Why choose liquid argon for DUNE detectors?

What can neutrinos — particles that are imperceptible to all but the most sensitive devices ever built — tell us about how matter triumphed over antimatter in the early universe? As head-in-the-clouds as this question sounds, real people working on the Deep Underground Neutrino Experiment are developing down-to-earth detectors, infrastructure and processes to run this very ambitious experiment to find out.

The liquid-argon-based technology chosen for the DUNE neutrino detectors promises to deliver stunning scientific insights and to be complementary to experiments pursuing similar goals that use the more traditional water-based technology. The U.S. Department of Energy’s Fermi National Accelerator Laboratory is the host for DUNE, in partnership with funding agencies and more than 1,400 scientists and engineers from around the world.

Atoms are mostly empty space with a nucleus in the middle and point-like electrons orbiting around it — like infinitesimal solar systems. The nucleus is held together by a force aptly named the “strong force” and it keeps its electrons in tow by the “electromagnetic force,” the familiar opposites-attract force. These are the two strongest of the four known fundamental forces of nature, and neutrinos are immune to both. They are, however, subject to what is called the “weak force,” which comes into play only if they get very close to a nucleus or an electron, and to gravity, which is negligible for this tiny particle.

Neutrinos — travelling at nearly the speed of light and basically blind to and unaffected by everything around them — therefore fly right through matter as though it’s not even there. Except that every once in a while a neutrino gets within weak force range of an atomic particle and crashes right into it. As in any collision, stuff sprays out — subatomic particles, in this case. DUNE wants to capture this once-in-a-while event in its detectors as many times as it can.

To improve the chances for neutrino interactions to both occur and then be detected, DUNE needs: (a) lots of neutrinos, (b) shielding from cosmic rays that would otherwise drown out the neutrino signals, and (c) lots of target material – the denser the better.

The Long-Baseline Neutrino Facility at Fermilab is building a beamline to send a prodigious flow of neutrinos (that’s part “a”) to DUNE’s two neutrino detectors, a smaller near detector at Fermilab, and a gigantic, modular far detector 800 miles downstream. DUNE’s far detector modules will be constructed and installed in South Dakota in a laboratory that is a mile underground (that’s “b,” the earth above the detector will absorb the cosmic rays). Finally, each detector module will be filled with kilotons of a quite well-endowed material, liquid argon (“c”).

In a detector known as a LArTPC, shorthand for liquid-argon time projection chamber, a bath of ultra-pure cryogenic liquid argon is subjected to a strong electric field created between a cathode and an anode, which are like the negative and positive terminals on a battery. Charged particles that emerge from neutrino-nucleus collisions strip electrons from argon atoms in the surrounding volume. These freed electrons, called ionization electrons, drift in the enormous electrified volume of argon, an inert element that won’t gobble them up enroute, to a multi-layered anode, which enables the time-projection aspect of the experiment. From the resulting 3D images, physicists can see how the event evolved and work back to understand the nature of the originating neutrino interaction.

“The ionization electrons carry the imprint of the neutrino interaction, said Hilary Utaegbulam, a graduate student at the University of Rochester. “They tell us where the neutrino interacted, how much energy it deposited, and depending on how the ionization patterns cluster, what type of neutrino it was. The electrons act as messengers that tell us a great deal about the interaction itself.”

The very fine-grained imaging from a liquid-argon time projection chamber makes it a desirable choice for neutrino experiments. Water Cherenkov technology, which relies on detecting photons generated when a charged particle moves faster than the speed of light in the water, has strengths that are complementary to those of LArTPCs, but lacks some of the latter’s capabilities. With an impressive history of discovery including that of neutrino oscillation about 25 years ago by the experiments SuperKamiokande in Japan and SNO in Canada, water Cherenkov is the technology choice of the other leading next-generation neutrino detector, HyperKamiokande.

