Neutron source

The United States Nuclear Regulatory Commission (NRC) defines a neutron source as any substance that releases enough neutrons to generate radiation of sufficient intensity that it can be measured when the neutrons are placed inside a nuclear reactor. Combining the elements of beryllium and radium will produce such a reaction, leading the NRC to cite beryllium-radium mixtures as an example of a neutron source. Other agencies, such as the National Institute of Standards and Technology (NIST), define neutron sources as systems or facilities that harvest neutrons from other substances for use in nuclear reactors and scientific experiments.

Many different neutron sources are used in research and industry. Neutrons play an important role in multiple fields, particularly in medicine and materials science. Many industrial applications include nuclear power generation, such as mineral resource exploration, oil well logging, moisture content measurements, nuclear materials analysis, neutron radiography, and the detection of contraband and explosive devices.

Background

Neutrons are subatomic particles contained within an atom’s nucleus. They carry a neutral electrical charge (neither positive nor negative) and are found in the nuclei of every element except hydrogen. Neutrons are one of two subatomic particles found in atomic nuclei, with positively charged protons being the other. Compared to protons, neutrons have a slightly higher mass.

Some processes, including nuclear fission and nuclear fusion, are capable of releasing neutrons from atomic nuclei. Nuclear fission occurs when a neutron collides with another atom, generating force that causes the atom to split into two smaller parts known as fission products. This atomic splitting releases large quantities of energy, which can be harvested and used to turn turbines and generate electricity.

Nuclear fusion is the opposite of fission. Rather than creating multiple smaller atoms from a single larger atom, fusion seeks to combine multiple smaller atoms into a single larger atom. This process, known as fusion, releases large quantities of energy that can be captured and used to generate electricity and for other applications. It also releases excess neutrons and is therefore described as a neutron source.

Neutrons have many different research and industrial applications. With respect to research, neutrons are particularly useful in medicine and materials science and are often deployed via a technique known as neutron scattering. Neutron scattering involves bombarding sample substances with neutrons, observing the angles at which the neutrons deflect off the substance, and drawing conclusions about the shape, size, and physical characteristics of the substance. Industrial applications of neutron techniques include quantifying the hydrocarbon content and porosity of oil deposits, detecting raw minerals and metallic ores, gauging the moisture content of materials as part of the civil engineering process, and performing quality control operations on materials that absorb neutrons during the fission process. Neutrons are also used to analyze the specific contents and characteristics of nuclear materials, in neutron radiography, and in situations where officials or agents seek to detect the presence of contraband or explosives without destroying the target substance, such as at airport security checkpoints.

Overview

Major neutron sources include alpha, gamma, and spontaneous fission sources along with fission reactors and particle accelerators. In industry, neutron sources such as radionuclide sealed sources and reactor beams are also common.

Alpha neutron sources, also known as alpha neutron materials, are a class of radioactive isotopes that give off alpha particles. Alpha particles are particles that consist of two neutrons and two protons, all of which are bound together in tight and strong configurations. Plutonium-carbon (PuC) and Americium-beryllium (AmBe) are examples of alpha neutron materials. Gamma neutron sources are similar, except that they emit gamma rays rather than alpha particles. Gamma rays are short-wavelength forms of electromagnetic radiation.

Spontaneous fission is a specific type of radioactive decay that occurs when heavy chemical elements with destabilized nuclei split into two fragments of roughly equal size without being externally induced. This process releases neutrons, which can then be harvested and used. Fission reactors are controlled environments in which scientists induce fission-based chain reactions, which generate thermal energy that is can be collected and used to create steam to spin turbines and generate electricity.

Particle accelerators are specialized pieces of scientific equipment that are designed to produce and speed up the motion of charged particles, usually to induce collisions or concentrate them on a target. These devices accelerate particles to velocities that can approach the speed of light. The atomic collisions they induce can release neutrons in large quantities.

The radionuclide sealed neutron sources commonly used in industrial applications are usually spontaneous fission sources that yield controllable amounts of energy, or alpha neutron sources such as AmBe. Reactor neutron beams are also used in both industry and research to source a specific type of neutron known as thermal neutrons, which are free neutrons (that are not bound to an atomic nucleus) and have varying levels of kinetic energy. These neutron sources have historically been used for radiography and for physics research on solid-state substances.

The National Institute of Standards and Technology maintains a list of systems and facilities serving as active neutron sources. In North America, such neutron sources include the NIST Center for Neutron Research along with Oak Ridge Neutron Facilities, the Los Alamos Neutron Science Center, the University of Missouri Research Reaction Center, and the Cyclotron Facility at Indiana University. NIST also recognizes neutron sources at institutions and research centers in European countries including France, Germany, Hungary, the Netherlands, Russia, Switzerland, and the United Kingdom, and in several other countries including India, Japan, South Korea, and Australia.

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