Quantitative U/Th deposition and cleanliness control strategies in the JUNO site air

To meet the ultra-low radioactive contamination requirements of the JUNO detector, the study details the implementation of a large-scale underground cleanroom system that maintains air cleanliness around class 74,000 and introduces a highly sensitive ICP-MS method for directly measuring and controlling U/Th deposition rates on detector surfaces.

The Gist

Imagine you are trying to catch a single, tiny firefly in a massive, pitch-black stadium. That firefly is a neutrino, a ghostly particle that rarely interacts with anything. The JUNO experiment is that stadium, a giant underground tank filled with 20,000 tons of special liquid that glows when a neutrino bumps into it.

But here's the problem: The universe is full of "noise." Just like trying to hear a whisper in a rock concert, the faint glow of the neutrino can be easily drowned out by background radiation from dust, dirt, and even the rocks around the cave.

This paper is the story of how the scientists built a "silent room" deep underground to hear that whisper, and how they measured every speck of dust to make sure it wouldn't ruin the party.

The Challenge: The "Radioactive Dust" Problem

Think of the air in a normal city as a fog filled with tiny, invisible radioactive specks (like uranium and thorium). In the JUNO cave, the walls are made of rock that is naturally radioactive. If even a tiny bit of this rock dust floats into the liquid, it acts like a thousand tiny flashlights, blinding the detectors to the real signal.

The scientists needed the liquid to be 12 orders of magnitude cleaner than the rock around it. To put that in perspective:

• If the rock dust were a mountain, the allowed dust in the liquid would be smaller than a single grain of sand.
• The entire 20,000-ton tank could only tolerate about 8 milligrams of dust. That's less than the weight of a single eyelash!

The Strategy: Building a "Clean Room" Underground

To achieve this, the team turned a massive underground cavern into the world's largest, most rigorous cleanroom.

1. The Air Shower: Imagine a car wash, but for people and boxes. Before anyone or anything could enter the main hall, they had to go through an "air shower" that blasted them with clean air to knock off any dust clinging to their clothes or equipment.
2. The Bubble Wrap: The giant glass ball (the acrylic sphere) holding the liquid was wrapped in special protective film, like a giant piece of bubble wrap, to keep it safe while it was being built.
3. The "Rain" Cleaning: Once the glass ball was built, they didn't just wipe it down. They used 3D nozzles to spray a fine mist of ultra-pure water inside the sphere. Think of it like a gentle rainstorm inside the ball. The water droplets caught the floating dust and washed it to the bottom, cleaning the air inside the sphere by a factor of 100.
4. The "No-Go" Zone: They banned welding, cutting, and even paper boxes (which shed dust) from the area. Everyone wore special "space suits" (cleanroom suits) to ensure no human hair or skin flakes escaped.

The Detective Work: Measuring the Invisible

Even with all these rules, the scientists needed proof. They couldn't just guess the air was clean; they had to measure exactly how much radioactive dust was landing on surfaces every day.

They set up "dust traps"—tiny, ultra-clean bottles and plates made of special plastic (PTFE and PFA) that are naturally very quiet (low radioactivity). They left these traps out in different spots:

• Open vs. Closed: They compared traps left open to the air vs. those covered up.
• Up vs. Down: They checked if dust fell faster on the top of a plate or the bottom.
• Shapes: They used cylinders and spheres to mimic the shape of the detector.

The Results:

• Gravity is the main culprit: Dust mostly falls straight down due to gravity. Surfaces facing up collected the most dust.
• Closed is better: Traps with lids collected far less dust than open ones.
• The "Acrylic Powder" Spike: During the construction, when workers polished the glass panels, tiny bits of acrylic flew into the air. This caused a temporary spike in dust counts, but the scientists knew this dust was "cleaner" than the rock dust, so it wasn't as dangerous.

The Final Verdict: Did They Succeed?

After three years of construction and cleaning, the team crunched the numbers:

1. The Liquid: The amount of radioactive dust that managed to sneak into the 20,000-ton liquid is so small that it contributes almost nothing to the background noise. It's like a single grain of sand in a swimming pool.
2. The Outside: The dust that settled on the outside of the glass ball (on the metal supports and cameras) does create some background noise, but the thick layer of water surrounding the ball acts as a shield, blocking most of it.
3. Radon: The only slight worry is "Radon" gas, which comes from the dust. The dust on the outside of the tank releases a tiny bit of radon into the surrounding water, but it's only about 20% of the maximum allowed limit.

The Takeaway

This paper is a testament to human precision. The scientists treated the construction of a neutrino detector like a surgeon performing heart surgery in a dusty workshop. By turning the workshop into a sterile operating room, using "air showers" for people, "rainstorms" for the glass, and "dust traps" to measure the air, they ensured that the only thing glowing in the dark is the neutrino they came to find.

They proved that with enough care, you can make a room so clean that even the universe's dirtiest particles can't find a place to hide.

Technical Summary

Here is a detailed technical summary of the paper "Quantitative U/Th deposition and cleanliness control strategies in the JUNO site air" by Zhao et al.

1. Problem Statement

The Jiangmen Underground Neutrino Observatory (JUNO) aims to detect neutrinos using a 20-kiloton liquid scintillator (LS) detector located 700 meters underground. To achieve its physics goals, the LS must possess an ultra-low radioactive background, specifically requiring a $^{238}\text{U}$ and $^{232}\text{Th}$ concentration of $10^{-17}$ g/g.

The primary challenge is that airborne dust in the experimental hall contains radioactive isotopes at levels roughly 12 orders of magnitude higher than the required purity of the LS. The underground rock walls, which were only sandblasted, release dust containing ppm-level uranium and thorium. If this dust settles on the detector components (the acrylic vessel, photomultiplier tubes, and stainless steel structures) or enters the LS during filling, it will introduce unacceptable background noise.

