Observation of light production by charged particles in WLS fibers

This study demonstrates that charged particles directly produce a significant amount of light (up to 23% relative to scintillating fibers) in wavelength-shifting fibers, a phenomenon previously ignored but now shown to be critical for accurate advanced detector simulations.

The Gist

The Big Idea: The "Glowing Wire" Surprise

Imagine you are building a giant, high-tech net to catch invisible particles (like tiny, fast-moving bullets) flying through the air. To see where these particles go, scientists use a special kind of net made of WLS fibers.

Think of these fibers like glow-in-the-dark fishing lines.

• How they usually work: You place a piece of glowing plastic (a scintillator) next to the line. When a particle hits the plastic, the plastic glows. The fiber line acts like a straw, sucking up that glow and carrying it to a camera (a sensor) at the end so you can see it.
• The old assumption: Scientists always assumed that if a particle hit the fiber line itself, it wouldn't produce any light. They thought the fiber was just a passive tube, like a clear glass straw that only carries light but doesn't make its own.

The Discovery:
This paper says, "Wait a minute! That's not true." The researchers found that when a charged particle hits the fiber line directly, the fiber actually glows on its own. It's not just a straw; it's a glowing straw!

The Experiment: Catching the Ghosts

To prove this, the team set up two different tests, kind of like a detective trying to solve a mystery.

Test 1: The Empty Net
They set up a grid of these fiber lines in empty space (no glowing plastic nearby). They shot a beam of particles through it.

• The Result: Even though there was no plastic to glow, the fibers lit up! When a particle passed through a fiber, it produced a small but clear signal. It was like walking through a dark room and seeing the floorboards light up under your feet.

Test 2: The Radioactive Source
They took a radioactive source (a safe, controlled emitter of electrons) and shot particles at the fibers from different angles. They compared three types of "straws":

1. The "Glowing" Fiber (WLS): The special fiber used in detectors.
2. The "Super-Glowing" Fiber (Scintillator): A fiber made of material that is designed to glow brightly when hit.
3. The "Clear" Fiber: A plain, clear plastic fiber that shouldn't glow at all.

The Findings:

• The Clear Fiber: When hit straight on, it stayed dark. But when hit at a sharp angle (45 degrees), it flashed briefly. This was Cherenkov light (a blue flash you get when something moves faster than light can travel in that material, like a sonic boom but for light).
• The WLS Fiber: When hit, it produced a steady glow.
• The Comparison: The light produced directly by the WLS fiber was surprisingly strong. It was about 23% as bright as the "Super-Glowing" fiber.

Why Does This Matter? (The "Recipe" Analogy)

Imagine you are a chef trying to bake a perfect cake (a particle detector simulation).

• The Old Recipe: You list the ingredients: "Flour (scintillator), Sugar (fiber), Eggs (particles)." You assume the sugar doesn't change the taste unless the flour is there.
• The New Reality: The researchers found out that the sugar (the fiber) actually adds its own flavor when mixed with the eggs (particles). If you don't account for this extra flavor, your cake (the simulation) won't taste right.

In the world of physics, if you are building a detector to measure energy or time very precisely, and you ignore the fact that the fibers glow on their own, your measurements will be slightly off. It's like weighing a bag of apples but forgetting to subtract the weight of the bag itself.

The "Why" Behind the Glow

Why does the fiber glow?

• Scintillation: The particle hits the plastic material of the fiber, exciting its atoms, which then release light.
• Cherenkov Radiation: If the particle moves fast enough, it creates a shockwave of light (like a boat creating a wake).

The researchers found that while the "Cherenkov" flash is visible at certain angles, the main glow comes from the fiber material itself reacting to the hit.

The Bottom Line

For decades, scientists ignored the light produced directly by particles hitting the fiber lines in their detectors. This paper proves that this light is real, measurable, and significant.

The Takeaway:
Next time you see a diagram of a particle detector, remember that those fiber lines aren't just passive pipes. They are active participants that light up when touched. If we want our "maps" of the subatomic world to be accurate, we have to update our computer models to include this extra glow.

In short: The fiber isn't just a messenger; it's also a performer. And now, the scientists know to give it a seat in the spotlight.

Technical Summary

1. Problem Statement

Wavelength-shifting (WLS) fibers are extensively used in particle physics detectors (e.g., DANSS, SFGD) to collect light from scintillators. In standard detector simulations, it is traditionally assumed that WLS fibers produce negligible light when directly traversed by charged particles; they are modeled solely as light guides for scintillation light originating from external scintillators.

However, preliminary observations from detector prototypes (DANSS and SFGD) exposed to a 730 MeV/c pion beam suggested that this assumption is incorrect. Significant signals were detected from WLS fibers that were not connected to scintillators, indicating that charged particles directly interacting with the fiber core generate a non-negligible amount of light. This paper aims to quantify this effect and determine its magnitude relative to standard scintillating fibers.

2. Methodology

The study employed a two-phase approach: qualitative observation using beam tests and quantitative measurement using a radioactive source.

