Physicists Develop New, Powerful Method to Detect Radioactive Material

At the University of Maryland (UMD), physicists have come up with a new, robust technique for detecting radioactive materials.

The novel method involves using an infrared laser beam to bring about a phenomenon called an electron avalanche breakdown close to the material and eventually detecting the protected material from a distance. The technique is better than present technologies that need close proximity to the radioactive material.

With further engineering developments, the technique can perhaps be scaled up and used for scanning shipping containers as well as at ports of entry, thus offering a new, robust tool for detecting dangerous and hidden radioactive materials. The research team detailed its proof-of-concept experiments in a research paper recently reported in the journal, Science Advances on March 22^nd, 2019.

Traditional detection methods rely on a radioactive decay particle interacting directly with a detector. All of these methods decline in sensitivity with distance. 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.

Robert Schwartz, Study Lead Author and Graduate Student, Department of Physics, University of Maryland.

Since decay particles are produced by radioactive material, electrons from nearby atoms are stripped or ionized by the particles, ultimately producing a tiny number of free electrons that rapidly adhere to oxygen molecules.

When Schwartz and his colleagues focused an infrared laser beam into this region, they were able to easily separate these electrons from their oxygen molecules, inducing an avalanche-like fast increase in free electrons that can be detected much more easily.

An electron avalanche can start with a single seed electron. 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. Electron avalanches were among the first demonstrations after the laser was invented. 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 laser’s infrared wavelength is important, because it can easily and specifically detach electrons from oxygen ions.

Howard Milchberg, Study Senior Author and Professor, Departments of Physics and Electrical and Computer Engineering, University of Maryland.

When a powerful infrared laser field is applied, the free electrons captured in the beam oscillate and strike nearby atoms. As soon as these collisions become sufficiently energetic, they can detach more numbers of electrons from the atoms.

“A simple view of avalanche is that after one collision, you have two electrons. 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,” stated Milchberg, who too has an appointment at the Institute for Research in Electronics and Applied Physics (IREAP) of UMD.

When ionization of the air in the laser’s path occurs, it causes a quantifiable impact on the infrared light backscattered, or reflected, toward a detector. Milchberg, Schwartz, and their colleagues monitored these changes and eventually established when the air started to ionize and the duration it takes to reach complete ionization.

The timing of the electron avalanche breakdown, or the ionization process, indicates the number of seed electrons that were available to start the avalanche. This estimate, consecutively, can give an indication of the amount of radioactive material existing in the target.

Timing of ionization is one of the most sensitive ways to detect initial electron density. 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.

Daniel Woodbury, Study Co-Author and Graduate Student, Department of Physics, University of Maryland.

When it comes to detecting radioactive materials, the investigators observed that their new technique is extremely specific and sensitive. In the absence of a laser pulse, radioactive material alone will not be able to trigger an electron avalanche. Likewise, without the seed electrons produced by the radioactive material, a laser pulse alone will not be able to promote an avalanche.

For now, the technique continues to be a proof-of-concept exercise; however, the investigators envision more engineering advancements that will lead to viable applications to improve security at ports of entry worldwide.

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. Anywhere you can park a truck, you can deploy such a system. This would provide a very powerful tool to monitor activity at ports.

Robert Schwartz, Study Lead Author and Graduate Student, Department of Physics, University of Maryland.

Apart from 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 graduate student.

The research paper titled, “Remote detection of radioactive material using mid-IR laser-driven avalanche breakdown,” by Robert Schwartz, Daniel Woodbury, Joshua Isaacs, Phillip Sprangle and Howard Milchberg, was published in the journal, Science Advances on March 22^nd, 2019.

The study was supported by the Defense Threat Reduction Agency (Award No. HDTRA11510002), the Air Force Office of Scientific Research (Award Nos. FA9550-16-10121 and FA9550-16-10259), the Office of Naval Research (Award No. N00014-17-1-2705), and the Department of Energy (Award No. DE-NA0003864).