NEUTRON DETECTORS

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Neutron detectors are special devices for detecting neutrons. Neutrons, as their name suggests, are neutral particles. They do not interact with electrons but they interact with various nuclei to produce charged particles. There are two types of neutron interactions;

  • Neutron colliding with a nucleus through elastic scattering, transferring some of its kinetic energy to the nucleus and creating ions which are detected.

  • Neutron can cause a nuclear reaction, then the products from these reactions, such as protons, gamma rays, alpha particles and fission fragments can initiate the detection process. The electrical signals produced by the charged particles can then be processed by the detection system.

Types of neutron detectors

Gas-filled detectors

These are the first detectors to be used in radiation detection. They are highly used in nuclear reactions and in recoil interactions to detect thermal neutrons and fast neutrons respectively. The gas detector is made of a metal cylinder with an electrical connector at one end. Detector walls are mostly made from aluminum because of their high detection efficiency. The interior walls are coated with activated charcoal to absorb electronegative gases that build up during neutron irradiation. The coating is used in tubes filled with boron trifluoride (BF3) gas and in Helium-3 (3He)-filled tubes operated in high neutron fluxes.

Helium-3 is obtained by separation from tritium produced in reactors. It reacts as follows by absorbing thermal neutrons producing a Proton (1H) and Triton (3H) ion, making it an effective neutron detector material.

3He + n → 3H + 1H + 765 keV

 Another nuclear reaction is that of Boron-10 (10B), whereby the boron nucleus breaks up into a helium nucleus (alpha particle) and a lithium nucleus.

B + n → 7Li* + He + 2310 keV

and 7Li* → 7Li + 480 keV

These reactions are exothermic and releases energetic charged particles into the gas. The ionization produced by these particles initiate the detection process.

Scintillation neutron detectors

Example of scintillation neutron detectors are the plastic and liquid (organic) scintillators. Plastic and liquid (organic) scintillators are often used for fast-neutron detection because of their fast response. But their limitation is that they have high gamma-ray sensitivity.

Neutron-sensitive scintillating glass fiber detectors

The scintillating glass fiber works by incorporating 6Li and Ce3+ into the glass bulk composition. 6Li has a high cross-section for thermal neutron absorption, and neutron absorption produces tritium ion, 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. This results in a flash of light of several thousand photons for each neutron absorbed.

Fast neutron detectors

Fast neutrons interact in scintillation through elastic scattering with the nuclei present. They are often detected by first moderating them to thermal energies. Fast neutron detectors differentiate themselves from one another by their sensitivity and capability to differentiate neutron and gamma. Excellent capabilities to distinguish between neutron and gamma are experienced in noble gas based 4He detectors due to their low electron density and great pulse shape.

Applications of neutron detectors

  1. Radiation safety. Neutron detectors are used in radiation areas associated with neutron sources such as in nuclear reactors, accelerators or space travel, to detect any neutron leakage for safety purposes.
  2. Cosmic ray detection. Neutron monitors are employed on the ground to detect the cosmic rays reaching the earth’s surface.
  3. Particle physics. Neutron detection has contributed in enhancing neutrino detectors in Particle Physics.
  4. Nuclear power. Nuclear detectors provide an important measure of power in nuclear power and research reactors.

REFERENCES

  1. Atkinson M. (1987). Nuclear Instruments and Methods in Physics Research.
  2. T.W Crane and M. P. Baker. Neutron Detectors.

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