Laser allows long-range detection of radioactive materials

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In a new breakthrough, a team of physicists from the United States has successfully demonstrated a new way to detect radioactive materials using carbon-dioxide lasers — from a distance. The potential applications of this innovative technique span national defence and emergency response, where rapid, accurate detection from safe distances is paramount.

At the core of the new technique is a phenomenon called avalanche breakdown. When some material undergoes radioactive decay, the charged particles it releases travel through the air and ionise it, i.e. separate its positive and negative charges and create a state of matter called plasma.

The negative charges, or electrons, can be accelerated to collide with other atoms and release even more electrons. This is avalanche breakdown. The researchers used a carbon-dioxide laser emitting long wave infrared radiation at a wavelength of 9.2 micrometres to accelerate the electrons, and were able to detect alpha particles from a radioactive source located 10 m away. This improves the range in previous experiments by a factor of 10. (An alpha particle is a bundle of two protons and two neutrons.)

The electrons that are accelerated in the first step of avalanche breakdown are called seeds. In this experiment, each seed electron resulted in distinct balls of microplasma in the air that generated a measurable optical backscatter. Crucially, the researchers were able to amplify this backscatter as it travelled back through the laser system, substantially improving detection sensitivity.

A compelling advantage of using long-wavelength lasers is their ability to drive electron avalanches, which in turn is crucial to detect very low concentrations of seeds. The laser’s longer wavelengths also reduce the likelihood of undesirable ionisation effects that could otherwise mask the detection signal.

In the experiment, the researchers also used fluorescence imaging to further illuminate the dynamics within the plasma created by the laser-induced avalanches, allowing them to characterise in detail the seed density profiles. Then they developed a mathematical model that accurately predicted the backscatter signals based on these seed densities, validating the technique.

The advance sets the stage to potentially expand avalanche-based laser detection techniques to identify gamma-ray radiation sources at greater stand-off distances. Gamma rays, which some radioactive nuclides like caesium-137 emit, travel much farther in air than alpha particles, reducing the density of the ionisation they produce. Despite this challenge, the researchers suggested that a Cs-137 source could be detected from about 100 m away provided the laser focusing optics are scaled up appropriately. This would greatly surpass current detection capabilities.

But extending the detection range further also introduces notable difficulties. Using longer focal lengths to reach distances of around 1 km or more would require even larger optics and higher laser energies due to diminishing signal strengths. At such extended distances, the laser backscatter method — the primary approach tested here — is limited because the signal could become saturated by background radiation and atmospheric interference.

The team’s findings were published in Physical Review Appliedon March 4.

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