An electrically neutral, elementary nuclear particle having a rest mass of 1.674×10^–24 gram or an atomic mass of 1.00898. (i.e., very nearly the same as that of a proton), which exists in all nuclei except that of hydrogen.

Neutrons exhibit a broad variation of kinetic energies ranging from as little as 0.025 eV to as much as 50 MeV. Neutrons are used in logging as a means of measuring the quantity of hydrogen important in the moderation process (energy transfer) or as a means to induce radiation in stable isotopes.