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 (the most common today is the scintillation detector) 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 to detect directly than charged particles. Further, their paths of motion are only weakly affected by electric and magnetic fields. Mass: The neutron mass of 1.0086649156(6) Da 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 −1.9130427(5) μN, 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 885.7±0.8 s (about 14 minutes, 46 seconds). Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay: n0 → p+ + e− + νe. Although the p+ and e− produced by neutron decay are detectable, the decay rate is too low to serve as the basis for a practical detector system.

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(n,p) 3H, 6Li(n,t) 4He, 10B(n,α) 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 (e.g., indium, gold, rhodium, iron (56Fe(n,p) 56Mn), aluminum (27Al(n,α)24Na), niobium (93Nb(n,2n) 92mNb), and silicon (28Si(n,p) 28Al)) 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 historic neutron exposures (e.g., forensic reconstruction of neutron exposures during an accidental criticality). Elastic scattering reactions (also referred to as proton-recoil) - 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 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 (i.e., neutrons that have slowed to equilibrium with their surroundings), 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 (i.e., essentially monoenergetic and lie within a narrow bandwidth of energies) 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 (count rate), 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 (which has a 12.3 year half-life); tritium is produced either as part of weapons programs as a booster for nuclear weapons or as a byproduct of reactor operation.

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