Neutron Generator Theory and Operation
Small neutron generators using the deuterium (D, hydrogen-2, 2H) tritium (T, hydrogen-3, 3H) fusion reactions are the most common accelerator based (as opposed to isotopic) neutron sources. In these systems neutrons are produced by creating ions of deuterium, tritium, or deuterium and tritium and accelerating these into a hydride target loaded with deuterium, tritium, or deuterium and tritium. The DT reaction is used more than the DD reaction because the yield of the DT reaction is 50–100 times higher than that of the DD reaction.
D + T → n + 4He En = 14.1 MeV
D + D → n + 3He En = 2.5 MeV
Neutrons produced from the DT reaction are emitted isotropically (uniformly in all directions) from the target while neutrons from the DD reaction are slightly peaked in the forward (along the axis of the ion beam) direction. In both cases, the associated He nuclei (alpha particles) are emitted in the opposite direction of the neutron.
The gas pressure in the ion source region of the neutron tubes generally ranges between 0.1–0.01 mm Hg. The mean free path of electrons must be shorter than the discharge space to achieve ionization (lower limit for pressure) while the pressure must be kept low enough to avoid formation of discharges at the high extraction voltages applied between the electrodes. The pressure in the accelerating region has however to be much lower, as the mean free path of electrons must be longer to prevent formation of a discharge between the high voltage electrodes.
The ion accelerator usually consists of several electrodes with cylindrical symmetry, acting as electric lenses. The ion beam can be focused to a small spot of the target that way. The accelerators usually have several stages, with voltage between the stages not exceeding 200 kV to prevent field emission.
In comparison with radionuclide neutron sources, neutron tubes can produce much higher neutron fluxes and monochromatic neutron energy spectrums can be obtained. The neutron production rate can also be controlled.
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