Prout's hypothesis being correct. Nevertheless, the rule was not found to predict isotope masses better than this for all isotopes, due mostly to mass-defects resulting from release of binding energy in atomic nuclei, when they are formed. Although all elements are the product of nuclear fusion of hydrogen into higher elements, it is now understood that atoms consist of both protons (hydrogen nuclei) and neutrons. The modern version of Prout's rule is that the atomic mass of an isotope of proton number (atomic number) "Z" and neutron number "N" is equal to sum of the masses of its constituent protons and neutrons,
Ultracold neutrons Ultracold neutrons Ultracold neutrons (UCN) are free neutrons which can be stored in traps made from certain materials. The storage is based on the reflection of UCN by such materials under any angle of incidence. The reflection is caused by the coherent strong interaction of the neutron with atomic nuclei. It can be quantum-mechanically described by an effective potential which is commonly referred to as the "Fermi pseudo potential" or the "neutron optical potential". The corresponding velocity is called the "critical velocity" of a material. Neutrons are reflected from a surface if the velocity component normal to the reflecting surface