Development and performance evaluation of a water-based liquid scintillator tracking detector with wavelength-shifting fiber readout

We have developed a novel tracking detector utilizing a water-based liquid scintillator (WbLS) for the accurate characterization of neutrino interactions on a water target. In this detector, the WbLS is optically segmented into small cells by reflective separators, and the scintillation light is read out in three directions using wavelength-shifting fibers coupled to silicon photomultipliers. We developed and optimized WbLS samples for this application and measured their light yield using cosmic-ray muons. Subsequently, we constructed a prototype of the WbLS tracking detector and evaluated its performance with a positron beam. The beam test demonstrated good tracking performance, although the light yield was lower than required. The result prompted a review of the surfactant used in the WbLS and the material of the optical separators, leading to a significant improvement in light yield. In this paper, we report on a design of the WbLS tracking detector, the development of the WbLS, the results of the beam test, and subsequent improvements to the WbLS and optical separators.

💡 Research Summary

This paper presents the comprehensive development and performance evaluation of a novel particle tracking detector that utilizes a Water-based Liquid Scintillator (WbLS) as its active medium. The primary goal is to create a detector capable of precisely characterizing neutrino interactions on a water target, thereby reducing systematic uncertainties in neutrino oscillation experiments that arise from using different target materials (like plastic) in near detectors.

The detector’s innovative design features a three-dimensional grid of optically isolated, 1 cm³ cubic cells filled with WbLS. Reflective separators, fabricable via 3D printing, define each cell. Wavelength-shifting (WLS) fibers are inserted through holes in these separators along three orthogonal axes within each cell. Scintillation light produced in the WbLS is absorbed by the fibers, re-emitted at a longer wavelength, and guided to silicon photomultipliers (SiPMs) at the fiber ends. This structure, inspired by the plastic-scintillator-based SuperFGD, enables high-granularity 3D tracking of charged particles in a water-equivalent medium.

The research followed a multi-stage experimental process. First, the team developed and optimized the WbLS formulation itself. Over 200 samples with varying concentrations of solvent (pseudocumene), fluor (PPO), wavelength shifter (Bis-MSB), and surfactant (primarily Triton X-100 with SDS) were produced, maintaining a water content above 65%. Initial light yield screening was performed using a simple PMT-based cosmic-ray muon system to identify compositional trends. Promising samples were then tested in a more relevant setup: small 3D-printed acrylic cells instrumented with a single WLS fiber and SiPM, again using cosmic rays. Interestingly, the light yield trends observed with the fiber readout (e.g., minimal gain from increased PC, a decrease with higher PPO) differed significantly from the PMT-based results, underscoring the importance of testing within the actual detection geometry.

Second, a prototype detector module was constructed using selected WbLS samples and evaluated with a 500 MeV positron beam at the Research Center for Accelerator and Radioisotope Science (RARiS). The beam test successfully demonstrated the detector’s core functionality: it clearly reconstructed positron tracks, proving the feasibility of the WLS fiber readout concept for tracking in a segmented WbLS detector. However, a critical shortcoming was identified: the measured average light yield per channel was lower than the requirement of 8.1 photoelectrons per MeV, which is needed for high detection efficiency of minimum ionizing particles.

Third, based on the beam test results, the team diagnosed the causes of the low light yield and implemented effective improvements. Analysis suggested that the surfactant Triton X-100 was absorbing ultraviolet light, thereby quenching the WbLS scintillation efficiency. Additionally, the reflectivity of the 3D-printed optical separators was suboptimal. To address these issues, the researchers replaced Triton X-100 with an alternative surfactant possessing UV-blocking properties and changed the material of the optical separators to one with higher reflectivity. The paper reports that these modifications led to a significant improvement in the overall light yield, bringing the detector performance closer to its target specification.

In conclusion, this work marks the first successful implementation of WbLS in a finely segmented tracking detector with WLS fiber readout. It validates the tracking capability of the design and demonstrates a rigorous, iterative development process where performance limitations identified in beam tests are systematically addressed through material science and optical engineering. This progress establishes a crucial foundation for developing a water-target near detector for future large-scale neutrino experiments like Hyper-Kamiokande.