The Dual Energy X-ray technique employs two X-ray projection images of an object with X-ray energy spectra at a low X-ray energy and a high X-ray energy. The two energies are both high enough to penetrate all cargoes. The endpoint energies for low and high will be approximately 5-6 MeV and 8-9.5 MeV respectively. These energies are chosen such that pair production is the dominant energy loss mechanism for the high energy mode. By defining the ratio of the transmitted X-ray photon R = Thigh/Tlow it can be shown that there is a difference in the ratio that will permit the detection of materials that are significantly higher in atomic number than the low to mid atomic numbered elements that normally appear in the stream of commerce. This difference can be used to assist in the automatic detection of high atomic numbered materials. These materials might be a WMD or dirty bomb. When coupled with detectors that can observe the delayed signature of photon induced fission a confirmation of a WMD may be made. The use of the delayed photons and neutrons from Photofission can confirm the presence of Special Nuclear Materials (SNM). The energy required to induce fission in SNM by a photon is approximately 6 MeV with the maximum fission production rate from X-ray photons in the energy range of 12-15 MeV.
High Energy X-ray cargo screening is a mature technology that has proven its value in the detection of contraband material hidden within cargo including fully loaded sea containers. To date high energy screening has been largely applied to manifest verification and to drug detection. However, the dramatic change in world terrorism has altered the application. Now it is essential that weapons of mass destruction (WMD’s) be interdicted with incredibly high accuracy. The implication of a missed detection has gone from loss of revenue or the lowering of the street price of drugs to potentially stopping, at least for some significant time, most world commerce. Screening containers with high energy x-rays (~250+ mm of steel penetration) is capable of detecting all nuclear threats at a fraction of the strategically important mass. The screening operation can be automated so that no human decisions are required with very low false alarms. Finally, the goal of 100% inspection of cargo inbound to the United States from the twenty largest international ports is an achievable goal with hardware costs in the area of that already spent on airport security.