Response of wavelength-shifting and scintillating-wavelength-shifting fibers to ionizing radiation

This study characterizes the light yield and transmission properties of Saint-Gobain's BCF-91A and Eljen Technology's new EJ-160 wavelength-shifting fibers under ionizing radiation, revealing that the EJ-160 variants offer significantly higher light yields (5–7 times greater) despite varying attenuation lengths.

The Gist

Imagine you are trying to listen to a whisper in a very long, noisy hallway. To hear it clearly, you need a special kind of "messenger" that can catch the whisper, translate it into a louder language, and run down the hallway to your ear without losing the message.

This paper is about testing three different types of special plastic fibers that act as these messengers. Scientists use them in giant particle physics experiments (like LEGEND-1000) to detect invisible particles like ghosts passing through walls.

Here is the breakdown of what they did, using simple analogies:

1. The Problem: The "Ghost" Hunters

In the world of subatomic physics, scientists need to detect tiny flashes of light created when radioactive particles (like alpha, beta, and gamma rays) hit a detector.

• The Challenge: These flashes are often too faint to see directly, and they happen deep inside massive detectors.
• The Solution: They use long plastic fibers. When a particle hits the fiber, the fiber glows (scintillates) and changes the color of that light (wavelength-shifting) so it can travel down the fiber to a sensor at the end.

2. The Contestants: The "Messenger" Fibers

The researchers tested three different fibers to see which one is the best messenger:

• The Veteran (BCF-91A): This is an old, reliable fiber used in many past experiments. It's like a standard delivery truck. It gets the job done, but it's not super fast or loud.
• The Newcomer I (EJ-160I): A brand-new fiber made by Eljen Technology. Think of this as a high-performance sports car. It's designed to be brighter and catch more light.
• The Newcomer II (EJ-160II): Another new fiber from the same company, but with a slightly different "engine" (chemical mix). This is like a drag racer. It is incredibly loud and bright, but it might run out of steam a bit faster over long distances.

3. The Race: How They Tested Them

The scientists set up a "track" (a dark box) and shot different types of radioactive "bullets" at the fibers:

• Beta particles: Like tiny, fast ping-pong balls.
• Gamma rays: Like invisible, high-energy lasers.
• Alpha particles: Like heavy, slow bowling balls (they don't travel far, so the scientists had to hit the fibers from the very end).

They measured how many "photons" (packets of light) arrived at the sensor at the end of the fiber.

4. The Results: Who Won?

The results were exciting for the new fibers:

• Brightness (Light Yield):

  • The Newcomer I was about 5 times brighter than the Veteran.
  • The Newcomer II was about 7 times brighter than the Veteran.
  • Analogy: If the Veteran whispered the message, the Newcomer II shouted it like a megaphone. This means scientists can detect much fainter signals with the new fibers.

• Distance (Attenuation Length):

  • The Veteran and Newcomer I could carry the light signal for about 4 meters before it got too dim.
  • The Newcomer II was so bright at the start that it could still be seen, but the signal faded faster, only traveling about 2.5 meters effectively.
  • Analogy: The Veteran and Newcomer I are like marathon runners who keep a steady pace. The Newcomer II is a sprinter who explodes with energy at the start but tires out sooner.

5. Why Does This Matter?

The scientists are building a massive detector called LEGEND-1000 to hunt for a rare event called "neutrinoless double beta decay" (which could explain why the universe exists).

• To build this detector, they need fibers that are extremely sensitive (to catch the faintest whispers) and very pure (so the fibers themselves don't create fake signals).
• The new Eljen fibers (EJ-160) are much better at catching the light than the old ones. Even though one of them fades a bit faster, its incredible brightness makes it a huge upgrade.

The Bottom Line

The team successfully tested new, super-bright fibers that are 5 to 7 times better at detecting radiation than the current industry standard. While one version fades a little faster over long distances, the massive gain in brightness makes them perfect candidates for the next generation of giant particle detectors. It's like upgrading from a bicycle to a rocket ship for delivering light messages.

Technical Summary

Here is a detailed technical summary of the paper "Response of wavelength-shifting and scintillating-wavelength-shifting fibers to ionizing radiation."

1. Problem Statement and Motivation

Plastic wavelength-shifting (WLS) and scintillating-wavelength-shifting (Sci-WLS) fibers are critical components in particle and nuclear physics experiments (e.g., MINOS, NOvA, LEGEND) for efficient light collection and transport. However, future experiments like LEGEND-1000 impose stricter requirements:

• Radiopurity: Fibers must minimize background noise from natural radioactivity.
• Self-Tagging: Ideally, fibers should be able to detect their own radio-impurities (e.g., cosmogenic $^{42}$Ar or $^{39}$Ar decays in liquid argon environments).
• Optimization: Existing market fibers may not be fully optimized for these specific high-sensitivity needs.

The authors sought to develop and characterize new fibers that offer superior light yield and are better optimized for upcoming experiments, specifically comparing them against the industry-standard BCF-91A fiber.

