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The Silence of the Xenon

Michelle Dolinski and an engineer during the installation of the EXO-200 detectorHalf a mile beneath the desert of Carlsbad, New Mexico, a once-dense tube of super-chilled liquid xenon has been drained, its contents warmed and waiting in temporary storage bottles.

Although xenon rarely does much anyway (it’s one of the famed “noble gases” that tend not to engage in chemical reactions), this batch of xenon is particularly quiet. Its purpose for being there – a massive, international scientific experiment called EXO-200 – has halted for the foreseeable future following a series of accidents at the site.

The accidents in Carlsbad, though unrelated to the EXO-200 scientific effort, have made it unsafe for researchers to access the site to monitor and maintain the experiment. While the recovery process is underway, the scientists wait, not knowing when they will be able to resume their experiments and hoped-for observations of a certain, particularly rare – perhaps impossibly so – radioactive decay of the xenon.

Michael Jewell, a senior physics major at Drexel working in the lab of EXO-200 project collaborator and Drexel assistant professor Michelle Dolinski, PhD, has worked on programming algorithms to analyze data from EXO-200, as well as spending time last year in the underground lab in New Mexico working on the detector itself while it was still up and running.

As he prepares to graduate from Drexel on June 13 and head to Stanford University for graduate school in physics, Jewell doesn’t know if he’ll be continuing to work on EXO-200 in another collaborator’s lab there or if he’ll start on a fresh, new project.

“I knew when I sent my acceptance into Stanford that the detector was shut down,” Jewell said. “But if it’s going to come back on, I definitely want to be there. I’d feel like I missed out if I didn’t go there.”

Seeking Very Small Clues to Answer Very Big Questions

At this point, it would be fair to stop and explain why the EXO-200 experiment even matters. Why are so many scientists going to these depths to try to find something nearly undetectable that may not exist at all? What is Jewell so eager not to miss?

They are looking for a rare type of radioactive decay called neutrinoless double beta decay that’s theorized to exist, but that scientists are not sure has ever been seen. Finding it could help answer some deep, fundamental questions of existence.

Here’s one of them: Why does matter exist? It sounds philosophical or rhetorical, but it’s something scientists are actively trying to figure out through experiments and particle physics theories. They know that matter and antimatter annihilate one another when they come together. And in the moments immediately after the Big Bang, when the universe began, everything was very, very close together. If matter and antimatter were present in equal measure, this whole “universe” thing would have ended almost as soon as it began. But it didn’t. We’re here, and we ourselves and the stars and planets and everything we routinely see and experience is made of matter.

“It’s less interesting if there was just an excess to start with,” Dolinski said. “We’re looking for a way to explain why matter might have been preferred over anti-matter, or how the excess could have been generated after the Big Bang.”

Drexel senior physics major Michael Jewell and assistant professor of physics Michelle Dolinski, PhD

Dolinski, Jewell and the rest of the EXO team are looking for that evidence in the properties of some of the smallest particles, neutrinos.

The EXO-200 experiment is focused on detecting two rare types of radioactive decay: Double beta decay is one that releases particles called neutrinos out of the decaying atom’s nucleus. And the so-rare-it-may-not-exist, theorized variation of that decay, neutrinoless double beta decay, doesn’t release neutrinos at all.

A simple version of the theorized reason why not: Neutrinos might be their own anti-particle, in which case, sometimes, they might never make it out of the nucleus.

For reasons that are still more complex, if indeed neutrinos are their own anti-particle, that could explain why there’s excess matter in the universe. (This article in Quanta magazine includes more explanation, if you want to dig deeper into how.) Confirming that neutrinos are their own anti-particle would also give scientists a better understanding of neutrinos’ mass, and help answer lot of other unresolved questions.

Neutrinoless double beta decay, if it happens, is the ultimate goal. Finding out if, and at what rate, it happens, is the ultimate challenge.

How to Detect the Virtually Undetectable?

This goal and challenge are the reason why EXO-200 and several other large-scale, high-sensitivity detectors around the world have been created to find out if neutrinoless double beta decay is possible.

The process of searching for neutrinoless double beta decay is far from simple. Neutrinos, by the way, are extremely hard to detect. They have no charge – making them less inclined to react with any other particles, including a detector. Neutrinos are also extra tiny even by the standards of fundamental particles –so small that their mass hasn’t been measured yet. So it requires a lot of sensitive equipment and very little interference from other sources to detect a reaction involving neutrinos with a high enough resolution to also be able to detect a similar, rarer reaction with their absence.

