Neutron Screening for Airborne Cargo.

There is currently a massive political and scientific focus on the need to develop new explosive and contraband detection technologies. Recent terrorist attacks, especially those of 11 September 2001, have identified vulnerabilities and stimulated activities to improve security in aviation and other transport industries.At present, the detection of concealed contraband in air cargo is based mainly on the use of X-ray imaging. The production and detection technologies are mature, while the machines are relatively inexpensive and simple to operate. However, X-rays suffer from the disadvantage of having a small interaction probability with the low electron density elements from which organic materials are composed. Most contraband, such as explosives and illicit drugs, are organic and consequently have undistinguishable X-ray absorption or incoherent scattering characteristics. In addition, although some X-ray scanners can produce a sharp image as well as a density-dependent shading of the interrogated object, explosives and illicit drugs and can be moulded or packed into any form. This makes their detection very difficult through simple shape recognition. Such limitations have stimulated the development of alternative methods, including those based on neutrons. Although there are many types of explosives and illicit drugs, most are easily distinguished from common materials in one or more elemental features. Explosives are distinguished by relatively high proportions of nitrogen and oxygen and relatively low proportions of carbon and hydrogen whilst illicit drugs are generally rich in hydrogen and carbon and poor in nitrogen and oxygen.Since neutrons are unaffected by electromagnetic forces they can penetrate deeply into matter, interacting only with nuclei. When a neutron interacts, the nuclide reacts in a unique way that depends on the incident energy of the neutron and the species of the target nuclide. Most nuclides exhibit distinctive structure in their scattering cross sections which enhances the probability of certain neutron interactions occurring at specific incident neutron energies and scattering angles. In particular, the principal elemental constituents of narcotic and explosive substances differ strongly from one-another in their interactions with neutrons and can therefore be characterized via these differences.We will build a build a cut down prototype of a neutron screening device that will integrate radiographic information from the generator gamma flash and the transmitted neutrons with the activation signatures resulting from neutron absorption in the scanned material. This fusion of information will provide image information as well as details of the elemental composition of the scanned cargo.