Gravity Imaging: A New Look at Border Security

February 2011
With current methods to detect radioactive materials in sealed containers ineffective or easily thwarted, MITRE researchers propose the use of gravity imaging, which uses sensors to detect the dense mass of radioactive materials.
Ship at sea.

A terrorist attack involving radioactive materials would result in mass casualties, cause widespread property destruction, and poison large areas. The current methods used to detect radioactive materials being smuggled across our borders in sealed containers are sometimes ineffective or can be thwarted. Gravity imaging, by which sensors detect the dense mass of radioactive materials, may provide a quick and reliable method for securing our ports.

Every year about 20 million large shipping containers enter the United States by sea or land. One container could deliver the uranium for an atomic bomb to a terrorist inside our borders. How do we screen all shipping containers, both reliably and efficiently? The U.S. Department of Homeland Security has established the goal of 100 percent container screening by 2012, a Herculean task.

Radiation detectors have been installed in our shipping ports, but they still produce too many false alarms to be dependable. The government needs alternative methods to detect fissile materials smuggled inside sealed containers—methods that can guard our nation's safety without harming our prosperity. Since radioactive materials are unusually dense, examining containers with gravity imaging is an intriguing solution.

Nano-gravity: An Untapped Natural Resource

Gravity is the most obvious force of nature, evidenced every time an apple drops from a branch. But gravity is also a very weak force; it requires the huge mass of Earth to pull that small apple down to the ground. The gravitational attraction between two apples on a branch is too small for us to notice.

But not small enough to escape our measurement. New instruments called gravity gradiometers, which measure the rate of change in the gravitation field, are sensitive enough to measure the nano-tugs of gravity between everyday objects. MITRE and collaborating researchers are developing methods to use gravity gradiometers to determine the contents of shipping containers.

Gravitational forces cannot be shielded or canceled out as electric forces can be. This means that even if a uranium bomb lies inside a steel shipping container, the gravity of the dense uranium can still be sensed outside the container. The steel adds to the total gravity sensed, but does not cancel or mask the uranium's gravity. So if we scan the gravity field outside of the container, we can detect the presence of materials of extraordinary density among its contents. The force generated by gravitons (particles that carry the gravitational force) can be used to form images.

Experimental Proof of Concept

Researchers Barry Kirkendall, Yaoguo Li, and D. W. Oldenburg demonstrated the theoretical possibility of 3-D gravity imaging in 2007. Their concept was to send shipping containers through a gravity gate, similar to airport passengers passing through a metal detector. The gravity gate would collect gravitational data as the container passes through. The data would then be translated into an image of the three-dimensional mass density distribution inside the container.

But such imaging has yet to be performed in the real world with real gravity data. MITRE is conducting experiments to prove the gravity gate concept. Our first experiment involves running a perimeter scan around an 8-foot-by-10-foot tabletop with test masses representing a rectangular slice of a cargo container's contents. One test mass is a tungsten brick with about the same density as uranium and plutonium.

This experiment is designed to recover a two-dimensional picture or mass density distribution of the slice contents. We have verified that the 2-D design is viable using simulated sensor data. The resolution is not perfect, but adequate to detect and locate tungsten, lead, and fissile materials.

The Right Tools

The experiment is a collaborative effort. We will use a state-of-the-art gravity-sensing instrument, invented at the University of Maryland and being refined for commercial applications by Gedex, Inc., located near Toronto, Canada. The Gedex instrument uses the motions of two small torsion bars, cryogenically cooled to take advantage of superconducting effects. We hope to repeat the experiment later, this time using a laser-cooled, cold-atom fountain interferometer being developed at the Jet Propulsion Laboratory in Pasadena, Calif., by a team led by Nan Yu.

David Warme of Group W, who specializes in solving complex optimization problems, developed the image-recovery method. MITRE designed the experiment, and developed the forward model of the gravity sensing instrument.

By demonstrating that gravity imaging of containers is feasible using available instruments, we hope to spur the development of compact, inexpensive, lightweight gravity sensors with the exquisite sensitivity required, and the image recovery methods to interpret the data that these sensors provide. Both areas are nascent, wide open for research, and may prove crucial to maintaining the safety of our ports.

—by Lang Withers and Lynn Cooper

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