Development and Testing of a Modular Large-Area Cosmic Ray Telescope Using Scintillator-Fiber Hybrid Design for Millimeter-Level Muon Tracking

This paper presents the development and testing of a cost-effective, meter-scale modular cosmic ray telescope utilizing a novel scintillator-fiber hybrid design that achieves millimeter-level muon tracking with better than 2 mm spatial resolution and approximately 85% detection efficiency while significantly reducing the number of readout electronic channels.

The Gist

Imagine you are trying to track a ghost. This ghost is a cosmic ray muon—a tiny particle from deep space that zips through the Earth at nearly the speed of light. It's invisible, it passes through walls, and it's everywhere. To catch it, you need a special "net."

This paper describes a new, super-smart net built by scientists at Shandong University. It's designed to catch these cosmic ghosts with incredible precision (down to the size of a grain of sand) but without needing a million dollars worth of expensive electronics.

Here is the story of how they built it, explained simply:

1. The Problem: Too Many Ghosts, Too Many Cameras

Usually, to see where a ghost passed through a room, you might put up a grid of thousands of tiny cameras. If the room is 1 meter by 1 meter, and you want to know exactly where the ghost went, you might need hundreds of cameras.

• The Issue: Each camera needs its own wire, its own computer channel, and its own power. This gets expensive and messy very quickly.
• The Goal: They needed a telescope (a giant detector) that could pinpoint a muon's location to within 2 millimeters (about the thickness of a credit card) but use far fewer wires.

2. The Solution: The "Barcode" System

The scientists invented a clever trick using two different types of "sensors" stacked on top of each other. Think of it like a barcode scanner.

• Layer 1: The "Flashlight" (Scintillator Bars)
  Imagine 18 long, thick plastic sticks (like oversized popsicle sticks) laid side-by-side. When a muon hits one of these sticks, it flashes with light.

  • Role: This tells you which stick the muon hit. It's a rough guess, like saying, "The ghost was in the 5th row of the library."

• Layer 2: The "Fiber Optic Net" (Scintillating Fibers)
  Underneath those sticks is a mat made of thousands of tiny glass fibers (like hair-thin straws). These are arranged in bundles.

  • The Magic: Here is the genius part. Instead of giving every single fiber its own camera, they grouped them. They took the 1st fiber from every stick, tied them together, and sent them to one camera. Then they took the 2nd fiber from every stick, tied them, and sent them to another camera.
  • Role: This acts like a barcode. If the "5th stick" flashed (from Layer 1) AND the "3rd bundle of fibers" flashed (from Layer 2), the computer knows the muon hit the exact spot where the 5th stick and 3rd bundle cross.

The Analogy:
Imagine a city grid.

• The Bars are the Streets (North-South).
• The Fibers are the Avenues (East-West).
• Normally, you'd need a camera at every single intersection to know where a car is.
• Their trick: They only need cameras at the ends of the streets and the ends of the avenues. If Street 5 and Avenue 3 both light up, the car must be at the intersection of 5 and 3. This saves them from needing thousands of cameras; they only need 36!

3. Building the Machine

They built two giant layers of this "net," stacked one meter apart.

• Size: Each layer is 1 meter by 1 meter (about the size of a large dining table).
• Construction: They carefully aligned 18 modules side-by-side. Inside each module, they glued a plastic stick on top of a mat of 54 tiny fibers.
• The Eyes: They used special light detectors called PMTs (Photomultiplier Tubes). Think of these as super-sensitive eyes that can see a single photon of light.
  • One eye watches each plastic stick.
  • One eye watches a whole bundle of fibers from all the sticks.

4. How It Works in Real Life

When a cosmic muon flies through the telescope:

1. It zips through the top layer, hitting a plastic stick (which flashes) and some fibers (which also flash).
2. The "eyes" (PMTs) catch the light.
3. The computer says: "Okay, Stick #12 flashed, and Fiber Bundle #7 flashed. The muon is at the intersection of 12 and 7!"
4. It does this for the bottom layer too.
5. By connecting the dots between the top and bottom layers, the computer draws a straight line showing exactly where the muon came from and where it's going.

5. The Results: Did It Work?

The scientists tested it, and it was a huge success:

• Precision: They could pinpoint the muon's path to within 1.89 millimeters. That's sharper than the width of a pencil lead!
• Efficiency: It caught about 85% of the muons that tried to pass through.
• Cost: Because they used the "barcode" trick, they used way fewer electronic channels than a traditional design would require. This makes the whole system much cheaper and easier to build.

Why Does This Matter?

This telescope isn't just a science toy. It's being used to calibrate a massive experiment called HERD, which will be on the Chinese Space Station.

• Space Station: The HERD experiment wants to study high-energy particles from the universe. To make sure its sensors are working perfectly, they need to shoot muons through it from the ground. This new telescope is the "ruler" they use to measure the space station's sensors.
• Future Uses: Because this design is cheap and precise, it could also be used to look inside volcanoes, check for hidden chambers in ancient pyramids, or scan shipping containers for nuclear material.

In a nutshell: The scientists built a giant, high-tech "crossword puzzle" detector. By matching the clues from two different layers of sensors, they can track invisible space particles with amazing accuracy, all while keeping the cost down by using a smart, efficient design.

