Effect of construction steels on PMTs detection efficiency at JUNO

Simulations confirm that the carbon steel rebars and TT bridge in the JUNO detector structure do not significantly compromise PMT detection efficiency, as the resulting residual magnetic fields remain within the experiment's acceptable limits of 10% for CD-PMTs and 20% for Veto-PMTs relative to the geomagnetic field.

The Gist

Imagine the JUNO experiment as a giant, ultra-sensitive camera trying to take a picture of the faintest, most elusive ghost in the universe. This "camera" is actually a massive tank filled with liquid scintillator, lined with thousands of special light-sensors called Photomultiplier Tubes (PMTs). These sensors are the camera's eyes, and they need to be perfectly still and focused to catch the tiny flashes of light from subatomic particles.

However, there's a problem: the Earth itself acts like a giant magnet. This natural magnetic field is like a strong wind that tries to blow the "light particles" (photons) off course before they hit the sensors. If the wind is too strong, the camera gets blurry, and the experiment fails.

To fix this, the scientists built a set of giant, invisible "magnetic umbrellas" (compensation coils) around the tank. These umbrellas are designed to cancel out the Earth's magnetic wind, creating a calm, still zone inside where the sensors can work perfectly.

The New Problem: The Construction Site
The paper asks a specific question: What happens when you build a massive concrete structure around this delicate setup? The water tank is surrounded by thick concrete walls and a heavy steel bridge (called the TT bridge) hanging above it. Inside that concrete are steel bars (rebars), and the bridge is made of heavy steel beams.

Think of these steel bars and the bridge like a bunch of iron filing scattered around a magnet. Even though the scientists built their "magnetic umbrellas" to cancel the wind, the steel in the building might get magnetized by the Earth's field and create its own little gusts of wind, potentially messing up the calm zone again.

The Investigation
The authors of this paper ran a detailed computer simulation to see if the steel in the construction would ruin the experiment. They modeled:

1. The Rebars: The steel mesh inside the concrete floor and walls of the water tank.
2. The TT Bridge: The heavy steel structure hanging above the tank.
3. The Coils: The magnetic cancellation system.

They used a special software tool (Radia) to calculate exactly how the steel would react to the Earth's magnetic field and whether it would disturb the sensors.

The Results: Good News
The simulation showed that while the steel does create some extra magnetic "wind," it isn't strong enough to break the experiment.

• The Goal: The scientists set a rule: the magnetic field inside the tank must be less than 10% of the Earth's natural field for the main sensors (CD-PMTs) and less than 20% for the outer sensors (Veto-PMTs).
• The Reality: Even with all the steel bars and the heavy bridge included, the "wind" inside the tank stayed well below the limit.
  • The main sensors experienced a magnetic field that was only about 9% of the Earth's natural strength.
  • The outer sensors experienced about 18%.

The Conclusion
The paper concludes that the construction steel acts like a slightly noisy neighbor, but not a loud one. It creates a little bit of extra magnetic disturbance, but the "magnetic umbrellas" (coils) are strong enough to handle it. The sensors will still see the light clearly, and the experiment's ability to detect particles will not be significantly harmed by the steel used to build the facility.

In short: The steel in the building is heavy and magnetic, but the scientists' magnetic cancellation system is strong enough to keep the "camera" focused, ensuring the experiment can successfully catch its ghostly targets.

Technical Summary

Technical Summary: Effect of Construction Steels on PMTs Detection Efficiency at JUNO

Problem Statement
The Jiangmen Underground Neutrino Observatory (JUNO) experiment relies on a central detector (CD) and a water Cherenkov veto system to achieve precise energy resolution (3% @ 1 MeV) and high photoelectron collection (1,200 p.e./MeV). These performance metrics require the photomultiplier tubes (PMTs)—specifically 20-inch Hamamatsu PMTs, 20-inch Microchannel Plate PMTs (MCP-PMTs), and 3-inch PMTs—to operate in a region where the residual magnetic field is minimized. The geomagnetic field at the JUNO site is approximately 50 µT; without mitigation, the Lorentz force would deflect photoelectron trajectories, causing significant losses in Photon Detection Efficiency (PDE), particularly for MCP-PMTs which can lose up to 15% efficiency at 20 µT.