The challenge, then, is to set up a batch of an eligible radioactive element – as large a sample as possible – in as quiet a location as possible, and watch and wait for as long as possible with the most sensitive equipment possible – and then filter out all the background radiation that could confuse the signal coming from the very rare reactions they want to detect.

The scientists gather a lot of an eligible radioactive element. Dolinski said that the EXO team chose xenon because its isotope, xenon-136, which can undergo double beta decay, is relatively easy to purify using a centrifuge. At a comparatively low cost, the team was able to purchase and fill its 200-kilogram detector with an enriched mix of xenon atoms, 80 percent of which were xenon-136. (That’s a “relatively” low cost; that xenon cost $1.2 million when purchased and would cost more now.)

During the installation of the EXO-200 detector, the time projection chamber for holding liquid xenon was transported in a protective box into a clean room to minimize its exposure to outside radiation.

Then they place that sample in the quietest possible location to minimize background radiation. Scientists build entire labs underground to minimize interference from cosmic radiation.

“It’s an active mine. You get there at five in the morning and take a 15-minute elevator ride underground,” Jewell said of his time working on-site at EXO last year. “We work in a clean room and wear a bunny suit because of the worry that dirt and dust from the mine would affect the purity of the experiment.”

The EXO-200 detector measures electrical ionization and light emission and cross references those signals with the time and space where they are detected throughout the xenon tube.

While the detector runs, the scientists wait, maintain the equipment, tinker with side experiments and improvements as things move along, and monitor the signals.

At least, they did until the accidents elsewhere at the site made it unsafe to continue working underground with the EXO-200 detector. Since that happened in February, the detector equipment was shut down and data collection ceased. All of the scientists have continued working on above-ground pursuits including data analysis while they wait to see if and when EXO-200 detection can resume.

The data recorded in the detector include tiny, subtle changes in light and ionization emitted from the liquid xenon. These energy signatures of radioactivity interacting with the detector are both the signal of beta decays and the noise of background radiation.

“We combine the light and ionization signals to understand the energy and location of every event,” Dolinski said.

Last Word from EXO-200?

A time projection chamber full of xenon was inserted into a cryostat in the EXO detector. When shut down for safety reasons, the gaseous xenon was recovered in high pressure gas bottles.

Today, in the journal Nature, they have published their analysis of the largest set of data yet from EXO-200, prior to the shutdown, spanning about two years of data.

Dolinski, Jewell, a graduating senior, and two graduate students from Dolinski’s lab were among the co-authors.

Jewell earned a spot as a co-author – a rare achievement for an undergraduate when the paper is in Nature – for his role in programming software used in data analysis. Jewell helped develop an algorithm to reduce noise from electrons hitting the light detector.

“Mike’s work was part of a larger effort to improve the energy resolution of our detector in software, by characterizing and subtracting noise in our electronics that was spoiling the resolution in our scintillation light channel,” Dolinski said.

In the end, the EXO team still hasn’t demonstrated whether neutrinoless double beta decay exists in reality. But the researchers did narrow down a better idea of just how exceptionally rare it must be, if it does exist: The half-life of the reaction must be at least 1.1 x 1025 years. In perspective: That’s approximately 100 trillion times longer than the universe has existed. But they can say, with 90 percent confidence—based on how long, how well and how closely they’ve monitored their experiment, and how robustly and carefully they detected and analyzed their data—that the reaction must be at least that rare for it not to have been detected yet.

An Uncertain Future

“We are anxious to get back underground, to repair any damage to equipment from soot (from the fire) and from neglect over the months of shutdown, and to resume data taking,” Dolinski said. “The longer we go without access the more difficult this will be.”

But while this detector remains inactive, Dolinski’s lab has plenty of other concurrent projects underway investigating fundamental questions about neutrinos– including building a detector to find “sterile neutrinos” that seem to disappear because they have no interactions, and figuring out how to make xenon ice cubes for a next-generation detector.

For Jewell, his time working in Dolinski’s lab as a full-time co-op employee and subsequently on his senior research project has given him plenty of experience in both the hardware and software work involved in neutrino detection experiments.

“I hope everything works out and I can continue working on this collaboration,” Jewell said, feeling optimistic that he can continue the work he began at Drexel using EXO to seek answers to some of the universe’s fundamental questions.

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