The BUTTON-30 detector at Boulby

This paper describes the design and construction of the BUTTON-30, a 30-tonne hybrid detector installed 1.1 km underground at the Boulby Laboratory to evaluate the potential of simultaneous Cherenkov and scintillation light detection for neutrino interactions in a low-background environment.

The Gist

Imagine you are trying to listen for a single, tiny whisper in the middle of a roaring stadium. That is essentially what scientists do when they try to detect neutrinos. These are ghostly particles that pass through almost everything without leaving a trace. To catch them, you need a massive, ultra-quiet listening post deep underground.

This paper describes the construction and setup of BUTTON-30, a new "listening post" built 1.1 kilometers (about 0.7 miles) underground in an old salt mine in Boulby, England.

Here is the story of BUTTON-30, broken down into simple concepts:

1. The Goal: Catching Ghosts with Two Eyes

Neutrinos are tricky. When they hit water, they usually create a tiny flash of light. Scientists have traditionally used two types of detectors:

• The "Speed Camera" (Cherenkov): Catches the flash of light that happens when a particle moves faster than light can in water. It's great at telling you which direction the particle came from.
• The "Motion Sensor" (Scintillator): Catches the light emitted when the particle slows down. It's very sensitive and tells you how much energy the particle had.

The Innovation: BUTTON-30 is a hybrid. It's like giving the detector two pairs of eyes. It uses a special liquid (a mix of water and a tiny bit of oil-based scintillator) that allows it to see both the direction and the energy of the particle at the same time. This is a test run for future, massive detectors that could revolutionize our understanding of the universe.

2. The Location: A Deep, Quiet Basement

Why go underground? Because the surface is noisy. Cosmic rays (particles from space) and natural radioactivity from the ground create a lot of "static" that drowns out the neutrino whispers.

• The Mine: The team built this in an active salt mine. Salt is naturally very clean and doesn't have much radioactivity, acting like a giant, natural shield.
• The Depth: Being 1.1 km down is like putting a soundproof blanket over the entire experiment. It blocks 99.9999% of the cosmic noise, leaving a perfectly quiet room to listen for the ghosts.

3. The Tank: A Giant, Clean Fishbowl

The detector is a 30-tonne tank (about the size of a small house) made of special stainless steel.

• The Liquid: They fill it with ultra-pure water. Think of it as water so clean that if you dropped a single speck of dust in, it would ruin the experiment. They have a complex filtration system (like a super-charged coffee maker) that scrubs the water of bacteria and chemicals.
• The "Gadolinium" Secret Sauce: They plan to add a special element called Gadolinium. Imagine this element as a "neutron magnet." When a neutrino hits the water, it sometimes releases a neutron. Gadolinium grabs that neutron and gives off a second, delayed flash of light. This acts like a "double-check" to confirm, "Yes, that was definitely a neutrino!"

4. The Eyes: 96 Giant Light Sensors

Inside the tank, there are 96 large cameras (called Photomultiplier Tubes or PMTs).

• The Problem: These cameras are sensitive to corrosion. If they touch the liquid directly, they might rust or leak chemicals.
• The Solution: The team put each camera inside a custom-made, waterproof acrylic "bubble" (like a diving bell for a camera). This keeps the camera dry and safe while still letting the light through.
• The Setup: These bubbles are arranged in a circle, looking inward, ready to catch any flash of light from the center of the tank.

5. The Calibration: Tuning the Radio

Before you can listen to music, you have to tune your radio so it doesn't sound fuzzy.

• The Tools: The team uses special radioactive sources (like a tiny, safe flashlight that emits particles) and lasers to test the cameras.
• The "Tagging" Trick: One of their sources is clever. It emits a gamma ray and a neutron at the exact same time. They have a special detector that "tags" the gamma ray. When the main cameras see the neutron flash right after the tag, they know the system is working perfectly.
• The FLASE Tool: They also built a device to measure how clear the liquid is. It's like shining a laser through a long tube of water to see how much the light dims or scatters, ensuring the "listening room" is crystal clear.

6. The Computer Brain: Sorting the Noise

When the experiment runs, it will generate a massive amount of data (about 2.5 terabytes a day!).

• The Trigger: The computer is programmed to ignore the background noise. It only saves data if it sees a specific pattern: at least four cameras flashing within a tiny fraction of a second. This is the "whisper" the scientists are looking for.
• The Clock: All the cameras are synchronized to the nanosecond (a billionth of a second). It's like having 96 musicians in an orchestra who must hit their notes at the exact same time, or the music falls apart.

Why Does This Matter?

BUTTON-30 is a prototype. It's a "technology demonstrator."

• The Risk: Building a detector the size of a skyscraper (kiloton-scale) is expensive and risky. If the technology doesn't work underground, you don't want to find out after you've spent billions.
• The Win: BUTTON-30 proves that this hybrid technology works in a real, deep-underground environment. If it succeeds, it paves the way for massive future experiments that could help us understand how stars burn, how nuclear reactors work, and what the fundamental building blocks of the universe are.

In short: The team built a super-clean, deep-underground listening post with 96 high-tech eyes to test a new way of catching invisible ghost particles. If this small test works, it will unlock the door to much bigger discoveries in the future.

Technical Summary

Based on the provided paper, here is a detailed technical summary of the BUTTON-30 detector project.

1. Problem Statement

The detection of low-energy neutrinos (∼3 MeV) is a significant challenge in particle physics due to low interaction cross-sections and high backgrounds from environmental radioactivity and cosmic-ray spallation products. Traditional detectors typically rely on either water Cherenkov or oil-based scintillator technologies, each with limitations:

• Water Cherenkov: Offers excellent directional reconstruction but low light yield.
• Scintillators: Offer high light yield but poor directional information.

