Neutron Converter – How neutrons can be detected

Detection of neutrons is very specific since the neutrons are electrically neutral particles. Thus, they are mainly subject to strong nuclear forces but not electric ones. Therefore, neutrons are not directly ionizing and usually have to be converted into charged particles before they can be detected. Generally, every type of neutron detector must be equipped with a converter (to convert neutron radiation to common detectable radiation) and one of the conventional radiation detectors (scintillation detector, gaseous detector, semiconductor detector, etc.).

Neutron Converters

Two basic types of neutron interactions with matter are for this purpose available:

• Elastic scattering. The free neutron can be scattered by a nucleus, transferring some of its kinetic energy to the nucleus. If the neutron has enough energy to scatter off nuclei, the recoiling nucleus ionizes the material surrounding the converter. Only hydrogen and helium nuclei are light enough for practical application. Charge produced in this way can be collected by the conventional detector to produce a detected signal. Neutrons can transfer more energy to light nuclei. This method is appropriate for detecting fast neutrons (fast neutrons do not have a high cross-section for absorption), allowing the detection of fast neutrons without a moderator.
• Neutron absorption. This common method allows the detection of neutrons in the entire energy spectrum. This method is based on various absorption reactions (radiation capture, nuclear fission, rearrangement reactions, etc.). The neutron is absorbed by the target material (converter), emitting secondary particles such as protons, alpha particles, beta particles, photons (gamma rays), or fission fragments. Some reactions are threshold reactions (requiring minimum energy of neutrons), but most reactions occur at epithermal and thermal energies. That means the moderation of fast neutrons is required leading to poor energy information of the neutrons. The most common nuclei for the neutron converter material are:
  • ¹⁰B(n,α). The neutron capture cross-section for thermal neutrons is σ = 3820 barns, and the natural boron has an abundance of ¹⁰B 19,8%.
  • ³He(n,p). The neutron capture cross-section for thermal neutrons is σ = 5350 barns, and the natural helium has an abundance of ³He 0.014%.
  • ⁶Li(n,α). The neutron capture cross-section for thermal neutrons is σ = 925 barns, and the natural lithium has an abundance of ⁶Li 7,4%.
  • ¹¹³Cd(n,ɣ). The neutron capture cross-section for thermal neutrons is σ = 20820 barns, and the natural cadmium has an abundance of ¹¹³Cd 12,2%.
  • ²³⁵U(n,fission). The fission cross-section for thermal neutrons is σ = 585 barns, and the natural uranium has an abundance of ²³⁵U 0.711%. Uranium, as a converter, produces fission fragments which are heavy charged particles. This has a significant advantage. The heavy charged particles (fission fragments) create a high output signal because the fragments deposit a large amount of energy in a detector-sensitive volume. This allows easy discrimination of the background radiation (e.i. gamma radiation). For example, this important feature can be used in a nuclear reactor power measurement, where the neutron field is accompanied by a significant gamma background.

Example – Neutron Converter

Fission Chamber – Wide Range Detectors

Fission chambers are ionization detectors used to detect neutrons. Fission chambers may be used as the intermediate range detectors to monitor neutron flux (reactor power) at the intermediate flux level. They also provide an indication, alarms, and reactor trip signals. The design of this instrument is chosen to provide overlap between the source range channels and the full span of the power range instruments.

In the case of fission chambers, the chamber is coated with a thin layer of highly enriched uranium-235 to detect neutrons. Neutrons are not directly ionizing and usually have to be converted into charged particles before they can be detected. A thermal neutron will cause an atom of uranium-235 to fission, with the two fission fragments produced having high kinetic energy and causing ionization of the argon gas within the detector. One advantage of using uranium-235 coating rather than boron-10 is that the fission fragments have much higher energy than the alpha particle from a boron reaction. Therefore fission chambers are very sensitive to neutron flux, and this allows the fission chambers to operate in higher gamma fields than an uncompensated ion chamber with boron lining.

Proton Recoil – Recoil Detectors

The most important type of detectors for fast neutrons are those that directly detect recoil particles, particularly recoil protons resulting from elastic (n, p) scattering. Only hydrogen and helium nuclei are light enough for practical application. In the latter case, the recoil particles are detected in a detector. Neutrons can transfer more energy to light nuclei. This method is appropriate for detecting fast neutrons and detecting fast neutrons without a moderator. This method allows the energy of the neutron to be measured together with the neutron fluence, i.e., the detector can be used as a spectrometer. Typical fast neutron detectors are liquid scintillators, helium-4-based noble gas detectors, and plastic detectors (scintillators). For example, plastic has a high hydrogen content. Therefore, it is useful for fast neutron detectors when used as a scintillator.

See above:

Detection of Neutrons