A new method to identify radioactive material employs an IR laser beam to detect shielded material from a distance. The method, developed by physicists at the University of Maryland, improves upon current detection technologies that require close proximity to the radioactive material. The remote detection method uses a mid-IR laser to induce electron avalanche breakdown near the radioactive material. As the material emits decay particles, the particles strip electrons from nearby atoms in the air, creating a small number of free electrons that quickly attach to oxygen molecules. By focusing an IR laser beam into this area, the researchers detached these electrons from their oxygen molecules, seeding an avalanche-like, rapid increase in free electrons that is relatively easy to detect. With additional engineering, a new method to detect radioactive material, developed by physicists at the University of Maryland, could be scaled up to scan shipping containers at ports of entry, providing a powerful new tool for security applications. Courtesy of USDA/APHIS. Applying an intense IR laser field causes the free electrons caught in the beam to oscillate and collide with atoms nearby. When these collisions gather enough energy, they can rip more electrons away from the atoms. “A simple view of avalanche is that after one collision, you have two electrons,” said professor Howard Milchberg. “Then, this happens again and you have four. Then the whole thing cascades until you have full ionization, where all atoms in the system have at least one electron removed.” As the air in the laser’s path begins to ionize, it has a measurable effect on the IR light that is backscattered toward a detector. By tracking these changes, the researchers were able to determine when the air began to ionize and how long it took to reach full ionization. “An electron avalanche can start with a single seed electron,” Milchberg said. “Because the air near a radioactive source has some charged oxygen molecules — even outside a shielded container — it provides an opportunity to seed an avalanche by applying an intense laser field. This is not a new phenomenon, but we are the first to use an infrared laser to seed an avalanche breakdown for radiation detection.” The timing of 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 researcher Daniel Woodbury. “We’re using a relatively weak probe laser pulse, but it’s ‘chirped,’ meaning that shorter wavelengths pass though the avalanching air first, then longer ones. By measuring the spectral components of the infrared light that passes through versus what is reflected, we can determine when ionization starts and reaches its endpoint.” The researchers said that their method is highly specific and sensitive to the detection of radioactive material and that 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 the method to be scaled up and used to scan trucks and shipping containers at ports of entry, providing a new tool to detect concealed radioactive material and enhance security around 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,” researcher Robert 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.” Traditional methods of detecting radioactive material rely on a radioactive decay particle interacting directly with a detector. “The benefit of our method is that it is inherently a remote process,” Schwartz said. “With further development, it could detect radioactive material inside a box from the length of a football field.” The research was published in Science Advances (https://doi.org/10.1126/sciadv.aav6804).