Although dark matter makes up roughly 85 percent of all matter, it's still never been directly detected. Ever more sensitive systems are in the works, watching and waiting for wandering dark matter particles to interact with them, and now researchers from the State University of New York at Albany have developed a new dark matter detector using supercooled water.
Contrary to popular belief, it's possible to chill water colder than 0° C (32° F) without it freezing solid. To form ice crystals, water molecules need solid surfaces to cling to. Dirt or other impurities can kickstart that process, and the more ice crystals form, the more surface area there is for new crystals. Under carefully controlled conditions, pure water can stay liquid far below its usual freezing point – a process known as supercooling.
But it doesn't take much for supercooled water to suddenly freeze. Ice crystals can quickly form if it's disturbed in some way – if it's shaken or poured out for instance, or in the case of supercooled clouds, if it's buzzed by a passing plane. Certain particles could also trigger freezing when they collide with the water molecules, and perhaps that includes elusive dark matter particles.
In their tests, the researchers supercooled water down to -20° C (-4° F), then exposed it to different types of particles. Sure enough, they found that some particles did trigger freezing on the subatomic level.
"We managed to discover a new property of supercooled water," says Matthew Szydagis, lead author of the study. "To our great surprise, we found that some particles (neutrons) but not others (gamma rays) trigger freezing. Some particles like neutrons can even scatter multiple times within the water. We were able to show this not only with commercially available sources of particles, but also a Fiestaware 'radioactive red' plate with orange uranium-based paint from the 1950s."
This could make a big tub of supercooled water a decent dark matter detector, which the team calls a "snowball chamber." Dark matter is thought to be floating around us all the time, but it hardly ever interacts with normal matter. If it happens to interact with molecules of supercooled water, it could trigger freezing that looks spontaneous – provided all other disturbances are ruled out. That's a similar mechanism to current detectors that use liquid xenon or superfluid helium, but supercooled water could be a much cheaper and easier substance to make in large quantities.
In addition to helping in the search for dark matter, the researchers say their discovery could also potentially be used to detect nuclear weapons in cargo, increase understanding of cloud formation, and help shed light on how some hibernating mammals, such as the Arctic ground squirrel, keep their blood liquid even below its freezing point.
The researchers presented their findings this weekend at the 2019 American Physical Society meeting in Denver. The study has been submitted for publication.
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