What Is a Neutron?

A neutron is a neutral subatomic particle found in the nucleus of every atom except the most common isotope of hydrogen. It carries zero net electric charge, has a rest mass of approximately 1.675 x 10^-27 kg (slightly greater than the proton mass), and belongs to the class of particles called baryons, composed of three quarks bound by the strong nuclear force. Neutrons were identified in 1932 by James Chadwick, who observed that bombarding beryllium with alpha particles produced a penetrating radiation that could eject protons from hydrogenous materials but could not be deflected by magnetic fields, demonstrating the absence of charge. The discovery resolved a longstanding inconsistency in nuclear mass calculations and earned Chadwick the Nobel Prize in Physics in 1935, as described in his Nobel lecture on the neutron and its properties.

Within the nucleus, neutrons act as a nuclear "glue," moderating the electrostatic repulsion between protons through the short-range strong force. Free neutrons outside the nucleus are unstable, decaying into a proton, an electron, and an electron antineutrino with a half-life of approximately 610 seconds.

Properties and Classification

Neutrons are characterized by their kinetic energy, and this energy classification governs which nuclear reactions and detection methods are applicable. Thermal neutrons, in equilibrium with a room-temperature moderating material, have kinetic energies near 0.025 eV. Epithermal neutrons span energies from roughly 1 eV to a few keV, and fast neutrons range from keV into the MeV regime. The distinction is consequential for nuclear engineering: most fission reactor designs rely on thermal neutrons because fissile materials such as uranium-235 and plutonium-239 have dramatically higher fission cross-sections at thermal energies. Light-water reactors use the water itself as a moderator to thermalize the fast neutrons produced by fission.

Neutrons also exhibit wave properties at low energies, enabling neutron diffraction and reflectometry techniques that probe the atomic and magnetic structure of condensed matter at angstrom length scales.

Interactions with Matter

Because neutrons carry no charge, they do not interact with the electron clouds of atoms and are not affected by the Coulomb force. Their principal interactions are with atomic nuclei, and these take two broad forms: scattering and absorption. In elastic scattering, a neutron transfers kinetic energy to a recoiling nucleus, which is the physical basis for neutron moderation. The most efficient moderation occurs with light nuclei, since a neutron transfers the greatest fraction of its kinetic energy in a head-on collision with a nucleus of equal mass, which is why hydrogen (proton) is the most effective moderator per unit mass.

Absorption reactions include radiative capture (in which the nucleus absorbs the neutron and emits one or more gamma rays), charged-particle reactions such as the boron-10 (n, alpha) reaction exploited in detectors, and fission, in which heavy nuclei split into two fission fragments while releasing additional neutrons. These interactions are detailed in the American Physical Society's historical account of neutron research and its applications, which connects Chadwick's original experiments to subsequent nuclear technology development.

Neutron activation, in which stable isotopes absorb neutrons and become radioactive, is the basis for neutron activation analysis, a technique for quantitative elemental characterization of materials without sample destruction. The same phenomenon is a concern in the design of shielding and structural materials for nuclear reactors, where long-term neutron exposure can embrittle steel pressure vessels.

Single event upsets in microelectronics, in which a neutron strike induces a transient change in the logic state of a semiconductor memory cell, represent an important reliability concern in avionics, space, and ground-level computing, as studied by researchers at Brookhaven National Laboratory's Nuclear Chemistry program.

Applications

Neutrons have applications in a wide range of fields, including:

• Nuclear fission reactors for power generation and research
• Neutron scattering and diffraction for materials structure analysis
• Neutron activation analysis for elemental composition measurement
• Radiation therapy, including fast neutron therapy for certain tumors
• Radiation effects testing for aerospace and nuclear system electronics
• Non-destructive testing and neutron imaging of industrial components