Neutron generator

This article is about generators employing accelerators. For more general sources, seeNeutron source.

Nuclear physicist at theIdaho National Laboratorysets up an experiment using an electronic neutron generator.

Neutron generatorsareneutron sourcedevices which contain compactlinear particle acceleratorsand that produceneutronsby fusingisotopes of hydrogentogether. Thefusionreactions take place in these devices by accelerating eitherdeuterium,tritium,or a mixture of these two isotopes into a metalhydridetarget which also contains deuterium, tritium or a mixture of these isotopes. Fusion of deuterium atoms (D + D) results in the formation of ahelium-3ion and a neutron with a kinetic energy of approximately 2.5MeV.Fusion of a deuterium and a tritium atom (D + T) results in the formation of ahelium-4ion and a neutron with a kinetic energy of approximately 14.1 MeV. Neutron generators have applications in medicine, security, and materials analysis.

The basic concept was first developed byErnest Rutherford's team in theCavendish Laboratoryin the early 1930s. Using a linear accelerator driven by aCockcroft–Walton generator,Mark Oliphantled an experiment that fired deuterium ions into a deuterium-infused metal foil and noticed that a small number of these particles gave offAlpha particles.This was the first demonstration of nuclear fusion, as well as the first discovery of Helium-3 and tritium, created in these reactions. The introduction of new power sources has continually shrunk the size of these machines, from Oliphant's that filled the corner of the lab, to modern machines that are highly portable. Thousands of such small, relatively inexpensive systems have been built over the past five decades.

While neutron generators do produce fusion reactions, the number of accelerated ions that cause these reactions is very low. It can be easily demonstrated that the energy released by these reactions is many times lower than the energy needed to accelerate the ions, so there is no possibility of these machines being used to produce netfusion power.A related concept,colliding beam fusion,attempts to address this issue using two accelerators firing at each other.

Neutristorin its simplest form as tested by the inventor at Sandia National Laboratories

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 radioactive isotopes) 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, 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 +⁴HeE_n= 14.1 MeV

D + D → n +³HeE_n= 2.5 MeV

Neutrons produced by DD and DT reactions are emitted somewhatanisotropicallyfrom the target, slightly biased in the forward (in the axis of the ion beam) direction. The anisotropy of the neutron emission from DD and DT reactions arises from the fact the reactions areisotropicin thecenter of momentum coordinate system (COM)but this isotropy is lost in the transformation from the COM coordinate system to thelaboratory frame of reference.In both frames of reference, the He nuclei recoil in the opposite direction to the emitted neutron consistent with the law ofconservation of momentum.

The gas pressure in the ion source region of the neutron tubes generally ranges between 0.1 and 0.01mm Hg.Themean free pathof 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, however, has 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 aneinzel lens.The ion beam can thus be focused to a small point at the target. The accelerators typically require power supplies of 100–500 kV. They usually have several stages, with voltage between the stages not exceeding 200 kV to preventfield emission.

In comparison with radionuclide neutron sources, neutron tubes can produce much higherneutron fluxesand consistent (monochromatic) neutron energy spectra can be obtained. The neutron production rate can also be controlled.

Sealed neutron tubes

The central part of a neutron generator is the particle accelerator itself, sometimes 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 medium 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 3–10 metres (10–30 ft) of the accelerator head in laboratory instruments, but may be severalkilometersaway inwell logginginstruments.

In comparison with their predecessors, sealed neutron tubes do not requirevacuum pumpsand gas sources for operation. They are therefore more mobile and compact, while also durable and reliable. For example, sealed neutron tubes have replaced radioactivemodulated neutron initiators,in supplying a pulse of neutrons to the imploding core of modernnuclear weapons.

Examples of neutron tube ideas date as far back as the 1930s, pre-nuclear weapons era, by German scientists filing a 1938 German patent (March 1938, patent #261,156) and obtaining a United States Patent (July 1941, USP #2,251,190); examples of present state of the art are given by developments such as theNeutristor,a mostly solid state device, resembling a computer chip, invented atSandia National Laboratoriesin Albuquerque NM.Typical sealed designs are used in a pulsed modeand can be operated at different output levels, depending on the life from the ion source and loaded targets.

Neutristorin an inexpensive vacuum sealed package ready for testing

Ion sources

Main article:Ion source

A good ion source should provide a strongion beamwithout consuming much of the gas. For hydrogen isotopes, production of atomic ions is favored over molecular ions, as atomic ions have higher neutron yield on collision. The ions generated in the ion source are then extracted by an electric field into the accelerator region, and accelerated towards the target. The gas consumption is chiefly caused by the pressure difference between the ion generating and ion accelerating spaces that has to be maintained. Ion currents of 10 mA at gas consumptions of 40 cm³/hour are achievable.

For a sealed neutron tube, the ideal ion source should use low gas pressure, give high ion current with large proportion of atomic ions, have low gas clean-up, use low power, have high reliability and high lifetime, its construction has to be simple and robust and its maintenance requirements have to be low.

