A Brazilian technological innovation will be used in the Deep Underground Neutrino Experiment (DUNE), a multi-billion dollar project led by Fermilab, the leading particle physics laboratory in the United States, which is expected to be operational by the end of this decade. With the support of other research institutions and national companies, a team of physicists from the University of Campinas (UNICAMP) has developed a filtering method that removes a type of impurity commonly found in liquid argon: nitrogen atoms.
Maintained in chambers at minus 184 degrees Celsius (oC), argon, a noble gas at room temperature, becomes liquid and can be used to achieve the experiment’s main objective: to detect neutrinos, mysterious subatomic particles that have almost no mass, no electrical charge, and very little interaction with any material. Because it has a relatively heavy atomic nucleus, this chemical element is more likely to interact with neutrinos, the second most abundant particle in the universe after photons (particles of light).
The liquid argon chambers are the most advanced way of detecting neutrinos. The larger the volume, the greater the probability of interacting with the particles. For this reason, DUNE will have four pools, each containing 17,000 tons of this liquified chemical element. However, there are a number of contaminants in the tank that could affect the experiment. The three most common are oxygen, water, and nitrogen. Effective molecular filters exist for the first two types of contaminants. But not for nitrogen—at least not until the Brazilian team came up with their invention.
Contaminants are generally found at levels of less than 10 parts per million (ppm), which is very few micrograms in each gram of argon. “This level of impurity makes it impossible to carry out the experiment, and liquid argon of a higher purity is not available on the market,” explains physicist Pascoal Pagliuso, head of the UNICAMP group that developed the new method. “DUNE requires very few molecules of impurities, in parts per trillion.”
The largest neutrino experiment underway, at a cost of US$3.3 billion, DUNE consists of a facility (the Long Baseline Neutrino Facility, LBNF) located at Fermilab, which is dedicated to producing a beam of trillions of these particles, and two detectors separated by a great distance. It all starts at Fermilab’s particle accelerator in Batavia, on the outskirts of Chicago, Illinois. Proton collisions produce smaller particles, which decay and create other particles. Neutrinos are one of the by-products of these collisions and transformations of matter that take place in the accelerator. The LBNF is responsible for collecting a beam containing only these particles and sending it underground to the two detectors.
The first and smallest detector will be located next door, near the neutrino source at Fermilab, in a shallow cave, 60 meters deep. The second, much larger, will be located 1,300 kilometers away inside an old abandoned mine in Lead, a town in the state of South Dakota. The Sanford Underground Research Facility (SURF), which will house the detector in a cave that is being excavated 1,500 meters underground, is currently located there. The facility is designed to prevent the neutrino beam in South Dakota from being disturbed by cosmic rays and neutrinos from space and surface disturbances.
In 2020, with support from FAPESP, Brazilian researchers began developing an efficient method of purifying argon using a porous molecular sieve called a zeolite, based on the mineral aluminosilicate (composed of aluminum, silicon, and oxygen). The basic research that led to this technology was a study by Pagliuso’s team at UNICAMP focused on the differences between nitrogen (N2) and argon (Ar) and their response to application of an electric field.
The study’s practical aim was to find a zeolite that could absorb (through adhesion or fixation) only nitrogen molecules, leaving them free of argon. Dilson Cardoso, a chemist at the Federal University of São Carlos (UFSCar) who specializes in zeolites, was essential in this pursuit. Computer simulations were carried out for materials that could be used as filters to separate nitrogen from argon. “The modeling allowed us to determine how argon circulation and purification systems behave, providing data that would help us design various parts of the system,” explains chemical engineer Dirceu Noriler, from UNICAMP. “We obtained information on the filters’ saturation time, the number of purifiers needed, and the number of cycles needed to achieve the desired purity.”
The most promising materials were then tested on a small scale in a controlled super-cold environment. To this end, UNICAMP set up the Liquid Argon Purification Test Cryostat (PuLArC). Made of stainless steel and capable of holding 90 liters of liquid to be purified, the equipment was built by the companies Equatorial Sistemas and Akaer. The team also drew on their experience at the cryogenic laboratory at the Brazilian Center for Research in Physics (CBPF), in Rio de Janeiro. The cryostat is like a double-walled thermos with a vacuum in the middle. It prevents the environment’s temperature from affecting the temperature inside the container.
According to materials engineer Fernando Ferraz, Akaer’s vice president of operations, the experiment enabled them to create 3D models of the entire purification plant. We carried out comprehensive simulations of the transport process, assembly, and installation of all necessary equipment for one of DUNE’s laboratories,” says Ferraz. “The process of controlling the purity of argon requires filtering cycles in the liquid and gaseous states, regeneration, and condensation.”
