Is there any solids that lets neutron pass through it (without reflecting or moderating them) that is not air?

Thanks in advance :]

EDIT: clarification

  • 1
    $\begingroup$ I think you should constrain a bit more your definition of "material". If air works, probably most gases do too, as well as plain vacuum. $\endgroup$
    – Frotaur
    Jun 23, 2022 at 17:44

1 Answer 1


All solids reflect and moderate (slow down) neutrons to some extent; both reflection and moderation depend on neutron scattering off the solid's nuclei.

An effective reflector has a high neutron scattering cross section and a low neutron capture (absorption) cross section. Typical reflector materials are: graphite, beryllium, steel, and tungsten carbide.

An effective moderator has a low mass number to increase the average neutron energy loss per scattering reaction, and a high moderating ratio (ratio of scattering to absorption cross sections). Heavy water has the highest moderating ratio. Although deuterium in heavy water has a higher mass number than hydrogen in light water, the moderating ratio of heavy water is higher than the moderating ratio of light water due to the higher neutron capture (absorption) cross section of light water.

  • $\begingroup$ All atoms can scatter neutrons. Air does just fine, leading to having to consider 'sky shine' as a source of dose from neutron sources - those are just neutrons that went up, and got reflected back down (at a lower energy) to you. Quite necessary to include for, e.g., 14MeV neutron experiments. $\endgroup$
    – Jon Custer
    Jun 23, 2022 at 19:09
  • $\begingroup$ Good point! Also, other reactions can be important. For example, for 14 MeV neutrons interacting with lead, the dominant reaction at 14 MeV is (n, 2n). A nice feature for the lead slowing down time spectrometer since it effectively doubles the neutron source. $\endgroup$
    – John Darby
    Jun 23, 2022 at 19:24

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