EACH year, millions of cargo containers from around the world are shipped to U.S. ports, holding in their metal “bellies” a variety of essential goods such as food and textiles. While this method of importing freight is necessary for the nation’s livelihood, monitoring the contents in such a vast volume of containers poses a challenge to homeland security experts. The events of September 11, 2001, brought transportation security issues into the limelight, including the need to ensure that cargo containers coming into U.S. ports are not carrying clandestine fissile materials.
One of the difficulties scientists face in developing detection technologies for homeland security is how to accurately and efficiently identify hidden nuclear materials without significantly slowing commerce or, worse, bringing it to a halt. With funding from a grant through the University of California (UC) Office of the President, Livermore physicist Marie-Anne Descalle and UC Berkeley collaborators are studying the effectiveness of a radiographic imaging technique for use as a primary screening tool to rapidly scan cargo shipments. “To be effective,” says Descalle, “the technology must be able to identify high-atomic-number elements [high-Z, where Z is greater than 72] within a minute or less.”
In previous modeling studies performed by the UC Berkeley collaborators, Monte Carlo simulations showed that the proposed radiographic method has the potential to identify small quantities—0.1 kilograms—of uranium and plutonium within containers filled with homogeneous cargo. The method, which measures high-energy photons transmitted through a material, could potentially detect other high-Z materials used as shielding for particular objects. Screening authorities applying the technique could greatly minimize the number of suspect containers, identify possible materials of interest, and then permit definitive searches as warranted.
Narrowing Down the Suspects
A more efficient approach for identifying illicit materials is being studied by the Livermore–UC Berkeley team. This method uses a new photon-based radiographic technique to rapidly scan each container, which would allow port authorities to narrow the number of suspect containers in a short time and thus facilitate the flow of commerce. Stanley Prussin, a professor of nuclear engineering who leads the UC Berkeley work, says, “Our proposed primary screening process has the potential to rapidly scan containers with a high probability that 99.9 percent of the containers will not require further inspection.” Containers that warrant closer examination would undergo a secondary screening during which authorities would either physically inspect the container or use other radiation detection techniques to definitively analyze the contents.
The team’s research builds on a previous Livermore–UC Berkeley collaborative project known as the “nuclear car wash.” (See S&TR, May 2004, Screening Cargo Containers to Remove a Terrorist Threat.) In this detection scheme, a container-laden truck passes over an underground generator that propagates neutrons through the cargo. Similar to driving through a car wash, the truck then proceeds through an array of large plastic scintillators that detect high-energy delayed gamma rays emitted when neutrons interact with fissile material. One concern surrounding this method is that the neutron irradiation would induce some radioactivity. According to Prussin, “Our new approach uses photons that are unlikely to produce radioactivity or would induce such low-intensity radioactivity that it would be negligible.”
Small Target, Big Container
The team’s detection method uses a photon source to direct a beam of high-energy bremsstrahlung photons (x rays) through the side of a container. Depending on the cargo, the photons will either pass through the container relatively unchanged, be completely absorbed by the material inside, or undergo Compton scattering. In the last scenario, lower energy photons are produced when high-energy photons collide with atoms and then lose energy as they “bounce” off the atoms in various directions from their original trajectory.
On the opposite side of the container is a detector with an array of pixelated scintillators that measure all photons emerging from the container. “The intensity, and to some extent the energy spectrum, of the detected photons will be quite different if a material of interest is present in a container,” says Prussin. “Those measurements will show us whether a container holds something of concern.”
Each photon that reaches the detector produces a signal on an individual pixel. The spatial distribution of the material inside the container as well as the energies of the photons are determined from these signals. Ultimately, researchers plan to place two pixelated detectors at different angles to the container, one at the side and the other at the top, to create a more detailed radiograph that will allow them to see an object of interest and determine its dimensions. They will then use the dimensions and the estimated intensity of the source photons that have passed through the container without any interaction to derive the object’s linear attenuation coefficient (a function of material density and atomic number). “The challenge is how to distinguish these photons from photons of the same energy that arrive at the detector after having been scattered one or more times,” says Prussin.
Proving the Theory
Simulations helped the team troubleshoot issues related to detector design. For example, they assessed the effectiveness of materials that could be used to shield each pixel within the detector array. Without shielding, photons coming into the detector would bounce between pixels, which would affect the team’s ability to distinguish where the photons originated. “The simulations helped us identify which materials would provide the best shielding and how much shielding would be necessary,” says Descalle. “We determined 1 millimeter of tungsten between each pixel would provide the most effective shielding.” A prototype detector is now being built that consists of 64 pixels with individual pixel sizes of 0.6 square centimeters.
With the detector design complete, the team is focused on simulating how the method will perform under less than ideal conditions. “We are now modeling the physics of the photons interacting with the cargo and the detector material,” says Descalle. “Using simulations, we can model spectra that resemble the energy spectra we expect to see in an actual detector.” Additional simulations will verify whether obtaining more images of the container at different angles would improve accuracy. The set of images could be combined
using reconstruction algorithms to better identify high-Z materials in three dimensions and approximate linear attenuation coefficients.
The success of the project thus far is very much a team effort. “We are making the best use of the expertise inside the Laboratory and the flexibility of academia to pursue an idea that is important to the public interest,” says Prussin. With a little time, hard work, and high-performance computing power, the nation may soon have a more effective mechanism for revealing what is hidden inside the dark recesses of cargo containers.
Key Words: Cargo Advanced Automated Radiography System, cargo screening, fissile material, fission, Monte Carlo modeling, photon, radiation detection, scintillator.
For further information contact Marie-Anne Descalle (925) 423-9660 (email@example.com).
Lawrence Livermore National Laboratory
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