https://doi.org/10.1140/epjc/s10052-026-15410-y
Regular Article - Experimental Physics
Characterization of the ionization response of argon to nuclear recoils at the keV scale with the ReD experiment
1
Gran Sasso Science Institute, 67100, L’Aquila, AQ, Italy
2
AstroCeNT, Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences, 00-614, Warsaw, Poland
3
Physics and Astronomy Department, Università degli Studi di Catania, 90123, Catania, Italy
4
Istituto Nazionale di Fisica Nucleare, Sezione di Catania, 90123, Catania, Italy
5
Instituto de Física, Universidade de São Paulo, 05508-090, São Paulo, Brazil
6
Istituto Nazionale di Fisica Nucleare, Sezione di Cagliari, 09042, Cagliari, Italy
7
Istituto Nazionale di Fisica Nucleare, Sezione di Genova, 16146, Genoa, Italy
8
Physics Department, Università degli Studi di Genova, 16146, Genoa, Italy
9
Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, 80126, Naples, Italy
10
INFN Laboratori Nazionali del Gran Sasso, 67010, Assergi, AQ, Italy
11
National Research Centre Kurchatov Institute, 123182, Moscow, Russia
12
National Research Nuclear University MEPhI, 115409, Moscow, Russia
13
Physics Department, Università degli Studi Federico II, 80126, Naples, Italy
14
APC, Université de Paris Cité, CNRS, Astroparticule et Cosmologie, 75013, Paris, France
15
Università di Enna KORE, 94100, Enna, Italy
16
Istituto Nazionale Fisica Nucleare, Laboratori Nazionali del Sud, 95123, Catania, Italy
17
Mines Paris, PSL University, Centre for Energy Environment Processes (CEEP), 77300, Fontainebleau, France
18
Department of Pharmacy, Università degli Studi Federico II, 80131, Naples, Italy
19
Budker Institute of Nuclear Physics, 630090, Novosibirsk, Russia
20
Novosibirsk State University, 630090, Novosibirsk, Russia
21
Istituto Nazionale di Fisica Nucleare, Sezione di Roma, 00185, Rome, Italy
22
Physics Department, Princeton University, 08544, Princeton, NJ, USA
23
Centro Siciliano di Fisica Nucleare e Struttura della Materia (CSFNSM), 90123, Catania, Italy
24
Insititute of High Energy Physics, 100049, Beijing, China
25
University of Chinese Academy of Sciences, 100049, Beijing, China
a
This email address is being protected from spambots. You need JavaScript enabled to view it.
Received:
27
October
2025
Accepted:
6
February
2026
Published online:
5
March
2026
Abstract
In the recent years, argon-based experiments looking for Dark Matter in the Universe have explored the non-standard scenario in which Dark Matter is made by low-mass Weakly Interacting Massive Particles, of mass in the range of 1–10 GeV instead of the canonical hundreds of GeV. Detecting such particles is challenging, as their expected signatures are nuclear recoils with energies below 10 keV, observable solely via ionization. This necessitates a precise understanding of the detector response in this energy regime, which remains incomplete for argon. To address this, the ReD experiment was developed within the framework of the DarkSide-20k Collaboration to produce and characterize few-keV nuclear recoils. A compact dual-phase argon Time Projection Chamber (TPC) was irradiated with neutrons from a 
Cf source, to produce Ar recoils in the energy range of interest via (n,n’) elastic scattering. A downstream spectrometer composed of 18 plastic scintillators detected the neutrons scattered off Ar nuclei, enabling recoil energy reconstruction via two-body kinematics. The ionization yield 
of argon, defined as the number of electrons produced per unit energy deposit, was measured in a model-independent way between 2 and 10 keV. These measurements extend direct experimental coverage well below the previous limit of approximately 7 keV. The results are consistent with existing data above 7 keV, while they indicate a higher at lower energies.
© The Author(s) 2026
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Funded by SCOAP3.
