The shipment of cargo containers is a critical component of international trade and plays a fundamental role in the global economy. According to the U.S. Bureau of Customs and Border Protection, about 90% of the world's trade is transported in cargo containers, with almost half of incoming U.S. trade arriving by containers aboard ships. As terrorist organizations have increasingly turned to destroying economic infrastructure to make an impact on nations, the vulnerability of international shipping has come under scrutiny.
Researchers at the University of Maryland, College Park, have developed a technique to detect use of dirty bombs. Ionization of air surrounding crates is a sign of radioactive material. If authorities can detect this ionization outside of the crate, they can select specific containers for inspection, improve security and avoid delays in commerce.
The concept is described in a recent article in the Journal of Applied Physics co-authored by Victor Granatstein, a professor in the A. James Clark School of Engineering's Department of Electrical and Computer Engineering and the Institute for Research in Electronics and Applied Physics (IREAP), and Gregory S. Nusinovich, a research scientist in IREAP.
Gamma-ray emission from concealed radioactive material would pass through shipping container walls and increase ionization of the surrounding air. If a device pointed a high-power, short-wavelength electromagnetic wave at the container, the breakdown in the air would be detectable.
"We would create a spark in the air at the focus of an antenna driven by a high power, coherent, electromagnetic-wave generator, such as a gyrotron or laser," explains Granatstein. "The formation of the spark would be facilitated if gamma radiation from the concealed radioactive material were present."
Detection of radioactive material concealed in shipping containers is important to the early prevention of "dirty" bomb construction. There is currently a strong interest in determining whether a container ship approaching the U.S. is transporting radioactive material that might be used in the construction of a "dirty" bomb. Since there is a very large number of container ships approaching the U.S. every day, this determination needs to be made without stopping and boarding each ship (e.g., from a helicopter flying overhead). This would require a detection system with a range of tens of meters and with adequate sensitivity to detect small but troubling amounts of radioactive material.
There is at present no detection scheme that can easily satisfy these requirements. Several approaches have been suggested and are being explored, including the method proposed by Granatstein and Nusinovich. Such an effect then could be detected and evaluated.
Granatstein and Nusinovich are currently improving analysis of air breakdown in the presence of both gamma radiation and a high power-density electromagnetic wave in the spectral range between millimeter-waves and infrared. They are also developing a 0.67 terahertz gyrotron that will be capable of producing 300 kW, 10 microsecond pulses to be used in the experimental evaluation of the range and the sensitivity of their detection scheme. The researchers expect the gyrotron to be operating by the end of 2011.
"It is not yet clear whether this approach to detection is practical," says Granatstein. "But it is worth pursuing, since it might impact an important need related to national security."
The Office of Naval Research is supporting this study under a five-year research grant that began in September 2009.