To counteract this, JUNO employs a system of 16 pairs of compensation coils designed to reduce the magnetic field to less than 10% of the geomagnetic field strength in the CD-PMT region and less than 20% in the Veto-PMT region. However, previous simulations (e.g., Ref. [5]) did not account for the magnetic influence of the carbon steel structures inherent to the detector's construction, specifically the reinforcement bars (rebars) within the water pool concrete and the steel Top Tracker (TT) bridge. The presence of these ferromagnetic materials could distort the magnetic field, potentially compromising the shielding effectiveness of the compensation coils.

Methodology
This study evaluates the impact of the carbon steel rebars and the TT bridge on the residual magnetic field using the Radia 3D magnetostatics code. Radia was selected for its integral boundary method, which is computationally efficient for geometries extending to infinity compared to Finite Element Method (FEM) software.

The simulation incorporated the following components and simplifications:

• Material Properties: The magnetic behavior of the steel was modeled using the B-H curve of HRB400 structural steel, sourced from the National Institute of Metrology, China (NIM).
• Geometry and Simplification:
  • Rebars: The rebars in the water pool base (4 layers) and walls (2 cylindrical layers) were modeled. To manage computational load, the wall rebars were simulated with 1/4 the actual count but with doubled volume per bar to maintain total magnetic mass. The bottom rebars were modeled as a dense mesh covering the 45 m diameter base.
  • TT Bridge: Due to the structural complexity of the TT bridge (comprising trusses, girders, floors, and supports totaling hundreds of tons), the geometry was simplified into three rectangular prisms. This simplification preserved the total weight of the structure (maintaining the magnetic mass) while reducing geometric complexity.
• Field Configuration: The simulation included the geomagnetic field (perpendicular to the y-axis), the field generated by the compensation coils, and the induced magnetization of the steel structures.
• Sampling: Magnetic fields were sampled at 20,000 uniformly spaced points on spherical surfaces representing the locations of the CD-PMTs and Veto-PMTs.

Key Results
The simulations compared the residual magnetic field in two scenarios: one with only the compensation coils and geomagnetic field, and one including the rebars and TT bridge.

1. Overall Impact: The inclusion of construction steel significantly increased the residual magnetic field compared to the coil-only scenario (where fields were <3%). However, the fields remained within the experiment's safety margins.
2. CD-PMTs (Central Detector):
   • The maximum residual magnetic field experienced by CD-PMTs was 9% of the geomagnetic field strength.
   • Specific regional maxima included: ~7.6% near the bottom rebars, 9% below the TT bridge, and 9% near the wall rebars.
   • All CD-PMTs remained below the 10% acceptance limit.
3. Veto-PMTs (Water Cherenkov Detector):
   • The maximum residual magnetic field experienced by Veto-PMTs was 18% of the geomagnetic field strength.
   • Specific regional maxima included: ~16% near the bottom rebars, 17% below the TT bridge, and 18% near the wall rebars.
   • All Veto-PMTs remained below the 20% acceptance limit.
4. Spatial Distribution: The highest residual fields were not necessarily at the points closest to the steel structures but were concentrated near the main axis of the compensation coils and the bottom of the water pool. This is attributed to the strong combined magnetic field intensity in these specific regions.

Significance and Claims
The paper concludes that despite the presence of substantial carbon steel structures (rebars and the TT bridge) within the JUNO detector vicinity, the residual magnetic field experienced by the PMTs remains within the acceptable limits established by the experiment (10% for CD-PMTs and 20% for Veto-PMTs).

Based on the measured PDE curves (Figure 2), where efficiency loss is minimal below 5 µT and the geomagnetic field is ~50 µT, the authors assert that the residual fields (max 9% and 18% of 50 µT) will have minimal impact on the PMTs' photon detection efficiency. The study validates that the current compensation coil design is robust against the magnetic perturbations introduced by the necessary construction steel, ensuring the JUNO experiment can meet its physics requirements for observing the inverse beta decay spectrum.