Like many people, physicist Matthew Szydagis has been amused by all those YouTube videos showing people banging on a bottle filled with water, causing it to quickly freeze in response to the blow. The trick is to supercool the water beforehand—that is, cool it below the freezing point without the water actually freezing. (Yes, it's possible.) But when he saw the same phenomenon depicted in Disney's 2013 animated film Frozen, he realized he might be able to exploit the effect to hunt for dark matter, that most elusive of substances.
The result is his so-called "snowball chamber," which relies on a newly discovered property of supercooled water. A professor at SUNY's University of Albany, Szydagis gave an overview of this research at the American Physical Society's annual April meeting, held earlier this month in Washington, DC. A draft paper can be found on arXiv, and a final version is being prepared for journal submission.
“All of my work is motivated by the search for dark matter, a form of matter we’re sure is out there because we can observe its indirect gravitational effects,” Szydagis said. “It makes up a significant fraction of the universe, but we have yet to uncover direct, conclusive and unambiguous evidence of it within the lab.” The detector could also be useful for detecting nuclear weapons in cargo, for understanding cloud formation, and for studying how certain mammals supercool their blood when they hibernate.
Dark matter is a mysterious substance that physicists believe comprises around 27 percent of the Universe. The most likely candidate is a class of particles known as "weakly interacting massive particles" (WIMPs), so named because they rarely interact with ordinary matter. There are numerous experiments around the world hunting for these elusive WIMPs, using several different methods. Their detectors are usually housed deep underground, the better to reduce interference from cosmic rays, which can mimic a dark matter signature in the data.
The detectors typically contain a target material (germanium, silicon crystals, or liquid xenon). Whenever an incoming dark matter particle collides with the nucleus of an atom in the target material, there should be a recoil effect, producing a tiny flash of light called a "scintillation." If the dark matter particle manages to transfer sufficient energy in that collision, the flash will be strong enough to be detected.
The snowball chamber complements existing particle detectors known as bubble chambers and cloud chambers. Like the YouTube water bottle trick, it relies on supercooling, which makes the water "metastable." It's the mirror image of superheating. "If you tried to boil water in a very clean microwave, [held in] a very pure, smooth mug, instead of boiling, it makes superheat," explained Szydagis. "This is the other way around." The water he used is very low in impurities like dust particulates, supercooled down to -4º F (-20º C).
Szydagis and his colleagues discovered that some types of incoming radiation—along with particles like neutrons—can deliver sufficient energy over a small enough distance to trigger a sudden phase transition, freezing the supercooled water. "It's more like snow than ice, because when supercooled water freezes, it's kind of disorganized," he said. "It's not like the clean formation you have in a normal freezing process." Since water has the strange property of expanding rather than contracting when it starts to freeze, the prototype detector incorporates extra space for expansion. This will avoid a reaction similar to a beer can exploding in the freezer, although supercooled water doesn't expand nearly as much when it freezes as does regular water.
When an incoming particle triggers the freezing, they just count the number of events.
The team alternated runs with no radioactive sources as a control, then measured how long the water stayed supercooled in the presence of a radioactive source. When an incoming particle triggered the freezing, they just counted the number of events, then melted it all down and reset the chamber for the next cycle. (They found the supercooled water also responds quite well to the "radioactive red" Fiestaware plates from the 1950s, which used orange uranium-based paint.) They captured the process on video with an iPhone at 100 frames per second.
That's how Szydagis et al. discovered a new property of supercooled water: it is sensitive to particles like neutrons but not sensitive to gamma rays. That new property is what makes the snowball chamber a potentially good dark matter detector—assuming the dark matter turns out to be some form of WIMP and not something more exotic like axions or a dense vacuum-borne superfluid. There are many different models, but if WIMPs turn out to be similar to neutrons, with a mass of around 1 GeV (giga-electron volts), the snowball chamber could possibly find them. "The advantage is potentially the ability to look for dark matter that's lighter than the standard paradigm by having a lower energy threshold for producing an event," said Szydagis.
How would they know if the freezing is triggered by a dark matter particle rather than a neutron? Neutrons will scatter and bounce around the chamber, and a WIMP would not. "You're not going to see a dark matter particle interact with any detector more than once, because it interacts so rarely," said Szydagis.
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