The NEXT-100 Detector

This paper presents a detailed description of the NEXT-100 detector's design, assembly, and radiopurity budget, alongside results from its commissioning run at the Laboratorio Subterráneo de Canfranc, which demonstrated stable operation and effective monitoring using alpha decays.

The Gist

Imagine a giant, high-tech "fish tank" buried deep underground, designed not to hold fish, but to catch a ghost.

This paper introduces NEXT-100, a massive scientific instrument built to hunt for a rare event called neutrinoless double beta decay. If scientists can find this, it proves that neutrinos are their own antiparticles and helps explain why the universe is made of matter instead of being empty.

Here is the story of the NEXT-100 detector, explained simply:

1. The Goal: Catching a Ghost

Neutrinos are tiny, invisible particles that pass through everything like ghosts. Usually, when a specific atom (Xenon-136) decays, it spits out two electrons and two neutrinos. But physicists suspect that sometimes, the neutrinos cancel each other out, and only two electrons are released. Finding this "ghostly" event is incredibly hard because it happens so rarely and looks just like background noise.

2. The Machine: A Giant Xenon Cloud

The NEXT-100 detector is essentially a Time Projection Chamber (TPC). Think of it as a 3D camera for particles.

• The Tank: It's a pressure vessel made of super-clean stainless steel, holding about 70 kilograms of Xenon gas compressed to 13.5 times the pressure of the atmosphere (like being 135 meters underwater).
• The Trigger: When a particle zips through this gas, it knocks electrons loose, creating a trail of "dust" (ionization).
• The Flash: The detector uses a strong electric field to pull these electrons to a special zone where they get accelerated and create a bright flash of light (electroluminescence). It's like turning a single spark into a firework display so we can see it clearly.

3. The Eyes: Two Different Cameras

To see what's happening, the detector has two "camera planes" on opposite ends of the tank:

• The Energy Plane (The Light Meter): This side has 53 large Photomultiplier Tubes (PMTs). Think of these as giant, super-sensitive eyes that count every photon of light. They tell us how much energy the particle had.
• The Tracking Plane (The Map Maker): This side has over 3,500 tiny Silicon Photomultipliers (SiPMs). These are like a high-resolution grid of tiny sensors. They tell us exactly where the light came from, creating a 3D map of the particle's path.

Why two cameras?

• Background Noise: Most radioactive background events look like a single, messy dot (a single electron track).
• The Signal: The "ghost" event (neutrinoless double beta decay) creates a specific shape: two electron tracks starting from the same point, looking like a "double hairpin" or a "V".
• By combining the energy reading from the PMTs and the 3D shape from the SiPMs, the detector can say, "That's just noise," or "That looks like the ghost we are looking for!"

4. The Construction: Building a Clean Room

Building this was like assembling a Swiss watch inside a clean room.

• Radiopurity: The biggest enemy is natural radioactivity (like tiny bits of uranium or thorium in the metal). If the detector itself is radioactive, it will drown out the signal. The team used ultra-pure copper, special plastics, and cleaned every screw and wire with soap and acid to ensure the machine is "quiet."
• The Gas System: The Xenon gas must be pure. The machine has a complex recycling system (like a high-tech air filter) that constantly cleans the gas, removing impurities that would eat the electrons before they reach the cameras.
• The Pressure: The tank holds high pressure. If a window breaks, the gas could rush out. The team designed a "safety valve" system that can instantly suck the gas into a backup tank if a leak is detected, saving the expensive Xenon and the delicate electronics.

5. The Journey: From Argon to Xenon

The detector started working in May 2024 at the Canfranc Underground Laboratory in Spain.

