Scatter in cargo radiography.

As a complement to passive detection systems, radiographic inspection of cargo is an increasingly important tool for homeland security because it has the potential to detect highly attenuating objects associated with special nuclear material or surrounding shielding, in addition to screening for items such as drugs or contraband. Radiographic detection of such threat objects relies on high image contrast between regions of different density and atomic number (Z). Threat detection is affected by scatter of the interrogating beam in the cargo, the radiographic system itself, and the surrounding environment, which degrades image contrast. Here, we estimate the extent to which scatter plays a role in radiographic imaging of cargo containers. Stochastic transport simulations were performed to determine the details of the radiography equipment and surrounding environment, which are important in reproducing measured data and to investigate scatter magnitudes for typical cargo. We find that scatter plays a stronger role in cargo radiography than in typical medical imaging scenarios, even for low-density cargo, with scatter-to-primary ratios ranging from 0.14 for very low density cargo, to between 0.20 and 0.40 for typical cargo, and higher yet for dense cargo.