Neutron generator

Neutron generators are neutron source devices which contain compact linear accelerators and that produce neutrons by fusing isotopes of hydrogen together. The fusion reactions take place in these devices by accelerating either deuterium, tritium, or a mixture of these two isotopes into a metal hydride target which also contains either deuterium, tritium or a mixture. Fusion of deuterium atoms (D + D) results in the formation of a He-3 ion and a neutron with a kinetic energy of approximately 2.5 MeV. Fusion of a deuterium and a tritium atom (D + T) results in the formation of a He-4 ion and a neutron with a kinetic energy of approximately 14.1 MeV.

Thousands of such small, relatively inexpensive systems have been built over the past five decades.

Neutron Generator

Small neutron generators using the deuterium (D, hydrogen-2, $^{2}$H) tritium (T, hydrogen-3, $^{3}$H) 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 mroe 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 + $^{4}$He   E_n = 14.2 MeV

D + D → N + $^{3}$He   E_n = 2.5 MeV

Neutrons produced from the DT reaction are emitted isotropicly (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.

Sealed Neutron Tubes

The central part of a neutron generator is the particle accelerator itself, sometiems called a neutron tube. Neutron tubes have several components including an ion source, ion optic elements, and a beam target; all of these are enclosed within a vacuum tight enclosure. High voltage insulation between the ion optical elements of the tube is provided by glass and/or ceramic insulators. The neutron tube is, in turn, enclosed in a metal housing, the accelerator head, which is filled with a dielectric media to insulate the high voltage elements of the tube from the operating area. The accelerator and ion source high voltages are provided by external power supplies. The control console allows the operator to adjust the operating parameters of the neutron tube. The power supplies and control equipment are normally located within 10-30 feet of the accelerator head in laboratory instruments but may be several kilometers away in well logging instruments.

Ion Sources

Cold Cathode (Penning)

The Penning source is a low gas pressure, cold cathode ion source which utilizes crossed electric and magnetic fields. The ion source anode is at a positive potential, either dc or pulsed, with respect to the source cathode. The ion source voltage is normally between 2 and 7 kilovolts. A magnetic field, oriented parallel to the source axis, is produced by a permanent magnet. 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. The gas pressure in the source is regulated by heating or cooling the gas reservoir element.

Targets

The targets used in neutron generators are thin films of metal such as titanium, scandium, or zirconium which are deposited onto 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.

High Voltage Power Supplies

One particularly interesting approach for generating the high voltage fields needed to accelerate ions in a neutron tube is to use a pyroelectric crystal. In April 2005 researchers at UCLA demonstrated the use of a thermally cycled pyroelectric crystal to generate high electric fields in a neutron generator application. In February 2006 researchers at Rensselaer Polytechnic Institute demonstrated the use of two oppositely poled crystals for this application. Using these low-tech power supplies it is possible to generate a sufficiently high electric field gradient across an accelerating gap to accelerate deuterium ions into a deuterated target to produce the D + D fusion reaction. These devices are similar in their operating principle to conventional sealed-tube neutron generators which typically use Cockcroft-Walton type high voltage power supplies. The novelty of this approach is in the simplicity of the high voltage source. Unfortunately, the relatively low accelerating current that pyroelectric crystals can generate, together with the modest pulsing frequencies that can be achieved (a few cycles per minute) limits their near-term application in comparison with today's commercial products (see below). Also see pyroelectric fusion. [1]

Other Neutron Generator Technologies

In addition to the conventional neutron generator design described above several other approaches exist to use electrical systems for producing neutrons.

Inertial Electrostatic Confinement/Fusor

Another type of innovative neutron generator is the inertial electrostatic confinement fusion device. This neutron generator differs from the conventional ion beam onto solid target types because it avoids using a solid target which will be sputter eroded causing metalization of insulating surfaces. Depletion of the reactant gas within the solid target is also avoided. Far greater operational lifetime is achieved. Originally called a fusor, it was invented by Philo Farnsworth, the inventor of electronic television. This type of neutron generator is manufactured by NSD-Fusion. (Note: An extensive discussion of this technology is available in the fusor wiki.)

Organizations that manufacture neutron generators

• Adelphi Technology, Adelphi Technology (USA)
• Baker Hughes , Baker Hughes (USA)
• EADS SODERN , EADS SODERN (France)
• Halliburton, Halliburton (USA)
• Hotwell, Hotwell GmbH (Austria) [Using neutron tubes from VNIIA]
• Lawrence Berkeley National Laboratory, Lawrence Berkeley National Laboratory (USA)
• NSD-Fusion, NSD-Fusion (Germany)
• Sandia National Laboratories , Sandia National Laboratories (USA)
• Schlumberger, Schlumberger (USA)
• Thermo Fisher Scientific, Thermo Fisher Scientific (USA)
• VNIIA, VNIIA All Russia Research Institute of Automatics (Russia)

See also

• Fast neutron
• Nuclear fission
• Nuclear fusion
• Neutron source
• Neutron moderator
• Radioactive decay
• Radioactivity
• Slow neutron

External links

• Compact Accelerator Neutron Generators, AIP, The Industrial Physicist