The LArTPC technology, together with DUNE’s longer 800-mile baseline and its companion neutrino beam that spans a wide range of energies, will uniquely enable DUNE to measure all the sought-after neutrino oscillation properties.

“The DUNE LArTPCs offer millimeter spatial resolution on a timescale of milliseconds,” said Afroditi Papadopoulou, an Oppenheimer postdoctoral fellow at Los Alamos National Laboratory. “They also provide excellent particle identification, energy precision, and low particle-detection thresholds. All these properties make LArTPCs the ideal detectors for performing the high-impact measurements essential for world-leading discoveries.”

In addition, liquid argon is a tremendous scintillator. This means that when energy from an interaction bumps a neighboring argon atom up to an unstable excited state, the atom emits a packet of light energy called a photon as it returns to its ground state. LArTPCs are therefore supplemented with photon detectors. The photons, naturally traveling at the speed of light in argon, are detected nearly instantaneously, providing precise timing information. Light detection enhances the detector’s capabilities for all of DUNE’s planned measurements and opens up prospects for further physics explorations.

Finally, as liquid argon is a byproduct of the large industrial production of liquid oxygen and nitrogen, and is itself used in industrial applications such as welding, it is abundantly available and inexpensive. The only other liquids that could offer similar performance are, like argon, found in the far-right column of the periodic table, and are more challenging to acquire.

“The LArTPC design for DUNE gives us the benefits of large and scalable detectors without sacrificing high-resolution energy measurement over a wide range of neutrino energies,” said Fermilab scientist Anne Norrick. “We will have some healthy competition from HyperK, but when all is said and done, for neutrino detection, DUNE’s LArTPC technology is simply unmatched.”

Andrzej Szelc, a professor at the University of Edinburgh, has been elected as co-spokesperson for the Short-Baseline Near Detector experiment. SBND plays an essential role in the Short-Baseline Neutrino Program at the U.S. Department of Energy’s Fermi National Accelerator Laboratory.

“I’ve seen SBND’s voyage from an idea to becoming a reality. It’s a great honor to be chosen as the first international spokesperson to co-lead this collaboration into this next stage of data analysis,” said Szelc.

Spokespeople for physics experiments are principal leaders of the collaborations conducting the research. They ensure experiments operate effectively to meet scientific goals and serve as the primary representatives in communications.

The SBND collaboration brings together 210 scientists from 40 institutions in Brazil, Spain, Switzerland, the U.K. and the U.S.

Szelc, who previously served as the experiment’s physics coordinator, will succeed Ornella Palamara — a senior Fermilab scientist recently appointed director of user facilities and experiments — who has co-led the collaboration since 2014.

“Ornella has been instrumental in building the fantastic SBND collaboration, and her scientific vision and contributions to the experiment go back to the very beginning when we just had a notion of building a near detector along the neutrino beamline,” said David Schmitz, professor at the University of Chicago and co-spokesperson for the SBND collaboration. “Andrzej’s experience with SBND and other experiments of this kind is very broad, and I look forward to working with him as we continue our first physics run.”

As the near detector in the Short-Baseline Neutrino Program, SBND observes the neutrinos as they are produced in the Fermilab beam. This enables the SBN Program to definitively know the composition of the neutrino beam before it has a chance to change, through a process called oscillation, giving the collaboration a better handle on testing for the existence of a new type of neutrino.

Since seeing their first neutrinos last year, the SBND collaboration started their first official physics run in December. “We have this detector that works fantastically well, and we can reconstruct the neutrino interactions very, very precisely,” said Szelc. “Already, we’re seeing about 7,000 neutrinos per day. That adds up to the largest sample of neutrino interactions on argon in the world.”

SBND’s large data sample will enable physicists to study neutrino interactions in unprecedented detail. The physics of these interactions is crucial for other neutrino experiments, such as the long-baseline Deep Underground Neutrino Experiment.

