A champion of physics in South Africa

The Fermilab-hosted Deep Underground Neutrino Experiment is an enormous international scientific effort. More than a thousand researchers aim to shed light on elusive subatomic particles known as neutrinos — and possibly the nature of matter itself.

It’s also going to be physically enormous. The experiment will send the world’s most intense high-energy neutrino beam from U.S. Department of Energy’s Fermilab in Illinois to huge particle detectors 800 miles away at the Sanford Underground Research Facility in South Dakota. Each of the four neutrino detector modules will be four-stories high and over 200 feet long. Construction crews will excavate almost 800,000 tons of rock to create the gigantic caverns of the Long-Baseline Neutrino Facility that will house these detectors.

The challenge? Everything required to build the LBNF caverns in South Dakota, as well as the future particle detectors, must be lowered a mile below the surface of the Earth through a 13- by 5-foot shaft compartment and then assembled underground, like a ship in a bottle. Even the large machines necessary to remove the rock must follow the process.

On April 5, Thyssen Mining, the company contracted to carry out the excavation, received the green light to start underground work. Thyssen will bring about 35 pieces of equipment underground — around 30 will need to be disassembled to some degree to fit down the shaft. It will take about three months to mobilize all of the heavy equipment underground.

An orange and silver drill rig (a tractor-like apparatus with two parallel arms that reach above the cab and then make a steep diagonal to the ground) and several red and silver drill rigs sit in the foreground of a silty construction site. Other equipment is in the midground and hills filled with evergreens and blue sky above in the background.

These two jumbo drill rigs are some of the equipment that construction crews will use for the excavation of the caverns for the Deep Underground Neutrino Experiment. Before being lowered underground through the mile-deep Ross Shaft, they first need to be partially disassembled. Photo: Matthew Kapust, Sanford Underground Research Facility

“These machines are designed for mines, so they come in components, and the contractor looks at what size components can fit inside the hoist cage,” said James Rickard, the Fermilab resident engineer managing the excavation construction. “They try to break it down as minimally as possible” to fit the pieces into the 12-foot-tall cage. Long, narrow pieces are slung underneath the cage.

A giant orange apparatus that looks somewhat like the body of an automobile hangs from a large beam in the center of a warehouse-like room full of windows. People in hard hats and reflective vests stand on the ground around it.

This drill rig has been disassembled to prepare it for delivery to the LBNF work area a mile underground. Prior to lowering any large piece of equipment, crews perform a test sling to understand how to rig the piece so it hangs properly while traveling through the shaft. Photo: Adam Gomez, SURF

One of the first machines that will be brought in pieces down the shaft is a raise bore machine. Starting in May, the raise bore will be used to drill a pilot hole for a 1,200-foot-long ventilation shaft to increase airflow and allow heat to escape from the underground lab. After the 13-inch pilot hole is drilled, the drill is attached to a 12-foot reamer that is then pulled back up the 1,200 feet creating the full-size shaft. The shaft will be completed in fall 2021.

Other machines Thyssen will move underground include extendable forklifts called telehandlers; multifunctional skid steers; durable load, haul and dump machines; and jumbo drills that will create blasting holes. The equipment is coming to South Dakota from Thyssen’s headquarters in Nevada and Saskatchewan, as well as from project sites in the United States and Canada.

An automated rock bolter is being shipped to the site directly from the manufacturer in Finland. Its role is to install 20-foot-long steel bolts into the cavern, reinforcing the roof and walls. The machine boasts an advanced computer control system to accurately position the bolts, as well as advanced safety features and lower emissions. It will be one of only two such machines in the world.

Machines that have already arrived are being stored at an offsite yard, waiting their turn to be brought to the subterranean construction site. Once underground, the equipment will be stored in existing drifts and tunnels until an equipment and maintenance shop can be established.

The first underground blast for LBNF by Thyssen is scheduled for June. The main cavern excavation work will begin in August and continue for two-and-a-half years.

“It’s a pretty exciting time,” said Andrew Hardy, Thyssen’s project manager for the excavation. “We thought we already had a lot of activity up to this point, but now it really begins.”

A large support beam, too long to fit inside the cage of the Ross Shaft, is prepped for underground delivery by slinging it underneath the cage. The beam will be used to support drilling equipment. Credit: Adam Gomez, SURF

Once the subterranean work gets going, Thyssen will use the cage hoist daily to transport not only machinery but also materials, safety supplies, and people. The company has contracted 90 miners, mechanics and electricians split into three rotating crews, along with a surface support team of engineers, planners, buyers, safety coordinators and administrators, to keep the work going 24/7.

There is much excitement about moving towards the more substantial construction work for LBNF after over three years of pre-excavation work and reliability projects such as refurbishing the nearly 90-year-old hoists.

