Performance study of 4-MU-loaded water for Cherenkov light detection

TL;DR

This work addresses the limited photon yield in water Cherenkov detectors by introducing 4-MU as a wavelength-shifting additive to convert UV Cherenkov light into blue photons more efficiently detected by PMTs. The study characterizes the optical properties of 4-MU in water, confirming UV absorption near 320 nm and blue emission near 450 nm, and demonstrates stable behavior at 0.5–1 ppm with ethanol as a solvent facilitator. In a prototype detector, 1 ppm 4-MU increases the detected light yield by approximately a factor of three and enhances muon-detection efficiency, with performance saturating beyond 1 ppm and maintaining stability over at least seven weeks. The findings indicate that 4-MU is a practical, scalable additive for boosting photon collection in water Cherenkov detectors, with potential benefits for active muon veto systems in rare-event experiments.

Abstract

We report on R&D study to improve the photon detection efficiency of water Cherenkov detectors by doping ultra-pure water with 4-methylumbelliferone (4-MU), a wavelength shifting additive. Cherenkov light yields from cosmic-ray muons were measured for various 4-MU concentrations and compared with those from pure water. At a concentration of 1 ppm, the detected light yield increased by approximately a factor of three. This enhancement can be attributed to wavelength shifting and improved photon collection efficiency. No noticeable degradation in optical transparency was observed across the tested concentrations of 0.5 and 1 ppm with different concentration of ethanol. These results suggest that 4-MU is a promising additive for improving the performance of water Cherenkov detectors.

Figures (6)

• Figure 1: Absorbance and fluorescence emission spectra of 4-MU in water. The absorbance spectrum (blue) shows strong ultraviolet absorption, while the fluorescence emission spectrum (red), excited at 320 nm, peaks in the blue region near 450 nm. These properties make 4-MU a suitable wavelength shifter for enhancing the sensitivity of bialkali PMTs.
• Figure 2: Time evolution of absorbance values for 4-MU solutions over a 4-month period: (a) 0.5 ppm of 4-MU, 0.1% Ethanol, (b) 1 ppm of 4-MU, 0.1% Ethanol, (c) 1 ppm of 4-MU, 0.5% Ethanol, (d) 1 ppm of 4-MU, 1% Ethanol
• Figure 3: A schematic diagram of a prototype water Cherenkov detector (left) and a photo of experimental setup (right)
• Figure 4: (a) Charge correlation between upper and lower scintillator counters used for muon tagging. The red dashed lines indicate the selection cuts for through-going muon events. (b) NPE distribution obtained in pure water, shown before (blue) and after (red) applying the muon selection criteria.
• Figure 5: (a) Average NPE detected from muon events as a function of 4-MU concentration in water. A rapid increase in light yield is observed up to 1 ppm, beyond which the performance saturates. (b) Muon detection efficiency as a function of PMT high voltage for three different conditions: DI water, 0.5 ppm 4-MU, and 1 ppm 4-MU.