Detecting shielded uranium in the field

Security personnel use several methods to detect suspect materials such as explosives and dangerous chemicals (see Physics Today, June 2014, page 62 ). But uranium—especially the highly enriched weapons-grade version—can be difficult to spot because it is easily shielded by a relatively small amount of lead, which absorbs the fissile material’s telltale emissions. However, neutrons directed at a suspect object can readily penetrate the lead shielding and produce enough fission neutrons and gamma rays to be detected. For field operations, small and light neutron sources are needed. Jennifer Ellsworth and a team of researchers at Lawrence Livermore National Laboratory (LLNL) have developed a suitcase-sized prototype neutron source that they say can be shrunk to a 5-kg, 20-W, lunchbox-sized device that could be easily transported to inspect a suspicious item. Central to the device is an array of 40 etched iridium tips (in the assembly at left in the figure). The 100-nm-radius tips concentrate the electric field from a compact 10- to 20-kV power supply; the concentrated field rips apart and ionizes the molecular deuterium gas near the tips. Impinging on a tritiated-titanium target (at right) that is biased to −100 kV, a pulsed D⁺ current of 120 nA in the prototype generates bursts of 10⁷ 14-MeV neutrons, a flux that is an order of magnitude higher than was previously available in field-ionization neutron sources. The team’s prototype has produced D⁺ currents up to 500 nA while consuming less than 10 W. The LLNL team is bringing the size and weight down and the output up, which will allow neutron analysis to join portable x-ray devices in the field of nondestructive materials interrogation. (J. Ellsworth et al., J. Appl. Phys. 116, 193301, 2014 doi:10.1063/1.4901830 .)