Design, construction, and operation of a 30-ton Water-based Liquid scintillator detector at Brookhaven National Laboratory

This paper presents the design, construction, and operational details of a 30-ton Water-based Liquid Scintillator (WbLS) prototype detector at Brookhaven National Laboratory, which was developed to explore the medium's unique capabilities for separating Cherenkov and scintillation signals and enabling versatile neutrino detection.

The Gist

Imagine you are trying to build a giant, underwater camera that can see the invisible ghosts of the universe: neutrinos. These tiny particles zip through everything, including the Earth, without leaving a trace. To catch them, scientists need a massive tank of liquid that glows when a neutrino bumps into it.

For decades, scientists have used two main types of "cameras":

1. Water Tanks: Like a giant swimming pool. When a particle zips through, it creates a cone of blue light (like a sonic boom, but for light). It's great for seeing direction, but the light is dim.
2. Liquid Scintillator Tanks: Like a tank of glowing oil. When a particle hits it, the whole thing lights up brightly. It's great for seeing energy, but it's hard to tell exactly where the particle came from.

The Big Idea:
What if we could mix the two? What if we could create a liquid that acts like water and oil at the same time? That is exactly what the scientists at Brookhaven National Laboratory (BNL) have done. They built a 30-ton prototype detector filled with a special mixture called Water-based Liquid Scintillator (WbLS).

Think of WbLS as a microscopic salad. You have a giant bowl of water, and floating inside are billions of tiny, invisible bubbles (micelles) filled with the glowing oil. These bubbles are so small they don't sink or clump together; they stay perfectly suspended.

Here is a simple breakdown of how they built and tested this giant "salad bowl":

1. The Tank (The Bathtub)

They built a massive stainless steel tank, about the size of a large shipping container, capable of holding 30 tons of liquid.

• The Challenge: Stainless steel is usually fine, but over time, it can "leak" tiny metal ions into the water, making it cloudy.
• The Fix: They polished the inside of the tank to a mirror shine and treated it with a special chemical "pickling" to stop it from leaking. They also made sure every pipe and pump was made of materials that wouldn't react with the liquid.

2. The Eyes (The Cameras)

Inside this tank, they installed 36 giant eyes (called Photomultiplier Tubes, or PMTs).

• The Arrangement: Imagine 12 eyes looking up from the floor of the tank, and 24 eyes looking in from the walls, arranged in four rows.
• Why? This is the magic trick. When a particle zips through, it creates a cone of blue light (Cherenkov) that hits the bottom eyes. But the glowing oil (Scintillation) lights up everywhere, hitting the wall eyes too. By comparing what the bottom eyes see versus what the wall eyes see, the computer can figure out exactly what kind of particle passed through and where it came from. It's like having a 3D movie camera that can tell the difference between a fast car and a slow bicycle just by how the headlights look.

3. The Heart (The Circulation System)

Keeping 30 tons of liquid perfectly clear is hard. Dust, rust, or tiny impurities can make it cloudy, ruining the experiment.

• The Solution: They built a sophisticated "kidney" system for the tank. The liquid is constantly pumped out, cleaned, and pumped back in.
• The Filters:
  • The Nanofiltration System: Think of this as a super-fine sieve. It separates the tiny glowing bubbles from the water, cleans the water of rust and dirt, and then puts the bubbles back in.
  • The "Band-Pass" Filter: This is a fancy filter that acts like a radio tuner. It only lets specific things (like Gadolinium, a metal used to catch neutrons) pass through while blocking everything else.
  • The Iron Remover: They used special resin beads (like a sponge) that suck up iron rust from the water, keeping the liquid crystal clear.

4. The Brain (The Control System)

You can't just leave a 30-ton tank of glowing liquid alone. It needs a brain.

• They built a "Slow Control System" that monitors everything: temperature, pressure, flow rates, and liquid levels.
• If a pump gets too hot or a tank starts to overflow, the system automatically shuts things down to keep the scientists and the facility safe. It's like a smart home system, but for a giant physics experiment.