• Constraint: The total permissible dust mass in the 20 kt LS is approximately 8 mg.
• Requirement: The environment inside the 35.4-meter acrylic sphere must reach Class 1,000 cleanliness before filling, while the broader experimental hall must be maintained between Class 10,000 and Class 100,000.

2. Methodology

The authors employed a multi-faceted approach combining environmental engineering, rigorous cleaning protocols, and quantitative experimental measurement.

A. Cleanliness Control Strategies

• Environmental Isolation: The 120,000 m³ underground hall was isolated from external traffic tunnels using fire doors and air shower rooms for personnel and cargo.
• Filtration: Four self-circulating filtration fans (200,000 m³/h capacity) equipped with three-stage filtration (primary, medium, and high-efficiency) were installed.
• Surface Cleaning: Extensive deep cleaning was performed, including water flushing of rock walls, vacuuming, and wiping of all surfaces. The stainless steel (SS) truss and acrylic sphere underwent multiple cleaning cycles, including rust removal and polyurea treatment.
• Operational Protocols: Strict protocols were enforced, including mandatory cleanroom suits, removal of paper packaging at air showers, and banning of welding/cutting operations.
• Acrylic Sphere Specifics: A protective film covered the acrylic panels. Before filling, a 3D water spray system was used to generate micron-level mist, causing dust to adhere to water droplets and settle, followed by high-pressure flushing of the inner surface.

B. Quantitative Deposition Measurement

To move beyond theoretical estimates, the team developed a direct measurement method for U/Th deposition rates:

• Sampling Devices: Custom containers were designed to simulate detector geometries:
  • PTFE/Acrylic Plates: To test orientation effects (upward, downward, sideways).
  • PFA Bottles (Cylindrical): To simulate tanks.
  • PFA Flasks (Spherical): To simulate the JUNO acrylic sphere.
• Sample Processing: Samples underwent rigorous cleaning (degreasing, acid washing) and were exposed for 9–14 days. Post-exposure, samples were dissolved using acid (HNO$_3$ + HF) and organic powders (acrylic dust) were removed via filtration or ashing.
• Analysis: Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to measure U/Th concentrations with sub-ppt ($<10^{-12}$ g/g) sensitivity.
• Validation: Blank tests and recovery efficiency tests (using $^{229}\text{Th}$ and $^{233}\text{U}$ standards) confirmed the method's accuracy, with recovery rates around 106–108%.

3. Key Contributions

1. Translation of Physics Requirements to Engineering Standards: The paper quantitatively links the $10^{-17}$ g/g LS purity requirement to specific air cleanliness classes (Class 1,000 inside the sphere, Class 10,000–100,000 in the hall) and calculates the maximum allowable dust mass (8 mg).
2. Development of a Direct Deposition Measurement Technique: The authors established a robust experimental framework to measure actual U/Th settling rates on various surfaces and geometries under real-world construction conditions, rather than relying solely on theoretical models.
3. Validation of Cleaning Efficacy: The study provides empirical data proving that the implemented cleanroom management and water-mist cleaning strategies successfully reduced airborne particulate levels to the required specifications.
4. Background Budgeting: The paper quantifies the contribution of installation-era dust to the detector's background, distinguishing between direct contamination of the LS and gamma-ray background from external components.

4. Key Results

• Environmental Monitoring: Since May 2022, continuous laser particle monitoring achieved an average cleanliness level of Class 74,000 in the main hall. During acrylic sphere assembly, the environment was maintained between Class 10,000 and Class 100,000.
• Deposition Rates:
  • Geometry Effect: Spherical containers (flasks) showed settling rates ~6 times lower than cylindrical bottles due to geometric shielding.
  • Orientation Effect: Upward-facing surfaces collected 10–40 times more dust than downward-facing surfaces, confirming that gravitational settling is the dominant mechanism.
  • Material Effect: PTFE and acrylic plates showed similar upward deposition rates (~1000–1800 pg/d/m²), while side/downward rates were significantly lower.
  • Open vs. Closed: Open spaces had settling rates two orders of magnitude higher than closed spaces.
• Impact on JUNO Detector:
  • Acrylic Inner Surface: Estimated U/Th deposition during the pre-filling and filling periods is 1.3–13 ng, contributing **$0.13\text{--}1.3 \times 10^{-18}$g/g** to the LS. This is well below the$10^{-17}$ g/g requirement.
  • Residual Film: A small amount of protective film (~280 g) remained on the inner surface. Aging tests showed a leaching rate of 0.6–1.3 $\times 10^{-19}$ g/g/year, which is negligible.
  • External Components (PMTs/Truss): Dust on the outer surfaces contributes a single-event rate of ~2 mHz in the fiducial volume (R < 17.2 m), accounting for only 0.03% of the total detector background (7 Hz).
  • Radon: Radon release from deposited U/Th into the 9-kt water shield is estimated at ~2 mBq/m³, which is 20% of the maximum permitted limit (10 mBq/m³).

5. Significance

This work is critical for the success of the JUNO experiment and serves as a benchmark for future ultra-low background experiments.

• Feasibility Proof: It demonstrates that achieving the extreme radiopurity required for next-generation neutrino physics is possible through rigorous on-site cleanliness control and quantitative monitoring.
• Methodological Standard: The developed method for directly measuring U/Th deposition rates on detector surfaces provides a vital tool for validating cleanliness strategies in other low-background experiments (e.g., dark matter searches, double-beta decay experiments).
• Risk Mitigation: By quantifying the background contribution from installation dust, the authors confirmed that the construction process would not compromise the detector's physics sensitivity, allowing the collaboration to proceed with confidence in the final data analysis.