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27 Aug 2012

Neutron generators provide materials analysis and non-destructive testing tools to many industries, including oilfield operations, heavy mechanical production, art conservancy, detective work, and medicine. Many of these applications have been limited by the rather large size of current industrial and medical neutron sources. Now Sandia National Laboratories (SNL), whose main job is to develop and support the non-nuclear parts (including neutron generators) of nuclear weapons, has invented a new approach toward building tiny neutron generators called neutristors.

The neutron was discovered as the product of an early radiochemical fusion reaction in 1932. Following a decade of mainly scientific use, the WWII nuclear bombs exploded over Japan each included a neutron generator to ignite the critical mass of fissionable material at the correct time. This event neatly split development of neutron generators between the secret and the open worlds.

The neutron sources available to science and industry included particle accelerators (at that time these filled large rooms), nuclear reactors (filling large buildings), and radioactive materials the size of your little finger. As most researchers and manufacturing companies did not have easy access to reactors and accelerators, a good deal of work toward developing practical applications for neutron sources was carried out with radioactive neutron generators.

There are three main approaches toward using radioactive isotopes to generate neutrons:
Radioactively-induced fusion neutrons
Neutrons from isotopes undergoing spontaneous fission
Photoneutron sources
An example of the radioactive fusion sources is plutonium mixed with beryllium. The plutonium emits alpha particles (helium nuclei) that fuse with the beryllium nuclei, forming a neutron and a pair of alpha particles. Spontaneous fission sources usually contain californium-252, a transuranic isotope that decays by splitting into two parts with several neutrons left over. In the photoneutron sources, energetic gamma rays break beryllium into the same products as seen from the fusion neutron sources.
Radioactive neutron generators usually emit fewer than a billion neutrons per second with a kinetic energy of a few MeV. The power of the emitted neutrons is only about a milliwatt, but the yield is sufficient for many applications.

The problem with radioactive sources is they are dangerous, can't be turned off, and may not always be used by people understanding the danger. In many cases the shielding required is very large compared to the size of the source. Although such sources are still used for certain tasks, in the end, miniaturized particle accelerators that drive low-level fusion reactions won out, and accelerator-based neutron generators about the size of a mailing tube tied to a suitcase-sized electronics package became available.

The miniaturized neutron generators accelerate deuterium (D) or tritium (T) ions to energies of 100 KeV (kiloelectron volts) or less, corresponding roughly to a temperature of about a billion degrees Kelvin. These ions are then directed into a beam that impacts onto a target containing deuterium. When deuterium is used in the ion beam, two deuterium ions fuse (D-D fusion), while if tritium is used, a deuterium and a tritium ion fuse (D-T fusion). In both cases, neutrons are by products of the fusion reaction.

There are two main problems with accelerator-based neutron generators – their size and their cost. There are applications for which a three inch (7.5 cm) cylinder is too large, either physically (implanted neutron cancer therapy), or when a point source of neutrons is desired (e.g., for neutron inspection of weld flaws). Also, accelerator-based generators start at about a hundred thousand U.S. dollars, which is too large a price for some uses. For example, a neutron generator is needed for neutron activation analysis, a technique for rapidly identifying the composition of a sample. This is the sort of technique that would be amazing to incorporate in a Star Trek-style tricorder, but has been far too large and expensive.

SNL's compact neutron generator

Now SNL has announced its development of a new type of neutron generator that solves many of these problems by putting a particle accelerator on a chip. As seen in the figure above, the neutristor is layered in ceramic insulation because of the large voltages being used. The unit shown here produces neutrons through D-D fusion. The D-T reaction is easier to initiate, but the decision was made to require no radioactive materials in the design of the generator.

A voltage is applied between the ion source and the deuterium target so that the deuterium ions from the source are attracted to the deuterium target. The ions accelerate in the drift region between the source and the target. The drift region must be in vacuum so the ions do not scatter from the air molecules. When the energetic ions hit the target, a small fraction of them will cause D-D fusion, thereby generating a neutron. Sandia did not announce typical acceleration voltages used with the neutristor, however, commercial neutron generators use around 100 kV, but significant neutron yields can be obtained at voltages under ten kV.

The ion lens modifies the electric field between the ion source and the target so that the accelerated ions are concentrated on the region of the target loaded with deuterium. The SNL disclosure does not mention how the deuterium gas is stored, but one common approach is to coat the ion source and/or the target with palladium or some other metal that readily forms hydrides, or in this case, deuterides. For example, a palladium coating can store nearly one deuterium atom for each palladium atom. The ion current is sufficiently low that even these small amounts of deuterium will last a very long time in the completed neutristor. Neuristors can be operated in continuous or pulsed mode as required.

Current neuristors have a drift region a few millimeters across, forming a sufficiently small package for many new applications. The estimated production cost for neutristors is in the neighborhood of US$2,000, about a fiftieth of the cost of current accelerator-based neutron generators. The next generation of entirely solid-state neutristors will not require a vacuum for operation, thereby reducing the cost and increasing the durability of the device. In addition, SNL is working on neutristors two to three orders of magnitude smaller that would be fabricated using MEMS (microelectromechanical systems) technology.

The following movie is an excellent introduction to how the development of neutristors came about, and a good account of the underlying technology.

Source: Sandia National Laboratories

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