5. The Test Drive (Commissioning)

Before pouring in the expensive glowing liquid, they filled the tank with pure water.

• The Goal: To make sure all the cameras, pumps, and computers worked together.
• The Result: They watched cosmic rays (particles from space) pass through the water. The system worked perfectly! They saw the expected "rings" of light on the cameras.

6. The Big Pour (Injection)

Once the water test was a success, they injected the WbLS.

• The Process: They didn't just dump it in. They added it slowly in stages (0.3%, then 0.75%, then 1%) while the circulation system mixed it like a giant blender.
• The Surprise: When they first added the liquid, the light didn't just get brighter immediately. It went up, then down, then up again. Why? Because the tiny bubbles clumped together near the injection point, creating a "fog" that scattered the light. As the circulation system mixed it all out, the fog cleared, and the light became bright and uniform. It was a perfect demonstration of how the liquid behaves.

Why Does This Matter?

This 30-ton tank is a prototype. It's a "proof of concept" to show that we can build a 1,000-ton (or even larger) detector in the future.

• The Future: If this works at 30 tons, scientists can build massive detectors to study neutrinos from the sun, from exploding stars (supernovas), and from particle accelerators.
• The Bonus: This technology can also help detect nuclear materials (for non-proliferation) and even search for dark matter.

In a Nutshell:
The scientists at Brookhaven built a giant, high-tech bathtub filled with a special glowing "salad." They proved they can keep it clean, mix it perfectly, and use it to take 3D pictures of invisible particles. This successful test paves the way for building the next generation of giant neutrino observatories that could unlock the secrets of the universe.

Technical Summary

1. Problem Statement

Neutrino physics requires detectors capable of operating at multiple scales (MeV to GeV) with high precision for energy resolution, particle identification (PID), and background rejection. Traditional detectors face a trade-off: Water Cherenkov detectors offer large volumes and good timing but poor energy resolution and limited PID; Liquid Scintillator detectors offer high light yield and PID but are expensive to scale to kiloton sizes and suffer from self-absorption.

Water-based Liquid Scintillator (WbLS) was proposed as a hybrid solution: a stable aqueous solution containing nanometer-scale micelles of liquid scintillator. This medium aims to combine the Cherenkov and Scintillation signals, allowing for their separation and tuning. However, the scalability of WbLS technology remained unproven. Key challenges included:

• Maintaining long-term liquid stability and optical clarity in large volumes.
• Preventing metal ion leaching (e.g., Iron, Chromium) from stainless steel tanks which degrade optical transparency.
• Developing effective purification and circulation systems to remove impurities without disrupting the micelle structure.
• Validating the ability to "metal-load" the detector (e.g., with Gadolinium) for neutron tagging.

2. Methodology

The authors designed, constructed, and commissioned a 30-ton WbLS prototype at Brookhaven National Laboratory (BNL) to bridge the gap between previous 1-ton proofs-of-concept and future kiloton-scale experiments (e.g., Theia).

Key Technical Components:

• Detector Tank: A cylindrical 316L stainless steel tank (radius ~1.6m, height ~3m) holding 30 tons of liquid. The tank was passivated to minimize leaching and operated without an internal liner, requiring direct material compatibility.
• Photomultiplier Tubes (PMTs): 36 Hamamatsu R16367 (10-inch) PMTs were submerged directly in the liquid.
  • Geometry: 12 PMTs in a spiral on the floor; 24 PMTs in four rows on the walls.
  • Strategy: This geometry distinguishes Cherenkov light (conical, hitting bottom/lower walls) from isotropic Scintillation light (hitting upper walls), enabling signal separation.
• Circulation & Purification System: A sophisticated loop designed to continuously filter and purify the WbLS.
  • Nanofiltration (NF): A two-stage system. Stage 1 removes micelles (using a 600-800 Da membrane) to prevent fouling; Stage 2 removes free organics (using a 200 Da membrane) while retaining Gadolinium ions.
  • Gd-H2O System: A "Molecular Band-pass Filter" system designed to selectively retain Gadolinium ions while removing other impurities, analogous to an electronic band-pass filter.
  • Sequential Exchange Array (SEA): A resin-based filtration unit specifically developed to remove leached metal ions (Fe, Cr, Ni) from the stainless steel tank without removing the Gadolinium.
• Data Acquisition (DAQ): Utilized CAEN V1730S (14-bit, 500 MSPS) and V1740 digitizers. Triggers were generated via cosmic muon coincidence (top/bottom scintillator paddles) and a majority trigger from bottom PMTs.
• Calibration: Used a $^{210}$Pb radioactive source (initially) and later a random NIM pulser method to calibrate PMT gain and Transit Time Spread (TTS).

