What happens after a highly energetic beam of protons from a particle accelerator slams into a target designed to generate particles for advanced physics experiments?
Within microseconds, the area of impact in the target heats up by hundreds of degrees, inducing rapid expansion of its inner material. This expansion is constrained by the still cold surrounding material of the target, resulting in a pressure wave akin to a hammer blow from within. As atoms are displaced, defects are created, thereby altering the physical properties of the target. Concurrently, nuclear reactions occur as atoms break down, producing helium and hydrogen gases within the target material. These gases can accumulate into bubbles that, in turn, inflict damage.
Researchers are designing targets to withstand the onslaught of protons at the U.S. Department of Energy’s Fermi National Accelerator Laboratory for various high-energy physics experiments, such as the Deep Underground Neutrino Experiment.
“If the target can’t survive the violent interaction with the beam, then the experiment is off and the experiment cannot take data,” said Patrick Hurh, who has been working as an engineer at Fermilab for 35 years. “As target engineers, we like to feel that we are the gears that connect the accelerators to the experiments, harnessing the power of the proton beam to create the science.”
One type of particle that researchers at Fermilab are studying is the elusive neutrino. These subatomic phantoms are neutrally charged and small enough to easily pass through most objects. As neutrinos travel, they can shape-shift among several types. Their unique characteristics may carry the key to revealing foundational secrets such as why there is more matter than antimatter in the universe.
“In the case of generating a beam of neutrinos, we use a graphite target, which is carbon,” said Hurh. “The protons basically collide with the carbon atoms and produce a variety of secondary particles coming out of the target.”
Some of these secondary particles generated from the collisions are electrically charged pions. Because these pions are charged, researchers can steer their trajectories with magnetic fields.
To produce these magnetic fields, current is pumped into a powerful electromagnet that surrounds the target and focuses the pions. The pions then travel down a several-hundred-meter-long pipe, where they decay and produce neutrinos.
The target and magnet are part of a larger facility called a target station, which sprays neutrinos like shining light from a flashlight. The target station and the decay pipe enable several neutrino experiments.
“At the exit of the decay pipe are detectors called hadron and muon monitors that can tell us about the health of the target equipment,” said Hurh. “If the target breaks and falls apart, then more protons will make it through and less muons will be generated. We can see that in these monitors and then investigate.”
The three challenges
Researchers need to overcome three challenges to make a lasting target: radiation damage, high temperatures and stress from thermal expansion.
Frederique Pellemoine is a scientist and engineer at Fermilab who leads Fermilab’s High-Power Targetry R&D group and coordinates the Radiation Damage in Accelerator Target Environments collaboration. The RaDIATE collaboration was established in 2012 by Patrick Hurh with researchers from four other institutions. This collaboration grew to 20 national and international institutions, bringing experts together to study the damage caused by accelerator beams and find ways to build better targets.
“At Fermilab, we study the effects of radiation damage and thermal shock resistance on graphite, beryllium, titanium alloys and novel materials,” said Pellemoine.
A nanofiber developed by Fermilab engineer Sujit Bidhar is being researched as a potential target material due to its ability to mitigate thermal shock and be more resistant to radiation damage. High-entropy alloys are also being investigated by Fermilab engineer Kavin Ammigan for a critical part of the target system that separates the target from the accelerator. Ammigan’s research earned him a Department of Energy Early Career Award in 2022.
“Through all this research, we support the design of future targets,” added Pellemoine. “The design considerations include production of particles, operational safety, storage and disposal.”
Researchers examine samples of targets that have been used in Fermilab’s accelerator complex. They also bombard samples under controlled beam parameters in facilities such as the Brookhaven Linac Isotope Producer that can replicate damage to materials from long exposure to Fermilab’s accelerator beam.
After recreating the long-term damage, researchers will send these samples to other laboratories for post-irradiation examinations of the material to assess physical or structural property changes.
“With each pulse from Fermilab’s accelerator, you have a rapid increase in temperature that creates a thermal shock,” said Pellemoine. “The expansion of the heated material within a cold core causes compressive or tensile stress. One year of operation can include millions of pulses.”
The samples will be sent to an accelerator facility at CERN in Geneva, Switzerland, where researchers will create thermal shocks in the material with very high intensity single pulses. RaDIATE researchers compare the shocks between pristine samples and damaged samples.
“Radiation damage defects in materials can be annealed, or self-healed, at higher temperatures. It is important to study the temperature effect in our radiation damage studies,” said Pellemoine. “We need to find the sweet spot to anneal those defects but avoid creating other damages at high temperature.”
Next-generation experiment
A next-generation neutrino experiment is under construction at Fermilab in Batavia, Illinois and Sanford Underground Research Facility in Lead, South Dakota.
The Long Baseline Neutrino Facility and Deep Underground Neutrino Experiment will involve propelling a beam of neutrinos from Fermilab’s campus in Illinois to massive detectors a mile underground in South Dakota. Since the neutrinos are so small and rarely interact with other forms of matter, they will travel through the earth and arrive at the detectors.