A. Beam Test Observations (Qualitative)

• Setup 1 (DANSS Prototype): A $4\times4$ grid of 1.2 mm diameter Kuraray Y11(200)M WLS fibers was placed perpendicular to a 730 MeV/c pion beam.
  • Result: Even with empty space between fibers, clear signals were observed when the beam crossed the fibers. For track lengths $>1.0$ mm, the most probable light yield (LY) was $\approx 3$ photo-electrons (p.e.). When filled with plastic scintillator granules, the LY increased to $\approx 39$ p.e., confirming the direct fiber contribution was significant.
• Setup 2 (SFGD Prototype): A $5\times5\times5$ scintillator cube array with orthogonal WLS fibers.
  • Result: Light yield maps showed clear signals from WLS fibers outside the scintillator volume. The most probable LY for direct interaction was $3.1 \pm 0.13$ p.e., compared to $\approx 50$ p.e. for a Minimum Ionizing Particle (MIP) crossing the scintillator cube.

B. Radioactive Source Measurements (Quantitative)

To obtain precise quantitative data, a controlled experiment was conducted using a $^{90}\text{Sr}$ source.

• Apparatus:
  • Source: 5 mCi $^{90}\text{Sr}$ collimated through a 1 mm hole.
  • Fibers Tested:
    • WLS: Kuraray Y11(200)MSJ ("Y11-1") and two samples of Y11(200)MS ("Y11-2", "Y11-3").
    • Scintillating Reference: Bicron BCF-12 (1 mm diameter).
    • Clear Reference: Bicron BCF-98 (1 mm diameter, no scintillator).
  • Readout: Silicon Photomultipliers (SiPMs) coupled to the fiber ends.
  • Geometry: Fibers were positioned at $90^\circ$ (orthogonal) and $45^\circ$/$135^\circ$ relative to the electron track direction to distinguish between scintillation and Cherenkov light.
• Procedure:
  • A trigger scintillating fiber ensured only events where the beam passed through the test fiber were recorded.
  • Pedestal noise was subtracted.
  • Measurements were repeated multiple times to assess systematic uncertainties (reproducibility, polishing quality).

3. Key Results

Light Yield (LY) Comparison

The study measured the average light yield in photo-electrons (p.e.) for different fiber types at various crossing angles:

Fiber Type	$90^\circ$ (p.e.)	$45^\circ$ (p.e.)	$135^\circ$ (p.e.)
BCF-12 (Scintillator)	$18.45 \pm 1.94$	$21.72 \pm 1.63$	$21.38 \pm 2.47$
BCF-98 (Clear)	$0.11 \pm 0.01$	$1.04 \pm 0.09$	$0.09 \pm 0.06$
Y11 (WLS) Average	$3.81 \pm 0.24$	$5.45 \pm 0.30$	$4.73 \pm 0.26$

Key Findings:

1. Significant Direct LY in WLS Fibers: The WLS fibers produced a substantial signal directly from charged particles. The average LY of the Y11 fibers was approximately 23% ($0.23 \pm 0.02$) of the LY produced by the standard BCF-12 scintillating fiber.
2. No Scintillation in Clear Fibers: The clear BCF-98 fiber showed negligible signal at $90^\circ$ ($0.11$ p.e.), confirming that the signal in WLS fibers is not merely background noise.
3. Cherenkov Light Detection:
   • In the clear fiber, a distinct signal increase was observed at $45^\circ$ ($1.04$ p.e.) compared to $135^\circ$ ($0.09$ p.e.), confirming the presence of Cherenkov radiation.
   • In WLS fibers, the LY also increased at $45^\circ$, but the contribution of Cherenkov light is difficult to isolate because the WLS dyes absorb and re-emit photons, effectively isotropizing the Cherenkov light and losing its directionality.
4. Consistency: No significant difference was found between the Y11(200)MSJ and Y11(200)MS fiber batches.

4. Significance and Implications

• Simulation Accuracy: The assumption that WLS fibers do not produce light when traversed by charged particles is incorrect. The direct light yield is approximately 23% of that of a standard scintillating fiber. This effect must be included in advanced detector Monte Carlo simulations (e.g., for JUNO TAO, DANSS, SFGD) to avoid biases in energy reconstruction and timing.
• Detector Design: For detectors requiring high energy or time resolution, the "background" signal from WLS fibers themselves is a non-negligible factor that must be accounted for during calibration and analysis.
• Cherenkov vs. Scintillation: While Cherenkov light is clearly present in clear fibers, its contribution to WLS fiber signals is complex due to the wavelength-shifting properties of the dye, which scrambles the directional information of the Cherenkov cone.

5. Conclusion

The paper definitively demonstrates that charged particles produce a sizable amount of light directly within Kuraray Y11(200) WLS fibers. This light yield is roughly one-quarter of that produced by a standard scintillating fiber of the same diameter. Consequently, future particle physics detector simulations and analyses must incorporate this direct light production mechanism to ensure accurate modeling of detector response.