Operation and performance of ProtoDUNE Dual Phase liquid argon time projection chamber
This paper reports on the 2019–2020 operation and performance of ProtoDUNE-DP, the largest dual-phase liquid argon time projection chamber built to date, which successfully validated key technologies like high-voltage delivery and photon detection despite significant technical challenges with its charge readout planes, ultimately informing improved designs for future vertical drift detectors.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Imagine the universe is filled with invisible messengers called neutrinos. These tiny, ghostly particles zip through everything—planets, stars, and even your body—without ever saying hello. They are so shy that catching one is like trying to grab a snowflake with your bare hands in a blizzard. To catch them, scientists build giant, deep-underground traps filled with super-cold liquid argon, a noble gas that acts like a giant, invisible camera. When a neutrino finally bumps into an argon atom, it leaves a tiny trail of electric charge and a flash of light. By reading these trails, scientists can figure out what the neutrino was doing, which helps them solve mysteries about why the universe exists and how it works.
The big challenge is building a detector big enough to catch enough of these ghosts to study them. Scientists have been testing two different ways to build these giant cameras: one where the charge is read directly in the liquid, and another where the charge is pulled out of the liquid into a gas layer to be amplified, like turning a whisper into a shout. This second method, called "Dual-Phase," was the star of the show in a massive experiment called ProtoDUNE-DP. The goal was to see if this "whisper-to-shout" trick could work on a scale large enough for the future Deep Underground Neutrino Experiment (DUNE).
This paper tells the story of that giant experiment, which was the largest Dual-Phase liquid argon detector ever built. It operated for about a year, catching cosmic rays (particles from space) to see how the machine performed. The team found that while the "whisper-to-shout" system had some serious technical glitches that made it too unstable for the final, massive version of the experiment, it still taught them incredible lessons. They successfully proved they could deliver a massive electrical voltage of -300 kV to the detector and that their light-sensing system worked perfectly. However, the main charge-readout parts, which were supposed to amplify the signal, kept breaking down due to electrical sparks and a "charging-up" effect that dulled their performance over time. Because of these issues, the team decided to switch to a different design for the final DUNE detector, but the knowledge gained from this giant test run was essential for building the next generation of neutrino traps.
The Story of the Giant Argon Bubble
The Setup: A Frozen Ocean in a Box
Imagine a giant, stainless steel bathtub, but instead of water, it's filled with 720 tons of liquid argon, chilled to a frosty -186°C (87 K). This is the ProtoDUNE-DP detector, sitting at CERN in Switzerland. It's like a massive, frozen ocean inside a box. At the bottom of this ocean, there are 36 super-sensitive eyes (photomultiplier tubes) waiting to catch any flashes of light. At the top, floating just above the liquid surface, is a complex net called the Charge Readout Plane (CRP).
In this "Dual-Phase" setup, when a particle (like a cosmic ray muon) zips through the liquid argon, it knocks electrons loose. These electrons are like tiny, invisible swimmers. In a normal liquid detector, you'd try to catch them right there in the water. But in this Dual-Phase design, the scientists apply a strong electric field to pull these swimmers up out of the liquid and into a thin layer of argon gas floating above.
The Magic Trick: Amplifying the Whisper
Once the electrons cross the invisible border from liquid to gas, something magical happens. They enter a region with a very strong electric field where they crash into gas atoms, creating a chain reaction called a "Townsend avalanche." Think of it like a snowball rolling down a hill, picking up more snow until it becomes a giant boulder. In this case, one single electron can multiply into thousands, turning a tiny, hard-to-detect whisper of charge into a loud shout that the electronics can easily hear. This is the job of the Large Electron Multipliers (LEMs), which are like perforated sheets of circuit boards with thousands of tiny holes.
The Problems: Sparks and Sticky Surfaces
The paper reports that while the idea was brilliant, the execution had some bumps in the road. The detector ran from August 2019 to September 2020, but it faced significant technical headaches.
- The High Voltage Short: The detector needed a massive electrical push of -300,000 volts (300 kV) to pull the electrons up. The team built a special cable system to deliver this power deep into the tank. Unfortunately, a part of this cable system developed a short circuit (a leak in the insulation) about a quarter of the way up. This meant they couldn't reach the full -300 kV they wanted for most of the experiment. They had to run at a much lower voltage, which made it harder to pull electrons all the way to the top, but they still managed to collect data.
- The Sparking LEMs: The LEMs, which are supposed to amplify the signal, started acting up. They began sparking (tiny electrical discharges) more and more often. It was like having a room full of lightbulbs that kept flickering and popping. Over time, more and more of these amplifiers had to be turned down or shut off to prevent damage.
- The "Charging-Up" Effect: Even when the LEMs weren't sparking, they got "tired." As they ran, positive and negative charges built up on the insulating surfaces inside the holes, changing the electric field and making the amplification weaker. The paper found that the gain (how much the signal was boosted) dropped by a factor of about 2.5 over time. It's like a microphone that slowly loses its volume the longer you talk into it.
The Successes: What Worked
Despite the trouble with the amplifiers, the experiment was a huge success in other ways:
- The Light System: The 36 "eyes" at the bottom worked perfectly. They caught the flashes of light from the particles and even helped measure how well different materials (called PEN and TPB) could change the color of the invisible light into something the cameras could see. They found that the PEN material worked, but the TPB coating was about three times better at catching the light.
- The High Voltage Delivery: Even though they had a short circuit, they proved that they could build a system to deliver -300 kV to a detector of this size. This was a critical milestone. It showed that the future, even larger detectors could handle this kind of voltage.
- The Longest Drift: In a later run in 2021, after fixing the short circuit, they managed to pull electrons all the way up through 6 meters of liquid argon. This is the longest distance anyone has ever successfully drifted charge in a liquid argon detector. It was a world record for this type of technology.
The Conclusion: A Pivot, Not a Failure
The paper concludes that while the Dual-Phase design with LEMs had some great features (like being able to swap out electronics easily), the combination of the sparking LEMs, the charging-up effect, and the instability of the liquid surface made it too risky for the final, massive DUNE detectors.
Because of these lessons, the DUNE collaboration decided to change their plan. Instead of pulling the charge out into gas, they will use a new design called Vertical Drift. This new design keeps the charge in the liquid but uses a different kind of readout that doesn't rely on the tricky gas amplification. The experience from ProtoDUNE-DP was vital; it proved the -300 kV system works and showed exactly what not to do, paving the way for the next generation of neutrino detectors that will be built in the coming years. The giant bubble didn't catch the neutrinos the way they hoped, but it taught them exactly how to build a better trap for the future.
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