Gas can be efficiently stored in a replenisher, an electrically heated coil of zirconium wire. Its temperature determines the rate of absorption/desorption of hydrogen by the metal, which regulates the pressure in the enclosure.

Cold cathode (Penning)

ThePenningsource is a low gas pressure,cold cathodeion 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 apermanent magnet.Aplasmais 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. This disadvantage is however compensated for by the other advantages of the system.

One of the cathodes is a cup made ofsoft iron,enclosing most of the discharge space. The bottom of the cup has a hole through which most of the generated ions are ejected by the magnetic field into the acceleration space. The soft iron shields the acceleration space from the magnetic field, to prevent a breakdown.

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 accelerating electrode has the shape of a long hollow cylinder. The ion beam has a slightly diverging angle (about 0.1radian). The electrode shape and distance from target can be chosen so the entire target surface is bombarded with ions. Acceleration voltages of up to 200 kV are achievable.

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.

Radio frequency (RF)

Ions can be created by electrons formed in high-frequency electromagnetic field. The discharge is formed in a tube located between electrodes, or inside acoil.Over 90% proportion of atomic ions is achievable.

Targets

The targets used in neutron generators arethin filmsof metal such astitanium,scandium,orzirconiumwhich are deposited onto asilver,copperormolybdenumsubstrate. Titanium, scandium, and zirconium form stable chemical compounds calledmetal hydrideswhen combined with hydrogen or its isotopes. These metal hydrides are made up of twohydrogen(deuteriumortritium) 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, e.g.uranium hydride,as the active material.

Titanium is preferred to zirconium as it can withstand higher temperatures (200 °C), and gives higher neutron yield as it capturesdeuteronsbetter than zirconium. The maximum temperature allowed for the target, above which hydrogen isotopes undergo desorption and escape the material, limits the ion current per surface unit of the target; slightly divergent beams are therefore used. A 1 microampere ion beam accelerated at 200 kV to a titanium-tritium target can generate up to 10⁸neutrons per second. The neutron yield is mostly determined by the accelerating voltage and the ion current level.

An example of a tritium target in use is a 0.2 mm thick silver disc with a 1 micrometer layer of titanium deposited on its surface; the titanium is then saturated with tritium.

Metals with sufficiently low hydrogen diffusion can be turned into deuterium targets by bombardment of deuterons until the metal is saturated. Gold targets under such condition show four times higher efficiency than titanium. Even better results can be achieved with targets made of a thin film of a high-absorption high-diffusivity metal (e.g. titanium) on a substrate with low hydrogen diffusivity (e.g. silver), as the hydrogen is then concentrated on the top layer and can not diffuse away into the bulk of the material. Using a deuterium-tritium gas mixture, self-replenishing D-T targets can be made. The neutron yield of such targets is lower than of tritium-saturated targets in deuteron beams, but their advantage is much longer lifetime and constant level of neutron production. Self-replenishing targets are also tolerant to high-temperaturebake-outof the tubes, as their saturation with hydrogen isotopes is performed after the bakeout and tube sealing.

High voltage power supplies

One approach for generating the high voltage fields needed to accelerate ions in a neutron tube is to use apyroelectric crystal.In April 2005 researchers atUCLAdemonstrated the use of a thermally cycledpyroelectriccrystal to generate high electric fields in a neutron generator application. In February 2006 researchers atRensselaer Polytechnic Institutedemonstrated the use of two oppositely poled crystals for this application. Using these low-tech power supplies it is possible to generate a sufficiently highelectric fieldgradient 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 useCockcroft–Waltontype 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 seepyroelectric fusion.

Other 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

Main article:Fusor

Another type of innovative neutron generator is theinertial electrostatic confinementfusion device. This neutron generator 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 byPhilo Farnsworth,the inventor of electronictelevision.

Applications

Neutron generators find application in semiconductor production industry. They also have use cases in the enrichment of depleted uranium, acceleration of breeder reactors, and activation and excitement of experimental thorium reactors.

In material analysisneutron activation analysisis used to determine concentration of different elements in mixed materials such as minerals or ores.

See also

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

External links

• Chichester, D. L.; Simpson, J. D. (2003)."Compact Accelerator Neutron Generators"(PDF).The Industrial Physicist.9(6): 22–25. Archived fromthe original(PDF)on 2013-09-08.
• Elizondo-Decanini, J. M.; Schmale, D.; Cich, M.; Martinez, M.; Youngman, K.; Senkow, M.; Kiff, S.; Steele, J.; Goeke, R.; Wroblewski, B.; Desko, J.; Dragt, A. J. (2012). "Novel Surface-Mounted Neutron Generator".IEEE Transactions on Plasma Science.40(9): 2145–2150.Bibcode:2012ITPS...40.2145E.doi:10.1109/TPS.2012.2204278.S2CID20593594.
• "Sandia National Laboratories".