Geiger counters are so last summer. Lasers can detect radioactive material too, y'know

Boffins show how to nab radioactive contraband, by looking for 'electron avalanches'

Lasers could be used to detect radioactive material secretly transported to and from ports one day, according to a group of physicists from the University of Maryland in the US.

The boffins describe a proof-of-concept method that can sniff out the particles emitted from radioactive decay using a technique known as an “electron avalanche”. The process is a little complicated and contains multiple steps.

It begins with shining an infrared laser beam near a radioactive source. Radioactive material like uranium and thorium decays over time by ejecting alpha particles from its nucleus. The alpha particles can ionise nearby air molecules by robbing them of their electrons. The researchers call these free electrons “seed electrons” and they quickly attach themselves to nearby oxygen molecules.

Here’s where the laser comes in. An infrared laser can ionise those seed electrons weakly attached to the oxygen molecules. The energy from the laser accelerates these electrons and they go on to collide with other molecules in the air, ionising them to create an avalanche of free electrons.

“A simple view of avalanche is that after one collision, you have two electrons,” said Howard Milchberg, co-author of the paper published this month in Science Advances and a professor of physics and electrical and computer engineering at the University of Maryland (UMD). “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.”

Now, the air in the laser’s path is ionised and affects the photons in the laser beam. Some of these photons are reflected and backscattered from colliding with the ionised air molecules to reach a silicon photodiode detector. The spectrum of the backscattered light is processed using software to reveal details about the density of the seed electrons, which in turn determines the level of radioactivity.

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“If there is no radioactivity, there are no seed electrons so no avalanche breakdown occurs,” Robert Schwartz, first author of the paper and a physics graduate from UMD, explained The Register. “When you introduce a radioactive source there are seed electrons so we can see the breakdown. It is a very sensitive technique. Sources as weak as 1 microgram of Cobalt-60 produces detectable levels of air ionization.”

At the moment the technique has only measured radioactive material from distances of up to one meter away, so it’s not really much of an improvement over current methods like using a Geiger counter yet.

But Schwartz reckoned that scaling up the process could increase that distance to ten meters or even 100 meters if a stronger laser pulse was used. “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."

He believed that such a system could be fitted inside a van maybe in ten years’ time or so. "Anywhere you can park a truck, you can deploy such a system. This would provide a very powerful tool to monitor activity at ports." ®

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