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Detecting NeutronsNeutrons have ~1 amu, but no electronic charge. Therefore, there is no columbic interactions, the primary method of energy loss for electrons and other charged particles. Epithermal and fast neutrons (>0.5 eV) may have long “path lengths”. Many centimeters of material may be penetrated. Neutrons are usually “invisible” to standard detectors. When neutrons do undergo interaction, it is most often with a nucleus of the absorber material. The energy and direction of neutron is drastically changed. If the neutron is captured, the capture reaction gives rise to reaction
products in the form of secondary radiations and charged particles. These
reaction products can be detected by conventional coulombic interactions within
a detector through excitation or ionization.
Reaction 1 with 10B
10BF3 gas in tubes (0.5 to 1.0 atm) Q = 2.792 MeV It is used for detecting thermal neutrons, but 10B does have a large cross
section for higher energy neutrons as well. 10B(n, α) reaction
10B + 1n -> 7Li + 4α Thermal neutron x-section of 3840 barns but gas is toxic.
Reaction 2 with 3He
3He gas in pressurized tubes (1.0 to 20.0 atm) Q = 0.763 MeV Most commonly used for thermal neutron detection systems 3He (n, p) reaction
3He + 1n -> 3H + 1p No solid state detectors are possible. Thermal neutron x-section of 5339 Barns, but costly to cover large areas.
Reaction 3 with 6Li
An attractive alternative for slow neutron detection is the 6 Li
reaction Q = 4.78 MeV 6Li (n, α) reaction
6Li + 1n -> 3H + 4α Reaction proceeds only to the ground state. Thermal neutron x-section of 940 Barns, but lower x-section is offset by the higher Q. There are more moles of 6Li in the glass fibers than there are moles of 3He in a pressurized gas tube. The atoms are also better spatially distributed to maximize sensitivity through optimization of the geometric efficiency.
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