Speaker
Description
The Hyper-Kamiokande (HK) experiment aims to discover CP violation in the lepton sector by measuring the neutrino oscillation probabilities through the detection of accelerator neutrinos at the near detectors located immediately downstream of the beam source and at the far detector located 295 km away. While the far detector measures neutrinos using a water target, the existing magnetized near detector (ND280)uses plastic scintillator targets. It is known that this difference in nuclear targets introduces a large systematic uncertainty in the modeling of neutrino-nucleus interactions. In order to reduce this uncertainty, the conceptual design of the upgrade of the near detector, called ND280++, is being prepared.
For this upgrade, we are developing a new near detector, the water-based liquid scintillator detector (WbLS detector), which acts as neutrino active water target. The WbLS used in our study is a mixture of water and liquid scintillator combined with a surfactant, and it exhibits scintillator properties despite being mainly composed of water. Our detector consists of cube cells of 1 cm on a side, formed by separating the WbLS volumes with reflective walls, arranged in a three-dimensional grid. Scintillation photons produced in each cell are captured by orthogonal wavelength-shifting fibers inserted through the cell, transported to the outside of the detector, and detected by silicon photomultipliers (SiPMs). Owing to the low optical cube-to-cube crosstalk, the scintillation light in each cell can be detected almost independently. Thus, this detector can reconstruct the charged-particle tracks with a granularity of 1 cm and nearly isotropic detection efficiency.
In this study, to increase the light yield of the WbLS detector and to suppress the optical leakage into adjacent cells, we introduced several upgrades: attaching reflective sheets to the inner walls of the cells, adding reflective treatment to the non-readout ends of the wavelength-shifting fibers, using different SiPM models, and increasing the fiber diameter from 1 mm to 2 mm.
Next, we constructed three types of prototypes incorporating these improvements and conducted a performance evaluation test using an electron beam. The light yield was 25–29 photon equivalent per fiber, and the crosstalk fraction was 3–6%, which varied depending on the prototype. We also investigated the variation in the light yield and crosstalk fraction depending on the beam crossing position. Based on these results, we performed a detector performance simulation and showed that the current light yield is sufficient to achieve the required level of tracking performance and particle identification capability.