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Cosmic-ray characterization of a timing detector based on extruded scintillators and wavelength-shifting fibers read-out through SiPMs

This paper presents the results of cosmic-ray tests on timing detectors utilizing extruded scintillators and wavelength-shifting fibers read out by SiPMs, detailing their performance in terms of light yield, time resolution, and longitudinal coordinate resolution.

Original authors: F. Anulli, A. Betti, C. Bini, M. Corradi, C. Luci, L. Martinelli, A. Nisati, M. Pantalena, S. Rosati, R. Vari, S. Veneziano

Published 2026-08-21

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: F. Anulli, A. Betti, C. Bini, M. Corradi, C. Luci, L. Martinelli, A. Nisati, M. Pantalena, S. Rosati, R. Vari, S. Veneziano

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the vast, high-stakes arena of particle physics, scientists are constantly designing machines to smash particles together at speeds approaching that of light. The goal is to recreate the conditions of the early universe and uncover the fundamental rules that govern matter. To do this, they need detectors that can not only see where a particle goes but also record exactly when it arrives. This timing is crucial. If a detector can measure the arrival of a particle with extreme precision, it can help distinguish between common, boring events and rare, exotic ones that might reveal new physics. For the next generation of these massive colliders, researchers are looking for detectors that are not only incredibly fast but also robust and affordable enough to cover huge areas. They need a system that can catch muons, a type of heavy electron that penetrates deep into matter, and tell their story with split-second accuracy.

A team of researchers at Sapienza University in Rome and the INFN laboratories set out to test a specific design for such a detector. They focused on a simple yet elegant concept: long, rectangular bars made of a special plastic that glows when a particle passes through it. These bars, known as extruded scintillators, act like light pipes. Inside each bar runs a thin, transparent fiber designed to catch the faint glow and carry it to the ends. At the ends of these fibers sit tiny, ultra-sensitive cameras called silicon photomultipliers, which convert the light into electrical signals. The researchers wanted to see if this combination could provide the precise timing and spatial measurements needed for future experiments. They tested various versions of these bars, changing their thickness and the type of fiber inside, to find the best recipe for performance.

The team built a test station using a natural source of particles: cosmic rays. These are high-energy particles from space that constantly rain down on Earth. To ensure they were measuring the right kind of particles, the researchers placed their test bars between two other detectors that acted as a gatekeeper. Only when a particle passed through the gatekeepers and the test bars in perfect alignment did the system record the event. They also placed a block of lead in the path to filter out slower particles, ensuring that only the fastest, most energetic muons were being studied. The entire setup was wrapped in a light-tight box to prevent any stray light from interfering with the delicate measurements.

The researchers experimented with two main sizes of scintillator bars: "thick" bars that were 16 millimeters wide and "thin" bars that were 6.5 millimeters wide. Inside these bars, they inserted different types of light-carrying fibers, varying both the material of the fiber and its thickness. They also tested whether using a special optical grease to connect the fiber to the camera improved the signal. By running thousands of cosmic rays through these different configurations, they could measure how much light was produced, how fast the signal traveled, and how precisely they could determine the particle's position and time of arrival.

The results showed that the thickness of the bar matters significantly. The thicker bars produced roughly two and a half times more light than the thinner ones, which makes sense given their larger volume. The thickness of the fiber also played a role; thicker fibers carried more light than thinner ones. However, the type of fiber material was perhaps the most critical factor for speed. One type of fiber, while producing a lot of light, was slower to respond, which blurred the timing measurements. Another type, which produced slightly less light, responded much faster and allowed for sharper timing. The researchers found that using the faster fiber in the thicker bars yielded the best results, achieving a time resolution of about 250 picoseconds. To put that in perspective, a picosecond is one trillionth of a second; this level of precision is like measuring the time it takes for a bullet to travel the length of a room with an error of less than the width of a human hair.

Positioning accuracy was also a key finding. By measuring the difference in arrival time of the light at both ends of the bar, the team could calculate exactly where along the bar the particle had passed. In the best configurations, they could pinpoint this location within about 4 centimeters. While this is not as precise as the timing, it is sufficient to help track the particle's path when combined with other detectors. The study also confirmed that the light signal travels through the fibers at a consistent speed, allowing for reliable calculations of position. Interestingly, they found that adding a layer of optical grease between the fiber and the camera improved the light collection by about 8 to 10 percent, a small but valuable gain.

The researchers concluded that this technology is a strong candidate for the massive muon detectors needed in future particle colliders. The system proved to be capable of delivering the high-speed timing and reasonable spatial resolution required to handle the intense environment of a next-generation collider. While the single-layer bars tested here cannot measure the full three-dimensional path of a particle on their own, they provide an excellent foundation. When combined with other high-precision detectors, these scintillator bars could form the backbone of a system capable of capturing the fleeting signatures of new physics. The study demonstrated that by carefully choosing the right combination of bar thickness and fiber type, scientists can build detectors that are both fast and cost-effective, ready to help unlock the secrets of the universe.

