Construction, commissioning, and beam test of a pilot 3D-projection opaque water-based liquid scintillator detector

This paper reports the successful design, construction, and beam testing of a pilot 3D-projection detector using opaque water-based liquid scintillator (oWbLS), demonstrating its effective optical confinement and high timing resolution to validate the technology as a scalable concept for future particle physics experiments.

The Gist

The Big Idea: A "Liquid Camera" for Particles

Imagine you want to take a 3D photo of a tiny, invisible bullet (a subatomic particle) flying through a room. Usually, to do this, you would build a wall out of millions of tiny, separate Lego bricks. Each brick is a sensor. If the bullet hits a brick, that brick lights up. By seeing which bricks lit up, you can figure out where the bullet went.

However, building a detector out of millions of individual Lego bricks is a nightmare. It takes years to build, it's hard to fix if one breaks, and once it's built, you can't change the size of the bricks.

This paper describes a new, smarter way to do it. Instead of millions of solid bricks, the scientists built a clear box filled with a special milky, glowing liquid. They threaded hundreds of fiber-optic "straws" through this liquid in three directions (up-down, left-right, and front-back).

How It Works: The "Foggy Room" Analogy

Think of the liquid inside the box as a very dense, foggy room.

1. The Particle: When a high-speed particle (like a proton) flies through this liquid, it bumps into the liquid molecules and creates a flash of blue light, like a sparkler being lit.
2. The Fog: In a clear room, that spark would fly everywhere, making it hard to tell exactly where it started. But this liquid is "opaque" (foggy). The light bounces around wildly and gets trapped in a tiny ball right where the spark happened. It doesn't spread out far.
3. The Straws: The fiber-optic straws (wavelength-shifting fibers) act like vacuum cleaners for light. They suck up the trapped blue light and turn it into green light, which travels down the straw to a sensor at the end.
4. The 3D Picture: Because the straws are arranged in a grid in three directions, the sensors can tell exactly where the "light ball" was. It's like having three cameras looking at the same object from different angles; by matching the dots, you can reconstruct the exact 3D path of the particle.

What They Built and Tested

The team built a small "pilot" version of this detector (about the size of a large shoebox: 8x8x16 cm).

• The Box: Made of clear acrylic plastic, glued together with a special solvent cement.
• The Straws: They threaded 320 tiny fibers through the box in a perfect grid.
• The Liquid: They filled it with their special "opaque water-based liquid scintillator." It looks like milk but glows when hit by radiation.
• The Sensors: At the ends of the straws, they attached tiny, super-sensitive light cameras (called MPPCs) connected to fast computer chips.

The "Stress Test" (Beam Test)

To see if this new idea actually works, they took the detector to a particle accelerator at NASA's Space Radiation Laboratory. They shot protons (particles found in atomic nuclei) at the detector at four different speeds: slow, medium, fast, and very fast. They also waited for cosmic rays (particles from space) to hit it naturally.

The Results:

1. It Works: The detector successfully took clear 3D "photos" of the particles. They could see the tracks of cosmic rays and the paths of the protons.
2. The Light Stays Put: They wanted to prove that the "foggy" liquid kept the light trapped in a tight ball. They compared their real data to a computer simulation. The simulation assumed the light could travel 2 cm before scattering. The real data showed the light stayed much tighter than that (much less than 2 cm). This proves the "fog" is doing its job perfectly, keeping the light confined so the detector can pinpoint the location accurately.
3. Super Fast Timing: They measured how fast the detector could react. It was incredibly quick. For a single sensor, it could time an event with a precision of about 0.17 to 0.28 nanoseconds (that's less than one-billionth of a second). When they combined data from multiple sensors, the timing got even sharper, down to 0.05 nanoseconds. To put that in perspective, light travels about 1.5 centimeters in that tiny fraction of a second.

Why This Matters (According to the Paper)

The paper concludes that this "liquid camera" approach is a viable, scalable technology.

• Scalable: Instead of gluing millions of plastic bricks together, you can just pour more liquid into a bigger tank and thread more straws through it. It's much easier to build larger detectors this way.
• Flexible: You can change the properties of the liquid (like how "foggy" it is) by changing the chemistry, whereas you can't change the size of a plastic brick once it's made.

The authors state this technology is ready to be tested in larger sizes for future experiments in particle physics, specifically for neutrino research, rare particle searches, and collider experiments. They plan to build bigger modules (about 20 cm on each side) and test them with even more types of particles.

In short: They proved that a box of milky liquid with fiber-optic straws can act as a high-speed, 3D camera for subatomic particles, offering a simpler and more flexible alternative to the traditional "Lego brick" detectors.

