ICARUS gets ready to fly

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.

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.

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.

“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.

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.

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.

Editor’s note: The Dark Energy Spectroscopic Instrument seeks to further our cosmic understanding by creating the largest 3D map of galaxies to date. Below is a press release issued by the Department of Energy’s Lawrence Berkeley National Laboratory announcing the official start of DESI’s five-year survey. DOE’s Fermi National Accelerator Laboratory is a key player in the construction of this instrument, drawing on more than 25 years of experience with the Sloan Digital Sky Survey and the Dark Energy Survey.

Fermilab contributed key elements to DESI, including the online databases used for data acquisition and the software that will ensure that each of the 5,000 robotic positioners are precisely pointing to their celestial targets to within a tenth of the width of a human hair. Fermilab also contributed the corrector barrel, hexapod and cage. The corrector barrel holds DESI’s six large lenses in precise alignment. The hexapod, designed and built with partners in Italy, focuses the DESI images by moving the barrel-lens system. Both the barrel and hexapod are housed in the cage, which provides the attachment to the telescope structure. In addition, Fermilab carried out the testing and packaging of DESI’s charge-coupled devices, or CCDs. The CCDs convert the light passing through the lenses from distant galaxies into digital information that can then be analyzed by the collaboration.

“It is very exciting to have reached this point in the project,” said Liz Buckley-Geer, Fermilab scientist and one of the DESI lead observers. “I am looking forward to participating in the exciting science results that will be produced in the coming years.”

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A five-year quest to map the universe and unravel the mysteries of “dark energy” is beginning officially today, May 17, at Kitt Peak National Observatory near Tucson, Arizona. To complete its quest, the Dark Energy Spectroscopic Instrument (DESI) will capture and study the light from tens of millions of galaxies and other distant objects in the universe.

DESI is an international science collaboration managed by the Department of Energy’s Lawrence Berkeley National Laboratory, or Berkeley Lab, with primary funding from DOE’s Office of Science.

By gathering light from some 30-million galaxies, project scientists say DESI will help them construct a 3D map of the universe with unprecedented detail. The data will help them better understand the repulsive force associated with “dark energy” that drives the acceleration of the expansion of the universe across vast cosmic distances.

Jim Siegrist, associate director for High Energy Physics at DOE, says “We are excited to see the start of DESI, the first next-generation dark energy project to begin its science survey. Along with its primary mission of dark energy studies, the data set will be of use by the wider scientific community for a multitude of astrophysics studies.”

What sets DESI apart from previous sky surveys? The project director, Berkeley Lab’s Michael Levi, said, “We will measure 10 times more galaxy spectra than ever obtained. These spectra get us a third dimension.” Instead of two-dimensional images of galaxies, quasars and other distant objects, he explained, the instrument collects light, or spectra, from the cosmos such that it “becomes a time machine where we place those objects on a timeline that reaches as far back as 11-billion years ago.”

“DESI is the most ambitious of a new generation of instruments aimed at better understanding the cosmos — in particular, its dark energy component,” said project co-spokesperson Nathalie Palanque-Delabrouille, a cosmologist at France’s Alternative Energies and Atomic Energy Commission, or CEA. She said the scientific program —  including her own interest in quasars — will allow researchers to address with precision two primary questions: what is dark energy; and the degree to which gravity follows the laws of general relativity, which form the basis of our understanding of the cosmos.

“It’s been a long journey from the first steps that we took almost a decade ago to design the survey, then to decide which targets to observe, and now to have the instruments so that we can achieve those science goals,” Palanque-Delabrouille, said. “It’s very exciting to see where we stand today.”

The formal start of DESI’s five-year survey follows a four-month trial run of its custom instrumentation that captured 4-million spectra of galaxies — more than the combined output of all previous spectroscopic surveys.

The DESI instrument resides at the retrofitted Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, a program of the National Science Foundation’s NOIRLab. The instrument includes new optics that increase the field of view of the telescope and includes 5,000 robotically controlled optical fibers to gather spectroscopic data from an equal number of objects in the telescope’s field of view.

“We’re not using the biggest telescopes,” said Berkeley Lab’s David Schlegel, who is DESI project scientist. “It’s that the instruments are better and very highly multiplexed, meaning that we can capture the light from many different objects at once.”

In fact, the telescope “is literally pointing at 5,000 different galaxies simultaneously,” Schlegel said. On any given night, he explains, as the telescope is moved into a target position, the optical fibers align to collect light from galaxies as it is reflected off the telescope mirror. From there, the light is fed into a bank of spectrographs and CCD cameras for further processing and study.

“It’s really a factory that we have — a spectra factory,” said survey validation lead, Christophe Yeche, also a cosmologist at CEA. “We can collect 5,000 spectra every 20 minutes. In a good night, we collect spectra from some 150,000 objects.”

“But it’s not just the instrument hardware that got us to this point — it’s also the instrument software, DESI’s central nervous system,” said Klaus Honscheid, a professor of physics at Ohio State University who directed the design of the DESI instrument control and monitoring systems. He credits scores of people in his group and around the world who have built and tested thousands of DESI’s component parts, most of which are unique to the instrument.

Spectra collected by DESI are the components of light corresponding to the colors of the rainbow. Their characteristics, including wavelength, reveal information such as the chemical composition of objects being observed as well as information about their relative distance and velocity.

As the universe expands, galaxies move away from each other, and their light is shifted to longer, redder wavelengths. The more distant the galaxy, the greater its “redshift.” By measuring galaxy redshifts, DESI researchers will create a 3D map of the universe. The detailed distribution of galaxies in the map is expected to yield new insights on the influence and nature of dark energy.

“Dark energy is one of the key science drivers for DESI,” said project co-spokesperson Kyle Dawson, a professor of physics and astronomy at University of Utah. “The goal is not so much to find out how much there is – we know that about 70% of the energy in the universe today is dark energy — but to study its properties.”

The universe is expanding at a rate determined by its total energy contents, Dawson explains. As the DESI instrument looks out in space and time, he says, “we can literally take snapshots today, yesterday, 1-billion years ago, 2-billion years ago — as far back in time as possible. We can then figure out the energy content in these snapshots and see how it is evolving.”

DESI is supported by the DOE Office of Science and by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. Additional support for DESI is provided by the U.S. National Science Foundation; the Science and Technologies Facilities Council of the United Kingdom; the Gordon and Betty Moore Foundation; the Heising-Simons Foundation; the French Alternative Energies and Atomic Energy Commission, or the CEA; the National Council of Science and Technology of Mexico; the Ministry of Economy of Spain; and by the DESI member institutions.

The DESI collaboration is honored to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.

DESI is supported by the DOE Office of Science.

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.