R&D of cosmic ray detection module with liquid2 scintillator and wavelength shift fiber

This paper presents the research and development of a cost-effective cosmic ray detection module utilizing liquid scintillator and wavelength-shifting fibers, demonstrating through prototype testing that it offers a viable solution for background rejection in neutrino physics and rare event searches.

The Gist

The Big Picture: Catching Invisible Rain

Imagine the Earth is constantly being rained on by invisible particles called cosmic rays (mostly high-speed protons and muons) from space. While scientists love studying these particles, they are also a nuisance. If you are trying to find something extremely rare and quiet underground (like a ghostly neutrino or a rare decay), these cosmic rays are like a noisy crowd at a library—they create "background noise" that hides the signal you are looking for.

To solve this, scientists need a way to spot these cosmic rays and say, "Ah, that's just a cosmic ray, ignore it." This paper describes a new, cost-effective "net" designed to catch these cosmic rays.

The Tool: A Liquid Scintillator Sandwich with Fiber Optics

The team built a prototype detector that works like a high-tech sandwich:

1. The Filling (Liquid Scintillator): Instead of solid plastic, they used a special liquid that glows (emits light) when a cosmic ray particle smashes into it. Think of this liquid as a pool of water that flashes brightly whenever a stone is thrown into it.
2. The Straws (Wavelength-Shifting Fibers): Inside this liquid pool, they threaded 32 thin optical fibers (like straws) in a grid pattern—16 running horizontally and 16 running vertically.
   • How it works: When a particle hits the liquid, the liquid flashes. The fibers act like light pipes, catching that flash and guiding it to the ends of the box.
   • The Twist: These fibers are special "wavelength-shifting" fibers. They catch the blue-ish light from the liquid and change it into a different color that is easier for the sensors to see, kind of like a translator converting a foreign language into English.
3. The Eyes (PMTs): At both ends of every fiber, there is a sensor called a Photomultiplier Tube (PMT). These are super-sensitive eyes that can detect even a single photon of light.

How They Tested It

The researchers built a 1-meter square box (about the size of a large coffee table) filled with this liquid and fibers. They tested it in three different "states":

• Air: Just the empty box.
• Water: The box filled with plain water.
• Liquid Scintillator: The box filled with the special glowing liquid.

They used a "coincidence" rule to filter out noise. Imagine you have four security guards (the sensors) watching the box. If only one guard sees something, it might just be a glitch. But if all four guards (or at least two) see a flash at the exact same time, they know it's a real cosmic ray passing through.

What They Found

The results were very promising:

• Clear Distinction: The detector could easily tell the difference between the "background noise" (natural radioactivity from the environment) and the "real signal" (cosmic muons).
  • Analogy: It's like being able to hear a loud drum beat (the muon) clearly over the soft hum of a refrigerator (the background noise).
• Thickness Matters: The thicker the layer of liquid, the more light the detector caught.
  • At 2 cm thick, the detector saw a blur.
  • At 3 cm and above, the "drum beat" became so loud that it was impossible to confuse with the "fridge hum."
  • At 8 cm thick, the detector caught about 125 flashes (photoelectrons) for every single cosmic ray that passed through.
• Counting the Rays: The detector successfully counted about 85 cosmic rays per second passing through the box. This matches what scientists expect to find on the ground, proving the detector is working correctly.
• Mapping the Path: Because the fibers are arranged in a grid, the detector can guess where the particle entered.
  • The Catch: While the computer simulation (the virtual test) showed they could pinpoint the location within about 6 centimeters, the real-world data was a bit messier. The real detector tended to guess the center of the box more often than the actual edge. The team admits they need to tweak their math to make the real-world tracking as sharp as the simulation.

The Bottom Line

This paper proves that a detector made of liquid scintillator and optical fibers is a viable, affordable, and effective way to spot cosmic rays.

• Why it matters: It offers a cheaper alternative to building massive, expensive detectors.
• The Verdict: It works well at distinguishing cosmic rays from background noise and can count them accurately. However, the team needs to do more work to perfect the "GPS" feature (reconstruction) that tells them exactly where the particle came from in the real world.

In short: They built a glowing, fiber-optic net that catches cosmic rays efficiently, and it's ready to be scaled up for future large-scale observatories.

