The discussion revolves around the behavior of fast moving thermal neutrons, particularly focusing on the possibility of slowing them down to increase absorption chances and whether they can be brought to a complete halt. Participants explore theoretical implications, interactions with materials like liquid helium, and the effects of quantum mechanics on neutron behavior.
The discussion contains multiple competing views regarding the behavior of neutrons in helium and the implications of quantum mechanics on their ability to be brought to a halt. There is no consensus on the feasibility of storing neutrons in liquid helium or the significance of gravity in their behavior.
Participants express uncertainty regarding the assumptions about neutron interactions with helium, the implications of thermal energy, and the calculations related to scattering cross sections and mean free paths. These factors contribute to the complexity of the discussion without resolving the underlying questions.
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.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)
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.
Superfluid helium? Sure.Would He climb the vessel walls to the height of at least 10 cm?
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)?
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: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.
Why vibrating?QuantumPion said:It would be like trying to contain sand in a vibrating pasta strainer.
snorkack said:Gravity. The neutrons at 10 neV would rise just 10 cm above the surface of He.
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.
When you bring a neutron almost to a halt then small forces, like gravity, become significant compared to the remaining thermal energy.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.
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).
An interesting question. I would expect a length of the order of the thermal de-Broglie wavelength, a few hundred nm.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?
mfb said:An interesting question. I would expect a length of the order of the thermal de-Broglie wavelength, a few hundred nm.
Where does that number come from?snorkack said:An electron, repulsive potential about 1 eV, displaces 500 He atoms from a bubble it forms when forced in.
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.What effect does a slow neutron, repulsive potential about 10 neV, have on the surrounding He atoms if forced into the liquid?
Here:mfb said:Where does that number come from?
mfb said: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.