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Neutrons are relatively massive particles that are one of the primary constituents of the nucleus. However, neutrons can be produced in a number of ways and can represent a significant source of indirectly ionizing radiation. Generally, neutrons are segregated into several categories on the basis of their energy. Thermal neutrons are those that are in thermal equilibrium with matter and, in special cases, have a Maxwellian distribution of velocities. In this distribution, the most probable velocity at 295 K is 2200 m/sec, corresponding to an energy of 0.025 eV.
Neutrons in the energy range 0.5–10 keV are called intermediate neutrons. These neutrons may also be called resonance or epithermal neutrons. Fast neutrons are those in the energy range 10 keV to 10 MeV. In this energy range, neutrons interact with matter through elastic collisions (i.e., billiard-ball–type collisions). Neutrons with energies >10 MeV are called relativistic neutrons.
Neutrons are uncharged particles, and therefore they do not participate in the electromagnetic interaction and do not produce ionization of the atoms. The interaction of a neutron magnetic moment with matter is very weak and unlikely.
All the main processes of interaction are caused by nuclear forces, as a result of various manifestations of which energetic charged particles appear in the substance. These are charged particles produced by neutrons that transmit their energy to matter, mainly due to ionization.
Unlike charged particles, which practically continuously lose energy in small portions, neutrons experience rare collisions with atoms, in which they can lose either all or a large part of their energy, which is caused by the short-range nature of nuclear forces.
The physical nature of the interaction of neutrons with atoms is fundamentally different from that of gamma quanta, but, formally, they are identical. Both gamma quanta and neutrons are penetrating radiations, whose fluxes are attenuated exponentially. For both types of radiations, it is possible to use the similar parameters—absorption and scattering coefficients.
Let us note that a free neutron is an unstable particle, it experiences a beta decay with a half-life of 614 s. But all the processes of neutrons passing through matter usually end up with the capture of a neutron by some nucleus in the time much shorter than a second. Therefore, analyzing all processes of neutron interaction with matter, the neutron instability can be ignored.
Because neutrons do not have an electric charge, they freely penetrate through the electron shells of atoms and are not repelled by the Coulomb field of the nucleus. Therefore, neutrons are an excellent tool with which you can study the nucleus, solids, biological structures, and create new elements that are absent in the surrounding world and are useful for medicine, industry, agriculture, and science.
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