• Phase 1 (The Test Drive): They first filled it with Argon gas (a cheaper, safe gas) to make sure there were no leaks and that all the electronics worked. They used natural radioactive "alpha" particles (like tiny bullets) to test the sensors.
• Phase 2 (The Real Deal): Once everything was stable, they switched to Xenon. They tested the drift of electrons and confirmed the machine could see the "tracks" clearly.
• The Result: The detector is now running smoothly. It has proven that the technology works at this massive scale. It can see the shape of particles and measure their energy with incredible precision (better than 1% error).

6. Why It Matters

The NEXT-100 is the largest high-pressure Xenon detector in the world. It is the "proof of concept" for the future.

• It shows that we can build detectors this big.
• It proves the technology is stable and sensitive enough.
• It paves the way for the next step: a ton-scale detector (1,000 kg of Xenon) that could finally catch the neutrinoless double beta decay and unlock the secrets of the universe's matter.

In short: The NEXT-100 is a giant, ultra-clean, high-pressure camera that takes 3D photos of invisible particles. It has successfully passed its "driver's test" and is now ready to hunt for the most elusive ghost in physics.

Technical Summary

1. Problem Statement

The primary scientific goal is the detection of neutrinoless double beta decay ($\beta\beta0\nu$) in the isotope $^{136}\text{Xe}$. Observing this decay would confirm that neutrinos are Majorana particles (their own antiparticles), violate lepton number conservation, and provide insights into the origin of neutrino mass and the matter-antimatter asymmetry of the universe.

The challenge lies in the extreme rarity of this event, requiring detectors with:

• Ultra-high energy resolution: To distinguish the signal peak from the continuous background of standard two-neutrino double beta decay ($\beta\beta2\nu$) and other radioactive backgrounds.
• Topological discrimination: The ability to distinguish the specific track topology of a $\beta\beta0\nu$ event (two electron tracks originating from a single vertex) from background events (single electron tracks or gamma interactions).
• Radiopurity: Minimizing internal and external radioactive backgrounds to achieve a background index below $1 \times 10^{-3}$ counts/(keV·kg·year).

Previous prototypes (NEXT-DEMO, NEXT-DBDM, and NEXT-White) demonstrated the feasibility of High-Pressure Xenon Time Projection Chambers (HPXeTPC) with electroluminescence (EL) amplification, but a larger, ton-scale detector was needed to reach the sensitivity required for a definitive discovery ($\sim 10^{25}$ years half-life sensitivity).

2. Methodology and Detector Design

The NEXT-100 detector is the third phase of the NEXT program, designed as a scalable HPXeTPC operating at the Laboratorio Subterráneo de Canfranc (LSC) in Spain.

• Active Volume: Contains $\sim 70.5$ kg of enriched $^{136}\text{Xe}$ gas at a pressure of 13.5 bar.
• Detection Principle:
  • S1 Signal: Primary scintillation (VUV light at 172 nm) provides the event start time ($t_0$).
  • S2 Signal: Ionization electrons drift toward an Electroluminescence (EL) region where they are accelerated, producing proportional secondary scintillation light. This provides the energy and spatial information.
• Dual-Sensor Architecture:
  • Energy Plane (EP): Located at the cathode end, equipped with 53 Photomultiplier Tubes (PMTs). It measures the total S2 light for energy reconstruction and the prompt S1 signal.
  • Tracking Plane (TP): Located at the anode end, equipped with 3,584 Silicon Photomultipliers (SiPMs). It captures the spatial distribution of the S2 light to reconstruct the 3D trajectory of the event, enabling topological discrimination.
• Key Components:
  • Field Cage: Constructed with HDPE struts and copper rings to ensure a uniform drift field ($\sim 118$ V/cm).
  • Light Tube: Lined with PTFE panels coated with Tetraphenyl Butadiene (TPB) to shift VUV light to blue wavelengths and maximize reflectivity.
  • Shielding: A multi-layer system comprising an outer lead shield and an inner ultra-pure copper shield (120 mm thick) to block external gamma radiation and X-rays.
  • Gas System: A sophisticated closed-loop system featuring cold and hot getters for continuous purification, a cryo-recovery system for xenon storage, and an emergency recovery tank to mitigate risks like PMT window rupture.
• Data Acquisition (DAQ): Inherited from NEXT-White but scaled up. It utilizes a dual-trigger scheme to handle high-rate calibration sources (e.g., $^{83m}\text{Kr}$) while collecting physics data, employing lossless compression to manage data throughput.

