Why moderator must contain low mass nuclei in neutron diffrction?
(usually H or D )
To maximize the kinetic energy transfer between the neutron and the moderator, the mass of colliding atom should be equal or similar to the neutron mass (momentum conservation). Also the neutron capture cross-section should be as low as possible.
To maximize the kinetic energy transfer between the neutron and the moderator, the mass of colliding atom should be equal or similar to the neutron mass (momentum conservation). Also the neutron capture cross-section should be as low as possible.
As Ján already stated, it´s a problem of momentum and energy transfer in collisons. Additional to p and d in form of light or heavy water H2O or D2O carbon (graphite) is used as moderator. In light water neutrons need 18 collisions, in D2O 25 and in carbon 114 collision to get thermal. Water in both forms is practical because it simultaneously works as cooling medium. And if this medium gets to hot and starts to vaporize, the moderation is diminuished (security).
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