“Our live time for capturing neutrinos has been 98.6%, which isn’t something every experiment can say for its first run,” said Schmitz. “And the quality of the data is extremely high, thanks to the international team of amazing scientists working on SBND, strong support from Fermilab for the experiment, and an incredibly stable beam delivery from the Fermilab Accelerator Complex, making this year the best yet for the Booster Neutrino Beam.”

With so many neutrino interactions, SBND is also advancing techniques for the analysis of scientific data, including machine learning methods, which can be applied at nearly every stage of the data analysis. The progress made with this kind of pattern recognition software can be used in other applications like medical physics — including analysis of images from X-rays, CT-scans and MRIs.

The Short-Baseline Near Detector international collaboration is hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory. The collaboration consists of 40 partner institutions, including national labs and universities from five countries. SBND is one of the particle detectors in the Short-Baseline Neutrino Program that provides information on a beam of neutrinos created by Fermilab’s particle accelerators.

Three photographers have captured winning shots in Fermilab National Accelerator Laboratory’s 2025 Photowalk competition. These photographers will move on to the international Photowalk, with their images competing with photos from laboratories around the world. 

The winning photos, in alphabetical order by photographer, are “The Underside of Quantum Computing” by Mark Kaletka of Batavia, Illinois, “SSR1” by Krsto Sitar of Lombard, Illinois, and “QUANTUM COMPUTING” by Perry Slade of Aurora, Illinois.

A panel of four Fermilab judges reviewed 63 photos submitted by 21 photographers. They selected three winning images that represented the science and spirit of America’s premier particle physics and accelerator laboratory.

On Saturday, July 26, 2025, two dozen photographers visited Fermilab from across the United States; two even came from Europe. Guided by scientists and staff, the photographers received exclusive, behind-the-scenes tours of areas and experiments that are typically not accessible to the public: the Quantum Garage at the Superconducting Quantum Materials and Systems (SQMS) Center, the Muon g-2 experiment hall, the Short Baseline Near Detector (SBND), the Fermilab Accelerator Science and Technology/Integrable Optics Test Accelerator (FAST/IOTA) facility and the Industrial Center Building.

“It was an honor and a privilege to have the opportunity to participate in the 2025 Fermilab Photowalk. Photography’s my way of showing how I see the world and this recognition inspires me to keep creating,” said winning photographer Perry Slade from Aurora, Illinois.

“I’m very familiar with Fermilab, so I especially enjoy seeing it revealed through new perspectives — an unusual angle, the play of light or a close-up detail that transforms the familiar,” says Georgia Schwender, visual arts coordinator at Fermilab. “What intrigues me most is the sheer range of possibilities; every photographer brings their own way of seeing, reminding us that even the most well-known places can surprise us when viewed through a fresh lens. Serving as a judge for this contest was an honor, and it gave me the chance to experience Fermilab through the creativity and vision of others.”

Kaletka’s, Sitar’s and Slade’s winning photos will now advance to the worldwide Global Physics Photowalk competition. A shortlist of global finalists will be announced by the Interactions Collaboration in September, followed by a final selection through a jury and public vote. The winners of the international competition will be featured in a future issue of the CERN Courier and in Symmetry magazine.

The Fermilab Photowalk is part of the global event organized by the Interactions Collaboration, an international group of science communicators dedicated to telling stories about particle physics research and achievements. Fermilab has taken part in previous Photowalks organized by Interactions, and this year is one of 16 participating particle physics laboratories on three continents. Winners from the local contests advance to the international Photowalk competition, where the final winners will be chosen later this fall.

Fermilab will display a selection of photos from the Photowalk in Wilson Hall’s second-floor Art Gallery in September. A reception for will be hosted from 3 p.m. to 5 p.m. on Sept. 5 at the Fermilab Art Gallery. No registration is required for this event. Wilson Hall is open to visitors on Monday through Friday from 7:00 a.m. to 5:00 p.m. All visitors age 18 and older must present a Real ID-compliant form of identification to enter.