Neutron Source

A neutron source is any device or system that emits free neutrons, regardless of the physical mechanism by which those neutrons are produced. Neutron sources are essential tools across a wide range of scientific, industrial, medical, and military fields, including physics, nuclear engineering, medicine, nuclear power, petroleum exploration, materials science, chemistry, biology, and nuclear weapons research. The choice of neutron source depends on several variables, notably the neutron energy spectrum, neutron emission rate, physical size, operational cost, maintenance requirements, and applicable regulatory controls.

General Characteristics and Classification

Neutron sources are commonly classified according to their scale and production mechanism. They range from compact radioactive sources producing relatively low neutron fluxes to large accelerator- or reactor-based facilities capable of generating extremely intense neutron beams. Important performance parameters include neutron flux, defined as the number of neutrons passing through a unit area per unit time, and neutron energy, which determines suitability for applications such as imaging, activation analysis, or inducing nuclear reactions.

Small-Scale Neutron Sources

Spontaneous Fission Sources

Some heavy isotopes undergo spontaneous fission, emitting neutrons without the need for an external trigger. The most widely used spontaneous fission neutron source is californium-252. This isotope is produced by neutron irradiation of uranium or other transuranic elements in nuclear reactors, followed by successive neutron captures and radioactive decay.
Californium-252 neutron sources are typically compact, often measuring only a few centimetres in length and diameter. A new source commonly emits between 10⁷ and 10⁹ neutrons per second. With a half-life of approximately 2.6 years, the neutron output decreases by half over this period, limiting the operational lifetime of the source. These sources are valued for their portability and reliability but are expensive and tightly regulated.

Alpha–Neutron Sources

Alpha–neutron sources produce neutrons when alpha particles emitted by a radioactive isotope interact with light nuclei, such as beryllium, carbon, oxygen, or lithium. These sources are created by combining an alpha-emitting radionuclide with a suitable target material, often in powdered or sintered form.
Common material combinations include plutonium–beryllium, americium–beryllium, and americium–lithium. Alpha–neutron sources typically emit between 10⁶ and 10⁸ neutrons per second. Their neutron yield depends on the activity and half-life of the alpha emitter. In terms of size and cost, they are comparable to spontaneous fission sources and are widely used for calibration, well logging, and laboratory applications.

Photodisintegration Sources

Neutrons can also be generated by photodisintegration, in which high-energy gamma rays eject neutrons from atomic nuclei. This occurs when the photon energy exceeds the neutron binding energy of the nucleus. Light nuclei such as beryllium and deuterium are commonly involved in such reactions.
Photodisintegration-based neutron sources are typically used in specialised research environments and are less common than radioactive alpha–neutron or spontaneous fission sources.

Sealed-Tube Neutron Generators

Compact sealed-tube neutron generators use accelerator-based techniques to induce fusion reactions, most commonly deuterium–deuterium or deuterium–tritium fusion. Ions are accelerated into metal hydride targets containing hydrogen isotopes, producing neutrons as a by-product of fusion. These devices offer controllable neutron output and on–off operation, making them attractive for industrial and security applications.

Medium-Scale Neutron Sources

Dense Plasma Focus Devices

The dense plasma focus is a pulsed fusion-based neutron source that generates extremely dense and hot plasma using high-voltage electrical discharges. Under appropriate conditions, deuterium or deuterium–tritium plasma undergoes fusion, producing short bursts of neutrons. These devices are primarily used for research into plasma physics and fusion processes.

Inertial Electrostatic Confinement

Inertial electrostatic confinement devices, such as the Farnsworth–Hirsch fusor, use electric fields to accelerate ions into a central region where fusion reactions occur. While often associated with experimental or educational use, commercial versions can produce neutron outputs approaching 10⁹ neutrons per second, making them suitable for certain analytical and testing applications.

Light-Ion Accelerators

Traditional particle accelerators using hydrogen, deuterium, or tritium ion beams can produce neutrons by directing the beam onto targets made of low atomic number materials such as lithium, beryllium, or tritium-containing compounds. These accelerators typically operate in the megaelectronvolt energy range and are widely used in applied nuclear research.

Bremsstrahlung-Based Systems

In bremsstrahlung neutron sources, high-energy electrons striking a target produce gamma radiation. When these photons exceed nuclear binding energies, they can induce neutron emission through giant dipole resonance or photofission. Such neutron production becomes significant at photon energies above several megaelectronvolts and is an important consideration in high-energy radiotherapy facilities.

Large-Scale Neutron Sources

Nuclear Fission Reactors

Nuclear fission reactors are among the most prolific neutron sources available. Each fission event releases multiple neutrons, enabling extremely high neutron fluxes. Research reactors are specifically designed to provide access to intense neutron fields for experimental purposes, such as materials testing, neutron activation analysis, and isotope production.

Nuclear Fusion Systems

Nuclear fusion reactions involving isotopes of hydrogen have the potential to produce very large numbers of neutrons. Small-scale fusion devices are commonly used in laboratories for plasma research. Large experimental facilities, such as inertial confinement fusion installations and magnetic confinement devices, can generate brief but intense neutron bursts. Although not yet used routinely as neutron sources, fusion systems hold significant future potential.

Spallation Neutron Sources

Spallation neutron sources produce neutrons by bombarding heavy metal targets with high-energy protons. Each proton impact ejects multiple neutrons, resulting in extremely high neutron fluxes. Modern spallation sources surpass the neutron output of even the most powerful research reactors.
The most intense neutron sources in operation today are spallation-based, providing fluxes on the order of 10¹⁷ neutrons per square centimetre per second. These facilities are central to advanced neutron scattering research in condensed matter physics, chemistry, and materials science.

Subcritical Reactor Systems

Proposed subcritical reactors combine spallation neutron sources with non-critical fissile assemblies. In these systems, an external neutron source sustains fission reactions without reaching criticality. Such designs are being explored for nuclear waste transmutation, isotope production, and potential energy generation with enhanced safety margins.

Laser-Driven Neutron Sources

An emerging technology involves laser-driven neutron sources, which use ultra-intense laser pulses to accelerate charged particles that subsequently induce neutron-emitting nuclear reactions. These sources are compact compared with reactor or spallation facilities and offer ultrashort neutron pulses with high brilliance, making them attractive for time-resolved experiments.