Technical Summary

Technical Summary: Construction, Commissioning, and Beam Test of a Pilot 3D-Projection Opaque Water-Based Liquid Scintillator Detector

Problem Statement
Three-dimensional fine-grained scintillator detectors, such as the Super Fine-Grained Detector (SuperFGD) used in the T2K experiment, have demonstrated the ability to achieve sub-centimeter position resolution and sub-nanosecond timing. However, these detectors rely on a mechanical segmentation approach involving millions of individual optically isolated scintillator cubes. This manufacturing process is labor-intensive, spans years, and permanently fixes detector properties (granularity, radiation length, yield) once cast. The paper addresses the need for a scalable alternative that retains the fine-grained capabilities of segmented detectors while utilizing a continuous, pumpable liquid medium.

Methodology
The authors designed, constructed, and tested a pilot detector based on opaque water-based liquid scintillator (oWbLS). The core concept replaces mechanical segmentation with optical confinement: a highly scattering liquid medium restricts scintillation photons to a small volume around the energy deposition point. A lattice of wavelength-shifting (WLS) fibers samples this volume, capturing the confined light to provide spatial information without physical boundaries.

• Detector Construction: The pilot detector features an active volume of $8 \times 8 \times 16 \text{ cm}^3$ housed in an acrylic frame. It is instrumented with 320 Kuraray Y11 multi-clad WLS fibers arranged in three orthogonal planes (X, Y, Z) with a 1 cm pitch. The fibers are read out by Hamamatsu S13360-1325CS Multi-Pixel Photon Counters (MPPCs). The readout electronics utilize CITIROC-based front-end boards (FEBs), originally developed for the Baby MIND and SuperFGD detectors of the T2K experiment.
• Medium: The detector was filled with oWbLS, a formulation where surfactant molecules encapsulate organic scintillator in nanometer-scale micelles within an aqueous mixture. This creates a milky, translucent medium with a short scattering length (order of mm), designed to confine light.
• Beam Test: The detector was commissioned at Brookhaven National Laboratory (BNL) and subsequently tested at the NASA Space Radiation Laboratory (NSRL). It was exposed to cosmic rays and proton beams with kinetic energies of 50, 100, 250, and 500 MeV.
• Analysis: The study employed a unified event selection and 3D reconstruction pipeline. Key analyses included:
  • Light Yield: Measured via cosmic muons and compared to reference liquid scintillators using a coincidence scintillation counter.
  • Optical Confinement: Quantified by analyzing transverse light spread via radial charge distributions and comparing data to Geant4 Monte Carlo simulations with varying scattering lengths.
  • Timing Resolution: Measured using 500 MeV proton beam data through three complementary methods: pair-difference, per-event peak (proton mean time), and multi-fiber averaging.

Key Contributions and Results

1. Proof of Concept for 3D-Projection oWbLS: The paper successfully demonstrates the viability of a 3D-projection detector using a continuous opaque liquid medium. The detector achieved 3D event reconstruction for both cosmic muons and proton beams, matching hits across orthogonal fiber views to reconstruct voxel positions.
2. Optical Confinement and Scattering Length:
   • The oWbLS medium demonstrated an intrinsic light yield of approximately 12,000 photons/MeV.
   • Radial charge distribution measurements showed that ~94% of the scintillation charge was confined within 1 cm of the beam axis.
   • Comparison with Geant4 simulations revealed that the experimental data exhibited significantly tighter light confinement than a simulation assuming a 2 cm scattering length. This places the effective scattering length of the oWbLS medium well below 2 cm, confirming the medium's ability to optically confine scintillation light.
3. Timing Performance:
   • Using 500 MeV proton beam data, the study achieved a single-channel timing resolution ($\sigma_t$) of approximately 0.17–0.28 ns, depending on the photoelectron (PE) statistics.
   • By aggregating statistics across multiple fibers (per-voxel), the resolution improved to ~0.16 ns.
   • For half-track timing (aggregating 7 voxels), the resolution reached **0.05 ns**.
   • These results indicate that the detector can timestamp track segments at the sub-100 ps level when sufficient photoelectron statistics are available, surpassing the intrinsic time-stamp quantization floor of the electronics (0.72 ns) through statistical averaging.
4. Scalability: The construction process utilized standard acrylic bonding and fiber threading techniques, demonstrating a path toward scalable manufacturing that avoids the complex assembly of millions of individual cubes.

Significance and Claims
The paper claims that these results validate the 3D-projection oWbLS technology as a scalable, fully-active detector concept suitable for next-generation particle physics experiments. The technology offers a distinct advantage over mechanically segmented detectors by allowing tunable detector properties (granularity via fiber pitch, radiation length via metal-loading) and eliminating the need for massive, labor-intensive cube assembly.

The authors position this work as a proof-of-concept for applications in neutrino physics, collider experiments, and rare-process searches. They note that future work will focus on constructing modular prototypes (approx. 20 cm scale) with heavy-metal-loaded formulations to study electromagnetic and hadronic calorimetry, but they do not claim immediate deployment in large-scale experiments. The study confirms that the optical confinement and timing performance of oWbLS meet the requirements for high-precision tracking and calorimetry, establishing a foundation for larger-scale detector development.