3. Key Contributions

The paper provides a comprehensive description of the NEXT-100 detector, marking a transition from prototype to a large-scale physics instrument.

• Scalability Demonstration: Successfully designed and assembled a meter-scale TPC (1.7 m$^3$ volume) operating at 13.5 bar, proving the technology can scale to the tonne-scale required for future experiments.
• Advanced Sensor Integration: Implemented a high-density SiPM tracking plane (3,584 channels) with custom feed-throughs capable of extracting signals from a high-pressure vessel without compromising vacuum integrity or radiopurity.
• Radiopurity Budget: Conducted an extensive screening campaign (using HPGe spectroscopy, ICP-MS, and GDMS) for all detector materials. The paper details the specific activities of $^{238}\text{U}$ and $^{232}\text{Th}$ chains, establishing a background model where the total expected background is $\sim 1$ count/year in the region of interest.
• System Upgrades: Introduced significant improvements over NEXT-White, including:
  • Enhanced gas circulation flow with dedicated injection ports to ensure uniform electron lifetime.
  • A robust emergency recovery system with a 20 m$^3$ tank to handle overpressure events or PMT failures.
  • Improved DAQ software (Go-based) and hardware (ATCA blades) for higher throughput.

4. Results and Performance

The detector began operations in May 2024 and completed a two-phase commissioning campaign:

• Phase 1 (Argon, May–Sept 2024):
  • Tested all subsystems (HV, gas, DAQ) at $\sim 4$ bar.
  • Confirmed leak-tightness and stable high-voltage operation (up to 14.7 kV cathode, 6.7 kV gate).
  • Measured electron lifetime and drift velocity, confirming system stability.
• Phase 2 (Xenon, Oct 2024–Feb 2025):
  • Operated with $^{136}\text{Xe}$-depleted xenon at $\sim 4$ bar.
  • Electron Lifetime: Derived from $^{222}\text{Rn}$ alpha decays, the electron lifetime was found to be sufficiently long to support the target energy resolution ($\le 1\%$ FWHM). No significant spatial variations were observed.
  • Drift Velocity & Diffusion: Measured drift velocities ($\sim 0.82$ mm/$\mu$s) and longitudinal diffusion coefficients matched Magboltz simulations and previous NEXT-White results.
  • Topological Imaging: Successfully reconstructed alpha decays (point-like) and double electron tracks (extended), demonstrating the detector's ability to distinguish signal topologies.
  • Stability: The detector operated stably with constant electron lifetime and light yield variations $< 1\%$ over 24-hour periods.

5. Significance

The successful commissioning of NEXT-100 represents a major milestone in the search for neutrinoless double beta decay:

1. Validation of Technology: It confirms that High-Pressure Xenon TPCs with EL amplification are scalable to large masses while maintaining excellent energy resolution and topological discrimination.
2. Path to Ton-Scale: The design and operational insights gained (handling meter-scale TPCs, stable HV at high pressure, efficient gas recovery) are directly applicable to the next generation of detectors (NEXT-1000 or tonne-scale experiments).
3. Physics Reach: With a projected sensitivity to a half-life of $\sim 10^{25}$ years, NEXT-100 will probe the inverted neutrino mass hierarchy region.
4. Operational Readiness: The detector is now taking data, with plans to transition to a high-pressure run (up to 13.5 bar) and a background-free campaign in 2025, paving the way for definitive physics measurements.

In summary, the paper details the construction, commissioning, and initial performance of the world's largest high-pressure xenon TPC, establishing a robust platform for the next decade of rare-event searches.