Performance study of a fiber-PMT water Cherenkov detector prototype without inner reflector

Water Cherenkov detectors (WCDs) are widely used in cosmic ray and gamma-ray experiments. We present a cost-effective WCD prototype that integrates a small photomultiplier tube (PMT) with wavelength-shifting (WLS) fibers. The detector was systematically tested with cosmic ray muons under different trigger configurations. The fiber-PMT design achieved an average light yield of about 30 photoelectrons (PE), representing nearly a 200% improvement compared to a standalone PMT, while maintaining a time resolution of 2–4 ns. Additional measurements performed without a reflective inner lining showed a significant reduction in light yield and a modest degradation in timing precision, confirming the critical role of reflective surfaces in the current configuration. These results demonstrate the feasibility of the fiber-PMT WCD approach and highlight the importance of detector optics. Future studies with larger detector volumes are planned to evaluate whether geometry can further enhance photon collection efficiency and compensate for the absence of reflectors.