Probing dark matter interactions with a RES-NOVA prototype cryogenic detector

TL;DR

This work tackles direct dark matter detection with ultra-low background, high-mass-number targets, and low energy thresholds by using a $13$ g PbWO$_4$ crystal grown from archaeological Pb as a cryogenic detector. It demonstrates a robust cryogenic platform at LNGS with effective vibration isolation and a Ge-based thermistor readout, enabling the derivation of the first dark matter exclusion limits for PbWO$_4$ as a target for both spin-dependent and spin-independent interactions. A comprehensive data-analysis chain with an Optimum Filter, pulse-shape consistency checks, and calibration anchored to $^{208}$Tl and $^{210}$Pb signatures establishes reliable energy reconstruction and efficiency estimates. The results validate the RES-NOVA concept and underscore the potential gains from larger target masses and TES readout for future DM and CE$ u$NS searches.

Abstract

We report on the operation of a 13 g PbWO$_4$ crystal, grown from archaeological Pb and operated as a cryogenic calorimeter in an underground environment. Read out with a Ge thermistor, the detector achieves a low energy threshold and, for the first time, enables the derivation of a dark matter exclusion limit using PbWO$_4$ as target material, for both spin-dependent interactions on neutrons and spin-independent interactions. Although limited in mass and not representative of the final RES-NOVA detector design, this prototype demonstrates effective control of mechanical vibrations and low-energy noise in a cryogenic system, which is a key requirement for rare-event searches. The experiment therefore provides a proof of principle for the RES-NOVA detection concept, validating the use of archaeological Pb-based PbWO$_4$ crystals, low-background operation, and robust data-analysis procedures. These results establish a solid technological and methodological foundation for future RES-NOVA detectors employing larger target masses and advanced thermal readout technologies.

Figures (8)

• Figure 1: Photograph of the Ieti cryogenic infrastructure used for the PbWO$_4$ detector operation. The internal structure of the dilution refrigerator is shown, with the main temperature stages labeled (4 K stage, Still stage, Cold stage, and mixing chamber (MC) stage). The detector is installed at the MC stage, corresponding to the lowest temperature point of the cryostat. Passive Pb shieldings are placed above the detector to mitigate environmental $\gamma$-ray backgrounds while maintaining compatibility with the cryogenic and mechanical constraints of the setup. The installation position of the geophones is also indicated.
• Figure 2: Ieti cryostat amplitude spectral density (ASP) of the axial displacement. This is measured at the Mixing Chamber plate during operation at 7 mK. The spectrum highlights the broadband vibrational background as well as narrow resonant features associated with mechanical modes of the cryogenic infrastructure.
• Figure 3: Schematic rendering of the cryogenic detector. The PbWO$_4$ crystal produced from archaeological Pb is held in position by PTFE clamps, which provide both mechanical support and a weak thermal coupling to the heat bath. The assembly is housed within a Cu frame that ensures mechanical stability and thermal anchoring to the cryogenic environment.
• Figure 4: Trace detector pulses at the end of the electronics read-out chain. The left (right) panel shows a typical 3 keV (8 keV) pulse. For each acquisition window, the raw trace (grey) is compared with the optimal-filter (OF) reconstructed trace (orange) and the corresponding signal template trace (dark blue). The comparison illustrates the improvement in signal-to-noise ratio achieved by the optimal filtering and the good agreement between the reconstructed pulses and the template shape. Signal-to-noise ratio in regions close to the DM analysis threshold (2.5 keV) is close to unity.
• Figure 5: Background energy spectra measured of the PbWO$_4$ cryogenic detector. Left: energy spectrum of all accepted events, including $e^{-}/\gamma$ interactions and nuclear recoils, measured up to 800 keV. Apart from the peak at 46 keV, attributed to the characteristic emission of $^{210}$Pb, no other clear spectral features are observed. The region of interest for the DM analysis (2.5--10 keV) is highlighted in orange. Data acquired over an exposure of 32.4 g$\cdot$d are reported as a green histogram. Right: zoom in up to 10 keV of the background energy spectrum. For comparison, the expected contribution from radioactive bulk contaminations of the crystal, derived from Ref. kg-scale, is shown as a grey histogram. The measured spectrum is dominated by external backgrounds originating from the cryogenic infrastructure (e.g. $^{210}$Bi bremsstrahlung from the internal Pb shielding) and from the surrounding environment (e.g. ambient $\gamma$ and neutron radiation).