A modular 2 MeV Shaped Energy™ system complemented with two 220 keV scanning pencil beam systems is described. With the scanning x-ray pencil beams providing backscatter imaging, this multi-source system has excellent detection capabilities, low radiation dose, and a small footprint for inspecting air cargo containers in a crowded airport environment. Its design is based on a prototype inspection system with a 3.5 MeV Shaped Energy source and segmented transmission detector complemented with two 450 keV scanning pencil beam systems. This higher energy system was designed for very high density cargo such as fully loaded ISO shipping containers. The unique modular design provides maximum detection with minimum radiation because both the Shaped Energy system and the lower energy systems can be independently optimized. Moreover, the combination of high-resolution transmission coupled with backscatter provides increased probability of detecting threats. A novel configuration of these same modules could be applied to a proposed CT air cargo inspection system with true density determining capabilities. Sample images from the existing 3.5 MeV prototype system will be presented along with recent test results.
A new intermediate energy x-ray source is described which uses a cw electron linear accelerator created specifically for this application. This source has been installed in the hub of a hollow-spoked rotation wheel to form a scanning beam of x-rays. As cargo is transported through the inspection tunnel at speeds up to 6 inches per second it is raster-scanned by this beam to form digital images of the backscattered as well as the transmitted x-rays. The system will be described in detail, and sample images of a heavily loaded 8 foot wide ISO container will be presented. Environmental radiation due to the x-rays scattered from the cargo itself will be discussed in the context of the tradeoffs between penetration, spatial resolution, x-ray energy, and x-ray flux.
Extensive testing and simulations have been used to evaluate the transmission imaging capabilities of practical systems for inspecting cargo using x-ray sources ranged from 0.45 MeV to 10 MeV. In particular, the distance between the source and the cargo was constrained to be large enough to insure that the x-ray beam covered the entire cargo, the power of the electron beam was kept constant, and the scan speed of the inspection was fixed. The increased cost of the x-ray generating systems and facilities with high energy have been ignored. The results indicate that there is greater penetration for detecting radiographically thick objects behind steel as the energy is increased to about 5 MeV. Above 5 MeV the operational penetration actually decreases. Even if the energy is sufficient to penetrate a thick cargo, relatively large fluxes in the high energy portion of the x-ray spectrum are required to produce sufficient contrast for detecting moderate quantities of contraband. The relative fluxes required to produce equivalent image quality (contrast) are calculated for energies between 0.45 MeV and 10 MeV.