Spotting Distant Threats

Physicists at the University of Maryland have developed a new method to detect radioactive material—one that could be scaled up and used to scan trucks and shipping containers at ports of entry, providing a powerful tool to fight smuggling of these dangerous materials.

By using an infrared laser beam to induce a phenomenon known as an electron avalanche breakdown near the material, the new technique is able to detect shielded material from a distance. To counter the threat of nuclear terrorism, federal law requires all cargo containers arriving at U.S. ports to be screened for radioactive threats; this method improves upon current technologies that require close proximity to the radioactive material.

The researchers described their proof-of-concept experiments in a research paper published Friday in the journal Science Advances.

“Traditional detection methods rely on a radioactive decay particle interacting directly with a detector. All of these methods decline in sensitivity with distance,” said Robert Schwartz, a physics doctoral student at UMD and the lead author of the research paper. “The benefit of our method is that it is inherently a remote process. With further development, it could detect radioactive material inside a box from the length of a football field.”

As radioactive material decays, it emits particles that strip electrons from—or ionize—nearby atoms in the air, creating a small number of free electrons that quickly attach to oxygen molecules. By focusing an infrared laser beam into this area, Schwartz and his colleagues easily detached these electrons from their oxygen molecules, sparking an avalanche-like rapid increase in free electrons that is relatively easy to detect.  

“This is not a new phenomenon, but we are the first to use an infrared laser to seed an avalanche breakdown for radiation detection,” said Howard Milchberg, a professor of physics and electrical and computer engineering at UMD and senior author of the research paper. “The laser’s infrared wavelength is important, because it can easily and specifically detach electrons from oxygen ions,” added Milchberg, who also has an appointment at UMD’s Institute for Research in Electronics and Applied Physics (IREAP).

As the air in the laser’s path begins to ionize, it has a measurable effect on the infrared light reflected, or backscattered, toward a detector. By tracking these changes, Schwartz, Milchberg and their colleagues were able to determine when the air began to ionize and how long it took to reach full ionization.

The timing of the ionization process, or the electron avalanche breakdown, gives the researchers an indication of how many seed electrons were available to begin the avalanche. This estimate, in turn, can indicate how much radioactive material is present in the target.

“Timing of ionization is one of the most sensitive ways to detect initial electron density,” said Daniel Woodbury, a physics doctoral student at UMD and a co-author of the research paper.

The researchers say that their method is highly specific and sensitive to the detection of radioactive material. Without a laser pulse, radioactive material alone will not induce an electron avalanche. Similarly, a laser pulse alone will not induce an avalanche, without the seed electrons created by the radioactive material.

While the method remains a proof-of-concept exercise for now, the researchers envision further engineering developments that they hope will enable practical applications to enhance security at ports of entry across the globe.

“Right now we’re working with a lab-sized laser, but in 10 years or so, engineers may be able to fit a system like this inside a van,” Schwartz said. “Anywhere you can park a truck, you can deploy such a system. This would provide a very powerful tool to monitor activity at ports.”

In addition to Milchberg, Schwartz and Woodbury, UMD-affiliated co-authors of the research paper include Phillip Sprangle, professor of physics and electrical and computer engineering with an appointment at IREAP, and Joshua Isaacs, a physics doctoral student.