Characterization of the ionization response of argon to nuclear recoils at the keV scale with the ReD experiment

TL;DR

The paper reports a direct, model-independent measurement of the argon ionization yield Q_y for nuclear recoils in the 2–10 keV range using the ReD experiment, a compact dual-phase LAr TPC irradiated by a Cf-252 neutron source. Recoil energies are reconstructed event-by-event via two-body kinematics, with neutron energy determined from time-of-flight between a gamma-tagged Cf-252 source and a downstream neutron spectrometer. Ionization signals are quantified through the S2 electron count, calibrated by the ionization gain g_2 using Am-241 gamma calibrations, and a likelihood analysis yields Q_y across five NR energy bins, showing an increase in Q_y at lower energies and consistency with prior measurements above 7 keV. These results provide essential inputs for low-mass WIMP searches in argon and set the stage for future ReD+ campaigns aiming to extend the measurement to sub-keV nuclear recoils.

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 Cf252 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 Qy 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 Qy at lower energies.

Figures (9)

• Figure 1: Schematic view (not in scale) of the experimental setup. On the left is the shielding structure housing the $^{252}$Cf source and the two BaF$_{2}$ scintillator taggers. The collimator opening angle $\theta_{C}$ of $\ang{2.6}$ allows the full illumination of the LAr TPC (depicted in blue) at 92.1 cm . On the right, a schematic representation of the neutron spectrometer, not showing the support frame. The two detector arrays are mounted symmetrically at $\pm 27.2$ cm above and below the TPC center, outside the cone of the direct neutron flux. The spectrometer covers a scattering angle range of $\theta_{S} = \ang{12} - \ang{17}$. The blue arrow indicates the path of an incoming neutron from the source prior to (n,n') interaction with Ar in the TPC, while the red arrow depicts a possible trajectory of the scattered neutron n' within the acceptance of the neutron spectrometer. Cryogenic infrastructure is not shown for simplicity.
• Figure 2: ToF distributions of $\gamma$-rays from the $^{252}$Cf SF detected in the neutron spectrometer, after the synchronization procedure described in the text, which sets the peak position at the expected $\Delta t_{\gamma} = 6.5$ ns. The distributions for BaF0 (blue solid) and BaF1 (dashed violet) are shown separately.
• Figure 3: ToF between BaF$_{2}$ and PScis vs. $f_{p}$ of the PSci. The dashed rectangle indicates the selection region for tagged neutron events; this is only for visual purposes, as the selection cut in $f_{p}$ is energy-dependent, see text for more details. The large majority of triggered events comes from SF $\gamma$-rays that reach the neutron spectrometer (ToF $\sim 6.5$ ns and $f_{p}$$\sim 0.75$) and $\gamma$-ray accidental coincidences.
• Figure 4: Drift time distribution from $^{241}$Am calibrations (data points), superimposed with the MC simulation assuming the best-fit TPC offset $\Delta z= 0.23$ cm (red line) and the $\pm 1 \sigma$ band (shaded red region).
• Figure 5: Distribution of the recoil energy $E_r$ (left panel), S2 (middle panel) and $T_\mathrm{drift}$ (right panel) for the final sample of candidate signal events. The S1+S2 (red triangles) and the S2-only (blue circles) populations are shown separately and superimposed with the corresponding MC simulation (solid histograms).