Neutrons can be created by either fission or fusion, but existing sources are inflexible, short-lived and expensive. Commercial neutron generators fire beams of hydrogen isotopes – deuterium and tritium – at targets that also contain deuterium and tritium. Deuterium atoms in the beam fuse with deuterium and tritium atoms in the target to produce neutrons, but these sources stop working when the isotopes in the target are used up.

By changing the geometry and the physics of the commercial generator, Leung’s team has now developed a device that is both portable and long-lasting. They replaced the conventional target with a layer of titanium that collects hydrogen isotopes produced by a ‘plasma electrode’. As the deuterium and tritium atoms accumulate, they fuse to form neutrons. Since there is a continuous stream of isotopes from the plasma electrode, the target never gets depleted.

The Berkeley team then revised the lay-out of the device, wrapping the titanium target around the cylindrical plasma electrode, which is driven by a radio wave antenna. This arrangement gives the target a large surface area and allows many more fusion events to occur, increasing the number of neutrons produced. “The beauty of the coaxial design is that you can easily increase production by lengthening the cylinders”, says Leung. “You can also nest ion sources and targets inside each other to increase the output further”.

The plasma electrode generates a high proportion of single hydrogen isotopes, which create more neutrons than two- or three-atom molecules do. In contrast, existing commercial neutron generators produce only a small proportion of single isotopes.

Since the instrument is less than two inches in diameter, it could easily rest on a laboratory bench or descend into a bore-hole. Leung and colleagues are also optimistic that it will provide neutrons for an experimental therapy for brain cancer.