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The neutron veto of the XENONnT experiment: Results with demineralized water

XENON Collaboration, E. Aprile, J. Aalbers, K. Abe, S. Ahmed Maouloud, L. Althueser, B. Andrieu, E. Angelino, D. Antón Martin, F. Arneodo, L. Baudis, M. Bazyk, L. Bellagamba, R. Biondi, A. Bismark, K. Boese, A. Brown, G. Bruno, R. Budnik, C. Cai, C. Capelli, J. M. R. Cardoso, A. P. Cimental Chávez, A. P. Colijn, J. Conrad, J. J. Cuenca-García, V. D'Andrea, L. C. Daniel Garcia, M. P. Decowski, A. Deisting, C. Di Donato, P. Di Gangi, S. Diglio, K. Eitel, S. el Morabit, A. Elykov, A. D. Ferella, C. Ferrari, H. Fischer, T. Flehmke, M. Flierman, W. Fulgione, C. Fuselli, P. Gaemers, R. Gaior, M. Galloway, F. Gao, S. Ghosh, R. Giacomobono, R. Glade-Beucke, L. Grandi, J. Grigat, H. Guan, M. Guida, P. Gyorgy, R. Hammann, A. Higuera, C. Hils, L. Hoetzsch, N. F. Hood, M. Iacovacci, Y. Itow, J. Jakob, F. Joerg, Y. Kaminaga, M. Kara, P. Kavrigin, S. Kazama, M. Kobayashi, D. Koke, A. Kopec, H. Landsman, R. F. Lang, L. Levinson, I. Li, S. Li, S. Liang, Y. -T. Lin, S. Lindemann, M. Lindner, K. Liu, M. Liu, J. Loizeau, F. Lombardi, J. Long, J. A. M. Lopes, T. Luce, Y. Ma, C. Macolino, J. Mahlstedt, A. Mancuso, L. Manenti, F. Marignetti, T. Marrodán Undagoitia, K. Martens, J. Masbou, E. Masson, S. Mastroianni, A. Melchiorre, J. Merz, M. Messina, A. Michael, K. Miuchi, A. Molinario, S. Moriyama, K. Morá, Y. Mosbacher, M. Murra, J. Müller, K. Ni, U. Oberlack, B. Paetsch, Y. Pan, Q. Pellegrini, R. Peres, C. Peters, J. Pienaar, M. Pierre, G. Plante, T. R. Pollmann, L. Principe, J. Qi, J. Qin, D. Ramírez García, M. Rajado, R. Singh, L. Sanchez, J. M. F. dos Santos, I. Sarnoff, G. Sartorelli, J. Schreiner, P. Schulte, H. Schulze Eißing, M. Schumann, L. Scotto Lavina, M. Selvi, F. Semeria, P. Shagin, S. Shi, J. Shi, M. Silva, H. Simgen, C. Szyszka, A. Takeda, Y. Takeuchi, P. -L. Tan, D. Thers, F. Toschi, G. Trinchero, C. D. Tunnell, F. Tönnies, K. Valerius, S. Vecchi, S. Vetter, F. I. Villazon Solar, G. Volta, C. Weinheimer, M. Weiss, D. Wenz, C. Wittweg, V. H. S. Wu, Y. Xing, D. Xu, Z. Xu, M. Yamashita, L. Yang, J. Ye, L. Yuan, G. Zavattini, M. Zhong

TL;DR

The study demonstrates the XENONnT neutron veto, a water Cherenkov detector surrounding the TPC, achieving a neutron-detection efficiency of $82\pm1\%$ with demineralized water and enabling a WIMP-like neutron tagging efficiency of $53\pm3\%$ within a $250\mathrm{\,μs}$ window, at a livetime loss of $1.6\%$ during SR0. Using AmBe and Th calibrations, the team established a detailed understanding of NV response, timing, and optical properties, validated by simulations for radiogenic neutrons. The analysis shows robust PMT performance, stable detector conditions, and a measured NV tagging efficiency consistent with expectations, with four NV-tagged events in the blinded WIMP region and a neutron background estimate of $1.1^{+0.6}_{-0.5}$ events. These results confirm the NV as an effective neutron-background mitigation tool and set the stage for the subsequent gadolinium-doped phase, which is expected to further reduce neutron-induced backgrounds by roughly a factor of two and enhance WIMP sensitivity. The work highlights the practical integration of a large-scale water Cherenkov NV with a dual-phase LXe TPC for next-generation dark matter searches.

Abstract

Radiogenic neutrons emitted by detector materials are one of the most challenging backgrounds for the direct search of dark matter in the form of weakly interacting massive particles (WIMPs). To mitigate this background, the XENONnT experiment is equipped with a novel gadolinium-doped water Cherenkov detector, which encloses the xenon dual-phase time projection chamber (TPC). The neutron veto (NV) tags neutrons via their capture on gadolinium or hydrogen, which release $γ$-rays that are subsequently detected as Cherenkov light. In this work, we present the key features and the first results of the XENONnT NV when operated with demineralized water in the initial phase of the experiment. Its efficiency for detecting neutrons is $(82\pm 1)\,\%$, the highest neutron detection efficiency achieved in a water Cherenkov detector. This enables a high efficiency of $(53\pm 3)\,\%$ for the tagging of WIMP-like neutron signals, inside a tagging time window of $250\,\mathrm{μs}$ between TPC and NV, leading to a livetime loss of $1.6\,\%$ during the first science run of XENONnT.

