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Characterization of argon recoils at the keV scale with ReD and ReD+

L. Pandola, P. Agnes, I. Ahmad, S. Albergo, I. Albuquerque, M. Atzori Corona, M. Ave, B. Bottino, M. Cadeddu, A. Caminata, N. Canci, M. Caravati, L. Consiglio, S. Davini, M. De Napoli, L. K. S. Dias, G. Dolganov, G. Fiorillo, D. Franco, M. Gulino, T. Hessel, N. Kemmerich, M. Kimura, M. Kuźniak, M. La Commara, J. Machts, G. Matteucci, E. Moura Santos, E. Nikoloudaki, V. Oleynikov, R. Perez Varona, N. Pino, S. M. R. Puglia, M. Rescigno, D. Sablone, B. Sales Costa, S. Sanfilippo, C. Sunny, Y. Suvorov, R. Tartaglia, G. Testera, A. Tricomi, M. Wada, Y. Wang, R. Wojaczyński, P. Zakhary

TL;DR

The paper addresses the need for a direct, model-independent measurement of the argon ionization yield $Q_y$ for nuclear recoils in the 2–10 keV range to calibrate detectors targeting low-mass WIMPs. It uses a compact dual-phase argon TPC irradiated with $^{252}$Cf neutrons and a downstream spectrometer to determine recoil energies via two-body kinematics $E_R = 2 E_n \frac{m_n m_{Ar}}{(m_n+m_{Ar})^2} (1-\cos\theta_S)$ and measures the ionization yield from the observed $N_e$ with a gain $g_2$, obtaining results that extend below 7 keV and show an increasing $Q_y$ toward lower energies. The study identifies ~800 NR candidates in 2–10 keV, with ~55% signal and ~70% S2-only events, and finds good agreement with existing data above 7 keV while challenging extrapolations at lower energies; the measured resolutions are $\sim$9% in $E_R$ at 2 keV and $\sim$7% at higher energies, with $N_e$ resolution around 12% at $N_e=10$ improving to 7% for larger $N_e$. These results inform global response-model fits and dark matter sensitivity projections, and the ReD+ program aims to push measurements toward sub-keV scales using upgraded geometry, higher activity sources, and eventually a tagged $^3$He neutron-detector setup for precise, event-by-event neutron identification.

Abstract

The ReD experiment measured the ionization yield Qy of argon for nuclear recoils in the 2-10 keV range using a dual-phase Time Projection Chamber irradiated with neutrons from a Cf-252 fission source. The measurement extends coverage below 7 keV, confirms consistency with previous data above 7 keV, and indicates a higher Qy at lower energies. These results are relevant for argon-based experiments searching for dark matter in the form of low-mass Weakly Interacting Massive Particles, which are very sensitive to the modeling of the detector response in this energy range.

Characterization of argon recoils at the keV scale with ReD and ReD+

TL;DR

The paper addresses the need for a direct, model-independent measurement of the argon ionization yield for nuclear recoils in the 2–10 keV range to calibrate detectors targeting low-mass WIMPs. It uses a compact dual-phase argon TPC irradiated with Cf neutrons and a downstream spectrometer to determine recoil energies via two-body kinematics and measures the ionization yield from the observed with a gain , obtaining results that extend below 7 keV and show an increasing toward lower energies. The study identifies ~800 NR candidates in 2–10 keV, with ~55% signal and ~70% S2-only events, and finds good agreement with existing data above 7 keV while challenging extrapolations at lower energies; the measured resolutions are 9% in at 2 keV and 7% at higher energies, with resolution around 12% at improving to 7% for larger . These results inform global response-model fits and dark matter sensitivity projections, and the ReD+ program aims to push measurements toward sub-keV scales using upgraded geometry, higher activity sources, and eventually a tagged He neutron-detector setup for precise, event-by-event neutron identification.

Abstract

The ReD experiment measured the ionization yield Qy of argon for nuclear recoils in the 2-10 keV range using a dual-phase Time Projection Chamber irradiated with neutrons from a Cf-252 fission source. The measurement extends coverage below 7 keV, confirms consistency with previous data above 7 keV, and indicates a higher Qy at lower energies. These results are relevant for argon-based experiments searching for dark matter in the form of low-mass Weakly Interacting Massive Particles, which are very sensitive to the modeling of the detector response in this energy range.
Paper Structure (5 sections, 1 equation, 2 figures)

This paper contains 5 sections, 1 equation, 2 figures.

Table of Contents

  1. Introduction
  2. Experimental Setup
  3. Data analysis and results
  4. Future improvements with ReD+
  5. Conclusions

Figures (2)

  • Figure 1: Schematic view (not in scale) of the experimental setup. The arrows indicate a neutron emitted from the source which undergoes (n,n') interaction with Ar in the TPC and is eventually scattered within the acceptance of the neutron spectrometer. See text and Ref. Agnes:2025rxi for more details.
  • Figure 2: Measured ionization yield for nuclear recoils in the 2–10 keV range from ReD Agnes:2025rxi. Literature values up to 120 keV from ARIS Agnes:2018mvl, SCENE Cao:2015ks, and Joshi et al. Joshi:2014fna are shown for comparison; the 6.7 keV point from Joshi et al. has been rescaled as described in Ref. PhysRevD.104.082005. Model‑based interpretations are discussed in Refs. newpaperlidine25davide. The region below 2 keV will be explored by the upcoming ReD+ project, see Sect. \ref{['sec:redplus']}.