Newswise — Boxes filled with carefully packed circuit boards and other electronic components left a loading dock of the physics building at the U.S. Department of Energy’s Brookhaven National Laboratory this past spring for a transatlantic journey. This was the last shipment of state-of-the-art components destined for a revolutionary particle detector now being assembled at CERN, the European laboratory for particle physics. By early fall, the circuit boards and chips should be reading out signals of charged particles traversing a tank filled with frigid liquid argon. This detector, called ProtoDUNE-SP (for single phase), is a proving ground for ideas and technologies developed at Brookhaven Lab and collaborating institutions for a mammoth international experiment to be built in the United States that seeks to answer some of the deepest questions in physics.
As the name implies, ProtoDUNE is a prototype for a much larger detector for the Deep Underground Neutrino Experiment (DUNE), an international endeavor to search for new subatomic phenomena and potentially transform our understanding of neutrinos, the most abundant particles of matter in the universe. Neutrino s generated at Fermi National Accelerator Laboratory (Fermilab) in Illinois will travel through Earth’s crust to the Sanford Underground Research Facility 800 miles away in South Dakota. By studying neutrinos near the start and end of this journey, DUNE scientists from institutions in 32 countries hope to learn more about these particles’ ability to switch identities among three known “flavors,” and what these mysterious particles can tell us about the evolution of the universe.
“We’ve been pushing for a long-baseline neutrino experiment like DUNE for 20 years,” said Milind Diwan, one of a team of Brookhaven physicists who’ve been applying lessons gleaned from decades of neutrino research to design and refine key technologies needed to achieve DUNE’s research goals. “We are excited to see all of these key design features being tested in ProtoDUNE.”
A whole of many parts and partners
DUNE’s massive far detector will be built in enormous underground caverns containing four vessels, each filled with 17,000 tons of liquid argon, a material that can be ionized—triggered to emit electrons—when charged particles are created by the occasional neutrino striking an argon atom. Sophisticated electronics designed to operate in the frigid liquid (-303 degrees Fahrenheit) will trace the tracks of the emitted electrons as they drift through the liquid between a negatively charged cathode and a positively charged anode—much like a giant battery—so scientists can capture 3D images of the neutrino interactions.
“Brookhaven scientists pioneered the development of liquid argon detectors and the associated sensitive electronics. These ideas have fed the development of liquid argon time projection chambers (LArTPC) initiated in Italy and further developed by Brookhaven and collaborating institutions. The DUNE LArTPC will be the pinnacle of achievement for this technology, which combines the detection medium and the electronics into one unit,” said Brookhaven Lab physicist Bo Yu, a leader in detector design.
The Brookhaven team also developed a “systems engineering” approach for building the detector in a modular fashion and how to put it all together in the underground laboratory—as well as the software needed to reduce electronic “noise” so physicists can extract subtle neutrino signals.
Iterations of these ideas and technologies have been incorporated in other neutrino experiments in Italy and the United States over the years—and refined by scientists at Brookhaven and other national laboratories and research institutions around the world.
“Our colleagues in the field have taken many of the concepts that originated at Brookhaven and run with them and improved them to make this world-class experiment a reality,” said Mary Bishai, another member of the Brookhaven team. “For DUNE, we are working with more than 1,000 scientists from over 30 countries.”
Scaled down test of full-size components
ProtoDUNE will be the ultimate test of the full array of technologies, giving scientists a chance to work out any kinks before construction starts on the full-size DUNE detector.
“With ProtoDUNE, we are testing components that we are going to use in DUNE,” said Brookhaven physicist and DUNE collaborator Elizabeth Worcester. “We are testing all the production techniques, all the quality control procedures. In many ways, ProtoDUNE is ‘module 0’—the first version of what we will be building for DUNE.”
The prototype has six “anode plane assemblies,” each wrapped with 15 miles of fine wires that will pick up signals of drifting electrons from both sides. These anode planes are the same size they’ll be in the DUNE detector, and the drift distance between cathode and anode is also the same.
“All the pieces for ProtoDUNE are the same size as for DUNE—which was determined by what could fit down the shaft into the underground area where the DUNE detector will be assembled. There will just be more of these pieces in the final detector,” Yu said—150 anode plane assemblies in each of four 10-kiloton detector modules.
Integrated cold electronics
Each anode plane assembly has an integrated design that includes the electronics for reading out signals from the electron-sensing wires, and photon detectors that track telltale flashes of light for each neutrino-argon interaction.
“The photon detector tells you when a neutrino interaction took place. Having that information, together with the time it takes electrons to drift to and strike the anode plane wires and where they hit, gives you the location and the ability to reconstruct these events in three dimensions and fully describe the interaction,” explained Bishai.
But neutrino interactions at DUNE will still be rare. The scientists expect to see only about 10 per day. And these subtle signals could be lost or distorted by electronic “noise”—interference generated by outside sources of electromagnetic radiation or heat—especially if the raw signals have to be transmitted outside the cold chamber before being read out. For a huge detector like DUNE, the challenges multiply as the distance signals have to travel to exit the detector increases.
