Neutrons together with protons are the constituents of atomic nuclei. The neutron was discovered after more than two decades of speculation that electrically neutral particles exist in atoms (see Chapter 3). Because the neutron is electrically neutral, it easily interacts with nuclei and does not interact directly with electrons. Since the nucleus of an atom is about one ten-thousandth the size of the electron cloud, the chance of neutrons interacting with a nucleus is very small, allowing them to travel long distances through matter. As a free particle, the neutron is an important and yet unique tool used for various applications: in medicine to initiate powerful nuclear interactions whose products can directly destroy cancer cells (neutron capture therapy for example), for research on physical and biological materials, for imaging through easy allocation of light atoms especially hydrogen, to investigate properties of magnetic materials (neutrons possess a magnetic moment and thus act as small magnets), to track atomic movement (thermal neutron energies almost directly coincide with the energies of atoms in motion) and to maintain the fission chain reaction in nuclear reactors. Free neutrons are unstable (see Chapter 3) and decay in short time through β– decay process into a proton, electron and neutrino. However, free neutrons will most likely interact with the surrounding matter and disappear through nuclear interactions long before they decay.