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

TL;DR

This work introduces a water-based liquid scintillator (WbLS) tracking detector with three-dimensional readout via wavelength-shifting fibers and silicon photomultipliers, aimed at precise neutrino interaction measurements on water targets. The authors developed and screened hundreds of WbLS samples, performed PMT-based and fiber-based light-yield measurements, and built a five-layer prototype to test tracking with a 500 MeV positron beam. Initial beam-test results showed track detection but light yield was below the 99% efficiency target, prompting improvements in WbLS composition and optical separator reflectivity. The study projects a multi-step path to higher light yields, estimating around 6.0 p.e./MeV per fiber after refinements, though further optimization remains necessary to meet the 8.1 p.e./MeV requirement for reliable single-cell MIP detection. Overall, the work demonstrates tangible progress toward a high-granularity, water-equivalent tracking detector for future neutrino experiments and identifies concrete avenues for further enhancements.

Abstract

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.

Figures (36)

• Figure 1: Schematic view of the WbLS tracking detector concept with orthogonal WLS fibers and optically segmented cells.
• Figure 2: Examples of produced WbLS sample (left) and pure water (right) exposed to an ultraviolet light.
• Figure 3: Setup of the light yield measurement with a PMT.
• Figure 4: Various PC ratio
• Figure 5: Various Triton ratio