Technical Summary

Technical Summary: R&D of a Cosmic Ray Detection Module with Liquid Scintillator and Wavelength-Shifting Fibers

Problem Statement
In the context of neutrino physics and rare event searches, cosmic muons present a significant background challenge that must be effectively identified and rejected. Concurrently, there is a critical need for large-scale cosmic ray detection arrays that balance high performance with cost-efficiency. While liquid scintillators (LS) have demonstrated advantages in large-scale projects like JUNO, and fiber-based plastic scintillators have been utilized in TAO and LHAASO, there remains a need to optimize and validate specific detector schemes combining LS with wavelength-shifting (WLS) fibers. Previous studies have explored such configurations, but uncertainties persist regarding detector response to varying particle energies, background rejection capabilities, environmental adaptability, and long-term stability.

Methodology
The authors developed and tested a prototype cosmic ray detection module designed to be modular, fast-responding, and cost-effective.

• Detector Design: The module consists of a 100 cm × 100 cm × 15 cm aluminum box housing a PVC container (80 cm × 80 cm × 12.5 cm) filled with liquid scintillator. Inside, 32 BCF-92 WLS fibers (1.5 mm diameter, 2.4 m length) are arranged in orthogonal layers (16 horizontal, 16 vertical) to provide two-dimensional position tracking. The fibers are secured by perforated PVC holding panels and coupled to 3-inch photomultiplier tubes (PMTs) at both ends using silicone oil and a four-stage polishing protocol to maximize photon collection.
• Readout System: Four Hainan Zhanchuang Photonics (HZC) 3-inch PMTs (type XP72B22), previously used in JUNO prototypes, were employed. They were calibrated to a gain of approximately $6 \times 10^6$ using single-photon electron testing. Signals were digitized using a CAEN DT5743.
• Simulation: A Geant4-based Monte Carlo simulation was constructed to match the experimental dimensions, incorporating LS properties, WLS fiber parameters (BCF-92), and PMT quantum efficiency. The simulation modeled muon energy deposition (Landau distribution) and scintillation photon production (Gaussian distribution).
• Experimental Setup: The prototype was tested in three media: air, pure water, and liquid scintillator. Measurements were conducted with varying liquid thicknesses (2 cm to 8 cm) and different coincidence trigger modes (1/4 to 4/4).

Key Contributions and Results

• Detector Response and Background Rejection: The prototype successfully distinguished between cosmic muons and natural radioactivity. With an 8 cm LS thickness, the muon signal produced a most probable value of approximately 125 photoelectrons (PE), clearly separated from the natural radioactivity peak. The study demonstrated that increasing LS thickness improves the separation between these two populations; a distinct double-peak spectrum emerged when thickness exceeded 3 cm.
• Light Yield Linearity: Experimental results confirmed a near-linear relationship between LS thickness and the number of collected photoelectrons. The detector generates approximately 16 PE per centimeter of LS thickness. This experimental slope ($k$) was consistent with Geant4 simulation data, though a discrepancy in the intercept was attributed to the physical fiber holding structures in the real detector, which blocked photons at lower thicknesses—a factor not fully modeled in the simulation.
• Muon Identification Rates: The module achieved a muon identification rate of approximately 85 Hz under 4/4 coincidence counting with an 8 cm LS thickness. This rate aligns with expected ground-level muon flux for the detector's dimensions. The identification efficiency improved significantly as LS thickness increased from 3 cm to 5 cm, stabilizing thereafter.
• Reconstruction Performance: A time-based reconstruction algorithm was proposed to determine the muon vertex. While simulations showed a strong correlation between true and reconstructed positions with a resolution of approximately 6 cm on both axes, the experimental data exhibited a centralized distribution toward the detector center. The authors note that the reconstruction algorithm performed better in simulation than with real data, indicating a need for further investigation.
• Noise Suppression: The 4/4 coincidence mode effectively suppressed noise. In air, the random coincidence rate was negligible (<0.001 Hz at a 6 mV threshold), confirming the system's ability to filter out dark counts.

Significance and Claims
The paper claims that the proposed liquid scintillator and WLS fiber module offers a viable, low-cost option for future large-scale cosmic ray observatories and underground rare-event detection experiments. The study highlights the feasibility of this specific detector schema in addressing current challenges in particle physics and astrophysics, particularly in differentiating cosmic muons from background radioactivity.

The authors modestly conclude that while the prototype demonstrates excellent muon identification capabilities and good photoelectron response, the long-term stability of the LS with fiber integration requires further investigation. Additionally, the discrepancy between simulation and data in vertex reconstruction suggests that the algorithm requires refinement before deployment in large-scale arrays. The work serves as a foundational step toward forming a candidate for a larger-scale cosmic observatory array.