Ka-Ngo Leung and his colleagues in Berkeley Lab’s Accelerator and Fusion Research Division (AFRD) have devised a series of neutron generators small enough to descend into a borehole, provide neutrons for brain-cancer therapy or look inside airport luggage.
The performance of the group’s compact generator is said to match the largest neutron sources now in use buts at a fraction of the cost. For this reason, said William Barletta, AFRD division director, ‘neutron tubes offer a means of providing a new generation of instruments on university campuses to provide a growing, vigorous user base for national user facilities.’
Although isotopic sources release small amounts of neutrons by radioactive decay, many research programs rely on fission reactors or on high-energy particle accelerators such as Los Alamos’s LANSCE, which uses proton beams to break up heavy nuclei in a target.
Neutrons are also freed in fusion reactions. Commercial ‘neutron tubes,’ are miniature, low-energy accelerators that produce neutrons by hitting a metal target with deuterium or tritium ions.
The fusion reaction occurs between the deuterium in the beam and deuterium and tritium in the target (D-D or D-T reactions). Commercial tubes, including electronics, typically cost $100,000; they cease to function once their target is depleted of hydrogen isotopes.
‘Our interest in neutron generators began about five years ago, when we worked with the Earth Sciences and Engineering divisions to build a downhole logging device,’ said Leung. To help study the geology of Yucca Mountain, the researchers developed a device that increased neutron output over commercial sources hundreds of times, but still fit inside a two-inch borehole.
Driven by a radio-frequency antenna, the miniaturised plasma source reportedly produces a high current of deuterium ions, more than 90 percent of them single atoms, much more likely to produce neutrons in the target than two – or three-atom molecules. Beams in commercial sources are typically only 20 percent monoatomic.
‘After we made the logging device, we asked ourselves if a larger tube could be made for boron neutron capture therapy, BNCT,’ said Leung. BNCT is an experimental treatment for an inoperable form of brain cancer that uses energetic neutrons from reactors or accelerators, but Leung’s calculations indicated that with a D-T reaction, a compact neutron generator could produce the requisite amount of neutrons at the right energy, around 10,000 electron volts.
The new design included a new kind of target containing no hydrogen isotopes. All the deuterium and tritium is said to come from the beam itself, hitting a thin layer of titanium bonded to a layer of copper that is pierced with water-cooling channels. Since the deuterium and tritium is continually loaded onto the titanium, the target cannot be depleted.
To get sufficient flux for BNCT, Leung said, ‘we needed D-T reactions, which are more efficient in neutron production than D-D. But the great majority of research programs and applications want much lower energies – so-called ‘thermal’ neutrons that are easily produced by D-D. One advantage is that deuterium is stable.’
Leung’s group found a way to multiply the neutron output of a compact source. Instead of a generator shaped like a TV picture tube – with a single beam of ions from a source striking a target plate – they built a coaxial cylinder, in which a rod-shaped ion source emits beams radially along its length, striking a large target wrapped around it.
‘The beauty of the coaxial design is that you can easily increase production by lengthening the cylinders,’ Leung said. ‘You can also nest ion sources and targets inside one another.’ The result: tens of trillions of neutrons per second.