Neutron detection

Neutron detection is the effective detection of neutrons entering a well-positioned detector. There are two key aspects to effective neutron detection: hardware and software. Detection hardware refers to the kind of neutron detector used and to the electronics used in the detection setup. Further, the hardware setup also defines key experimental parameters, such as source-detector distance, solid angle and detector shielding. Detection software consists of analysis tools that perform tasks such as graphical analysis to measure the number and energies of neutrons striking the detector.

Basic physics

Signatures by which a neutron may be detected

Atomic and subatomic particles are detected by the signature they produce through interaction with their surroundings. The interactions result from the particles' fundamental characteristics.

Charge: Neutrons are neutral particles and do not ionize directly; hence they are harder than charged particles to detect directly. Further, their paths of motion are only weakly affected by electric and magnetic fields.
Mass: The neutron mass of is not directly detectable, but does influence reactions through which it can be detected.
Reactions: Neutrons react with a number of materials through elastic scattering producing a recoiling nucleus, inelastic scattering producing an excited nucleus, or absorption with transmutation of the resulting nucleus. Most detection approaches rely on detecting the various reaction products.
Magnetic moment: Although neutrons have a magnetic moment of μ_N, techniques for detection of the magnetic moment are too insensitive to use for neutron detection.
Electric dipole moment: The neutron is predicted to have only a tiny electric dipole moment, which has not yet been detected. Hence it is not a viable detection signature.
Decay: Outside the nucleus, free neutrons are unstable and have a mean lifetime of . Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay:
: → + +.
Classic neutron detection options

As a result of these properties, detection of neutrons fall into several major categories:

Absorptive reactions with prompt reactions - low energy neutrons are typically detected indirectly through absorption reactions. Typical absorber materials used have high cross sections for absorption of neutrons and include helium-3, lithium-6, boron-10, and uranium-235. Each of these reacts by emission of high energy ionized particles, the ionization track of which can be detected by a number of means. Commonly used reactions include ³He ³H, ⁶Li ⁴He, ¹⁰B ⁷Li and the fission of uranium.
Activation processes - Neutrons may be detected by reacting with absorbers in a radiative capture, spallation or similar reaction, producing reaction products that then decay at some later time, releasing beta particles or gammas. Selected materials have extremely large cross sections for the capture of neutrons within a very narrow band of energy. Use of multiple absorber samples allows characterization of the neutron energy spectrum. Activation also enables the reconstruction of an historic neutron exposure.
Elastic scattering reactions - High energy neutrons are typically detected indirectly through elastic scattering reactions. Neutrons collide with the nuclei of atoms in the detector, transferring energy to those nuclei and creating ions, which are detected. Since the maximum transfer of energy occurs when the mass of the atom with which the neutron collides is comparable to the neutron mass, hydrogenous materials are often the preferred medium for such detectors.
Types of neutron detectors

Gas proportional detectors

can be adapted to detect neutrons. While neutrons do not typically cause ionization, the addition of a nuclide with high neutron cross-section allows the detector to respond to neutrons. Nuclides commonly used for this purpose are helium-3, lithium-6, boron-10 and uranium-235. Since these materials are most likely to react with thermal neutrons, they are typically surrounded by moderating materials to reduce their energy and increase the likelihood of detection.
Further refinements are usually necessary to differentiate the neutron signal from the effects of other types of radiation. Since the energy of a thermal neutron is relatively low, charged particle reactions are discrete while other reactions such as gamma reactions will span a broad energy range, it is possible to discriminate among the sources.
As a class, gas ionization detectors measure the number, and not the energy of neutrons.

³He gas-filled proportional detectors

Helium-3 is an effective neutron detector material because it reacts by absorbing thermal neutrons, producing a ¹H and ³H ion. Its sensitivity to gamma rays is negligible, providing a very useful neutron detector. Unfortunately the supply of ³He is limited to production as a byproduct from the decay of tritium ; tritium is produced either as part of weapons programs as a booster for nuclear weapons or as a byproduct of reactor operation.