3. Key Contributions

• Scale-Up Validation: Successfully demonstrated the engineering feasibility of a 30-ton WbLS detector, an order of magnitude larger than previous prototypes.
• Novel Purification Architecture: Developed and tested a multi-stage purification strategy (NF + Band-pass + SEA) capable of handling the unique chemistry of WbLS, specifically addressing the challenge of removing metal ions without losing the Gadolinium dopant or disrupting micelles.
• In-Situ Circulation: Proved that continuous circulation and purification can be maintained in a large volume without degrading the optical properties of the liquid.
• PMT Integration: Validated the long-term stability of submerged PMTs in WbLS, including soak tests and gain monitoring over months of operation.
• Mixing Dynamics Analysis: Provided the first detailed observation of WbLS mixing behavior, identifying non-uniform dispersion phases (micelle-dense regions) that affect trigger rates and light yield during injection.

4. Results

• Commissioning: The detector was successfully commissioned with pure water (Omega-18) starting July 2024, establishing a stable baseline.
• PMT Performance:
  • Average Transit Time Spread (TTS) was measured at 2.8 ns (consistent with manufacturer specs).
  • Gain stability was maintained at ~1 pC (Single Photoelectron) throughout the operation, with daily variations under 10%.
  • Two-month soak tests confirmed no degradation in PMT seals or performance.
• WbLS Injection & Mixing:
  • WbLS was injected to reach a 1% concentration in May 2025.
  • Light Yield: A significant increase in light yield was observed on side PMTs (out-of-Cherenkov-cone) compared to the water phase, confirming the scintillation component.
  • Mixing Dynamics: During injection, a transient drop in trigger rate was observed. This was attributed to a "micelle-dense" region near the injection point causing high scattering, which temporarily reduced the number of PMTs hitting the Cherenkov cone. As circulation homogenized the mixture, the trigger rate and light yield stabilized at expected higher levels.
• Purification: The Sequential Exchange Array (SEA) demonstrated a capacity to remove ~14.33 mg of Iron per gram of resin, effectively mitigating metal leaching from the tank.
• Stability: The detector operated stably for several months in pure water and several days post-injection, with environmental factors (temperature, flow) monitored for correlation with data stability.

5. Significance

This work represents a critical milestone in the global R&D program for WbLS technology.

• De-risking Future Experiments: The successful operation of the 30-ton prototype validates the scalability of WbLS, providing essential data to de-risk the design of future multi-kiloton detectors like Theia and BUTTON 1kT.
• Physics Potential: By proving the ability to separate Cherenkov and Scintillation light and to effectively load the medium with Gadolinium (for neutron tagging), the technology opens new avenues for:
  • Neutrino Physics: Precision measurements of oscillations (GeV scale) and solar/supernova neutrinos (MeV scale).
  • Nuclear Non-Proliferation: Enhanced capabilities for reactor monitoring via neutron tagging.
  • Dark Matter: Potential for active shielding in direct detection experiments.
• Technological Legacy: The developed purification systems (NF, Band-pass, SEA) and the operational experience with large-scale liquid handling provide a blueprint for future large-volume liquid detectors.

The paper concludes that the 30-ton detector is fully operational and stable, with detailed performance analyses and comparisons to Monte Carlo simulations (RATPAC-TWO) reserved for future publications. The next phase involves full Gadolinium doping and the deployment of an AmBe calibration system for neutron-gamma coincidence studies.