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Read the travelogue of a xenon atom as it journeys from the air we breathe to a dark matter detector a mile underground.
Erin Lorraine Broberg

This article was originally published in Symmetry Magazine. Read it here

 

Xenon Travelogue, Entry 001

I’ve never thought of myself as a particularly remarkable atom.

Sure, I was born inside the blast of a supernova. But what element doesn’t have an explosive, cosmic origin story? I’m a rare type of atom, I guess, but xenon is also pulled from the air on a regular basis as part of the process of making steel.

I know I have the potential to do interesting stuff. Xenon atoms have been used to propel spacecrafts and create fancy blue headlights. They’re even used as a general anesthetic.

But I’ve never done any of those things. I’ve spent the last few billion years just floating listlessly through Earth’s atmosphere as a colorless, odorless gas.

Today, all of that changes. I’ve been chosen to take part in an experiment seeking one of the most mysterious substances in the universe.

 

In the early 2000s, a group of physicists was thinking about how to detect dark matter, a mysterious substance that outweighs ordinary matter 5 to 1. The physicists were specifically looking for a way to discover theoretical dark matter particles called WIMPs, or weakly interacting massive particles.

As they scanned the periodic table, their eyes landed on xenon, a naturally occurring and very stable gas—or, under the right conditions, liquid or solid—made up of atoms with large nuclei. If a WIMP happened to bump into one of those nuclei in a particle detector full of liquid xenon, it could knock several xenon electrons from their shells. This would result in detectable flashes of light.

After a series of small experiments, the scientists eventually built a detector called LUX, which they filled with 350 kilograms of liquid xenon. But to catch something as elusive as dark matter, they needed a bigger machine. Now they’re working on a new detector, called LUX-ZEPLIN (or LZ), which has a capacity of 10 metric tons.

“It was very important for us to increase to 10 tons, because as we increase the target mass, we have a higher probability of a dark matter particle interacting in our detector,” says Carmen Carmona-Benitez, assistant professor at Penn State University who oversees development and implementation of the LZ circulation and cryogenics systems.

 

two researchers in clean suits work on experiment components in a cleanroom
Carmen Carmona-Benitez and David Woodward worked on the internal components of a test cryostat for LZ, inside the cleanroom laboratory at Penn State University. These components were used to test the experiment’s circulation and cryogenics system. Photo courtesy Carmen Carmona-Benitez

 

The LZ collaboration is made up of 250 scientists, engineers, and technicians from 34 institutions in the US, UK, Portugal and Korea. The experiment is supported by the US Department of Energy Office of Science, the UK Science & Technology Facilities Council, Portuguese Foundation for Science and Technology, and the Institute for Basic Science, Korea.

To fill the LZ detector, the collaboration had to secure a quarter of the world’s annual supply of xenon—without letting the increased demand drastically inflate the price of the rare element. With the help of a $8.6 million investment from the South Dakota Science and Technology Authority, the collaboration purchased xenon from three international manufacturers over a period of four years.

Xenon Travelogue, Entry 002

I just heard that I’m considered the “ideal candidate” for finding dark matter. Me? Ideal? I blush. I just hope I can live up to the hype!

 

“I do think that xenon is an ideal target for dark matter, and there are many reasons,” Carmona-Benitez says.

For one, liquid xenon is dense—so dense that solid aluminum can float on it. This density allows xenon to self-shield. This will create a quiet region at the center of the LZ detector, where researchers can look for WIMP signals without the interference of other particles, called background.

Xenon is also radiopure, meaning it doesn’t contain radioactive isotopes that will decay and obscure other reactions in the detector.   

It’s possible to keep xenon in liquid form in a laboratory. Xenon is liquified at minus 162.6 degrees Fahrenheit—which may sound extremely cold but isn’t even as chilly as hospitals need to keep the magnet in the middle of an MRI machine.

But the greatest attribute of xenon? It’s a noble gas.

“Noble gases have a property where they produce two signals when an interaction happens,” says David Woodward, postdoctoral researcher at Penn State University. “The power of LZ is that we can detect both of those signals and combine them to very accurately distinguish a dark matter signal from other interactions. The intrinsic nature of xenon gives us a good, clean signal to work with.”

Xenon Travelogue, Entry 003

Just when I start feeling appreciated, I find out I’m not yet considered up to the job. I’m heading to California to undergo a preparation process, which somehow involves a maze?

 

“When the xenon arrives at our lab, impurities account for only 10 parts per billion,” says Christina Ignarra, a project scientist at SLAC National Accelerator Laboratory. But still, “at the parts-per-billion level, backgrounds would light up the detector like a Christmas tree, and we wouldn’t be able to see any real signals in the detector.”

The LZ detector needs to be filled with xenon clean enough to have its impurities measured in parts per quadrillion.

The most troublesome impurity is a particularly radioactive isotope of krypton. Krypton, like xenon, is a noble gas. Because noble gases are nearly chemically inert, they stubbornly refuse to be separated through typical filtration processes. In fact, these two elements have been intermingled for epochs.

“Xenon and krypton have been together a long time,” says Dan Akerib, a professor of particle physics and astrophysics at SLAC who directs the purification effort. “They were created together in supernova explosions and have floated in the atmosphere for billions of years.”