2. Methodology

The study involved a comprehensive experimental characterization of two new fibers developed in partnership with Eljen Technology against the standard BCF-91A (Saint-Gobain/Luxium).

• Fiber Samples:
  • BCF-91A: Standard WLS fiber (1 mm square cross-section, single PMMA cladding).
  • EJ-160I & EJ-160II: New Sci-WLS fibers (1 mm square, single PMMA cladding) featuring different fluor mixtures to enhance scintillation.
• Experimental Setup:
  • Length: Fibers were approximately 1.4 m long (matching the design for LEGEND-1000).
  • Readout: Both ends of the fibers were optically coupled to Hamamatsu S13360-3050CS SiPMs (Silicon Photomultipliers) using optical grease.
  • Irradiation Sources:
    • Beta ($\beta$): $^{90}$Sr source (4.8 $\mu$Ci).
    • Gamma ($\gamma$): $^{22}$Na source (3.4 $\mu$Ci, 511 keV).
    • Alpha ($\alpha$): $^{241}$Am source (1.0 $\mu$Ci, 5.486 MeV).
  • Measurement Technique:
    • For $\beta$ and $\gamma$: The source was moved along the fiber length (5 cm to 133 cm) to measure light transmission and attenuation.
    • For $\alpha$: Due to the short range of alpha particles, fibers of varying lengths (5 cm to 138 cm) were irradiated at one end while the other end was coupled to a SiPM.
  • Data Analysis: Mean photoelectron (p.e.) counts were recorded. Light yield and attenuation were modeled using double-exponential fits ($I = I_{long}e^{-x/\lambda_{long}} + I_{short}e^{-x/\lambda_{short}}$) to separate core-guided and cladding-guided light components.

3. Key Contributions

• Development of Sci-WLS Fibers: Successfully characterized two new variants (EJ-160I and EJ-160II) that combine scintillation and wavelength-shifting capabilities, a significant step beyond standard WLS fibers.
• Comprehensive Comparative Study: Provided a direct, side-by-side performance comparison of the new Eljen fibers against the established BCF-91A standard under three distinct types of ionizing radiation ($\alpha, \beta, \gamma$).
• Methodological Rigor: Established systematic uncertainty protocols by reconstructing the experimental setup multiple times and utilized double-exponential modeling to accurately describe light transport in short fibers where single-exponential fits often fail.

4. Key Results

Light Yield Performance

The new Sci-WLS fibers demonstrated significantly higher light yields compared to the standard BCF-91A:

• Beta ($\beta$) Irradiation:
  • EJ-160I: ~5.0x higher yield than BCF-91A (64.0 p.e. vs. 12.7 p.e.).
  • EJ-160II: ~6.9x higher yield than BCF-91A (87.7 p.e. vs. 12.7 p.e.).
• Gamma ($\gamma$) Irradiation:
  • EJ-160I: ~5.6x higher yield (55.8 p.e. vs. 10.0 p.e.).
  • EJ-160II: ~7.7x higher yield (76.9 p.e. vs. 10.0 p.e.).
• Alpha ($\alpha$) Irradiation:
  • The enhancement was more modest due to quenching effects in polystyrene-based fibers for heavy charged particles.
  • EJ-160I: ~2.9x higher (81.6 p.e. vs. 28.5 p.e.).
  • EJ-160II: ~3.6x higher (103 p.e. vs. 28.5 p.e.).

Attenuation Lengths ($\lambda_{long}$)

• BCF-91A: 3.80 m
• EJ-160I: 4.00 m (Slightly better than BCF-91A)
• EJ-160II: 2.50 m (Shorter, indicating a trade-off where higher light yield comes at the cost of reduced transmission distance).

Light Transport Characteristics

• All fibers exhibited a double-exponential decay behavior.
• A "short" component ($\lambda_{short} \approx 0.1$ m) was observed, attributed to cladding-guided light which attenuates rapidly.
• A "long" component ($\lambda_{long}$) corresponds to core-guided light.
• The study confirmed that even standard WLS fibers (BCF-91A) produce detectable signals from ionizing radiation due to intrinsic scintillation in the polystyrene core, though the new Sci-WLS fibers are explicitly optimized for this.

5. Significance and Conclusion

• Performance Leap: The EJ-160 series offers a massive improvement in light collection efficiency (up to 7x) compared to the current standard, which is crucial for improving signal-to-noise ratios in low-background experiments.
• Design Trade-offs: The results highlight a trade-off between light yield and attenuation length. EJ-160II offers the highest yield but shorter range, while EJ-160I offers a balanced improvement with the longest attenuation length.
• Future Applications: These fibers are highly suitable for next-generation experiments like LEGEND-1000, where detecting rare events (like neutrinoless double-beta decay) requires maximizing light collection while managing radiopurity. The ability of these fibers to self-tag radio-impurities could relax manufacturing radiopurity constraints.
• Ongoing Work: This paper serves as a foundation for further development, including radio-purity improvements and advanced simulation frameworks to model photon transport in these fibers.