Design and development of optical modules for the BUTTON-30 detector

TL;DR

BUTTON-30 demonstrates a watertight acrylic optical-module design for 10'' PMTs to operate in gadolinium-loaded water-based liquid scintillator media. The design is DOM-inspired, with UV-transparent front acrylic and RTV27905 gel for optical coupling, and a robust penetrator-based cable feed for high-voltage and signals. Optical characterisation confirms adequate transmission down to the PMT's sensitive range and an overall module transmittance compatible with the PMT QE, while pressure qualification shows the final blow-moulded housing can withstand 3 bar hydrostatic pressure. A rigorously documented assembly and QA workflow yielded 98% first-pass deployment across 99 modules, enabling safe operation in WbLS and gadolinium-loaded media and informing future large-scale detectors. The work provides a practical blueprint for scalable PMT encapsulation in challenging media and underground environments, with planned long-term monitoring of acrylic stability and in-situ performance once data-taking begins.

Abstract

BUTTON-30 is a neutrino detector demonstrator located in the STFC Boulby underground facility in the north-east of England. The main goal of the project is to deploy and test the performance of the gadolinium-loaded water-based liquid scintillator for neutrino detection in an underground environment. This will pave the way for a future large-volume neutrino observatory that can also perform remote monitoring of nuclear reactors for nonproliferation. This paper describes the design and construction of the watertight optical modules of the experiment.

Figures (9)

• Figure 1: Schematic illustration of the acrylic optical housing with an encapsulated 10-inch Hamamatsu R7081-100 PMT. The main components are labelled, including the hemispherical housing halves, flange with O-ring, washer plates, penetrator assembly, and optical coupling gel. This design ensures watertight operation with various fill media. The interior of the back half of an actual housing is painted black.
• Figure 2: Measured transmittance of the RTV27905 optical gel (200–800 nm) and the selected UV-transparent acrylic, compared with the Hamamatsu R7081-100 photocathode quantum efficiency curve R7081datasheet. The transparency of both materials aligns well with the PMT spectral response, ensuring efficient photon collection.
• Figure 3: Finite-element analysis (COMSOL Multiphysics) of a thermoformed acrylic housing under 3 bar pressure. Left: stress distribution across the hemisphere. Right: zoomed region near the dome-to-flange transition, where maximum stress ($\sim$60 MPa) is observed due to abrupt thinning of the thermoformed geometry.
• Figure 4: Finite-element analysis (COMSOL Multiphysics) of the blow-moulded acrylic housing. Left: stress distribution across the hemisphere. Right: region near the flange where stress concentrations occur. The blow-moulded design exhibits reduced maximum stress ($\sim$20 MPa) and improved uniformity compared to the thermoformed version (Figure \ref{['fig:6-COMSOL-old-3D']}).
• Figure 5: Workflow summary of the optical module assembly procedure, The steps include quality assurance of components, preparation of housing and penetrator, degassing of the optical gel, precise positioning of the PMT, closure of housing halves, controlled torqueing of bolts, and final verification checks.