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Researchers use purified liquid xenon to search for mysterious dark matter particles

Researchers at SLAC use purified liquid xenon to search for mysterious dark matter particles

Xenon purification system at SLAC. The 2 central columns are every stuffed with nearly half a ton of charcoal, which is used to supply ultra-clean xenon for the LUX-ZEPLIN (LZ) darkish matter experiment. Credit score: Jacqueline Ramseyer Orrell/SLAC Nationwide Accelerator Laboratory

Sitting a mile under floor in an deserted gold mine in South Dakota is a big cylinder holding 10 tons of purified liquid xenon carefully watched by greater than 250 scientists around the globe. That tank of xenon is the center of the LUX-ZEPLIN (LZ) experiment, an effort to detect darkish matter—the mysterious invisible substance that makes up 85% of the matter within the universe.

“Folks have been looking for darkish matter for over 30 years, and nobody has had a convincing detection but,” stated Dan Akerib, professor of particle physics and astrophysics on the Division of Vitality’s (DOE) SLAC Nationwide Accelerator Laboratory. However with the assistance of scientists, engineers, and researchers across the globe, Akerib and his colleagues have made the LZ experiment one of the vital delicate particle detectors on the planet.

To succeed in that time, SLAC researchers constructed on their experience in working with liquid nobles—the liquid types of noble gases resembling xenon—together with advancing the applied sciences used to purify liquid nobles themselves and the methods for detecting uncommon darkish matter interactions inside these liquids. And, Akerib stated, what researchers have realized will support not solely the seek for darkish matter, but additionally different experiments looking for uncommon particle physics processes.

“These are actually profound mysteries of nature, and this confluence of understanding the very giant and really small on the similar time may be very thrilling,” Akerib stated. “It is potential we might be taught one thing utterly new about nature.”

In search of darkish matter deep underground

A present main candidate for darkish matter is weakly interacting large particles, or WIMPs. Nevertheless, because the acronym suggests, WIMPs barely work together with unusual matter, making them very tough to detect, even though there are theoretically lots of them passing by us on a regular basis.

To cope with that problem, the LZ experiment first went deep underground within the former Homestake gold mine, which is now the Sanford Underground Analysis Facility (SURF) in Lead, South Dakota. There, the experiment is nicely protected against the fixed bombardment of cosmic rays on Earth’s floor—a supply of background noise that might make it arduous to pick hard-to-find darkish matter.

Even then, discovering darkish matter requires a delicate detector. For that cause, scientists look to noble gases, that are additionally notoriously reluctant to react with something. This implies there are only a few choices for what might occur when a darkish matter particle, or WIMP, interacts with the atom of a noble gasoline, and due to this fact a decrease likelihood of scientists lacking an already tough-to-find interplay.

However which noble? Because it seems, “xenon is a very good noble for detecting darkish matter,” Akerib stated. Darkish matter interacts most strongly with nuclei, and the interplay turns into even stronger with the atomic mass of the atom, Akerib defined. For instance, xenon atoms are a bit of greater than thrice as heavy as argon atoms, however they’re anticipated to have interactions with darkish matter which might be greater than ten occasions as sturdy.

One other profit: “When you purify different contaminants out of the liquid xenon, it may be very radio quiet by itself,” Akerib stated. In different phrases, the pure radioactive decay of xenon is unlikely to get in the way in which of detecting the interactions between WIMPs and xenon atoms.

Simply the xenon, please

The trick, Akerib stated, is getting pure xenon, with out which all the advantages of the noble gasoline are moot. Nevertheless, purified noble gases aren’t available—the truth that they do not work together with a lot of something additionally means they’re typically fairly tough to separate from each other. And, “sadly you possibly can’t simply purchase a air purifier off the shelf that may purify noble gases,” Akerib stated.

Akerib and his colleagues at SLAC due to this fact had to determine a technique to purify the entire liquid xenon they wanted for the detector.