Technical Summary

1. Problem Statement

The High Energy cosmic-Radiation Detection (HERD) facility, a future space experiment for the Chinese Space Station, requires a ground calibration system for its core 3D imaging calorimeter (CALO). This calibration relies on cosmic-ray muons and demands a telescope with millimeter-level spatial resolution over a large area (meter-scale).

Existing solutions face a trade-off:

• High Resolution: Typically requires small scintillator elements, leading to a massive number of readout electronic channels, which increases cost and complexity.
• Low Cost/Channel Count: Often sacrifices spatial resolution.
• Challenge: The project needed a solution that achieves sub-2 mm spatial resolution over a $1 \text{ m} \times 1 \text{ m}$ area while minimizing the number of readout channels to keep manufacturing costs low.

2. Methodology and Design

The authors developed a modular, meter-scale cosmic-ray muon telescope featuring a Scintillator-Fiber Hybrid Design.

A. Geometric Structure

• Configuration: The telescope consists of two parallel super-layers, vertically separated by 1 meter. Each super-layer contains two orthogonal detection layers (totaling 4 detection layers).
• Modularity: Each detection layer comprises 18 modules arranged side-by-side in a close-packed configuration ($1 \text{ m} \times 1 \text{ m}$ total area).
• Module Composition:
  • Scintillating Bar: A plastic scintillator bar ($55 \text{ mm} \times 10 \text{ mm} \times 1000 \text{ mm}$) coupled to a dedicated Photomultiplier Tube (PMT). This provides the trigger signal and coarse positioning.
  • Scintillating Fiber Mat: Located beneath the bar, consisting of two layers of $1 \text{ mm}$ diameter Kuraray SCSF-78 fibers. Fibers are grouped into bundles (3 upper + 3 lower fibers = ~3 mm width).
• Hybrid Readout Logic (The Innovation):
  • Each scintillator bar has its own PMT (18 PMTs per layer).
  • Fiber Multiplexing: Fiber bundles at the same relative position across all 18 modules are bundled together and connected to a single PMT.
  • Position Reconstruction: When a muon passes through, it triggers a specific scintillator bar (identifying the module) and a specific fiber bundle (identifying the position within that module). By matching the "Bar ID" and "Fiber Bundle ID," the system achieves millimeter precision using only 36 PMTs per layer (18 for bars + 18 for fibers).

B. Data Acquisition (DAQ) System

• Signal Processing: Uses Time-Over-Threshold (TOT) circuits to digitize negative pulses from 144 PMTs (total for the telescope).
• FPGA Logic: A Field Programmable Gate Array (FPGA) manages coincidence windows (60 ns) and validates hits (pulse width > 45 ns).
• Triggering: A valid trigger requires at least one hit in each of the two super-layers. Data is transferred to a PC via USB or Ethernet.

C. Fabrication and Calibration

• Fiber Mat: Fibers were aligned, coated with $TiO_2$-doped epoxy for reflection, and cured to ensure tight packing (positional deviation < 0.01 mm).
• Optical Coupling: PMTs were coupled to fibers and bars using optical grease and black foam shielding to prevent light leakage.
• Simulation: Geant4 simulations were used to optimize the number of fiber layers (determined to be 2 layers for ~99% efficiency).

3. Key Contributions

1. Novel Encoding Scheme: Introduced a method where the number of readout channels scales inversely with the square root of the scintillator size ($N \propto 1/\sqrt{x}$), rather than linearly. This allows for high spatial resolution without a proportional explosion in channel count.
2. Scalable Modular Design: The telescope is built from 72 identical modules, facilitating mass production, easy replacement, and uniformity.
3. Cost-Effective High Precision: Successfully achieved millimeter-level tracking with a significantly reduced number of electronic channels (144 total for a 4-layer, $1 \text{ m}^2$ system), offering an economical solution for large-area muon tracking.

4. Experimental Results

The telescope underwent rigorous testing, including component characterization, single-module tests, and full-system performance evaluation.

• PMT Performance: Gain measurements at 1400 V showed a stable Gaussian distribution with a mean gain of $\approx 1.22 \times 10^7$.
• Spatial Resolution:
  • Using a self-calibration method (reconstructing trajectories from 3 layers to test the 4th), the system achieved a position resolution of 1.89 mm (Gaussian $\sigma$).
  • This meets the requirement of better than 2 mm.
• Detection Efficiency:
  • Fiber Layer Efficiency: Exceeded 96% per layer.
  • Overall Telescope Efficiency: Approximately 85% (product of four layers).
• Uniformity: Tests showed detection efficiency variations of only ~2% for scintillator bars and ~6% for fibers across the active area, confirming the uniformity of the design.

5. Significance and Impact

• HERD Calibration: The telescope is currently operational and serving as the ground calibration tool for the HERD CALO, validating its ability to track muons with the precision required for space-based calorimeter calibration.
• Broader Applications: The design is highly adaptable for other fields requiring large-area, high-resolution muon tracking, such as:
  • Mineral exploration.
  • Archaeological void detection.
  • Customs inspection for nuclear materials.
• Technological Advancement: The paper demonstrates that hybrid scintillator-fiber designs can break the traditional trade-off between spatial resolution and channel count, providing a blueprint for future large-scale particle detectors that are both high-performance and cost-effective.