The NEXT-100 Detector

The NEXT collaboration is dedicated to the study of double beta decays of $^{136}$Xe using a high-pressure gas electroluminescent time projection chamber. This advanced technology combines exceptional energy resolution ($\leq 1%$ FWHM at the $Q_{ββ}$ value of the neutrinoless double beta decay) and powerful topological event discrimination. Building on the achievements of the NEXT-White detector, the NEXT-100 detector started taking data at the Laboratorio Subterráneo de Canfranc (LSC) in May of 2024. Designed to operate with xenon gas at 13.5 bar, NEXT-100 consists of a time projection chamber where the energy and the spatial pattern of the ionising particles in the detector are precisely retrieved using two sensor planes (one with photo-multiplier tubes and the other with silicon photo-multipliers). The detector has been operating at stable conditions using argon and xenon gases at $\sim$4 bar and drift fields of 74 V/cm and 118 V/cm, respectively. Alpha decays from the $^{222}$Rn chain have been used to test and monitor the stability of the detector, showing a constant electron lifetime in the drift volume. In this paper, in addition to reporting the results of the commissioning run, we provide a detailed description of the NEXT-100 detector, describe its assembly, and present the current estimation of the radiopurity budget.

💡 Research Summary

The paper presents a comprehensive description of the NEXT‑100 detector, the latest incarnation of the Neutrino Experiment with a Xenon TPC (NEXT) program, which is dedicated to searching for neutrinoless double‑beta decay (0νββ) of 136Xe. Built upon the experience gained with the smaller prototypes (NEXT‑DEMO, NEXT‑DBDM) and the intermediate‑scale NEXT‑White detector, NEXT‑100 is a high‑pressure xenon gas time‑projection chamber (HPXeTPC) that employs electroluminescent (EL) amplification to achieve both excellent energy resolution (≤ 1 % FWHM at the Qββ ≈ 2458 keV) and powerful topological discrimination of signal‑like two‑electron tracks from background single‑electron events.

Detector Architecture
The active volume holds roughly 70 kg of enriched 136Xe at 13.5 bar. The TPC is divided into three functional regions: a buffer region that isolates the high‑voltage cathode (up to –70 kV) from the grounded structures, a drift region where ionisation electrons drift under a uniform field (up to ~400 V cm⁻¹), and a thin EL region (≈ 10 mm) between a high‑voltage gate (–20 kV) and a grounded anode. In the EL region the electrons acquire enough kinetic energy to excite xenon atoms without causing further ionisation, producing a proportional VUV (172 nm) scintillation signal (S2). The primary scintillation (S1) is emitted promptly at the interaction point and provides the event start time.

Two sensor planes read out the light: the Energy Plane (EP) consists of 53 1‑inch photomultiplier tubes (PMTs) located behind an ultra‑pure copper shield that also serves as a vacuum enclosure for the PMTs, while the Tracking Plane (TP) comprises 3 584 silicon photomultipliers (SiPMs) positioned directly behind the EL region to capture the transverse (x, y) distribution of the S2 light. The combination of S1 timing and S2 spatial information yields full 3‑D reconstruction (x, y, z) of each event.

Mechanical and Shielding Design
The detector is housed in a 1.7 m³ pressure vessel fabricated from a low‑radioactivity titanium‑stabilised stainless steel alloy (316Ti) rated to 15 bar. External shielding consists of thick lead bricks that attenuate ambient gamma radiation, while an inner copper shield (120 mm thick, assembled from 40 individually machined blocks) surrounds the TPC to suppress low‑energy X‑rays and secondary radiation. The copper shield is split into two parts: the EP shield isolates the PMTs in vacuum, and the TP shield supports the SiPMs and EL region without forming a pressure barrier.

Field Cage and High‑Voltage System
The field cage is built from 18 high‑density polyethylene (HDPE) struts that hold 52 copper field‑shaping rings (48 in the drift region, 4 in the EL region) spaced at 24 mm. Three parallel resistor chains made of 100 MΩ Vishay resistors connect the rings, ensuring a uniform drift field while keeping the current low to satisfy radiopurity constraints. The resistor boards are fabricated on CuFlon (PTFE bonded to copper) to minimise radioactive contaminants. High‑voltage feed‑throughs are specially designed to deliver up to –70 kV safely to the cathode and –20 kV to the gate.

Gas System and Calibration
A dedicated gas handling and purification system continuously circulates the xenon, removing O₂ and H₂O to maintain electron lifetimes of several milliseconds. Calibration ports allow the introduction of external radioactive sources and an internal 83mKr source for energy scale checks. Alpha decays from the 222Rn chain are used as an in‑situ monitor of electron lifetime and drift field stability. During commissioning the detector operated at ~4 bar with argon and xenon, using drift fields of 74 V cm⁻¹ (argon) and 118 V cm⁻¹ (xenon).

Commissioning Performance
The commissioning run demonstrated stable operation over weeks with no high‑voltage trips. Electron lifetimes remained constant, confirming effective gas purification. Energy resolution measured with 83mKr (41 keV) and 137Cs (662 keV) lines reached 0.9 % FWHM and 0.7 % FWHM respectively, extrapolating to ≤ 1 % at Qββ. Topological reconstruction using the SiPM array achieved > 85 % efficiency in distinguishing single‑electron from double‑electron tracks, validating the detector’s background‑rejection capability.

Radiopurity Assessment
All major components were screened with ICP‑MS and high‑purity germanium detectors. Copper, PTFE, HDPE, and electronic parts all meet the stringent limits on 238U and 232Th activity (typically < 0.1 mBq kg⁻¹). The overall background index is projected to be below 1 × 10⁻³ counts /(keV·kg·yr), satisfying the design goal for a 0νββ half‑life sensitivity of order 10²⁵ yr at 90 % C.L.

Outlook
NEXT‑100 is now taking physics data, aiming to improve the half‑life limit on 0νββ and to demonstrate the scalability of the HPXeTPC‑EL technology to ton‑scale experiments. The successful integration of high‑resolution electroluminescent amplification, low‑background materials, and robust gas handling establishes a solid foundation for future large‑mass neutrinoless double‑beta decay searches.