Neutrons may be produced using a number of techniques including isotopic sources, small deuterium-tritium neutron generators, and large accelerators.

Isotopic neutron sources produce continuous fluxes of neutrons. The most common isotopic source of neutrons is from spontaneous fission of Californium-252 (²⁵²Cf). The average energy of neutrons from ²⁵²Cf is 2.3 MeV. The half life is 2.3 years. Neutrons may also be produced by mixing an isotope which emits a particle with beryllium 9. Neutrons are produced by the (a, n) reaction with beryllium. Common (a,n) sources are:

• ²³⁹Pu with ⁹Be,                
• ²²⁶Ra with ⁹Be,                
• ²⁴¹Am with ⁹Be

Isotopic neutron sources have the advantage having a long useful life and producing a relatively constant flux of neutrons. They may also be relatively inexpensive for low flux (<10⁸ neutrons per second) sources. However, isotopic sources have several disadvantages. The neutron output can not be turned off, requiring that they be contained within bulky shielding at all times. Isotopic neutron sources can not be pulsed and the energy spectrum of the emitted neutrons is broad and peaks at energies below the threshold for some important reactions.

D + T→ N + ⁴He         En = 14.2 MeV

D + D→ N + ³He        En = 2.5 MeV

Yield(D,T) ~ 100 x Yield(D,D)

The basic features of a sealed neutron tube are illustrated in the schematic. This is typical of the neutron tubes used in the Thermo MF Physics A325, A-320 and A-210/211 neutron generators.

The gas pressure in the source is regulated by heating or cooling the gas reservoir element.

A plasma is formed along the axis of the anode which traps electrons which, in turn, ionize gas in the source. The ions are extracted through the exit cathode. Under normal operation, the ion species produced by the Penning source are over 90% molecular ions.

Ions emerging from the exit cathode are accelerated through the potential difference between the exit cathode and the accelerator electrode. The schematic indicates that the exit cathode is at ground potential and the target is at high (negative) potential. This is the case in many sealed tube neutron generators. However, in cases when it is desired to deliver the maximum flux to a sample, it is desirable to operate the neutron tube with the target grounded and the source floating at high (positive) potential. The accelerator voltage is normally between 80 and 180 kilovolts.

The ions pass through the accelerating electrode and strike the target. When ions strike the target, 2 - 3 electrons per ion are produced by secondary emission. In order to prevent these secondary electrons from being accelerated back into the ion source, the accelerator electrode is biased negative with respect to the target. This voltage, called the suppressor voltage, must be at least 500 volts and may be as high as a few kilovolts. Loss of suppressor voltage will result in damage, possibly catastrophic, to the neutron tube.

Some neutron tubes incorporate an intermediate electrode, called the focus or extractor electrode, to control the size of the beam spot on the target. Both the A-711 neutron generator and the A-910/920 neutron generators incorporate sealed neutron tubes which have focus electrodes.

The target is a thin film of a metal such as titanium, scandium, or zirconium which is deposited on a copper or molybdenum substrate. Titanium, scandium, and zirconium form stable chemical compounds called metal hydrides when combined with hydrogen or its isotopes. These metal hydrides are made up of two hydrogen (deuterium or tritium) atoms per metal atom and allow the target to have extremely high densities of hydrogen. This is important to maximize the neutron yield of the neutron tube. The gas reservoir element also uses metal hydrides as the active material. All MF Physics neutron tubes are designed such that the gas reservoir element and the target each incorporate equal amounts of deuterium and tritium. In these mixed gas tubes, both the ion beam and target contain 50% deuterium and 50% tritium. This allows the tubes to have very stable neutron yields over their operational life.

Neutron generators possess none of the disadvantages of isotopic neutron sources.   Sealed tube neutron generators can be turned off. They may be operated either as continuous or pulsed neutron sources. The neutrons produced are monoenergetic (2.5 MeV or 14 MeV). The 14 MeV neutrons are sufficiently energetic to excite n,n’g reactions in nitrogen and oxygen which are particularly important to many applications.