BF₃ gas-filled proportional detectors

As elemental boron is not gaseous, neutron detectors containing boron may alternately use boron trifluoride enriched to 96% boron-10. Boron trifluoride is highly toxic. The sensitivity of this detector is around 35-40 CPS/nv the sensitivity of boron-lined detectors is approximately 4 CPS/nv. This is because in boron-lined detectors, neutrons react with boron to produce ion pairs inside the boron layer and so charged particles produced may lose some of their energy inside that layer. This means that low-energy charged particles may be unable to reach the ionization chamber's gas environment resulting in a lower number of ionizations produced. In BF₃ gas-filled detectors, on the other hand, neutrons react with ¹⁰B atoms inside the detector gas volume, so charged particles produced are more likely to deposit their energy in the gas volume, producing more ionizations and therefore higher signal.

Boron-lined proportional detectors

Boron-lined gas-filled proportional counters react similarly to BF₃ gas-filled proportional detectors, but instead of containing boron-rich gas, the walls are coated with ¹⁰B with another fill gas. In this design, since the reaction takes place on the surface, only one of the two particles will escape into the proportional counter.

Scintillation neutron detectors

Scintillation neutron detectors include liquid organic scintillators, crystals, plastics, glass and scintillation fibers.

Organic Scintillators

Organic scintillators are materials that has the property to emit light when exposed to ionizing radiation. They generally have fast response time to radiation and can detect a wide range of radiation. As the name suggests, they are usually made of organic materials, mostly carbon-based materials.
The organic scintillators comes in many forms such as plastic scintillators, liquid scintillators, and crystal scintillators. The plastic scintillators are made from polymers like polyvinyltoluene or polystyrene. The Liquid scintillators are simply made by dissolving the organic scintillators in suitable solvents. The crystal scintillators are made of solid crystals like anthracene or stilbene.
When a scintillator is exposed to ionizing radiation, the molecules within them are excited to a higher energy state through the interaction with the incoming radiation. These excited molecules tries to become stable by emitting a photon. The emitted light can be detected by amplifying it in a photo-multiplier and converted to an electric signal. These signals are proportional to the energy of the radiation, which helps in identifying the radiation that was exposed to.
The neutrons can be detected by the organic scintillators using the Pulse Shape Discrimination technique, which can differentiate between the radiation based on their pulse/signal shape.

Neutron-sensitive scintillating glass fiber detectors

Scintillating ⁶Li glass for neutron detection was first reported in the scientific literature in 1957 and key advances were made in the 1960s and 1970s. Scintillating fiber was demonstrated by Atkinson M. et al. in 1987 and major advances were made in the late 1980s and early 1990s at Pacific Northwest National Laboratory where it was developed as a classified technology. It was declassified in 1994 and first licensed by Oxford Instruments in 1997, followed by a transfer to Nucsafe in 1999. The fiber and fiber detectors are now manufactured and sold commercially by Nucsafe, Inc.
The scintillating glass fibers work by incorporating ⁶Li and Ce³⁺ into the glass bulk composition. The ⁶Li has a high cross-section for thermal neutron absorption through the ⁶Li reaction. Neutron absorption produces a tritium ion, an alpha particle, and kinetic energy. The alpha particle and triton interact with the glass matrix to produce ionization, which transfers energy to Ce³⁺ ions and results in the emission of photons with wavelength 390 nm – 600 nm as the excited state Ce³⁺ ions return to the ground state. The event results in a flash of light of several thousand photons for each neutron absorbed. A portion of the scintillation light propagates through the glass fiber, which acts as a waveguide. The fibers ends are optically coupled to a pair of photomultiplier tubes to detect photon bursts. The detectors can be used to detect both neutrons and gamma rays, which are typically distinguished using pulse-height discrimination. Substantial effort and progress in reducing fiber detector sensitivity to gamma radiation has been made. Original detectors suffered from false neutrons in a 0.02 mR gamma field. Design, process, and algorithm improvements now enable operation in gamma fields up to 20 mR/h.
The scintillating fiber detectors have excellent sensitivity, they are rugged, and have fast timing so that a large dynamic range in counting rates is possible. The detectors have the advantage that they can be formed into any desired shape, and can be made very large or very small for use in a variety of applications. Further, they do not rely on ³He or any raw material that has limited availability, nor do they contain toxic or regulated materials. Their performance matches or exceeds that of ³He tubes for gross neutron counting due to the higher density of neutron absorbing species in the solid glass compared to high-pressure gaseous ³He. Even though the thermal neutron cross section of ⁶Li is low compared to ³He, the atom density of ⁶Li in the fiber is fifty times greater, resulting in an advantage in effective capture density ratio of approximately 10:1.