Neutron generator
Fusion of deuterium atoms (D + D) results in the formation of a helium-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 helium-4 ion and a neutron with a kinetic energy of approximately 14.1 MeV.
A related concept, colliding beam fusion, attempts to address this issue using two accelerators firing at each other.
The anisotropy of the neutron emission from DD and DT reactions arises from the fact the reactions are isotropic in the center of momentum coordinate system (COM) but this isotropy is lost in the transformation from the COM coordinate system to the laboratory frame of reference.
In both frames of reference, the He nuclei recoil in the opposite direction to the emitted neutron consistent with the law of conservation of momentum.
The gas pressure in the ion source region of the neutron tubes generally ranges between 0.1 and 0.01 mm Hg.
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.
In comparison with their predecessors, sealed neutron tubes do not require vacuum pumps and gas sources for operation.
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 the Neutristor,[3] a mostly solid state device, resembling a computer chip, invented at Sandia National Laboratories in Albuquerque NM.
[citation needed] Typical sealed designs are used in a pulsed mode[4] and can be operated at different output levels, depending on the life from the ion source and loaded targets.
A plasma is formed along the axis of the anode which traps electrons which, in turn, ionize gas in the source.
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.
Loss of suppressor voltage will result in damage, possibly catastrophic, to the neutron tube.
[2] Input - target for hyperneutron decay detector: e.g. could be tungsten cones in nested pre-cylinder pre-diodes out of wolfrum The targets used in neutron detector itself are thin films of metal such as titanium, scandium, or zirconium which are deposited onto a silver, copper or molybdenum substrate.
Titanium, scandium, and zirconium form stable chemical compounds called metal hydrides when combined with hydrogen or its isotopes.
A 1 microampere ion beam accelerated at 200 kV to a titanium-tritium target can generate up to 108 neutrons per second.
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.
[2] One approach for generating the high voltage fields needed to accelerate ions in a neutron tube is to use a pyroelectric crystal.
In February 2006 researchers at Rensselaer Polytechnic Institute demonstrated the use of two oppositely poled crystals for this application.
These devices are similar in their operating principle to conventional sealed-tube neutron generators which typically use Cockcroft–Walton type high voltage power supplies.
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).
Another type of innovative neutron generator is the inertial electrostatic confinement fusion device.
This neutron generator avoids using a solid target which will be sputter eroded causing metalization of insulating surfaces.
