Moderation of Fast moving thermal neutron

In summary, thermal neutrons in a nuclear reactor can be slowed down to increase absorption chances. It is possible to produce ultra-cold neutrons, but they cannot form a liquid and will bounce around until they either decay or scatter out of the container. Helium-4 has a positive optical potential for neutrons, but it cannot contain them indefinitely due to their 1/v scattering cross section. Gravity does not bind neutrons to helium and they will diffuse out of the container in all directions.
  • #1
EinsteinII
35
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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)
 
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  • #2
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).
 
  • #3
EinsteinII said:
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)

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.
 
  • #4
EinsteinII said:
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)
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
 
  • #5
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?
 
  • #6
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.
 
  • #7
QuantumPion said:
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.

How would they scatter out of the container?

On examination, it turns out that helium does possesses 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)?
 
  • #8
Would He climb the vessel walls to the height of at least 10 cm?
Superfluid helium? Sure.

A container of superfluid helium for ultracold neutrons... sounds interesting.
 
  • #9
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?
 
  • #10
snorkack said:
How would they scatter out of the container?

On examination, it turns out that helium does possesses 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)?

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.
 
  • #11
QuantumPion said:
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.
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.
QuantumPion said:
It would be like trying to contain sand in a vibrating pasta strainer.
Why vibrating?
 
  • #12
snorkack said:
Gravity. The neutrons at 10 neV would rise just 10 cm above the surface of He.

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.

snorkack said:
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.

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).
 
  • #13
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.
 
  • #14
QuantumPion said:
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.
When you bring a neutron almost to a halt then small forces, like gravity, become significant compared to the remaining thermal energy.
QuantumPion said:
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).

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?
 
  • #15
snorkack said:
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?
An interesting question. I would expect a length of the order of the thermal de-Broglie wavelength, a few hundred nm.
 
  • #16
mfb said:
An interesting question. I would expect a length of the order of the thermal de-Broglie wavelength, a few hundred nm.

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.
 
  • #17
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.
 
  • #18
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?
 
  • #19
snorkack said:
An electron, repulsive potential about 1 eV, displaces 500 He atoms from a bubble it forms when forced in.
Where does that number come from?

What effect does a slow neutron, repulsive potential about 10 neV, have on the surrounding He atoms if forced into the liquid?
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.
 
  • #20
  • #21
A very interesting concept for a neutrino detector, thanks for the link.

Is there a significant repulsion at all? The nucleus has no bound states for the neutron, that is not the same as a significant repulsion.
 

1. What is fast moving thermal neutron moderation?

Fast moving thermal neutron moderation is the process of reducing the energy of fast moving neutrons to the thermal energy level (around 0.025 electron volts) by collisions with a moderator material. This allows the neutrons to be more easily absorbed by a target material in a nuclear reaction.

2. Why is moderation of fast moving thermal neutrons important?

Moderation of fast moving thermal neutrons is important in nuclear reactors because it allows for a sustained nuclear chain reaction to occur. Without moderation, the fast moving neutrons would pass through the target material without being absorbed, making the reactor inefficient.

3. What materials are commonly used as moderators?

Some common materials used as moderators in nuclear reactors include water, graphite, and heavy water (deuterium oxide). These materials are effective at slowing down fast moving neutrons due to their relatively low atomic masses and high hydrogen content.

4. How does the choice of moderator affect the efficiency of a nuclear reactor?

The choice of moderator can greatly affect the efficiency of a nuclear reactor. Materials with high hydrogen content, such as heavy water, are more efficient at slowing down neutrons and therefore allow for a higher rate of fission reactions. Additionally, the type of moderator can also affect the type of nuclear reactions that occur in the reactor.

5. Are there any safety concerns related to the moderation of fast moving thermal neutrons?

Yes, there are safety concerns related to the moderation of fast moving thermal neutrons. If the moderator is not properly controlled or if there is a sudden increase in the rate of nuclear reactions, it can lead to a runaway chain reaction and potentially a nuclear meltdown. Therefore, proper monitoring and control of the moderator is crucial in nuclear reactors.

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