“I can’t wait,” said Fermilab’s Michael Gemelli, the LBNF Far-Site Conventional Facilities project manager. “I’m looking forward to the next stage of this project. The site project team has done so much great work to set the stage for the excavation work to commence.”

Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov

The ICARUS detector, part of Fermilab’s Short-Baseline Neutrino Program, will officially start its hunt for elusive sterile neutrinos this fall. The international collaboration led by Nobel laureate Carlo Rubbia successfully brought the detector online and is now collecting test data and making final improvements.

When teams began cooling the ICARUS neutrino detector and filling it with 760 tons of liquid argon in early 2020, few people knew how much the world would change in the two months that the fill would take.

“In an ideal world, as soon as the filling is complete and the cryogenic plant is stabilized, then we can activate the detector and start looking for particle tracks basically immediately,” said Angela Fava, the ICARUS commissioning coordinator and deputy technical coordinator.

The ICARUS collaboration includes more than 150 scientists from 23 institutions in Italy, Mexico, Switzerland and the United States. The detector is located at the U.S. Department of Energy’s Fermi National Accelerator Laboratory, located near Chicago.

Restrictions on international travel instituted last year due to the COVID-19 pandemic meant that many European experts could not come to Fermilab in person as planned to start up the detector components. Researchers restructured their plans to get the detector up and running with much of the team working remotely.

The collaboration successfully activated ICARUS in August 2020 and recorded the first particle tracks — from cosmic rays, particles from space that constantly bombard Earth — soon after. Exposed to both the Booster and NuMI neutrino beams at Fermilab, the ICARUS detector has recorded the first muon and electron neutrinos, demonstrating the high-level detection capabilities of the liquid-argon time projection chamber technique.

Two graphs on bright blue background with different scales of 40 centimeters. The one of the left is labeled electron neutrino. The chart on the right is labeled muon neutrino.

The ICARUS detector has been collecting test data in preparation for the official start of the physics data collection later this year. The left panel shows an electron neutrino interaction that produced a proton (top track) and an electron, which produced an electromagnetic shower with photons and electrons (bottom tracks). The right panel shows a muon neutrino interaction that produced a proton (short track, top left) and a muon (3.4-meter-long track); a cosmic-ray track independent of the muon neutrino interaction is also visible in the lower half of the image. In both panels, the neutrino beam came from left. Image: ICARUS collaboration

The team is now working on finishing the system to identify and exclude cosmic-ray signals. They are also making final improvements to the neutrino data acquisition system to prepare the detector for its official first data collection run in fall 2021.

“We’ve been able to do our jobs with most people not moving from their local offices or homes,” said Claudio Montanari, the ICARUS technical coordinator. “Everybody contributed to the best of their ability, which was key to the success of the operation.”

Searching for stealth particles

When the ICARUS detector was originally assembled at the laboratories of the Italian National Institute for Nuclear Physics in Pavia in the early 2000s, it was the largest liquid-argon detector in the world. It began its neutrino-hunting career at Italy’s Gran Sasso National Laboratory in an experiment that ran between 2010 and 2014.

After the experiment in Italy concluded, scientists realized that the ICARUS detector could have a second life at Fermilab, searching for a new type of particle: the sterile neutrino.

An illustration labeled Short-Baseline Neutrino Program at Fermilab shows from left to right: 1) The word Target above a gray building that says protons with an arrow to neutrinos, with horn + decay pipe and 0 below. 2) SBND above a blue building with 270 tons of argon and 110 meters below. 3) MicroBooNE with an orange building and below, 170 tons of argon and 470 meters, and ICARUS with a yellow building and 760 tons of argon and 600 meters below.

ICARUS will be the largest and farthest detector in the Short-Baseline Neutrino program at Fermilab, which examines neutrino oscillations over short distances and looks for hints of elusive sterile neutrinos. Graphic: Fermilab

Scientists already know of three types, or flavors, of neutrinos. The particles are notoriously hard to catch because they interact through only two of the four known forces: gravity and the weak force. But this potential fourth kind of neutrino — if it exists — may not even be sensitive to weak interaction, making detection even trickier. Scientists will have to look carefully at how the different flavors of neutrinos morph into one another, a phenomenon called neutrino oscillation.

Previous experiments saw hints of unusual oscillation, but researchers need more data to determine if sterile neutrinos were responsible for the results. Finding evidence of sterile neutrinos would advance scientists’ knowledge about physics beyond the Standard Model, the theoretical framework that has accurately described almost all known subatomic particle interactions for over 50 years.

To make this happen the ICARUS detector’s two school-bus-size modules were shipped from Gran Sasso to CERN for upgrades. In 2017, the two modules travelled by truck and ship to Fermilab, where they will soon begin hunting for ultra-elusive sterile neutrinos.

