How to Peek Inside Containers and Detect Bombs? Try Lasers

By Sarah Zhang | March 2, 2012 7:56 am

spacing is important
The Raman spectrometer emits a laser beam.

What’s the News: Using a laser, a super-strong telescope, and some physics know-how, researchers say they have impressive power to look through solid barriers. Scientists have developed a technique to do so using Raman scattering, which is the change in energy of photons bouncing off a material. The technique could be used to detect hidden explosives or do geological analysis.

How the Heck? 

  • A clever set-up and powerful detector telescopes were the key advancements in this study. The physics of Raman scattering itself is nothing new—a Nobel Prize was awarded in 1930 for its discovery.
  • To understand what’s clever about the study though, we do have to get into how Raman scattering works. Photons usually bounce off a molecule unchanged in “elastic scattering“—same energy, same color. In a very small number of cases—and that’s why you need powerful telescopes to detect it—you have Raman scattering: The photon actually picks up or loses some energy from the molecule and scatters in a different color. Different materials cause different shifts in energy, which can be a signature used to detect explosives.
  • The researchers put sodium chlorate, a white powdery chemical, in white plastic bottles and blasted a laser at it. Now the bottle itself will scatter some light, but a small proportion of the light will also penetrate the bottle and be scattered by the sodium chlorate, and a tiny proportion of that light will have an energy change per Raman scattering.
  • Because the laser hits the container in a very small area, any light scattered off the container will be concentrated around one spot. Raman scattering from the sodium chlorate, however, goes out uniformly in all directions. The researchers found that by placing the light detectors offset from the laser, they are able to distinguish the scattering fingerprint of the container material from hidden material inside. No hiding in the dark.

What about Fluorescence? 

The more physics-inclined among you may be wondering how Raman scattering is different from fluorescence and if it’s possible to foil this detection method using fluorescence. Yes, and here’s the explanation:

  • Fluorescencing materials actually absorb the photon and re-emit a new photon of a different wavelength. Fluorescence is as if you throw a ball to me, and I catch it and throw a different ball back at you at a different speed. Raman scattering is as if you throw a ball at me, and it bounces off me at a different speed.
  • Imagine balls bounce off different people at different speeds. Now if you see a ball leaving a person but you don’t know whether that person threw it or if it just bounced off, you still can’t identify the person based on the speed of the ball.
  • The researchers tried this but with photons. They smeared some motor oil—a fluorescent material—on the opaque plastic bottle, and detecting Raman scattering was possible but more difficult. If the inner material itself is fluorescent though, the technique falls apart.

The Future Holds 

  • To detect explosives from expert bomb-makers, scientists will definitely need to figure out how to deal with the issue of fluorescence. (The ball is obviously a simplified analogy, so this isn’t necessarily impossible with photons.) It’s still a neat way of peering inside something solid—whether that’s a container of explosives or something buried under rock.
  • There’s also research into how Raman scattering can be used to detect trace gases of explosives in the air from roadside bombs. While it’s not a  new phenomenon, the potential of Raman scattering has not been fully explored.

Reference: Zachhuber et al. Stand-Off Spatial Offset Raman Spectroscopy for the Detection of Concealed Content in Distant Objects. Analytical Chemistry. 3 November 2011. DOI: 10.1021/ac2021008

Image courtesy of TU Vienna

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