A team of Russian scientists has designed a device that they claim would provide a quick and easy way of detecting explosives, drugs and other illegal substances hidden inside enclosed spaces such as trucks and shipping containers. The device would provide a 3D image and information on the chemical composition of the material inside a vehicle by bombarding that material with neutrons, allowing authorities to combat criminals and terrorists without the need for expensive and time consuming manual inspections.
The design, produced by researchers at the Institute of Solid State Physics , the Joint Institute for Nuclear Research, and the company Aspect, all based near Moscow, involves taking deuterons (nuclei comprising one neutron and one proton) from high-pressure deuterium gas and accelerating them in a sapphire tube. These deuterons collide with a tritium target, producing high-energy neutrons that leave the tube, penetrate any casing and interact with the nuclei of the material under inspection. Gamma rays produced in this interaction are then detected by an array of fibre optic scintillator detectors.
Intersecting trajectories
This device, say the researchers, would create a 3D image of a container’s contents, with a resolution of a few centimetres, and identify its various chemical components — allowing inspectors, for example, to identify heroin or the explosive trinitrotoluene within bags of sugar. The image is created by intersecting the trajectory of each emitted gamma ray with that of its associated neutron. Although the direction and speed of each neutron cannot be measured directly, these data are worked out by detecting the alpha particles that are generated alongside each neutron and which travel away from the tritium target in the opposite direction. The chemical composition, meanwhile, is revealed by the energy of the emitted gamma rays, since this energy depends precisely on which element was involved in the interaction.
Such neutron-based detectors have been built by other research groups in the past. They have the disadvantage that inspectors must keep their distance during irradiation. Furthermore, according to Dennis Slaughter of the Lawrence Livermore National Laboratory in the US, who has developed such a device, these kinds of detector sometimes struggle to distinguish between explosives and common materials such as wool, leather and many hydrocarbons, because the relative proportions of hydrogen, carbon and oxygen inside each are quite similar.
Crystalline structure
Nikolai Klassen of the Institute of Solid State Physics, a member of the Russian team, acknowledges Slaughter’s criticisms but he points out that the new device, in addition to identifying the chemical composition of any substance, would also be able to identify its crystalline structure via neutron scattering. He points out that the characteristics of two different crystalline forms of the same chemical can often be very different.
Klassen also points to a number of advantages that he says his group’s device would have over existing instruments. One of these is compactness, since sapphire can accelerate deuterons to a given energy in a shorter distance than the more commonly used quartz — resulting in a total volume of some 3–4 m3, allowing the device to be put in a medium-sized truck and taken to wherever it is needed. Klassen also maintains that his group’s device would require less power and operate more quickly, since its array of scintillation detectors would require less time to scan an object than would the single row of detectors employed up to now.
Unfortunately, the Russian researchers need more funding to realize their design. They have constructed a prototype version, which fired gamma rays instead of neutrons, but they have still to perfect the technologies needed to build the sapphire tube and scintillation array.
