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

SLAC’s xenon purification system. Each of the two central columns is filled with nearly half a ton of coal, which is used to produce ultra-pure xenon for the LUX-ZEPLIN (LZ) dark matter experiment. Credit: Jacqueline Ramsir Orel/SLAC National Accelerator Laboratory

Located a mile underground in an abandoned gold mine in South Dakota, a giant cylinder contains 10 tons of purified liquid xenon, and is closely watched by more than 250 scientists around the world. This xenon tank is the heart of LUX-ZEPLIN (LZ) An attempt to discover dark matter – the mysterious, invisible substance that makes up 85% of the matter in the universe.

“People have been looking for dark matter For more than 30 years, no one has had a convincing discovery yet, said Dan Akrip, a professor of particle physics and astrophysics at the Department of Energy’s (DOE) SLAC National Accelerator Laboratory. But with the help of scientists, engineers, and researchers around the world, Akrip and his colleagues have made the LZ experiment one of the most sensitive particle detectors on the planet.

To get to this point, SLAC researchers built on their experience working with liquid nobles – the liquid forms of noble gases such as xenon– Including the development of techniques used to purify the liquid nobles themselves and systems for detecting rare darkness issue interactions within those fluids. What the researchers have learned will help not only the search for dark matter, Akrip said, but also other experiments looking for rare particle physics processes.

“These are really deep mysteries of nature, and this confluence of understanding what is so big and so small at the same time is very exciting,” Akrip said. “It is possible that we can learn something completely new about nature.”






The search for dark matter in the depths of the earth

The current leading candidate for dark matter is a weak interaction between massive particles, or WIMPs. However, as the acronym suggests, WIMPs hardly interact with ordinary matter, which makes them very difficult to detect, despite the fact that many of them pass by us in theory all the time.

To tackle this challenge, the LZ experiment first went deep underground at the former Homestake gold mine, which is now the Sanford Underground Research Facility (SURF) in Leed, South Dakota. There, the experiment is well shielded from the constant bombardment of cosmic rays on Earth’s surface – a source of background noise that can make hard-to-find dark matter hard to pick up.

However, finding dark matter requires a sensitive detector. For this reason, scientists look to the noble gases, which are also notorious for reluctance to react with anything. This means there are very few options for what might happen when a dark matter particle, or WIMP, interacts with a noble gas atom, so scientists have less chance of missing out on an already hard-to-find interaction.

But which noble? As it turns out, “Xenon is an especially good noble for detecting dark matter,” Akrip said. Akrip explained that dark matter interacts strongly with nuclei, and the interaction becomes stronger with the atomic mass of an atom. For example, xenon atoms are slightly more than three times heavier than argon atoms, but they are expected to have interactions with dark matter ten times stronger.

Another benefit: “Once the other contaminants are filtered out of the liquid xenon, it will be very quiet by itself,” Akrip said. In other words, the natural radioactive decay of xenon is unlikely to get in the way of detecting interactions between WIMPs and xenon atoms.

Only xenon, please

The trick, Akrib said, is to get pure xenon, without which all the benefits of the noble gas are moot. However, purified noble gases are not readily available – and the fact that they don’t react with much of anything also means that they are generally very difficult to separate from one another. And “Unfortunately, you can’t buy an air purifier off the shelf to purify water noble gasesAkrib said.

So Akrip and his colleagues at SLAC had to figure out a way to purify all the liquid xenon they needed for the reagent.

The biggest contaminant in xenon is krypton, which is the second lightest noble gas and has a radioactive isotope, which could mask the reactions researchers are already looking for. To prevent krypton from becoming a particle detector kryptonite, Akrip and his colleagues spent several years perfecting a technique for purifying xenon using what’s called gaseous coal chromatography. The basic idea is to separate the components in a mixture based on chemical properties Where the mixture is transported through some kind of medium. Gaseous coal chromatography uses helium as the carrier gas for the mixture, and coal as the separation medium.

“You can think of helium as a steady breeze through coal,” Akrib explained. “Each xenon and krypton atom spends part of the time stuck to the coal and some time unbound. When the atoms are in the unstuck state, they are swept by a helium breeze down the shaft.” The atoms of the noble gases are less viscous the smaller they are, which means that krypton is somewhat less viscous than xenon, so it is removed by the non-stick helium “breeze”, thus separating the xenon from the krypton. The researchers could then capture the krypton, throw it away, and then retrieve the xenon, Akrip said. “We did that for something like 200 cylinders of xenon gas – it was a pretty big drive.”

The LZ experiment is not the first that SLAC has been involved in trying to research new physics using xenon. The Enriched Xenon Observatory Experiment (EXO-200), which ran from 2011 to 2018, isolated a specific xenon isotope to search for a process called neutrinoless double-beta decay. The results of the experiment indicated that the process is unimaginably rare, but the new proposed research called Next EXO (nEXO) will continue to search with a detector similar to LZ.

Different type of electrical network

No matter what noble liquid fills the detector, a complex detection system is critical if scientists hope to find something like dark matter. above and below tower liquid xenon For the LZ experiment, there are large, high-voltage grids that create electric fields in the detector. If a particle of dark matter collides with a xenon atom and hits some electrons, it will release some of the electrons from the atom and separately create a burst of light that can be detected by photo-detectors, recently explained Ryan Linehan, Ph.D. Graduated from SLAC’s LZ Group who helped develop high voltage networks. Electric fields that pass through the detector and then push free electrons It reaches a thin layer of gas at the top of the cylinder where they create a second optical signal. “We can use that second signal along with the original signal to learn a lot of information about location, energy, particle type, and more,” Linehan said.

But these aren’t your average electrical grids — they carry tens of thousands of volts, and they’re so high that any microscopic bits of dust or debris on the wire mesh can cause spontaneous reactions that rip electrons off the wire itself, Linehan said. “And these electrons can create signals that are similar to the electrons that come from xenon,” thus masking the signals they are trying to detect.

Linehan said researchers have come up with two main ways to reduce the chances of getting false signals from networks. First, the team used a chemical process called passivation to remove iron from the surface of the mesh wires, leaving a chromium-rich surface that reduces the wire’s tendency to emit electrons. Second, to remove any dust particles, the researchers sprayed the grids with deionized water thoroughly – and very carefully – just before installation. “Together, these processes helped us get the networks to a state where we can actually get clear data,” he said.

Publish their own LZ team first results Online in early July, after it pushed the search for dark matter even further.

Linehan and Akrip said they are impressed with what the LZ global collaboration has been able to achieve. “Together we learn something fundamental about the universe and the nature of matter,” Akrip said. “We have just started.”

LZ’s efforts at SLAC are being led by Akrip, along with Maria Elena Monzani, Principal Scientist at SLAC and LZ Deputy Operations Director for Computing and Software, and Thomas Schott, who was the founding speaker for the LZ Collaboration.


A global team of scientists has finished assembling the next generation of dark matter detectors


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