How to detect radioactive material remotely

March 21, 2016

Researchers have proposed a new way to detect radioactive material using two co-located laser beams that interact with elevated levels of oxygen ions near a gamma-ray emitting source (credit: Joshua Isaacs, et al./University of Maryland)

University of Maryland researchers have proposed a new technique to remotely detect the radioactive materials* in dirty bombs or other sources from up to a few hundred meters away based on ion density. The technique might be used to screen vehicles, suspicious packages, or cargo.

The researchers calculate that a low-power laser aimed near the radioactive material could free electrons from the oxygen ions. A second, high-power laser could energize the electrons and start a cascading breakdown of the air. When the breakdown process reaches a certain critical point, the high-power laser light is reflected back. The more radioactive material in the vicinity, the more quickly the critical point is reached.

“We calculate we could easily detect 10 milligrams [of cobalt-60] with a laser aimed within half a meter from an unshielded source, which is a fraction of what might go into a dirty bomb,” said Joshua Isaacs, first author on the paper and a graduate student working with University of Maryland physics and engineering professors Phillip Sprangle and Howard Milchberg. Lead could shield radioactive substances, but most ordinary materials like walls or glass do not stop gamma rays.

In 2004 British national Dhiren Barot was arrested for conspiring to commit a public nuisance by the use of radioactive materials, among other charges. Authorities claimed that Barot had researched the production of “dirty bombs,” and planned to detonate them in New York City, Washington DC, and other cities. A dirty bomb combines conventional explosives with radioactive material. Although Barot did not build the bombs, national security experts believe terrorists continue to be interested in such devices for terror plots.

The lasers themselves could be located up to a few hundred meters away from the radioactive source, Isaacs said, as long as line-of-sight was maintained and the air was not too turbulent or polluted with aerosols. He estimated that the entire device, when built, could be transported by truck through city streets or past shipping containers in ports. It could also help police or security officials detect radiation without being too close to a potentially dangerous gamma ray emitter.

The proposed remote radiation detection method has advantages over two other approaches. Terahertz radiation, proposed as a way to breakdown air in the vicinity of radioactive materials, requires complicated and costly equipment. A high-power infrared laser can strip electrons and break down the air, but the method requires the detector be located in the opposite direction of the laser, making it impractical as a mobile device.

The new method is described in a paper in the journal Physics of Plasmas, from AIP Publishing.

* Radioactive materials are routinely used at hospitals for diagnosing and treating diseases, at construction sites for inspecting welding seams, and in research facilities. Cobalt-60, for example, is used to sterilize medical equipment, produce radiation for cancer treatment, and preserve food, among many other applications. In 2013, thieves in Mexico stole a shipment of cobalt-60 pellets used in hospital radiotherapy machines, although the shipment was later recovered intact.

Cobalt-60 and many other radioactive elements emit highly energetic gamma rays when they decay. The gamma rays strip electrons from the molecules in the surrounding air, and the resulting free electrons lose energy and readily attach to oxygen molecules to create elevated levels of negatively charged oxygen ions around the radioactive materials.

Abstract of Remote Monostatic Detection of Radioactive Materials by Laser-induced Breakdown

This paper analyzes and evaluates a concept for remotely detecting the presence of radioactivity using electromagnetic signatures. The detection concept is based on the use of laser beams and the resulting electromagnetic signatures near the radioactive material. Free electrons, generated from ionizing radiation associated with the radioactive material, cascade down to low energies and attach to molecular oxygen. The resulting ion density depends on the level of radioactivity and can be readily photo-ionized by a low-intensity laser beam. This process provides a controllable source of seed electrons for the further collisional ionization (breakdown) of the air using a high-power, focused, CO2 laser pulse. When the air breakdown process saturates, the ionizing CO2 radiation reflects off the plasma region and can be detected. The time required for this to occur is a function of the level of radioactivity. This monostatic detection arrangement has the advantage that both the photo-ionizing and avalanche laser beams as well as the detector can be co-located.