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Effect of energy on x-ray penetration and image quality

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.