ICARUS is one of three particle detectors at Fermilab that will look for indicators of sterile neutrinos as part of the laboratory’s Short-Baseline Neutrino Program, along with the Short-Baseline Neutrino Detector and MicroBooNE. Together, the detectors will analyze how neutrinos oscillate as they travel along their straight beamline path through these detectors.

SBND, situated 110 meters from the start of the neutrino beamline, will provide a snapshot of the neutrinos right after they’re produced. MicroBooNE, located 360 meters farther down the beamline, will provide a second look at the beam composition. The final checkpoint is ICARUS, 600 meters from the start of the beamline. If ICARUS picks up fewer muon neutrinos and more electron neutrinos than expected based on data from SBND and MicroBooNE, “the combination of these things would be the unique signature of the oscillation and therefore of the existence of the sterile neutrino,” said Fava.

Preflight checklist

Getting ICARUS ready to search for signs of sterile neutrinos at Fermilab has involved three distinct stages: installation, activation and commissioning. Installation started in 2018 and included set up of the vacuum chambers, insulation, cryostats and various electronics used to power the detector and collect data.

After electrical safety checks, making sure the vacuum chambers were leak-free and testing the components’ basic functionality, it was time to get the detector ready for activation. Technicians started up the filters, pumps and condensers for the cryogenic systems and began adding the liquid argon in early 2020.

Collaborators from CERN and INFN with historical knowledge of the detector were present for the beginning of the fill. They left with plans to return to Fermilab in April 2020 to help wrap up the process and see the detector through to activation. While they were unable to return in person, the group successfully coordinated with the Fermilab branch of the team to complete the activation last summer.

At an angle from the second floor looking down into a rectangle of multi-colored, interconnected pipes.

ICARUS was filled with 760 tons of super-pure liquid argon in early 2020 and activated in August. Photo: Lynn Allan Johnson, Fermilab

“We were lucky enough not to have any showstoppers,” said Montanari.

With the detector activated, the international collaboration turned its attention to debugging and optimizing the equipment. For example: To capture good neutrino data, the liquid argon inside the detector has to be ultra-pure. When researchers found the argon was less pure than expected, they traced the problem back to slow gaseous argon movement through the recirculation system and took steps to address the flow.

“That’s the life of a physicist — dealing with problems and finding a way of overcoming them,“ Fava said.

Since last year, ICARUS has been in the commissioning phase. The team is testing all of the subsystems to make sure they are in sync and calibrated to collect quality data with minimal noise before the start of official data collection.

Getting ready for takeoff

ICARUS began taking test data from the booster neutrino beam in December 2020. That data is now being used to refine the triggers for deciding what type of signal constitutes a particle “event” worthy of recording.

“The trigger system is one of the most critical components to commission, because it brings together all the other subsystems,” said Fava.

The trigger rate — how frequently the system records an event — must be finely tuned. If it’s too high, the researchers end up sifting through more data than they need to, wasting time and computing power. Too low, and they might miss recording particle interactions that are crucial to making a discovery. The team plans to test the next iteration of trigger logic in May.

In addition to refining the trigger, the ICARUS team will install a final set of cosmic-ray trackers. Roughly 10 cosmic rays hit the detector during each 1.6-millisecond time window used to record a potential neutrino interaction. The cosmic-ray trackers are used to sort out which signal is which.

“If there is an external signal and the timing is correct, we can reject that event on the basis that it was induced by a particle that was coming from outside,” said Montanari. Trackers on the bottom and sides have already been installed — all that’s needed now is to finish the top.

With everything expected to be in place this fall, the experiment will move into the next exciting stage: collecting high-quality data that will be used in scientists’ search for sterile neutrinos.

“I’m really looking forward to making a nice data analysis and seeing what nature is willing to tell us,” Montanari said.

ICARUS is supported by the U.S. Department of Energy Office of Science, the Italian National Institute for Nuclear Physics (INFN) and CERN, the European Organization for Nuclear Research.

Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance LLC.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

The Argonauts of Greek mythology braved sharp rocks, rough seas, magic and monsters to find the fabled Golden Fleece. A new robotics project at the Department of Energy’s Fermi National Accelerator Laboratory will share that same name and spirit of adventure.

Argonaut’s mission will be to monitor conditions within ultracold particle detectors by voyaging into a sea of liquid argon kept at minus-193 degrees Celsius — as cold as some of the moons of Saturn and Jupiter. The project, funded in March, aims to create one of the most cold-tolerant robots ever made, with potential applications not only in particle physics but also deep space exploration.

A person wearing a hardhat stands inside a bright yellow honeycomb-like box that appears to be two or three times their height.

