Optical characterization of UV-transmitting acrylics for the Hyper-Kamiokande multi-PMT module

The Hyper-K project aims to construct a large-volume underground water Cherenkov detector to address unresolved questions in fundamental particle physics. The Hyper-Kamiokande far detector (HK) will incorporate various types of photodetectors, among which the multi-photomultiplier tube (mPMT) optical module providing critical directional information. To house the electronics and photodetectors, the mPMT vessel must meet specific criteria: (i) transparency in the visible and ultraviolet ranges, covering the spectrum of Cherenkov photons; (ii) an airtight seal to maintain optimal functionality in a dry, controlled environment; and (iii) resilience to the hydrostatic pressure of the HK water tank, which reaches 0.8 MPa. While glass has traditionally been used to fulfill these requirements, its intrinsic radioactivity may compromise the sensitivity of Hyper-K measurements. Acrylic that is transparent, impermeable and robust, by contrast, offers the advantage of a low-radioactivity. Optical characterizations were performed on various acrylic samples, revealing that optical performance is influenced not only by the material composition but also by the production processes used to achieve different thicknesses. Our findings support the use of acrylic as a viable alternative, but it is essential to rigorously monitor the material's performance during mass production to ensure optimal and uniform detector performance.