SLAC’s xenon purification lab boasts two massive cylinders of charcoal, which is valuable for a specific characteristic: It’s covered in tiny pores that give it an enormous surface area.

“Charcoal has a surface area that is absolutely freakish,” says Thomas Shutt, a professor of particle physics and astrophysics at SLAC who developed this filtration system. “Every gram of activated charcoal has about 1000 square meters of surface area.”

 

two large cylinders in a lab space at the SLAC laboratory
At the SLAC xenon purification lab, two central columns are each filled with nearly half a ton of charcoal, which is used to produce ultra-clean xenon for the LZ dark matter experiment. Photo by Jacqueline Orrell, SLAC National Accelerator Laboratory

 

With all of the ins and outs of their surfaces, these charcoal cylinders serve as a giant maze. Lightweight elements like helium navigate the maze in seconds, but heavier elements like krypton and xenon could take years to plod through on their own. To speed up the process, researchers give the gases a boost with a breeze of lightweight helium.

Krypton is slightly quicker than xenon. When krypton arrives at the finish line, researchers give it a blue ribbon and direct it off to the side. When xenon finally reaches the end, researchers divert it to a recovery chamber.

 

an animated gif demonstrates this filtration process
This animation shows how krypton (red) is removed from xenon gas (blue) by flowing the combined gases through a column of charcoal (black specks). Both elements stick to the charcoal, but krypton is not as strongly attached and gets swept out first when the column is purged with helium gas. Animation by Greg Stewart, SLAC National Accelerator Laboratory

 

Researchers separate the speed-boosting helium from the xenon by turning the temperature down to minus 169.2 degrees Fahrenheit. At that temperature, the xenon freezes, but the helium remains in its gas phase and is pumped away, leaving behind a chamber of ultra-pure xenon icicles.

“But how do we even know that we got the krypton out of the xenon? How do we know it's purified? We need to sample the xenon and measure its content,” Ignarra says.

A team of researchers from the University of Maryland are tasked with testing small “sips” of xenon every day, sometimes every hour. When the xenon has reached certified purity, it is secured in storage packs, tagged with a GPS tracker and loaded onto a truck.

 

a researcher turns a dial on a complicated testing system
John Armstrong, a graduate student at the University of Maryland, measures xenon purity to the parts per quadrillion, ensuring the gas is clean enough for the LZ experiment. Photo by Adam Gomez, Sanford Underground Research Facility

 

Xenon Travelogue, Entry 004

I made it through the maze! I didn’t win the race, but that didn’t seem to matter. Wow, it was cold in there. At least now I’m clean, warmed back up and on my way to this journey’s final destination: Lead, South Dakota.

 

When the xenon storage packs arrive at Sanford Underground Research Facility, they travel down a mile-deep shaft and are stored in a drift adjacent to the Davis Cavern, where in the 1960s physicist Ray Davis used chlorine atoms to count neutrinos in his Nobel-winning Solar Neutrino Experiment.

 

close up of the compressed air cylinders holding gaseous xenon underground at SURF
Compressed xenon gas is stored in a drift on the 4850 Level of Sanford Underground Research Facility. Photo by Adam Gomez, Sanford Underground Research Facility

 

Meanwhile, in the Davis Cavern, physicists put the final pieces of the LZ detector in place.

Xenon Travel Log, Entry 005

My life has certainly changed. I live underground now. Most of the time, I’m just chilling in the LZ detector—and I do mean chilling, it’s close to freezing in here!—completely cut off from the rest of the world.

But it never lasts long. Every two days, I’m whisked away through a network of piping. First, I’m a liquid, then I’m a gas—next thing I know, I’m in another charcoal maze or zooming over a superheated plate.

 

While xenon spends the majority of its time in a supercooled liquid state in the heart of the detector, it needs to be cleaned regularly from impurities that might sneak in.

“We send the xenon through a labyrinth of tubes and vessels outside the detector, cleaning and pumping it, before returning it into the detector,” says Woodward, who coordinates LZ’s cryogenics and circulation systems. “It’s all part of one closed loop system, called the circulation system.”

 

a researcher turns a dial on a complex wall of controls
David Woodward directs the flow of xenon from a control panel in the Davis Campus. Photo by Adam Gomez, Sanford Underground Research Facility

 

The full volume of LZ’s xenon is filtered every two and a half days, meaning xenon doesn’t get to rest long before starting the cycle again. “Keeping the xenon pure is an extra challenge, but that's what the science demands,” Woodward says.

Xenon Travelogue, Entry 006

I’m back in the LZ inner detector again. It’s quiet. And dark. If I’m honest, it gets a smidge boring in here. Sometimes I miss just floating around in the sky. At least up there—OOOF! What was that?!

 

LZ researchers hope that someday soon, a WIMP will bump into one of the billions of xenon atoms inside the LZ detector. Such a collision would produce two unique bursts of light, which, in the otherwise pitch-black detector, would alert researchers that something interesting is going on.

If enough of these collisions occur, the LZ collaboration will have good evidence that they’ve discovered dark matter (and, of course, they’ll thank xenon for its assistance).