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

This paper details the design and construction of the watertight optical modules for BUTTON-30, a neutrino detector demonstrator located in the STFC Boulby underground facility that aims to test gadolinium-loaded water-based liquid scintillator for future large-volume observatories and nuclear reactor monitoring.

The Gist

Imagine trying to build a giant, high-tech underwater camera to catch tiny, ghostly particles called neutrinos. These particles are so elusive that they usually pass right through everything without leaving a trace. To catch them, scientists need a massive tank filled with a special, glowing liquid. But there's a catch: the "cameras" (which are actually giant light-sensitive tubes called PMTs) are very delicate and cannot touch the liquid directly, or they would short-circuit or corrode.

This paper describes how the team built a custom "diving suit" for these cameras so they could survive underwater in a special chemical soup.

The Mission: BUTTON-30

The project is called BUTTON-30. It's a test run for a future, much bigger neutrino detector. It's located deep underground in a salt mine in England (the Boulby Underground Laboratory). Being deep underground is like wearing a heavy lead blanket; it blocks out the "noise" of cosmic rays from space, allowing the scientists to hear the faint whispers of neutrinos.

The tank is filled with 30 tons of a special liquid called Water-based Liquid Scintillator (WbLS) mixed with Gadolinium. Think of this liquid as a high-tech, glowing water that flashes when a neutrino bumps into it.

The Problem: The Delicate Cameras

The "cameras" are 96 large glass tubes (10-inch Photomultiplier Tubes, or PMTs). They are incredibly sensitive to light but very sensitive to chemicals.

• The Issue: The scientists wanted to use the new WbLS liquid, but tests showed that the liquid would eat away at the electrical parts of the camera tubes.
• The Solution: They needed to put each camera inside a waterproof, transparent bubble that keeps the liquid out but lets the light in.

The Design: The "Acrylic Bubble"

The team designed a custom housing that looks like a giant, clear plastic snow globe.

• The Shell: It's made of two halves of a clear acrylic sphere (like a giant fishbowl). The front half is made of a special type of plastic that lets ultraviolet light through (which the camera needs to see), while the back half is painted black inside to stop light from bouncing around confusingly.
• The Seal: The two halves are pressed together with a giant rubber O-ring (like the seal on a Tupperware container) to make it watertight.
• The Glue: Inside the bubble, the camera is glued to the plastic shell using a special clear gel. This gel acts like a bridge, letting light pass from the plastic to the camera without losing any.
• The Umbilical Cord: A cable exits the bubble through a special "penetrator" system (a high-tech cork) that keeps water out while letting electricity in.

The Stress Test: Can It Hold?

Before building the real thing, the team had to make sure the plastic bubbles wouldn't crush under the weight of the water.

• The Simulation: They used computer models (like a video game physics engine) to simulate the pressure. They found that an early design (made by heating and stretching the plastic) had weak spots where the plastic was too thin.
• The Fix: They switched to a "blow-molding" technique (like blowing up a balloon to shape it). This made the plastic thicker and stronger at the edges.
• The Result: The new design is strong enough to handle the pressure of 3 meters of water (about 3 times the pressure you feel diving to the bottom of a swimming pool) with a huge safety margin.

The Assembly: Building the Bubbles

Putting these together was like a precise assembly line, similar to how the IceCube detector in Antarctica was built.

1. Preparation: They painted the inside of the back half black and cleaned the camera tubes.
2. The Gel: They mixed the special glue and removed all air bubbles from it (using a vacuum, like sucking the air out of a bag of chips) so the glue was perfectly clear.
3. The Drop: They carefully lowered the camera into the gel-filled front half, making sure it was perfectly centered.
4. The Cure: They let the glue harden for 24 hours.
5. The Seal: They screwed the back half on, tightening the bolts in a specific pattern (like tightening the lug nuts on a car tire) to ensure an even seal.
6. The Check: Every single bubble was dunked in a water tank to check for leaks. They even froze one to make sure it wouldn't crack in the cold.

The Outcome

The team successfully built 99 of these custom "diving suits." 98% of them worked perfectly on the first try. They were shipped to the underground mine and installed in the giant tank.

In short: The paper explains how the team engineered a robust, transparent, and watertight "bubble" to protect sensitive light detectors, allowing them to operate safely in a new, glowing chemical liquid deep underground. This successful test paves the way for even larger neutrino detectors in the future.