The neutron veto of the XENONnT experiment: Results with demineralized water

TL;DR

The study demonstrates the XENONnT neutron veto, a water Cherenkov detector surrounding the TPC, achieving a neutron-detection efficiency of with demineralized water and enabling a WIMP-like neutron tagging efficiency of within a window, at a livetime loss of during SR0. Using AmBe and Th calibrations, the team established a detailed understanding of NV response, timing, and optical properties, validated by simulations for radiogenic neutrons. The analysis shows robust PMT performance, stable detector conditions, and a measured NV tagging efficiency consistent with expectations, with four NV-tagged events in the blinded WIMP region and a neutron background estimate of events. These results confirm the NV as an effective neutron-background mitigation tool and set the stage for the subsequent gadolinium-doped phase, which is expected to further reduce neutron-induced backgrounds by roughly a factor of two and enhance WIMP sensitivity. The work highlights the practical integration of a large-scale water Cherenkov NV with a dual-phase LXe TPC for next-generation dark matter searches.

Abstract

Radiogenic neutrons emitted by detector materials are one of the most challenging backgrounds for the direct search of dark matter in the form of weakly interacting massive particles (WIMPs). To mitigate this background, the XENONnT experiment is equipped with a novel gadolinium-doped water Cherenkov detector, which encloses the xenon dual-phase time projection chamber (TPC). The neutron veto (NV) tags neutrons via their capture on gadolinium or hydrogen, which release -rays that are subsequently detected as Cherenkov light. In this work, we present the key features and the first results of the XENONnT NV when operated with demineralized water in the initial phase of the experiment. Its efficiency for detecting neutrons is , the highest neutron detection efficiency achieved in a water Cherenkov detector. This enables a high efficiency of for the tagging of WIMP-like neutron signals, inside a tagging time window of between TPC and NV, leading to a livetime loss of during the first science run of XENONnT.

Paper Structure

This paper contains 12 sections, 3 equations, 20 figures.

Table of Contents

  1. Introduction
  2. The XENONnT neutron veto
  3. Detector description
  4. Light calibration tools
  5. Electronics, DAQ, and time synchronization
  6. Analysis software and data reconstruction
  7. PMT performance and detector stability
  8. Neutron veto efficiency
  9. Calibration setup
  10. Neutron veto detection efficiency
  11. Neutron veto tagging efficiency
  12. Impact of the NV in SR0 and conclusion

Figures (20)

  • Figure 1: CAD rendering of the NV surrounding the TPC cryostat in the center of the XENONnT water tank. Its main elements are the support structure (grey), reflector panels (white), and the 120 PMTs (orange). The main components of the calibration system entering the NV are also shown: neutron generator pipe (purple), I-belt (blue), and U-tubes (red and green). The reflector foils attached to the cryostat were deliberately removed in this drawing.
  • Figure 2: NV DAQ scheme. PMT signals are digitized by the V1730S modules with a sampling rate of 500MHz. Data is read out from the digitizers by the reader server and written to a common (Ceph) storage disk available to the event-builder processing. The busy logic and the acquisition monitor functionalities are handled by V1495 and V1724 modules, respectively.
  • Figure 3: Waveform of a single photoelectron signal recorded in PMT 2087. The black solid and dashed lines represent the baseline of the waveform as estimated by the digitizer and the processing software, respectively. The black-shaded region indicates the baseline RMS. The blue horizontal line shows the hitfinder threshold, set to 15ADCc above baseline. All consecutive samples below this line are marked as a hit. The blue shaded region indicates the hit including the left and right extension, named "hitlet" in the straxen framework, as explained in the text.
  • Figure 4: Event display of a 4.4MeV $\gamma$-ray recorded during [241]AmBe calibration. Top: Two-dimensional projection of the NV. The outer wall of the cryostat (black circle), the four diffuser balls (purple dots) and the NV walls (black octagon) are shown. Each circle next to the octagon represents one of the NV PMTs with the innermost circle corresponding to the lowest PMT in a column. The size of the dot indicates the integrated charge detected by the PMT in the displayed event. The color encodes the arrival time of the first detected photon in each respective channel. Center: Arrival time of the individual hitlets for the given event, using the same legenda as in the top panel. Bottom: Summed waveform of the event in the NV. The event display in XENON:2024InstrumentPaper shows for the same event the corresponding neutron interaction inside the TPC.
  • Figure 5: Evolution of the dark rate of six randomly chosen PMTs during SR0. The colors indicate different PMT channels. The temperature measured inside the demineralized water purification plant is shown as a blue-dashed line.
  • ...and 15 more figures