本文发表在 rolia.net 枫下论坛The neutrons emitted in fission reactions have high energies, typically in the range of 1 MeV. But the cross section for neutron capture leading to fission is greatest for neutrons of energy around 1 eV, a million times less. It is necessary to slow down the neutrons for efficient operation of a nuclear reactor, a process called moderation. While neutrons are efficiently slowed by inelastic scattering from U-238 , the non-fissionable isotope of uranium, when their energies are higher than 1 MeV, the remainder of the process of slowing them down must be done by elastic scattering from other nuclei. When a neutron collides elastically with another nucleus at rest in the medium, it transfers some of its energy to it. The maximum transfer of energy occurs when the target nucleus is comparable in mass to the projectile. Water and carbon (graphite) are commonly used moderators. Water is a good moderator, but the hydrogens in the water molecule have a fairly high cross section for neutron capture, removing neutrons from the fission process. Heavy water, used as moderator in Canadian reactors, avoids this loss.更多精彩文章及讨论，请光临枫下论坛 rolia.net