Technical Summary

Technical Summary: Design and Development of Optical Modules for the BUTTON-30 Detector

Problem Statement
The BUTTON-30 project aims to deploy a neutrino detector demonstrator in the Boulby Underground Laboratory to test the performance of gadolinium-loaded water-based liquid scintillator (WbLS) for neutrino detection and reactor monitoring. A critical engineering challenge identified was the incompatibility of standard photomultiplier tube (PMT) bases with WbLS; soak tests revealed that the waterproof bases supplied by Hamamatsu react chemically with the scintillator medium. Consequently, a custom encapsulation solution was required to allow 10-inch Hamamatsu R7081-100 PMTs to operate within this novel, chemically aggressive fill media while maintaining watertight integrity and optical transparency.

Methodology
The authors designed, fabricated, and assembled custom optical modules based on a spherical acrylic housing concept inspired by the IceCube Digital Optical Modules (DOMs). The methodology encompassed three primary phases:

1. Design and Material Selection:

   • Housing: The module consists of two acrylic hemispheres (radius 48.6 cm) joined by a flat flange. The front half utilizes UV-transparent acrylic, while the rear half uses standard acrylic painted black internally to suppress stray light.
   • Sealing: A Viton O-ring within a flange groove, compressed by stainless steel washer plates, provides the watertight seal. All steel components are marine-grade 316 stainless steel, pickled, and electropolished to prevent corrosion and leaching.
   • Optical Coupling: The PMT is optically coupled to the housing using Techsil RTV27905, a two-component silicone gel. This gel was selected for its UV transmission, low cost, and refractive index (1.405) matching the PMT requirements.
   • Penetrator: A custom penetrator system allows the high-voltage and signal coaxial cable to exit the housing while maintaining a seal via ferrules and O-rings. A silica desiccator bag controls internal humidity.

2. Characterization and Simulation:

   • Optical: Transmittance of the acrylic and gel was measured (200–800 nm) and compared against the PMT quantum efficiency. The system maintains approximately 88% transparency at 400 nm relative to a bare PMT.
   • Pressure: Finite-element analysis (COMSOL Multiphysics) and direct pressure testing were conducted. Initial prototypes used vacuum thermoforming, which resulted in thickness variations and high stress concentrations (~60 MPa) at the dome-flange transition. To improve durability, the final production switched to a blow-moulding process, which increased flange thickness and reduced maximum stress to ~20 MPa.

3. Assembly and Quality Assurance (QA):

   • A rigorous assembly workflow was established, involving component cleaning, gel degassing under vacuum, precise PMT alignment (maintaining a 4 mm gap), and staged torqueing of bolts (initial 4.5 Nm, followed by two passes at 6 Nm).
   • QA procedures included immersion tests in deionized water (30 minutes to 2 weeks), thermal cycling (0°C), and electrical testing (dark count and current draw) in light-tight tents.

Key Results

• Production Efficiency: Ninety-nine optical modules were successfully designed and produced. The process achieved a 98% success rate on the first deployment attempt, with only a handful of housings requiring rework, primarily due to torque application variations.
• Performance Validation:
  • The blow-moulded housings demonstrated superior handling characteristics compared to thermoformed prototypes, eliminating the fragility issues observed in early iterations.
  • Modules passed all watertightness tests, including extended submersion and thermal shock tests, with no signs of water ingress.
  • Electrical testing confirmed that encapsulated PMTs maintained performance metrics consistent with pre-encapsulation characterizations.
  • A spare module operated in 1% WbLS at Brookhaven National Laboratory for several weeks with no visible signs of corrosion, degradation, or cracking.
• Deployment: Sixty-four radial and sixteen bottom-mounted modules were successfully installed in the BUTTON-30 detector tank at Boulby.

Significance and Claims
The paper claims that the developed acrylic housing design successfully enables the operation of standard PMTs in chemically reactive, gadolinium-loaded water-based liquid scintillators. The authors state that the production process was efficient and robust, resulting in a high yield of deployable modules. The housing design is presented as capable of withstanding 3 bar of pressure, making it suitable not only for the BUTTON-30 demonstrator but also for future large-volume neutrino observatories. The work paves the way for the BUTTON-30 experiment to begin data-taking, which will provide critical data on the long-term stability of acrylic in WbLS and the in-situ optical performance of the detector system.