“Some cables will have to be 25 meters or longer, which would distort the signal and add too much noise if we were reading out the raw analog signals. Digitized signals won't suffer from this problem,” said Matt Worcester, a Brookhaven physicist who is working to address this challenge.
The plan for DUNE is to have integrated analog-to-digital converters for digitizing the signals right inside the detector. Converting signals to digital format eliminates electromagnetic interference no matter how far the signals have to travel. A version of this next-generation analog-to-digital converter that can operate in the cold was designed at Brookhaven and will be used in ProtoDUNE.
"We’re working with our colleagues at Fermilab, SLAC National Accelerator Laboratory, and Lawrence Berkeley National Laboratory to develop an improved analog-to-digital converter that will be used in DUNE,” Matt Worcester said. Meanwhile, other groups at Brookhaven, SLAC and other institutions are engaging in active research and development on alternate designs. Ultimately, whatever works best will be used in DUNE.
This animation shows how the ProtoDUNE-SP detector picks up signals from charged particles and the drifting electrons that they release to create 3D particle tracks. The tracks reveal details about the neutrinos interacting with argon atoms in the detector.
Reducing noise and assessing performance
But as Bishai pointed out, “It is not enough to have low-noise electronics if you are going to put all of these electronics in a noisy environment. Experience has shown that a low-noise chip is not sufficient. You need an integrated system that includes the chips, the circuit boards, the cables, the electrical environment in which these sit, the electrical grounding to the thermos-like cryostat and the anode plane assembly, as well as the shielding that keeps external electromagnetic signals from coming in.”
For instance, the high voltage on the cathode—which sets up the powerful electric field that causes the electrons triggered by neutrino interactions to drift—can introduce noise. So too can the power supplies that provide the high voltage in the field and on the sensing wires.
“All of that has to be tested,” Bishai said.
The Brookhaven scientists also developed components of the “warm electronics”—those that connect the detector’s internal cold components to the ordinary temperatures outside the cryostat. Some of these components provide “on-board” diagnostics.
“These sit right outside the cryostat and will allow us to do some debugging on what’s going on inside the cryostat—check that channels are reading out the way they’re supposed to be, do noise checks, voltage checks, and so on,” Matt Worcester explained. “You can just plug in your laptop and run software written by our engineers that interprets the data to do a quick verification that things are working—or identify connections that need to be fixed.”
Pre-prototype testing
A lot of testing took place long before anything was shipped to CERN, and the testing continues there, even before ProtoDUNE becomes fully operational.
“Last summer, we had undergrads who were doing internships at universities and other national labs and even two local high-school students do a lot of the testing—plus grad students and postdocs from DUNE-collaborating institutions. It was a huge effort,” said Bishai. “They were testing the analog-to-digital converters and the analog chips in the cold—dunking them into liquid nitrogen to see how they would perform.”
“We’ve tested things on lab benches, we’ve tested in liquid nitrogen at Brookhaven, and we’ve tested in a cold box with cold gaseous nitrogen at CERN—all with huge help from our collaborators,” Elizabeth Worcester elaborated.
So far, the results look good.
“We took a fully assembled anode-plane-assembly system with all the readout electronics, the data acquisition system, the power supplies—everything—and put it in a box with cold gaseous nitrogen and were able to measure noise as we expected or even better than the design,” Bishai said. “That was a major achievement.”
Next steps
Testing of the full ProtoDUNE detector will continue through the fall. Those tests will use a beam of charged particles to simulate the charged-particle signals that will result from neutrino interactions with argon.
“We will use these tests to calibrate the detector response to the charged particles that are produced by neutrino interactions—electrons, pions, protons,” Bishai said.
That way, when neutrinos come along in DUNE and produce these types of particles, the scientists will know from the controlled tests what the signals mean.
“Neutrino interactions with argon are sufficiently complicated that we don’t know exactly how many of each type of particle will be produced. But if we know how our detector responds, we can work our way backwards to understand the neutrino interactions,” Bishai said.
The scientists are ready to get going.
“The team at CERN has been phenomenally successful,” Elizabeth Worcester said—erecting a new building, constructing the cryostats, and completing the beamlines that will deliver particles to the detector. Then, after receiving the final components, they installed the detector.
“They closed the cryostat in early July, and they’ve been cooling it down and filling it with 145,000 gallons of liquid argon over the last two months,” she said.
“At every step we need to continue to check for other sources of noise, so there’s still a lot of work to do,” noted Bishai. “We want to be sure to work all that out before we get to DUNE, because it will take a full year to fill those tanks (six tanker trucks a day for a full year pumping argon down the mile-long shaft!).”
The collaborators expect to turn on the ProtoDUNE detector in early in September and complete all possible testing before CERN shuts down for a major upgrade to the Large Hadron Collider in 2019.
As Diwan summed it up, “After many years of effort, the Brookhaven team is excited to see the results from these ProtoDUNE tests—so we can move on to incorporating what we learn into the design of detectors for DUNE, the most ambitious neutrino experiment ever undertaken.”
Brookhaven Lab’s contributions to ProtoDUNE and DUNE are funded by the DOE Office of Science.
Brookhaven National 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.
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