A neutron in the liquid would get scattered around, of course, until it decays or reaches the border of the helium.
     
  15. When you bring a neutron almost to a halt then small forces, like gravity, become significant compared to the remaining thermal energy.
    And what is the mean free path of visible photons in helium with respect to Rayleigh scattering?

    Yet photons can be and are refracted on the surface of helium, and that includes complete reflection. Though photons in dielectric are reflected internally.

    Photons that reflect do penetrate the interface as evanescent waves, and can tunnel through thin layers of unsuitable media. How deep will neutrons penetrate into helium they are unable to enter?
     
  16. mfb

    Staff: Mentor

    An interesting question. I would expect a length of the order of the thermal de-Broglie wavelength, a few hundred nm.
     
  17. The wavelength grows without limit with the speed of the neutron. Again comparing with photons, long radio wave are stopped in short distance relative to their wavelength; electrostatic and magnetostatic fields by definition have infinite de Broglie wavelength, and are stopped by thin layers of conductor and superconductor respectively.

    So I would expect a length which is determined by the properties of the interaction of the substance with neutrons.
     
  18. mfb

    Staff: Mentor

    It should be similar to the solutions for a potential well - the energy barrier is 10neV, so neutrons are somewhere between 0 and 10 neV below the potential of the wall. Some of them (just below the energy threshold) will extend deeply into the wall, but that is rare.

    For photons, you have to consider the electromagnetic properties in the material. Ideal superconductors have "zero wavelength" if I remember correctly, real superconductors will still lead to a very short length scale.
     
  19. So, if the neutron does get forced into helium...

    An electron, repulsive potential about 1 eV, displaces 500 He atoms from a bubble it forms when forced in.

    What effect does a slow neutron, repulsive potential about 10 neV, have on the surrounding He atoms if forced into the liquid?
     
  20. mfb

    Staff: Mentor

    Where does that number come from?

    The whole point of the effective potential are coherent effects in the medium - if the neutron would be localized somewhere, it would not feel an effective potential.
     
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