A single short-pulse laser-driven neutron source could make searching for nuclear materials smuggled in shipping containers much easier, an international experiment has demonstrated. Detecting nuclear material using active interrogation previously had proved impractical because it required a giant, stationary facility or long measurement times with less powerful neutron generators. Researchers at Los Alamos National Laboratory used the short-pulse laser at the Trident facility to demonstrate a laser-generated neutron beam with novel characteristics that interrogated a closed container to confirm the presence and quantity of nuclear material. The successful experiment paves the way for creation of a tabletop-sized or truck-mounted neutron generator that could be installed at strategic locations worldwide to thwart smugglers trafficking in nuclear materials. During a recent experiment at Los Alamos National Laboratory’s Trident laser facility, a burst of laser energy 50 times greater than the worldwide output of electrical power slams into an extremely thin foil target, producing neutrons and proving that laser-driven neutrons can be used to detect and interdict smuggled nuclear materials. Images courtesy of Los Alamos National Laboratory. “We have demonstrated for the first time a novel approach for generating a record number of neutrons driven by a laser directed into a beam over a very small area that could provide proof positive of a large variety of nuclear items,” said Los Alamos physicist Andrea Favalli, an Italian researcher who led the experiment conducted in February. In the experiment, an extremely short and intense laser pulse was focused onto an ultrathin foil of deuterated plastic — a material in which hydrogen atoms within the foil were replaced with deuterium isotopes. When the tremendous laser burst — 50 times greater than the entire production of worldwide electrical power packed into a burst of energy lasting only one-half of one-thousandth of a billionth of a second (0.5 ps) — hit the foil target, it transferred much of its energy into the deuterium nuclei — the hydrogen isotope with one proton and one neutron. It then accelerated those particles into a beam traveling at about one-tenth the speed of light into a second metal target located 5 mm beyond the foil. When the fast-moving deuterons slammed into the secondary target, they created a blizzard of very fast moving neutrons (up to 40 billion, moving at a fraction of the speed of light) in a directed bunch lasting a billionth of a second, in a cone with an angle of about 30°. Neutrons are a good choice as a probe because they can penetrate most materials very easily. If they encounter some nuclear material (such as uranium or plutonium), they cause fission, and more neutrons are released. Some of the released neutrons are produced in seconds, enabling them to be measured after the initial neutron burst is finished. These “delayed neutrons” are a good measure of the presence of nuclear material because very few other materials produce them. An experimental apparatus used by scientists to prove that laser-driven neutrons can be used to stop nuclear smugglers. The Los Alamos researchers interrogated two containers, an empty one and a closed one containing nuclear material. After comparing the results, they discovered that the laser-driven-neutron-interrogation method confirmed not only the presence of nuclear material, but also the quantity. “This is something that has never been demonstrated before,” Favalli said. “Up until this experiment, nuclear material detection with a single laser-generated neutron pulse was merely an idea. Our team invented the concept, fabricated all the materials necessary for the experiment and confirmed our results within three weeks from start to finish. It is very exciting!” Neutron interrogation also could find use in materials and biological science applications, and the use of lasers could make the technique practical in other laboratories and universities. For more information, visit: www.lanl.gov