Operating a large-diameter dual-phase liquid xenon TPC in the unshielded PANCAKE facility

This paper reports the successful stable operation of a large-diameter, shallow dual-phase liquid xenon time projection chamber within the unshielded PANCAKE facility, demonstrating that sensitive performance characterization is achievable in a high-background environment despite a relatively high energy threshold.

The Gist

Imagine trying to build a giant, ultra-sensitive camera that can see the faintest whispers of light from invisible particles. To do this, scientists usually need to bury their equipment deep underground to block out the "noise" of cosmic rays raining down from space. But what if you wanted to test a huge, new camera lens before you built the whole camera, and you didn't have a deep underground lab handy?

That is exactly what this paper describes. A team of physicists in Freiburg, Germany, built a massive, shallow "test tank" called PANCAKE right on the surface of the Earth. They filled it with liquid xenon (a heavy, cold gas turned into a liquid) and ran a giant, flat detector inside it, all while surrounded by the noisy, unshielded background of the everyday world.

Here is a breakdown of what they did and found, using simple analogies:

1. The "Swimming Pool" Test Tank

Think of the PANCAKE facility as a giant, high-tech swimming pool, but instead of water, it holds liquid xenon.

• The Size: It's huge. The tank is about 9 feet (2.75 meters) wide.
• The "Swimmer": Inside this tank, they floated a very flat, pancake-shaped detector. It was about 4.5 feet (1.33 meters) wide but only about an inch (3 cm) tall.
• The Challenge: Usually, these detectors are buried deep underground to avoid cosmic rays (particles from space). This facility was on the surface, meaning it was bombarded by cosmic rays constantly. It was like trying to listen to a whisper in the middle of a rock concert.

2. The "Pancake" Detector

The detector itself was a Time Projection Chamber (TPC).

• How it works: Imagine a sandwich. The bottom slice is a "cathode" (negative), the top is an "anode" (positive), and in the middle is a "gate." When a particle hits the liquid xenon, it creates a flash of light (S1) and frees some electrons.
• The Drift: The electric field pulls those electrons up toward the top. When they hit the gas layer above the liquid, they create a second, bigger flash of light (S2).
• The Goal: By measuring the time between the first flash and the second, and how bright they are, scientists can figure out exactly where the particle hit and what kind of particle it was.

3. The "Noise" Problem and the Solution

Because they were on the surface, the detector was flooded with background noise.

• The Analogy: Imagine trying to hear a single drop of water fall in a stadium full of cheering fans.
• The Result: Despite the noise, the team proved the detector worked. They used a special "muon telescope" (like a pair of binoculars looking up at the sky) to tag when a cosmic ray passed through. They found that the detector could still distinguish real events from the noise, even without the usual underground shielding.

4. Testing the "Wires" and "Cables"

The detector uses thousands of tiny wires to create the electric fields.

• The Stress Test: The team wanted to see if these wires would snap or sag when cooled down to -100°C (the temperature of liquid xenon).
• The "Guitar String" Test: They used a special device to pluck the wires (like a guitar string) and listen to the vibration. By measuring the pitch, they could tell how tight the wire was.
• The Finding: After running the detector for weeks in the freezing cold, the wires were just as tight as they were before. They didn't break or loosen up.

5. Cleaning the "Water"

For the detector to work, the liquid xenon must be incredibly pure. If there are tiny impurities (like oxygen or water), they act like "sponges" that catch the electrons before they reach the top, ruining the signal.

• The Purification: They ran the xenon through a giant filter system (a "getter") to suck out the impurities.
• The Proof: They measured how long the electrons survived before being caught. At first, they died quickly (10 microseconds). After cleaning, they lived much longer (25 microseconds). This proved their cleaning system worked, even in a dirty, unshielded environment.

6. The "Flashlight" Calibration

To test how sensitive the detector was, they injected a tiny amount of a radioactive gas called Krypton-83.

• The Test: This gas decays in two quick steps, creating two flashes of light very close together in time. It's like a strobe light flashing twice.
• The Result: In "light-only" mode (no electric field pulling electrons), they could clearly see these double flashes. This told them the detector could see energy levels as low as about 15 keV (a very small amount of energy).
• The Limitation: When they turned on the electric fields (TPC mode), the signal got weaker, and the low-energy flashes became harder to see. This is because the electric field "quenches" (dampens) the light, similar to how a strong wind might blow out a candle flame.

The Bottom Line

This paper is a "proof of concept." It shows that you can build and operate a giant, 100-kilogram scale detector on the surface of the Earth, without expensive underground shielding, and still get useful, high-quality data.

They proved that:

1. The massive wires and cables can survive the extreme cold.
2. You can clean the xenon effectively even in a noisy environment.
3. You can detect particle interactions and measure their properties.

This success is a crucial step for future, even bigger projects (like the proposed XLZD detector) that will need to test massive components before they are buried deep underground to hunt for dark matter. They built the "pancake" to prove the recipe works before baking the whole cake.

