Seeing the wood for the trees: could forests be used as neutrino detectors?

21 Feb 2024

Treemendous: trees could detect radio waves at frequencies of interest for tau neutrinos (courtesy: Shutterstock/Conny-Sjostrom)

Trees could shed light on some of the most cataclysmic events in the universe, according to a particle physicist at the University of Kansas in the US. Steven Prohira thinks these woody objects could function as radio antennas to spot neutrinos, with forests forming large detector arrays. Prohira argues that such detectors could be cheaper and easier to deploy than arrays of artificial antennas (arXiv: 2401.14454).

Neutrinos are difficult to observe directly because they interact so fleetingly with matter. Some detectors use thousands of photomultiplier tubes to spot the Cherenkov radiation produced on the rare occasions when a neutrino collides with ice or liquid water. Others, like the proposed Giant Radio Array for Neutrino Detection (GRAND), use radio antennas to detect the secondary particles created from collisions between high-energy neutrinos and particles in the atmosphere.

As neutrinos are so elusive, detectors need to cover a huge area. GRAND, for example, would need 20 giant radio antenna arrays around the world, each covering 10,000 square kilometres.“The big question with high-energy neutrino detectors is how to instrument a large enough volume to actually detect a measurable flux of neutrinos,” says Prohira, who hit upon the idea of using trees as radio antennas when thinking how to construct a large-scale instrument using infrastructure that already exists.

Wood you believe it

The idea of trees as radio antennae is not new, dating back to the early 1900s. But it only gained traction in the 1960s and 1970s during the Vietnam War when the US Army wrapped large magnetic induction coils around trees to improve the audibility of radio signals in the jungle. Its work showed that instrumented trees produced stronger and clearer signals than manufactured antennas. “The preliminary studies that I can find in the literature seemed quite promising,” Prohira notes.

Although trees work across a wide range of radio frequencies, Prohira thinks much more work will be needed to explore how they perform at frequencies of interest to tau neutrino detectors, which are higher than those normally used for radio communications. Researchers will also need to investigate the properties of the radio waves detected by trees and see how easy it is to reconstruct the neutrino signals from them. Other questions focus on the uniformity of the signals between different trees and the performance of various types and species of tree.READ MORE

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The beauty of trees, however, is that they are already in place. Prohira argues that the natural, often uniform spacing of trees in forests should make it feasible to create an array of tree antennas with a very similar layout to that proposed by the GRAND experiment. If such an array is possible, it could be used to study events such as gamma-ray bursts or collapsing stars.

“It is definitely worth exploring, because if it turns out to be relatively easy to instrument the trees and they work fairly well as antennas that could open up the potential to being able to instrument a large-scale array in an efficient array,” Prohira says. He warns, however, that there maybe “particular logistical challenges to a forest detector that make it unfeasible”.Prohira appeared on the Physics World Weekly podcast last year, when he explained how radio waves can be used to detect neutrinos: “Using radar to detect cosmic neutrinos in ice sheets…“.

Michael Allen is a science writer based in the UK.

FROM PHYSICSWORLD.COM    30/3/2024