Argonaut is a robotic system being designed to monitor the interiors of liquid-argon particle detectors, which are kept at minus-193 degrees Celsius. The ProtoDUNE neutrino detector at CERN uses fixed internal cameras to look for issues like bubbles and sparks when filled with 800 tons of liquid argon. Photo: CERN

Argon, an element commonly found in the air around us, has become a key ingredient in scientists’ quests to better understand our universe. In its liquid form, argon is used to study particles called neutrinos in several Fermilab experiments, including MicroBooNE, ICARUS, SBND and the next-generation international Deep Underground Neutrino Experiment. Liquid argon is also used in dark matter detectors like DEAP 3600, ARDM, MiniCLEAN and DarkSide-50.

Liquid argon has many perks. It’s dense, which increases the chance that notoriously aloof neutrinos will interact. It’s inert, so electrons knocked free by a neutrino interaction can be recorded to create a 3D picture of the particle’s trajectory. It’s transparent, so researchers can also collect light to “time stamp” the interaction. It’s also relatively cheap — a huge plus, since DUNE will use 70,000 tons of the stuff.

But liquid-argon detectors are not without their challenges. To produce quality data, the liquid argon must be kept extremely cold and extremely pure. That means the detectors must be isolated from the outside world to keep the argon from evaporating or becoming contaminated. With access restricted, diagnosing or addressing issues inside a detector can be difficult. Some liquid-argon detectors, such as the ProtoDUNE detectors at CERN, have cameras mounted inside to look for issues like bubbles or sparks.

“Seeing stuff with our own eyes sometimes is much easier than interpreting data from a sensor,” said Jen Raaf, a Fermilab physicist who works on liquid-argon detectors for several projects including MicroBooNE, LArIAT and DUNE.

The idea for Argonaut came when Fermilab engineer Bill Pellico wondered if it would be possible to make the interior cameras movable. A robotic camera may sound simple — but engineering it for a liquid-argon environment presents unique challenges.

All of the electronics have to be able to operate in an extremely cold, high-voltage environment. All the materials have to withstand the cooling from room to cryogenic temperatures without contracting too much or becoming brittle and falling apart. Any moving pieces must move smoothly without grease, which would contaminate the detector.

“You can’t have something that goes down and breaks and falls off and shorts out something or contaminates the liquid argon, or puts noise into the system,” Pellico said.

Pellico received funding for Argonaut through the Laboratory Directed Research and Development program, an initiative established to foster innovative scientific and engineering research at Department of Energy national laboratories. At this early stage of the project, the team — Pellico, mechanical engineers Noah Curfman and Mayling Wong-Squires, and neutrino scientist Flavio Cavanna — is focused on evaluating components and basic design aspects. The first goal is to demonstrate that it’s possible to communicate with, power and move a robot in a cryogenic environment.

“We want to prove that we can have, at a bare minimum, a camera that can move around and pan and tilt in liquid argon, without contaminating the liquid argon or causing any bubbles, with a reliability that shows that it can last for the life of the detector,” said Curfman.

The plan is to power Argonaut through a fiber-optic cable so as not to interfere with the detector electronics. The fist-sized robot will only get about 5 to 10 watts of power to move and communicate with the outside world.

The motor that will move Argonaut along a track on the side of the detector will be situated outside of the cold environment. The camera will be inside the cold liquid and move very slowly; but that’s not a bad thing — going too fast would create unwanted disturbances in the argon.

“As we get more advanced, we’ll start adding more degrees of freedom and more rails,” said Curfman.

A 3D rendering of Argonaut, which appears to be a silver right angle on top of a black back with a lens on the front and silver studs around the lens.

To keep power requirements low and avoid disturbances in the liquid argon, Argonaut will move slowly along tracks on the side of the detector. Its main function is a movable camera, but the engineers working on it hope to add other features like extendable arms for minor electronics repair. Image courtesy of Bill Pellico, Fermilab

Other future upgrades to Argonaut could include a temperature probe or voltage monitor, movable mirrors and lasers for calibrating the light detectors, or even extendable arms with tools for minor electronics repair.

Much of the technology Argonaut is advancing will be broadly applicable for other cryogenic environments — including space exploration. The project has already garnered some interest from universities and NASA engineers.

Deep space robots “are going to go to remote locations where they have very little power, and the lifetime has to be 20-plus years just like in our detectors, and they have to operate at cryogenic temperatures,” Pellico said. The Argonaut team can build on existing robotics know-how along with Fermilab’s expertise in cryogenic systems to push the boundaries of cold robotics.

Even the exteriors of active interstellar space probes such as Voyagers 1 and 2 don’t reach temperatures as low as liquid argon — they use thermoelectric heaters to keep their thrusters and science instruments warm enough to operate.

“There’s never been a robotic system that operated at these temperatures,” said Pellico. “NASA’s never done it; we’ve never done it; nobody’s ever done it, as far as I can tell.”

Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.