Technical Summary

Technical Summary: Operating a Large-Diameter Dual-Phase Liquid Xenon TPC in the Unshielded PANCAKE Facility

Problem and Motivation
Future liquid-xenon (LXe) observatories for rare processes, such as the proposed XLZD (Xenon Large Detector), require target masses and exposures significantly larger than current generation detectors (e.g., PandaX-4T, XENONnT). To achieve exposures of up to 1000 t×y and explore the full parameter space before the "neutrino floor," detectors with linear dimensions of approximately 3 meters are required. Developing and testing critical components for these multi-ton scale instruments under realistic cryogenic conditions necessitates large-scale test facilities. However, many existing facilities are either underground (shielded) or lack the capacity to test large, flat components. The PANCAKE facility at the University of Freiburg offers a solution as an unshielded, above-ground platform with a large internal diameter (2.75 m), but its suitability for operating a large-diameter TPC in a high-background environment without dedicated low-background materials had not been demonstrated.

Methodology
The authors operated a shallow, dual-phase LXe Time Projection Chamber (TPC) within the PANCAKE facility to test its diagnostic capabilities and operational stability.

• The TPC: The detector featured a cylindrical active volume with an inner diameter of 133.4 cm and a height of 3.1 cm, containing approximately 127 kg of active LXe (out of a total 340 kg xenon inventory). It utilized three stainless steel electrodes (cathode, gate, and anode) originally designed for the XENONnT upgrade. The electrodes were biased to create a drift field of 140 V/cm and extraction fields of 3.7 kV/cm (liquid) and 7.2 kV/cm (gas).
• Instrumentation: The setup included a hexagonal array of 19 Hamamatsu R11410-21 PMTs suspended in the gas phase above the TPC for light detection. Cryogenic cameras (overview and grid cameras) monitored the interior, and a muon telescope installed below the facility tagged cosmic ray events. High-voltage systems provided bias to the electrodes, and a wire-tension measurement system verified mechanical integrity.
• Environment: The experiment was conducted in an unshielded environment with no underground suppression of cosmic-ray backgrounds. The facility was equipped with a purification system (SAES purifier) and a trace gas analyzer (HALO+) to monitor xenon purity.
• Data Acquisition: A self-triggered DAQ system (based on XENONnT's strax framework) recorded PMT signals. Data analysis involved reconstructing S1 (prompt scintillation) and S2 (delayed electroluminescence) signals, correlating them with muon tags, and analyzing electron drift times and lifetimes.

Key Contributions and Results
The paper presents the first successful operation of a ~1.33 m diameter dual-phase LXe TPC in the unshielded PANCAKE facility. Key results include:

• Stable Operation: The TPC operated stably for approximately 60 days over a four-month period. The system maintained stable PMT gains (within ±10%) and demonstrated that a large-diameter TPC can function effectively despite the high background rates inherent to an unshielded, above-ground location.
• Event Reconstruction: Despite the high background, the authors successfully reconstructed events by pairing S1 and S2 signals. The spatial correlation between the centroids of S1 and S2 signals on the PMT array confirmed the validity of the event matching, even with a small PMT array and no light reflectors.
• Purity and Electron Lifetime: Xenon purification was monitored via the HALO+ sensor and TPC data. Over a ten-day period, the electron lifetime ($\tau_e$) increased from 10 $\mu$s to ~25 $\mu$s. This value exceeds the maximum electron drift time of the TPC (20 $\mu$s), demonstrating that the purification system is sufficient to achieve high charge collection efficiency in this geometry.
• Drift Velocity: The electron drift velocity was measured at drift fields of 140 V/cm. The results agreed with established field-dependent measurements from previous experiments, particularly those taken at similar LXe temperatures (~−106°C).
• Calibration and Thresholds: Using a $^{83m}$Kr source, the authors identified the 32.1 keV and 9.4 keV transitions in "light-only" mode. The 9.4 keV signal was detectable above the threshold. In TPC mode, field quenching reduced the light yield, making the 9.4 keV line difficult to distinguish from background, implying an effective electronic recoil threshold of approximately 15 keV.
• High-Voltage Diagnostics: The system identified limitations in the high-voltage range due to cable breakdowns (observed as light emission along cables at high cathode voltages) and successfully used cameras to locate these discharges.

Significance and Claims
The authors claim that this work demonstrates the diagnostic capabilities of the PANCAKE platform for testing large-scale TPC components. Specifically, the paper establishes that:

1. Sensitive measurements characterizing TPC performance (electron lifetime, drift velocity, event reconstruction) are feasible in a high-background, unshielded environment using a basic PMT-based light detection system.
2. The PANCAKE facility can support the testing of large-diameter (1.3 m) components required for future multi-ton detectors like XLZD.
3. The platform is suitable for testing high-voltage systems and studying mechanical stress on large components during cooling to cryogenic temperatures.

The paper concludes modestly, noting that while the current light collection efficiency was low (due to a lack of reflectors), future runs with optimized light detection will lower the energy threshold and enable more precise calibration measurements. The work serves as a proof-of-concept for using large, shallow TPCs in unshielded environments to validate components for next-generation dark matter and neutrino experiments.