To overcome these limitations, the field is moving toward hybrid detectors using Water-based Liquid Scintillator (WbLS) doped with Gadolinium (Gd). This technology aims to simultaneously capture the directional topology of Cherenkov light and the high light yield of scintillation, while using Gd to capture neutrons for interaction tagging. However, scaling this technology to kilotonne volumes requires rigorous validation of material stability, purification, and detector performance in a low-background environment before full-scale deployment.

2. Methodology

The BUTTON-30 (Boulby Underground Technology Testbed for Observing Neutrinos) project was established to serve as a 30-tonne technology demonstrator to validate this hybrid approach.

• Site Selection: The detector is installed at the Boulby Underground Laboratory (BUL) in the UK, located 1.1 km deep (2.8 km water equivalent). This depth attenuates cosmic muon flux by a factor of $10^6$, and the surrounding polyhalite salt rock provides an ultra-low natural radioactivity background.
• Detector Design:
  • Tank: A 3.6 m diameter, 3.2 m high cylindrical tank made of marine-grade 316 stainless steel, holding approximately 30 tonnes of liquid.
  • Optical System: 96 Hamamatsu R7081-100 10-inch Photomultiplier Tubes (PMTs) are mounted on a stainless steel support structure (PSUP). To ensure compatibility with both Gd-doped water and WbLS, PMTs are encapsulated in custom acrylic housings with Viton O-rings and silicone gel coupling.
  • Liner: A black polyethylene liner is mounted on the PSUP to reduce light scattering from the support structure.
  • Filtration: A sophisticated water purification system (Reverse Osmosis, Deionization, UV sterilization) maintains high optical transparency (18 MΩ cm resistivity, <5 ppb TOC). The system is designed to be reconfigured for WbLS operation to prevent damage to the scintillator or RO membranes.
• Material Selection & Radioassay: Strict material selection criteria were applied to prevent leaching and corrosion. All materials underwent rigorous radioassay using HPGe detectors (BUGS facility) to quantify radioactive impurities ($^{238}$U, $^{232}$Th, $^{40}$K). Components were double-bagged and handled in a Class 10,000 cleanroom environment to prevent contamination.
• Calibration Systems:
  • Radioactive Sources: A deployment system on the tank lid houses AmBe sources (both untagged and tagged with a BGO crystal/PMT for neutron-gamma coincidence).
  • Light Injection: A laser system (405 nm) with diffusers monitors PMT single-photoelectron response and timing.
  • FLASE: A dedicated device (Florida-Livermore Attenuation and Scattering Experiment) measures attenuation and scattering lengths of the liquid fill in situ.
• Data Acquisition (DAQ): The system uses 112 ADC channels (CAEN V1730 digitizers) running at 500 MS/s. Data is processed by a local server running ToolDAQ, utilizing a majority trigger (4+ PMT hits in a 100 ns window) to reconstruct events.

3. Key Contributions

• Construction of a Hybrid Testbed: BUTTON-30 is the largest detector constructed in the Boulby mine, providing the first deep-underground benchmark for Gd-WbLS technology.
• Encapsulated PMT Technology: The development of acrylic-encapsulated PMT housings allows the use of standard low-radioactivity PMTs in chemically aggressive WbLS environments, a critical innovation for future large-scale detectors.
• Integrated Calibration Infrastructure: The paper details the design of a multi-modal calibration suite (radioactive, optical, and fluid-dynamic) capable of characterizing the detector's response to both Cherenkov and scintillation light.
• Background Modeling: The authors provide a comprehensive radioassay of all detector components and a simulation-based background model (using RATPAC-Two/GEANT4) predicting the detector's noise floor.

4. Results

• Material Purity: Radioassay results confirmed that most detector materials have radioactivity levels below the sensitivity limits of the assay systems. The estimated total background activity from the tank, liner, and support structures is low, with the dominant contributions coming from the cavern rock rather than the detector materials themselves.
• Background Rates: Simulations predict a background trigger rate of approximately 120 Hz, dominated by the $^{238}$U and $^{232}$Th decay chains in the surrounding rock (specifically $^{214}$Bi and $^{208}$Tl). The detector materials themselves contribute minimally to this rate.
• System Performance: The DAQ system is fully integrated, capable of collecting ~2.5 Terabytes of data per day. The calibration systems (laser and radioactive sources) are designed to achieve timing precision of 0.5 ns and single-photoelectron peak monitoring with ~5% uncertainty.
• Operational Status: The detector installation is complete. The experiment is scheduled to begin data collection in Autumn 2025, starting with pure water fills to establish baselines, followed by WbLS and Gd-doped fills.

5. Significance

The BUTTON-30 experiment is a critical stepping stone for the next generation of neutrino observatories (such as THEIA and Hyper-Kamiokande upgrades). Its significance lies in:

1. Risk Mitigation: It validates the stability and purification of Gd-WbLS in a realistic underground environment, de-risking the scaling to kilotonne detectors.
2. Technology Validation: It benchmarks advanced photodetectors (like LAPPDs) and hybrid light detection techniques in a low-background setting.
3. Scientific Output: Beyond technology demonstration, the experiment will provide unique data on atmospheric neutrino flux and cosmogenic neutron production in the Boulby environment.
4. Broader Applications: The technology developed supports non-proliferation efforts, nuclear safeguards, and remote reactor monitoring by enabling precise neutrino detection.

In summary, BUTTON-30 represents a comprehensive engineering and physics validation of the hybrid Cherenkov-scintillator approach, paving the way for future large-scale experiments capable of probing fundamental physics, solar processes, and nuclear security.