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Detecting Neutrons

Neutrons 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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