How is a Neutron able to be reflected? An Electron or Proton is repelled by the Electromagnetic force, but what is a Neutron repelled by?
Someone will probably be along with a more detailed answer soon, but for starters, neutrons are made of quarks, which can interact via nuclear forces (strong nuclear force, weak nuclear force).
Also, the quarks themselves have electric charges, it just happens that their charges cancel out in the case of a neutron. So it’s possible for neutrons to experience electromagnetic forces (e.g., dipole forces), but I think these would typically be quite weak since the quarks are all tightly bound together.
Your choices (combine as needed) –
(1) Magnets. Neutrons have a magnetic moment, so a suitable spatially varying magnetic field will trap neutrons of one spin state.
(2) Gravity. Many neutron experiments are done with very low energy neutrons, and gravity can keep them from drifting up out of the experimental vessel. (Not really the sort of “reflection” you had in mind, but it’s no different fundamentally.)
(3) Bottles. Low energy neutrons can have a de Broglie wavelength much longer than the interatomic spacing of the “bottle” material. Thus, the neutron can’t sneak through the cracks, so to speak, and will face a barrier whose reflectivity is governed by the strong force. (Typical applications achieve virtual total reflection.)
Neutrons have, as best as we can tell, zero electric dipole moment. If they had any, it would violate both time and parity invariance for the electromagnetic interaction. The current best measurement puts the dipole moment at less than 10[sup]-25[/sup] e cm, or the equivalent of an electron/positron pair separated by a ludicrously small 10[sup]-25[/sup] cm.
Short answer: The buggers bounce off other nuclei or free nucleons.
Now, a lot of this is going to depend on what you mean by reflected. I’m working off the assumption that you’ve heard the term “neutron reflector” as it pertains to the nuclear power field, so that may limit the usefulness of my answer.
Light atoms make for good neutron reflectors and moderators - they’ll absorb some energy from the collisions, and reflect the neutron’s trajectory back the way it, came. Often towards the source of the neutron emission. When dealing with a nuclear pile this is very useful, as it allows the neutron to pass through the fuel more than just once after it has been emitted.
Water, for the hydrogen atoms/protons, and graphite are two of the more common neutron reflectors in use in nuclear power.
Beryllium, in addition to being a low atomic number element that can backscatter neutrons, also has the property of emitting neutrons when bombarded by high energy particles (such as other neutrons). So it’s a particularly good choice for use in nuclear weapons designs.
OK, I don’t see the answer I’m looking for yet.
Yes, I know that a neutron can be reflected from an atomic nucleus - that’s what the U238 tamper / neutron reflector in a fission bomb is for.
What I’m looking for is WHY the neutron bounces off of the nucleus. What is going on at the sub-atomic level that causes a neutral particle to be repelled?
That’s what tim314’s answer said: the strong nuclear force. To a pretty good approximation, you can treat any two nucleons (protons or neutrons) as exchanging particles called pions when they interact, the same way two charged particles will exchange photons when they interact. The fact that these pions have mass is what leads to the interaction being “short-ranged”, as opposed to the massless photon, which gives rise to a long-range force.
If you “zoom in” more on the interaction, though, what you’d notice is that the nucleons and the exchanged pion are all made of quarks. The individual quarks kind of get swapped back & forth between the nucleons when they interact; take a look at the diagrams in this Wikipedia article for a taste of how complicated it can get.
Hmm… The Strong Nuclear force is an attractive force, so I would think that something else must be responsible…
Even an attractive force can lead to scattering states as well as bound states.
Imagine a meteoroid flying by the Earth. If it’s going slowly, it might spiral down to Earth, or even end up in orbit around the Earth. But a faster moving object would whip part way around the Earth and then go flying back off into space.
I think this is essentially what you have happening in neutron scattering (leaving out the complication that particles behave like waves in quantum mechanics). The neutron gets near the atomic nucleus, it interacts via the strong nuclear force, and then flies off at a different trajectory. Thinking of the neutron actually bouncing off the nucleus like a billiard ball may not be so accurate.
OK, I can believe that.
So it’s not a classical mechanism at all, it’s explicitly a quantum phenonemon? Something like “Once a neutron approaches a nucleus closely enough that there’s a signficant statistical overlap in their wave functions, then if the neutron is not absorbed it’s trajectory will be altered by statistical probability <boldface greek symbol>”?
I’m starting to feel like the wizards of Discworld, where “quantum” means “because it works that way”.
You can do a lot of the modeling entirely classically if you’re working with particle energies that are much less than the pion energy (about 100 MeV). The way it’s done is to assume that your “central particle” creates a potential well, with the potential energy of an incoming particle depending solely on its distance from the central particle. Once you’ve got that down, you can in principle figure out the complete trajectory of a particle coming in from with infinity with a certain energy and angular momentum. In particular, you can figure out what angle its path gets deflected by. The only difference between nucleon scattering and tim314’s gravitational example is that the form of the potential created by the central particle is different — instead of the familiar 1/r gravitational potential energy, you would use the Yukawa potential to compute the trajectories.
My point in saying it’s not like “billiard balls” wasn’t that it can only be understood with quantum mechanics. Rather, I was just saying that you don’t have to have the neutron “striking something” to be deflected, any more than the meteroid in my example has to actually hit the Earth to have its path deflected by Earth’s gravity. (That example is of course a classical one, not a quantum one.)