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With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a nucleus is the atomic number and defines the type of element the atom forms. — “Neutron - Wikipedia, the free encyclopedia”, en.wikipedia.org
A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from both stable molecules and isotopes, and these free neutrons react with nuclei of other stable molecules to form new isotopes of previously non-isotopic molecules, which in turn produce radiation. — “Neutron radiation - Wikipedia, the free encyclopedia”, en.wikipedia.org
The reflection is caused by the coherent strong interaction of the neutron with atomic nuclei. Neutrons are reflected from a surface if the velocity component normal to the reflecting surface is less or equal the critical velocity. — “Ultracold neutrons - Wikipedia, the free encyclopedia”, en.wikipedia.org