The most important contaminant in xenon is krypton, which is the subsequent lightest noble gasoline and has a radioactive isotope, which might masks the interactions researchers are literally on the lookout for. To forestall krypton from changing into the particle detector’s kryptonite, Akerib and his colleagues spent a number of years perfecting a xenon purifying approach utilizing what’s referred to as gasoline charcoal chromatography. The fundamental concept is to separate elements in a combination based mostly on their chemical properties because the combination is carried via some form of medium. Gasoline charcoal chromatography makes use of helium because the provider gasoline for the combination, and charcoal because the separation medium.

“You possibly can consider the helium as a gradual breeze via the charcoal,” Akerib defined. “Every xenon and krypton atom spends some fraction of time caught on the charcoal and a while unstuck. When the atoms are in an unstuck state, the helium breeze sweeps them down the column.” Noble gasoline atoms are much less sticky the smaller they’re, which suggests krypton is considerably much less sticky than the xenon, so it will get swept away by the non-sticky helium “breeze,” thus separating the xenon from the krypton. The researchers might then seize the krypton and throw it away after which get better the xenon, Akerib stated. “We did that for one thing like 200 cylinders of xenon gasoline—it was a fairly large marketing campaign.”

The LZ experiment is not the primary experiment SLAC has been concerned in an try and seek for new physics with xenon. The Enriched Xenon Observatory experiment (EXO-200), which ran from 2011 to 2018, remoted a particular xenon isotope to seek for a course of referred to as neutrinoless double beta decay. Outcomes from the experiment steered the method is unimaginably uncommon, however a brand new proposed search dubbed Subsequent EXO (nEXO) will proceed the search utilizing a detector just like LZ’s.

A distinct form of electrical grid

It doesn’t matter what noble liquid fills the detector, a classy detection system is essential if scientists ever hope to search out one thing like darkish matter. Above and under the tower of liquid xenon for the LZ experiment are giant, high-voltage grids that create electrical fields within the detector. If a darkish matter particle collides with a xenon atom and knocks a couple of electrons off, it should free some electrons from the atom and individually create a burst of sunshine that may be detected by picture detectors, defined Ryan Linehan, a latest Ph.D. graduate from SLAC’s LZ group who helped develop the excessive voltage grids. Electrical fields working via the detector then drive the free electrons up into a skinny layer of gasoline on the high of the cylinder the place they create a second gentle sign. “We will use that second sign along with the unique sign to be taught plenty of details about place, power, particle sort, and extra,” Linehan stated.

However these aren’t your common electrical grids—they’re carrying tens of hundreds of volts, so excessive that any microscopic bits of mud or particles on the wire grid may cause spontaneous reactions that rip electrons out of the wire itself, Linehan stated. “And people electrons can create indicators that look similar to the electrons that got here from the xenon,” thus masking the indicators they’re attempting to detect.

The researchers got here up with two predominant methods to attenuate the possibilities of getting false indicators from the grids, Linehan stated. First, the group used a chemical course of referred to as passivation to take away iron from the floor of the grid wires, leaving a chromium-rich floor that reduces the tendency of the wire to emit electrons. Second, to take away any mud particles, the researchers totally—and really fastidiously—sprayed the grids with deionized water instantly earlier than set up. “These processes collectively helped us get the grids to a state the place we might truly get clear information,” he stated.

The LZ group revealed their first outcomes on-line in early July, having pushed the seek for darkish matter additional than it is ever gone earlier than.

Linehan and Akerib stated they’re impressed by what LZ’s world collaboration has been in a position to accomplish. “Collectively, we’re studying one thing basic concerning the universe and the character of matter,” Akerib stated. “And we’re simply getting began.”

The LZ effort at SLAC is led by Akerib, along with Maria Elena Monzani, a lead scientist at SLAC and LZ deputy operations supervisor for computing and software program, and Thomas Shutt, who was the founding spokesperson of the LZ collaboration.

World group of scientists end assembling next-generation darkish matter detector

Supplied by SLAC Nationwide Accelerator Laboratory

Quotation: Researchers use purified liquid xenon to seek for mysterious darkish matter particles (2022, September 15) retrieved 15 September 2022 from

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