Technical Summary: Cosmic-ray Characterization of a Timing Detector Based on Extruded Scintillators and WLS Fibers

Problem and Motivation
The proposed FCC-ee electron-positron facility at CERN requires high-performance muon detection systems capable of precise spatial and temporal measurements. While precision drift tubes are often combined with fast plastic scintillators for these purposes, there is a need to evaluate the competitiveness of scintillator-based detectors regarding cost, robustness, time resolution, and rate capability for large surface areas. Specifically, the physics program at FCC-ee demands muon reconstruction with time resolutions on the order of a few hundred picoseconds (ps) for triggering and time-of-flight (ToF) measurements, alongside spatial resolutions of a few millimeters for a second coordinate and a few centimeters for the orthogonal direction to reduce tracking ambiguities. This paper addresses the performance characterization of extruded scintillator bars coupled with wavelength-shifting (WLS) fibers and read out by Silicon Photomultipliers (SiPMs) to determine their viability for such applications.

Methodology
The study utilized a cosmic-ray telescope located at the INFN laboratories at Sapienza University in Rome. The experimental setup consisted of two 1-meter bulk EJ-200 scintillation counters serving as a trigger, separated by a 5 cm lead absorber to select high-momentum cosmic muons. The detectors under test were extruded scintillator bars placed between the upper trigger counter and the absorber.

Two geometries of extruded bars were tested, both 1 meter long and provided by the FNAL-NICADD Extrusion Line Facility:

"Thick" bars: 16 mm $\times$ 19 mm cross-section with a 2 mm central hole.
"Thin" bars: 6.5 mm $\times$ 25 mm cross-section with a 1.5 mm central hole.

Three types of Kuraray WLS fibers (YS-2, YS-4, and YS-6) with varying diameters (1.0 mm, 1.5 mm, and 2.0 mm) and decay constants were inserted into the bars. The fibers were polished and inserted directly into the holes, with their ends in direct contact with Hamamatsu S136360-3075PE SiPMs (3x3 mm $^2$ sensitive area). No optical grease was used in the primary data set, though a specific test was conducted to evaluate its effect.

Signals were amplified by custom high-frequency amplifiers and digitized by a CAEN DT5743 module (3.2 GS/s). The analysis involved:

Calibration: Using a white LED source to determine SiPM gain and linearity, establishing the relationship between signal amplitude and the number of photoelectrons (p.e.).
Signal Processing: Offline evaluation of time (threshold crossing) and amplitude (signal integral). A slewing correction algorithm ( $a/x + b$ ) was applied to correct time measurements based on signal amplitude.
Resolution Calculation: By pairing identical bars and comparing the differences in their measured times ( $T$ ) and longitudinal coordinates ( $Z$ ), the time resolution ( $\sigma_T$ ) and coordinate resolution ( $\sigma_Z$ ) were extracted using Gaussian fits, effectively canceling trigger jitter.

Key Contributions and Results
The paper presents a systematic evaluation of light yield, attenuation, and resolution across seven distinct configurations (varying bar thickness and fiber type/diameter).

Light Yield:
- Light yield scales linearly with scintillator thickness and fiber diameter. Comparisons between "Thick" (16 mm) and "Thin" (6.5 mm) bars yielded ratios consistent with their thickness ratio (~2.5).
- Increasing fiber diameter from 1.5 mm to 2.0 mm (YS-2) increased light yield by a factor of ~1.34, consistent with the diameter ratio.
- YS-2 fibers provided approximately 15% higher light yield than YS-4 fibers of the same diameter.
- The application of optical grease between the fiber and SiPM resulted in an 8–10% improvement in light yield.
- At a bias voltage of $V_{break} + 3$ V, the best configuration (Thick bar, YS-2, 2.0 mm fiber) achieved an average light yield of $145 \pm 12$ p.e., while the lowest (Thin bar, YS-6, 1.0 mm fiber) yielded $17.2 \pm 0.5$ p.e.
Attenuation and Light Velocity:
- Light yield as a function of distance showed a single exponential decay for "Thin" bars, consistent with manufacturer specifications. "Thick" bars exhibited a double-exponential behavior, suggesting a need for further investigation.
- The effective light propagation velocity in the fibers was measured to be between 17.4 and 17.7 cm/ns.
Time and Coordinate Resolution:
- Time Resolution: The best performance was achieved with "Thick" bars instrumented with 1.5 mm YS-4 fibers, reaching a time resolution of approximately 250 ps at a baseline voltage (55 V) and thresholds between 1–4 p.e. Interestingly, despite higher light yield, the larger diameter YS-2 fibers (2.0 mm) resulted in worse time resolution due to their longer decay constant. "Thin" bars achieved a best resolution of approximately 350 ps.
- Coordinate Resolution: Longitudinal coordinate resolutions ( $\sigma_Z$ ) down to 4 cm were achieved in the best configurations.
- Scaling: Time resolution was found to scale reasonably well with $1/\sqrt{N_{pe}}$ . A constant term of approximately 100 ps was observed for all fiber types.

Significance and Claims
The paper concludes that extruded scintillator detectors coupled with fast WLS fibers and SiPMs are competitive candidates for large-area muon systems at next-generation colliders. The study demonstrates that a single layer of 1-meter extruded bars can achieve time resolutions down to 250 ps and longitudinal coordinate resolutions down to 4 cm.

The authors explicitly state that while these detectors provide excellent timing and one-dimensional spatial resolution, achieving the required transverse coordinate resolution (1 mm) necessitates either different geometries (such as the multi-layer or crossed-bar approaches proposed in other literature) or multi-layer structures. The work validates the technology's robustness and cost-effectiveness for providing the necessary timing and trigger capabilities for the FCC-ee muon system.

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