“About every second, a high-intensity pulse of protons will hit the target for the LBNF/DUNE experiment, and billions of neutrinos will be produced,” said Chris Densham, the high-power targets group leader at Rutherford Appleton Laboratory in Oxfordshire, England. “You create a very short pulse so that your detectors can know when the particles came from Fermilab.”
For LBNF, Densham is leading Rutherford Appleton’s contribution to create the target. The graphite target for LBNF/DUNE will be 1 1/2 meters long and will operate at a high temperature, allowing the material to partially repair itself as it is being irradiated.
“Graphite is a strange material; it’s quite happy being hot,” said Densham. “When it gets hot, it causes the structure to jumble around. If there’s an atom that gets knocked out of place, there is another one that’s able to fill it in.”
Due to the thermal shock that will be produced by quick pulses, Densham and his team developed a unique temperature control system to help equalize the temperatures within the target and the target surface which can help increase the lifespan of the target.
“We are going to use gaseous helium flowing over at high velocity around 440 meters per second,” said Densham. (Essentially, the helium could travel around a school’s running track in a second). “The end of the target looks like the front of a jet engine. Helium is a gentle coolant that doesn’t take too much heat out, so the target runs hot.”
“There’s a surprising amount of engineering details in making these targets and integrating them together,” added Densham.
Fermi National Accelerator Laboratory 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, please visit science.energy.gov.
The Spanish Ministry of Science, Innovation and Universities signed a memorandum of understanding with the U.S. Department of Energy’s Fermi National Accelerator Laboratory to further their participation in the development and production of advanced technologies for the international Deep Underground Neutrino Experiment. DUNE is an international megascience experiment that will use enormous particle detectors to study the behavior of neutrinos, which might indicate why we live in a matter-dominated universe.
The MOU signed by the Secretary of State for Science, Innovation and Universities, Juan Cruz Cigudosa, and the Director of Fermilab, Lia Merminga, formalizes the shared interest between both parties to work together on the construction of the DUNE detectors.
“This Memorandum of Understanding illuminates and expands on Spain’s long-time partnership with Fermilab in working together on high-energy physics research. Their neutrino physics work and contributions to DUNE are valuable to the project’s physics program and the necessary analysis tools using the latest software technologies,” said the Director of Fermilab, Lia Merminga.
As a founding member of the international collaboration, Spain has participated in DUNE since its inception in 2015 through six research groups from the Center for Energy, Environmental and Technological Research (CIEMAT), the Institute of Corpuscular Physics (CSIC-UV), the Institute of Theoretical Physics (CSIC-UAM), the Galician Institute of High Energy Physics (IGFAE-USC), the University of Granada and the University of Vigo.
As part of the MOU, Spain’s contributions to DUNE include the light-detection and temperature-monitoring system for the CERN prototypes and the massive liquid argon detectors that will be installed deep underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota.
Additional contributions from Spain include the coordination of the DUNE physics program, technical leadership of the large liquid argon detectors as well as the coordination of DUNE’s Phase II detector research and development. They also lead key groups related to light-detection and temperature-monitoring systems, the physics of low-energy neutrinos and physics beyond the standard model.
“The signing of the Memorandum of Understanding for the DUNE experiment is an important step for Spain, strengthening its role in particle physics and reaffirming bilateral relations with the United States,” said Juan Cruz Cigudosa, the Secretary of State for Science, Innovation and Universities. “This agreement recognizes the valuable contributions of Spanish researchers and institutions to DUNE and reinforces a shared commitment to international scientific collaboration, encouraging the exchange of knowledge and resources in this ambitious project.”
DUNE will study neutrino oscillation, a phenomenon in which a neutrino’s property, called flavor, changes as it travels. DUNE will probe this oscillation by shooting a beam of neutrinos 1,300 kilometers straight through the earth, from Fermilab’s accelerator complex in Illinois, through the Near Detector to the Far Detectors located a mile underground at SURF in South Dakota.
DUNE will be the world’s most comprehensive experiment to study neutrinos: tiny, lightweight particles that permeate the universe but rarely interact with anything. The experiment will seek to determine whether neutrinos could be the reason the universe is made of matter, look for neutrinos emitted from exploding stars to learn more about the formation of neutron stars and black holes and watch for a rare subatomic phenomenon that could explain the unification of nature’s forces.
The DUNE collaboration represents scientists from dozens of countries around the world who will contribute to the construction of detectors at two sites in the United States: one at Fermilab, the host lab for DUNE, 40 miles west of Chicago, and the other at SURF in Lead, South Dakota.
The science of DUNE is a global endeavor, and the partnership with funding agencies, scientists and engineers from around the world make it the first truly international megascience experiment to be hosted on U.S. soil. Additionally, hundreds of students from around the world will begin their careers in science, engineering and computing on DUNE.
Fermi National Accelerator Laboratory 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, please visit science.energy.gov.