Xenon: the porridge of particle physics | Local News

LEAD – When the Large Underground Xenon (LUX) experiment completed its 300-day run in 2016, it had failed in its mission to prove the existence of dark matter in the universe, but it proved to the world that xenon might be our best bet at finding the elusive particle.

“So what you can do is keep running this experiment for another 50 years, or you build one 50 times bigger and run it for a few years,” explained researcher Dr Markus Horn. Sanford Lab. “So obviously, since we all want to be alive when we get our Nobel Prize, we decided to build a bigger one.”

Where LUX used approx. 300 kg of liquid xenon to try to detect dark matter, the tank of LUX-ZEPLIN (LZ) will contain 10 tons of super-cooled fluid and will be 100 times more sensitive.

“(Detecting dark matter is) a game of statistics, so you have to have a huge amount of xenon atoms for possibly a dark matter particle (could hit) one of them,” Horn said. . “To increase the amount of xenon atoms, you build a bigger detector and liquefy the xenon so you have more xenon atoms in the same volume.”

In addition to dark matter detection experiments, xenon can also be used in light bulbs, as an insulating gas in the manufacture of microchips, and as an anesthetic.

Horn said xenon is a naturally occurring noble gas that makes up about 0.005% of the atmosphere. The xenon distillation process is complicated and requires several distillation cycles to achieve the desired level of purity. About 18 million liters of xenon are distilled each year.

Collecting, distilling, storing and shipping xenon can be an expensive business. In 2016, when Sanford Lab purchased the 1.5 million liters that will be used for LZ, the price was $5-6 per liter, and that number has only increased since. Horn said the price of xenon is currently around $10 to $15 per liter.

Although this is a significant investment, Horn said the value of the xenon used in LZ may actually increase as the experiment unfolds.

“Even if you have a super pure ship (like the tank that will hold the 10 tons of xenon used in LZ), you still outgas in whatever you put in it,” he said. “We don’t like any contaminants in our xenon, we want our xenon to be as pure as possible.”

For LZ, the xenon will be cooled to -160 degrees, turning it from a gas to a liquid and increasing its density. As the liquid xenon passes through the filtration and cooling system, it will be continuously purified maintaining its quality throughout the experiment.

Several properties of xenon make it ideal for detecting dark matter. Horn said it’s already a very “radio-pure” element, meaning it doesn’t produce internal background noise through radioactive decay like many other elements do. Combined with the protection against cosmic particles from the laboratory located almost a mile underground, xenon becomes a very suitable medium for the detection of dark matter.

“You have a mile of rock above you which helps you shield anything that comes down from the atmosphere, but the rock itself is kind of radioactive so it has an intrinsic background so you have to shield (the xenon) of that, therefore, you’re building more shielding around your detector on top of that, xenon itself is so heavy and so dense in its liquid form that it’s self-shielding,” explained Horn .

Once as many outer and inner particles as possible have been removed, anything left to interact with xenon atoms is likely to be a candidate for dark matter.

Horn said xenon is ideal for these types of detection because elements with lower atomic density are easier for the particles they seek to pass through undetected. Elements with higher atomic densities than xenon tend to be more radioactive, making it more difficult to detect the reaction they are looking for.

Another reason why xenon’s atomic weight makes it ideal for LZ is its similar mass to theorized models of dark matter itself. Although dark matter has yet to be observed, the gravitational effects of dark matter are seen throughout the galaxy.

“You have a spiral galaxy but you don’t see enough mass in the galaxy to explain why the outer stars are moving so fast because they should actually be flying away. So you need more mass in the spiral galaxy to keep these outer stars in line,” Horn explained.

Based on what is already known about how particle physics affects gravitational distortion, Horn said scientists are able to establish certain parameters for how they think dark matter is built. , including its approximate atomic weight.

“The various observations we have; we use to build a model of dark matter. … If you build those parameters into your model, you realize there’s only a certain range that makes the whole system work,” he said. “So you need them to be a bit heavier and a bit slower to really confine them to the galaxy.”

Horn said many models of dark matter list it as having an atomic weight somewhere in the range of 50 times the mass of a proton, which would indicate that a dark matter particle should be about the same size. than a xenon atom.

“If you then have two atoms of similar size or heavy, then the (reaction) works better.”

Another quality of xenon is that it produces a flash of visible light when ionized by another particle.

“We’re trying to see this dark matter particle hitting a xenon atom and when it hits it, it produces light,” Horn said.

As LZ’s lifeblood, xenon could be the key to detecting dark matter in the universe.

“In the grand scheme of things, that’s always how you create progress — you try to do something to push the boundaries of your knowledge a little bit further,” Horn said. “We physicists do this on a very remote scale, but it still ultimately has an effect on your daily life.”

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