Introduction

This project relates to an R&D program aimed at developing next-generation systems for detection of standard and improvised explosives as well as firearms concealed in cargo. Currently-employed detection techniques: Cargo security inspection at border crossings, seaports and aviation terminals is a complex issue and presents a formidable challenge. At present, the governing bodies responsible for cargo inspection at such crossings utilize several techniques: X-rays, physical search, decompression chambers and trace detection.

Fig.1 Inspection setup

The principal shortcomings of these techniques are: Non-automatic detection: With the exception of coherent X-ray scattering (which has penetration problems with massive and large size cargo), none of the above-mentioned explosives detection systems (EDS) is automatic; i.e., they rely heavily on operator skills and the process is usually time-consuming. Although X-ray-based technologies provide high spatial resolution and are useful for visual detection of guns and knives, they are not helpful in detecting liquid or plastic explosives, which do not possess characteristic densities or recognizable shapes. The task of screening cargo is rendered even harder because individual items are usually grouped into larger batches that are more difficult to screen. Consequently, in certain cases, the cargo shipment needs to be broken down into smaller units (break-bulk), in order to permit inspection using available screening technologies.

Automatic detection techniques: To address this problem, several fast-neutron-based techniques have been developed over the last two decades for the purpose of automatic cargo inspection. They include: Pulsed Fast Neutron Analysis (PFNA), Associated Particle Imaging (API) and Pulsed Fast Neutron Transmission Spectroscopy (PFNTS). These methods are indeed capable of automatically detecting a broad range of hazardous materials, by exploiting their specific elemental composition. Detection of small quantities: The challenge of cargo inspection is further compounded by the requirement of detecting ever-smaller quantities of explosives concealed in mailbags, pallets or aviation containers: for example, even small amounts could bring an aircraft down, if they are located in strategic places within the hold. However, concealing small amounts of thin explosive-sheet in mailbag cages, pallets or aviation containers is so easy that the hazard is very acute. Thus, in order to effectively counter this threat, the EDS needs to possess sufficient granularity (small voxel dimensions) in order to resolve compact, thin objects automatically. By their very nature, both PFNA and API are limited to granularities of several cm, as the uncertainty in neutron-interaction location (within the inspected object) grows with the depth of material traversed and depends on its composition. It is further restricted by the Time-of-Flight (TOF) resolution (1-2 ns). Thus, these methods are not well-suited for detecting small amounts of explosives in cargo. An example for a inspection setup is given in fig. 1.