Ryan Postel / Fermilab DUNE experiment cave in an old South Dakota mine where one of the neutrino detectors will be locatedRyan Postel / Fermilab
The results of the PuLArc tests were published in August 2024 in the Journal of Instrumentation. According to the publication, a filter made of a material known as Li-FAU, which contains lithium in addition to aluminosilicate, was the most efficient at capturing nitrogen molecules in liquid argon. With its use, the contamination in 100 liters of argon, which was initially between 20 and 50 ppm, fell to between 0.1 and 1 ppm in less than two hours. The filter was also tested by the DUNE team in a larger 3,000-liter tank and the results were equally good.
The Li-FAU-based process is currently in the final stages of testing at ProtoDUNE, the prototype for DUNE at the European Organization for Nuclear Research (CERN), on the France-Switzerland border. There, the amount of liquid argon to be purified exceeds several tons. The new process has been patented and could be used for other purposes in the future. It appears to be versatile and has the potential to be used to purify other gases, perhaps carbon dioxide, and liquids on an industrial scale.
The filter to remove contaminants from liquid argon is the second major contribution stemming from Brazil’s participation in DUNE, which has attracted 1,400 scientists and engineers from 200 institutions and 35 countries. The first contribution was a photon trap that captures the flashing light produced by the interaction of neutrinos with argon atoms. Invisible to the human eye, the light has a wavelength of 127 nanometers. By storing this data, the trap enables researchers to study the properties of neutrinos and reconstruct their trajectory in three dimensions. The device, called X-Arapuca, was created in the middle of the last decade by physicists Ettore Segreto and Ana Amélia Machado, from UNICAMP. Its latest version, 2.0, is already in use in the United States.
The neutrinos beamed at Fermilab will travel through Earth’s crust and reach the liquid argon tanks. Interaction with the argon releases electrons and produces flickers of light. A uniform electric field directs them towards the electron detectors. The photons produced by the scintillations are captured by the X-Arapuca traps. “The photons produced in the scintillation allow me to calculate when the neutrinos arrived, which direction they came from, and how they interacted with the argon,” explains Machado. We still do not know the mass of each of the three known types of neutrinos—muon, tau, and electron—or why they oscillate with each other as they move.
At the Sandford Research Center, where DUNE’s largest detector will be located, at least two of the experiment’s four modules will have X-Arapuca traps. The traps will form a photodetection system around the liquid argon pools. With funding from FAPESP, Brazil will be responsible for building some of the components and assembling and installing 6,000 X-Arapuca traps in one of the DUNE modules by the start of data collection, scheduled for 2029. “The biggest challenge will be to coordinate the process of building the traps in Brazil and receiving the rest of the components from abroad, without jeopardizing the experiment’s schedule,” says Segreto. “In Brazil, we will produce the mechanical parts and the optical filters, which are the most important elements for the device to function.”
According to Sylvio Canuto, a physicist at the University of São Paulo (USP), it is important to invest in DUNE, which should reveal details about neutrinos and, by extension, the formation of the universe. One of the most intriguing questions is why there are more particles than antiparticles in the cosmos. “In theory, we expected particles and antiparticles to have been created in the same proportion at the beginning of time. But today we see that the universe is mostly made up of particles. The origin of this mystery is attributed to the role of neutrinos, and today we are closer to unravelling it,” says Canuto, who has followed Brazil’s participation in DUNE since the beginning of the project and is an advisor to FAPESP’s Scientific Directorate. The next step, according to the USP physicist, is to ensure Brazilian participation in the work of analyzing the data generated by DUNE, thus creating a reference center in the country for Latin America.
The story above was published with the title “Closer to neutrinos” in issue 344 of October/2024.
Projects
1. Advanced instrumentation for large collaborations in high-energy physics: Air purification and photodetection for LBNF-Dune (n° 24/07128-7); Grant Mechanism Research Grant ‒ Special Projects; Principal Investigator José Pagliuso (UNICAMP); Investment R$84,484,851.05.
2. Light detection system for the Deep Underground Neutrino Experiment (n° 21/13757-9); Grant Mechanism Thematic Project; Principal Investigator Ettore Segreto (UNICAMP); Investment R$17,916,736.09.
3. Light detection system for the Dune-X-Arapuca experiment (n° 19/11557-2 ); Grant Mechanism Young Investigator Award; Principal Investigator Ana Amélia Bergamini Machado (UNICAMP); Investment R$2,992,720.82.
Scientific article
CARDOSO, D. et al. Innovative proposal for N2 capturingin Liquid Argon using the Li-FAU molecular Sieve. Journal of Instrumentation. Vol. 19. Aug. 2024.
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