Researchers are using purified liquid xenon to search for mysterious dark matter particles

A mile underground in an abandoned gold mine in South Dakota is a gigantic cylinder containing 10 tons of purified liquid xenon that is being closely watched by more than 250 scientists around the world. This tank of xenon is at the heart of the LUX-ZEPLIN (LZ) experiment, which aims to detect dark matter – the mysterious invisible substance that makes up 85% of the matter in the universe.

“People have been searching for dark matter for over 30 years, and no one has come up with convincing evidence,” said Dan Akerib, professor of particle physics and astrophysics at the DOE’s SLAC National Accelerator Laboratory. But with the help of scientists, engineers, and researchers around the world, Akerib and his colleagues turned the LZ experiment into one of the world’s most sensitive particle detectors.

To get to this point, SLAC researchers built on their expertise working with liquid noble gases — the liquid forms of noble gases like xenon — including advancing the technologies for cleaning liquid noble gases themselves and systems for detecting rare interactions with dark ones Matter within these liquids. And, Akerib said, what the researchers learned will aid not only the search for dark matter, but other experiments looking for rare particle physics processes.

“These are really profound mysteries of nature, and this confluence of understanding the very big and the very small at the same time is very exciting,” Akerib said. “Possibly we could learn something completely new about nature.”

Searching for dark matter deep underground

A current leading candidate for dark matter are weakly interacting massive particles, or WIMPs. However, as the acronym suggests, WIMPs hardly interact with ordinary matter, making them very difficult to detect, although in theory many of them are constantly passing us by.

To meet this challenge, the LZ experiment first went deep underground at the former Homestake gold mine that is now the Sanford Underground Research Facility (SURF) in Lead, South Dakota. There, the experiment is well protected from the constant bombardment of cosmic rays on Earth’s surface – a source of background noise that could make it difficult to detect hard-to-find dark matter.

Even then, finding dark matter requires a sensitive detector. For this reason, scientists are looking for noble gases, which are also known to be reluctant to react with anything. This means there are very few possibilities of what might happen when a dark matter particle, or WIMP, interacts with an atom of a noble gas, and therefore less chance of scientists missing an already elusive interaction.

But which noble? As it turns out, “xenon is a particularly good noble metal for detecting dark matter,” Akerib said. Dark matter interacts most strongly with nuclei, and the interaction gets even stronger with the atomic mass of the atom, Akerib explained. For example, xenon atoms are a little over three times heavier than argon atoms, but are expected to have more than ten times stronger interactions with dark matter.

Another benefit: “Once you’ve cleaned other contaminants out of the liquid xenon, it will be very quiet on its own,” Akerib said. In other words, the natural radioactive decay of xenon is unlikely to stand in the way of detecting the interactions between WIMPs and xenon atoms.

Just the xenon, please

The trick, Akerib said, is to get pure xenon without all of the noble gas’s benefits being in dispute. However, purified noble gases are not readily available – the fact that they hardly interact with anything also means that they are generally quite difficult to separate from one another. And “unfortunately, you can’t just buy an off-the-shelf purifier that cleans noble gases,” said Akerib.

Akerib and his colleagues at SLAC therefore had to find a way to purify all the liquid xenon they needed for the detector.

The main impurity in xenon is krypton, the second lightest noble gas and a radioactive isotope that could mask the interactions it is actually looking for. To prevent krypton from becoming the particle detector’s kryptonite, Akerib and his colleagues spent several years perfecting a xenon purification technique using what is known as gas-carbon chromatography. The basic idea is to separate ingredients in a mixture based on their chemical properties while carrying the mixture through some sort of medium. In gas charcoal chromatography, helium is used as the carrier gas for the mixture and activated charcoal as the separation medium.

“You can think of the helium as a steady breeze through the charcoal,” Akerib explained. “Each xenon and krypton atom spends some time sticking to the charcoal and some time detaching. Noble gas atoms are less sticky the smaller they are, meaning krypton is slightly less sticky than the xenon so it gets swept away by the non-sticky helium “breeze”, separating the xenon from krypton. Researchers could then capture and discard the krypton and then recover the xenon, Akerib said. “We did that for about 200 bottles of xenon gas – it was quite a big campaign.”

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

A different kind of power grid

No matter what noble liquid fills the detector, a sophisticated detection system is crucial if scientists ever hope to find anything like dark matter. Above and below the tower of liquid xenon for the LZ experiment are large high-voltage grids that create electric fields in the detector. If a dark matter particle collides with a xenon atom and knocks off a few electrons, it will liberate some electrons from the atom and separately produce a burst of light that can be detected by photodetectors, explained Ryan Linehan, a recent PhD researcher. Graduated from SLAC’s LZ group, which helped develop the high-voltage grids. Electric fields passing through the detector then propel the free electrons up into a thin layer of gas at the top of the cylinder, where they produce a second light signal. “We can use this second signal along with the original signal to learn a lot of information about position, energy, particle type and more,” Linehan said.

But these are no ordinary electrical grids — they carry tens of thousands of volts, so high that microscopic dust or dirt particles on the wire grid can trigger spontaneous reactions that rip electrons out of the wire itself, Linehan said. “And those electrons can create signals that look just like the electrons that came from the xenon,” masking the signals they’re trying to detect.

The researchers found two main ways to minimize the chance of getting false signals from the grids, Linehan said. First, the team used a chemical process called passivation to remove iron from the surface of the grid wires, leaving a chromium-rich surface that reduces the wire’s tendency to emit electrons. Second, immediately before installation, the researchers sprayed the grids thoroughly – and very gently – with deionized water to remove dust particles. “These processes together helped us get the grids into a state where we could actually get clear data,” he said.

The LZ team published their first results online in early July, having pushed the search for dark matter further than ever.

Linehan and Akerib said they are impressed with what LZ’s global collaboration has achieved. “Together we learn something fundamental about the universe and the nature of matter,” said Akerib. “And we’re just getting started.”

The LZ effort at SLAC is led by Akerib along with Maria Elena Monzani, a senior scientist at SLAC and LZ associate operations manager for computers and software, and Thomas Shutt, founding spokesperson for the LZ collaboration.