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Moderation of neutrons

  1. Feb 4, 2017 #1

    As we know, neutron does not have charge so it can not interact with any other particle with coulombic force. But in the reactor core fast neutrons can be slowed down. Does strong interaction govern this slowing down process?

    Thank you.
  2. jcsd
  3. Feb 4, 2017 #2


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    Staff: Mentor

    Have you found anything helpful via Google? :smile:
  4. Feb 4, 2017 #3


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    As berkeman indicated, one can use Google to search on "neutron moderation", or "neutron moderation process".

    However, the strong interaction does not play a significant role, until it is absorbed by a nucleus. Basically, moderation is achieved by classical elastic collision process. In light water reactors (LWRs), neutrons are primarily moderated by collisions with protons. Since the neutron has about the same mass as a proton, it may lose nearly all of it's energy/momentum to the proton; however, that requires a center-on-center direct hit, which is highly unlikely. Most of the time, neutrons will strike a glancing blow, and lose some of the energy.

    One should be able to locate a textbook on introductory nuclear reactor physics in which one should find a discussion of neutron moderation.
    Last edited: Oct 24, 2017
  5. Oct 15, 2017 #4
    Imagine billiard balls. Neutron is the cue ball. Protons of hydrogen atoms in the moderator are the other balls. If the cue ball impacts a larger object, like the wall, It's likely to just bounce off. If it impacts smaller objects it'll just plow through. But if it impacts similar sized objects there will be a significant energy transfer causing the cue ball to slow down.
  6. Oct 23, 2017 #5


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    Yes, elastic scattering with nuclei is governed by (residual) strong nuclear force; different kinds of scattering can be found [1]. The nuclear potential can be assumed to be a Wood-Saxon potential [2]. The cross section includes the "nuclear information", so with the cross section you can get information about the potential parameters. I have never done neutron scattering calculation, but in general, for nuclear reaction, you uses quantum scattering; maybe for neutrons in reactors semiclassical approximation can be enough (quantum scattering is required if the DeBroglie wavelength of the incident particle is comparable to the scatterer).

    Also, one can think that the neutron can only interact via strong force because neutron is neutral... but it has magnetic moment: if the speed of the neutron is low enough, they will be able to respond to magnetic fields (see "neutron spectroscopy" technique).

    [1] http://www.nuclear-power.net/nuclea...nuclear-reactions/neutron-elastic-scattering/
    [2] https://en.wikipedia.org/wiki/Woods–Saxon_potential
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