Neutron Detector

a device to detect neutrons. The operation of a neutron detector is based on the detection of the secondary particles formed from the interaction of neutrons with atomic nuclei. A number of nuclear reactions are used to detect slow neutrons. Among them are (1) the splitting of light nuclei on exposure to neutrons [¹⁰B (n,α)⁷Li, ⁶Li (n,α)³H, and ³He (n,p)¹H] with detection of the alpha particles and protons, (2) the fission of heavy nuclei with detection of the fission fragments, (3) the radiative capture of neutrons by nuclei (η,γ) with detection of the gamma quanta, and (4) the generation of artificial radioactivity.

Ionization chambers and proportional counters can be used to detect alpha particles, protons, and fission fragments. In both types of devices, the enclosure of the device is filled with gaseous BF3 and other gases that contain B or ³H, or the walls of the enclosure are coated with a thin layer of solid B, Li, or a fissionable material. The design and dimensions of such chambers and counters are diverse. Proportional counters may be as large as 50 mm in diameter and 2 m in length (the SNM-15). Neutron counters that contain ¹⁰B or ³He are most efficient with respect to thermal neutrons. Slow neutrons can also be detected by scintillation counters, which use Lil crystals with a trace of Eu, scintillating lithium glasses, or mixtures of boron-containing substances and the scintillator ZnS. The efficiency of detection of thermal neutrons may reach 40–60 percent in this case. A neutron scintillation counter in which radiative-capture events are detected has been constructed at the Joint Institute for Nuclear Research. It is designed for neutrons with an energy up to 10 kiloelectron volts (keV) and has an efficiency of approximately 20–40 percent.

The efficiency of detection with these detectors is hundreds of times lower for fast neutrons than for slow neutrons; therefore, fast neutrons are first slowed down in a paraffin block that surrounds the neutron detector. Blocks of specially selected shapes and sizes are used to obtain a virtually constant efficiency of neutron detection throughout an energy range running from several kiloelectron volts to 20 MeV (a long, or flat-response, detector). For direct detection of neutrons with energies of approximately 100 keV, usually either the elastic scattering of neutrons in hydrogen or helium is used or the recoil nuclei are detected. Since the energy of such recoil nuclei depends on the neutron energy, neutron detectors of this type make it possible to measure the neutron energy spectrum. Neutron scintillation counters can also detect fast neutrons on the basis of recoil protons in organic and hydrogen-containing liquid scintillators. Some heavy nuclei, such as ²³⁸U and ²³²Th, undergo fission only on exposure to fast neutrons. This makes it possible to design threshold neutron detectors that are used to detect fast neutrons against a background of thermal neutrons.

Nuclear photographic emulsions are also used to detect the products of nuclear reactions of neutrons with B and Li nuclei, recoil protons, and fission fragments. This method is especially convenient in dosimetry, since it makes possible the determination of the total number of neutrons during the irradiation period. When nuclei undergo fission, the fragments have an energy large enough to produce noticeable mechanical failures. One method of detecting the fission fragments is to slow them down in glass, which is then etched with hydrofluoric acid; as a result, the tracks of the fragments can be observed under a microscope.

The generation of artificial radioactivity upon exposure to neutrons is used for the detection of neutrons, especially in measurements of the neutron flux density, since the number of decays (the activity) is proportional to the flux of neutrons passing through the substance. (The activity can be measured after neutron irradiation is halted.) There are a large number of different isotopes that can be used as radioactive indicators of neutrons of various energies ε₀. For thermal energies, the isotopes ⁵⁵Mn, ¹⁰⁷Ag, and ¹⁹⁷Au are most widely used; for detecting resonance neutrons, the isotopes ⁵⁵Mn (ε₀ = 300 eV), ⁵⁹Co (ε₀ = 100 eV), ¹⁰³Rh and ¹¹⁵In (ε₀ = 1.5 eV), ¹²⁷I (ε₀ = 35 eV), and ¹⁰⁷Ag and ¹⁹⁷Au (ε₀ = 5 eV) are used. The threshold detectors ¹²C (ε_0thr = 20 MeV), ³²S (ℰ,_thr = 0.9 MeV), and ⁶³Cu (ℰ_thr = 10 MeV) are used at higher energies (see).