The answer is: nucleus of an atom.
Neutron (n°) is a subatomic particle with no net electric charge.
Protons and neutrons are found in nucleus of an atom (atomic nuclei).
They are called nucleons and have masses of approximately one atomic mass unit.
Mass number (A) is sum of protons and neutrons in nucleus of an atom.
Protons and neutrons are hadrons composed of three valence quarks.