This article appears in the Witness section of the spring 2020 issue of the New Humanist. Subscribe today.
Our eyes are fantastic detector systems. Light is focused onto an area at the back of the eyeball called the retina, where special cells are stimulated to convert the incoming light into electrical pulses. These signals are then transported via neuron cells to the brain, where they are computed to form an image.
Attempts are being made to replicate this kind of process in physical devices, to detect radiated particles or light that is outside the rainbow part of the spectrum. In other words, to make what is invisible to the human eye become visible. But as with most attempts at replicating nature, it’s hard for us to make something that performs so well. The challenge lies in finding materials that have this useful behaviour of absorbing radiation and stimulating electrical signals. It’s especially difficult when the species of radiation you are trying to track is a heavy, neutrally charged particle – a neutron – that whizzes straight through most materials without being absorbed.
Construction of a neutron semiconductor out of the few materials that do absorb neutrons, like hydrogen, helium, lithium and cadmium, hasn’t been successful so far, as the combination of materials needed to make an efficient device tends to be unstable – and therefore not practical. That is until very recently, when a team from Northwestern University and Argonne National Laboratory created a recipe for the world’s first stable neutron semiconductor – made of lithium-indium-phosphorous-selenium. The results were presented in the journal Nature, and show how this new material is so effective that it can detect very weak neutron signals in just a few billionths of a second.
This bonus feature of rapid detection has real-world applications. It is particularly useful for cross-border cargo security, where inspectors are on the look-out for signs of smuggled special nuclear materials – uranium and plutonium – with the tell-tale signature being neutron decay emission. The quicker the detection process, the less disruption to cargo flow, the more secure we are against the threat of these dangerous materials getting across borders undetected.