Development of a Hybrid Water Cherenkov and Liquid Scintillator Detector for Cosmic Ray Detection

This study presents a hybrid detector design integrating water Cherenkov and liquid scintillator technologies for future ultra-high-energy cosmic ray (UHECR) and gamma-ray observatories. The detector unit comprises a cylindrical structure (25 cm radius, 115 cm height) with a 7 mm or 10 mm-thick liquid scintillator layer from the JUNO experiment at the top, coupled with a purified water volume. A 3-inch PMT is precisely positioned 40 cm above the tank bottom, encircled by light-blocking material to optically separate water sections, enabling direct Cherenkov light reception. Signal acquisition is performed using a CAEN V1743 digitizer (3.2 GS/s, 12-bit) for high-precision waveform analysis. We systematically evaluated key performance parameters, including light yield, time resolution, and spatial uniformity, comparing liquid scintillator (LS) with plastic scintillator (PS). While PS showed higher light yield, 117.7 p.e. and better time resolution (0.73 ns) than LS (72.49 p.e., 0.93 ns) in individual tests, the basic performance of plastic scintillator was confirmed as stable and viable for energy measurement. Moreover, 7 mm liquid scintillator exhibited more pronounced spatial non-uniformity, that number of photoelectrons (NPE) drops from 89.65 p.e. to 47.45 p.e. compared to PS. Crucially, the hybrid water-based liquid scintillator (WbLS) configurations demonstrated significantly enhanced performance. Specifically, the combination of 2cm LS and 30cm water achieved a high light yield of 199.36 p.e. and an excellent time resolution of 0.91 ns. Optical optimization, such as reflective cones, proved vital in mitigating inherent non-uniformities, ensuring consistent detector response across its volume. This hybrid WbLS design effectively merges the Cherenkov timing precision of water with the high light yield of scintillator, offering superior performance for energy and direction reconstruction. It addresses limitations of traditional detectors like LHAASO-ED and represents a strong, viable, and cost-effective candidate for next-generation observatories like SWGO.