How do neutrons gain energy from phonon scattering?

In summary: This is because the neutron will transfer energy to the crystal lattice, creating phonons, and losing some of its own energy in the process. This process is known as inelastic scattering and it is the main mechanism for neutron-phonon interactions in a crystal.
  • #1
FillBill
7
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So during neutron scattering in a crystal, a neutron can interact with 0, 1, or more phonons. First of all, what is the actual mechanism by which they interact? My textbook just kind of glosses over that.

Second of all, when a neutron goes in it can absorb a phonon and come out with more energy than it came in with. This must mean the energy comes from the crystal, right? But what energy, just the thermal energy of the atoms?

So, if you scattered neutrons off a solid at 0K, would it only give energy to the crystal, not take it at all?
 
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  • #2
FillBill said:
So during neutron scattering in a crystal, a neutron can interact with 0, 1, or more phonons. First of all, what is the actual mechanism by which they interact? My textbook just kind of glosses over that.
Simple nucleon-nucleus scattering - same as when neutron scatters in a gas. Mainly strong interaction, with some contributions from electromagnetic and weak.
FillBill said:
Second of all, when a neutron goes in it can absorb a phonon and come out with more energy than it came in with. This must mean the energy comes from the crystal, right? But what energy, just the thermal energy of the atoms?
Yes. Thermal energy in a crystal consists of phonons.

Basically, whenever a neutron scatters off a loose nucleus in a gas, it transfers some momentum to the nucleus. But the effect on neutron energy depends on the velocity of the gas. In a gas where temperature is 0 and all nuclei are stationary in one frame, in that frame the neutrons can only lose energy, not gain.

In a crystal, the nuclei are unable to have a free choice of momentum and energy. The neutron may scatter off a nucleus so that the whole crystal takes the rebound momentum. Or else the neutron creates phonons - quantized movements inside the crystal. Or else phonons give energy to the neutrons.
FillBill said:
So, if you scattered neutrons off a solid at 0K, would it only give energy to the crystal, not take it at all?

Yes. There is no energy to take at all.
 
  • #3
Phonons are the vibrational modes of the crystal lattice. The mechanism for inelastic scattering of neutrons may be very simply described as this: When the neutron hits the nucleus of an atom in the crystal, energy may be transferred from the neutron to the nucleus and it will start oscillating. Due to EM interactions this motion will make the nearby atoms oscillate as well, the motion will spread out over the lattice and the effect is that a phonon is excited. Depending on the properties (interatomic distances) of the lattice only phonons of certain frequencies can be created.

If the nuclei are already oscillating (i.e. there is a phonon) the neutron may steal the vibrational energy upon collision.

FillBill said:
So, if you scattered neutrons off a solid at 0K, would it only give energy to the crystal, not take it at all?

Yes! At temperatures below ~10 K there are no thermally excited phonons present, therefore you will only see scattered neutrons with less energy than the incoming neutrons.
 

1. How do neutrons interact with phonons to gain energy?

Neutrons can interact with phonons, which are quantized lattice vibrations in a solid material. This interaction occurs through the process of phonon scattering, where a neutron transfers energy to or from a phonon, resulting in a change in its energy level.

2. What is the mechanism behind neutron-phonon interactions?

The mechanism behind neutron-phonon interactions is based on the laws of conservation of energy and momentum. When a neutron collides with a phonon, both energy and momentum are exchanged, resulting in an increase or decrease in the neutron's energy level.

3. How does the energy of the phonon affect the energy gain of the neutron?

The energy of the phonon affects the energy gain of the neutron through the process of phonon scattering. The higher the energy of the phonon, the greater the energy transfer to the neutron, resulting in a larger increase in its energy level.

4. What types of materials are most effective for phonon scattering to increase the energy of neutrons?

Materials with a high phonon density, such as metals and insulators, are most effective for phonon scattering to increase the energy of neutrons. This is because they have a larger number of phonons available for interaction with the neutrons.

5. Can neutrons gain energy through other processes besides phonon scattering?

Yes, neutrons can also gain energy through other processes, such as nuclear reactions or interactions with other particles. However, phonon scattering is a common and important mechanism for energy transfer in certain materials and is often used in neutron experiments for this purpose.

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