First use of large area SiPM matrices coupled with NaI(Tl) scintillating crystal for low energy dark matter search

TL;DR

This work tackles the challenge of independently verifying the DAMA dark matter signal by developing NaI(Tl) detectors with significantly improved low-energy sensitivity. The authors demonstrate, for the first time, a cryogenic NaI(Tl) detector of about 360 g read out by a large-area SiPM matrix (64 devices on a 5 cm × 5 cm area) and coupled to a fused-silica crystal shell, operated in a custom cryostat at ~80 K. They measure a net photoelectron yield of roughly $4.5\\mathrm{phe/keV}$ after crosstalk correction, with strong suppression of low-energy noise and a sub-keV energy threshold candidate, indicating a potential path to higher signal-to-noise and lower thresholds compared to PMT-based NaI(Tl) detectors. The results establish a proof-of-principle for PMT-free, low-radioactivity, cryogenic light readout and lay out a clear roadmap for scaling to multi-face readout, veto integration, and underground deployment to enhance dark matter sensitivity.

Abstract

The long-standing claim of dark matter detection by the DAMA experiment remains a crucial open question in astroparticle physics. A key step towards its independent verification is the development of NaI(Tl)-based detectors with improved sensitivity at low energies. The majority of NaI(Tl)-based experiments rely on conventional photomultiplier tubes (PMTs) as single photon detectors, which present technological limitations in terms of light collection, intrinsic radioactivity and high noise contribution at keV energies. ASTAROTH is an R&D project developing a NaI(Tl)-based detector where the scintillation light is read out by silicon photomultipliers (SiPMs) matrices. SiPMs exhibit high photon detection efficiency, negligible radioactivity, and, most importantly, a dark noise nearly two orders of magnitude lower than PMTs, when operated at cryogenic temperature. To this end, ASTAROTH features a custom-designed cryostat based on a bath of cryogenic fluid, able to safely operate the detector and the read-out electronics down to about 80 K. This work reports the first experimental characterization of an approximately 360 g NaI(Tl) detector read out by a large area (5 cm x 5 cm) SiPM matrix. The net photoelectron yield obtained with a preliminary configuration is approximately 4.5 photoelectrons/keV after crosstalk correction, which is rather promising in light of several planned developments. The signal-to-noise ratio and the energy threshold attainable with SiPMs is expected to improve the sensitivity for dark matter searches beyond the reach of current-generation PMT-based detectors. This result is the first proof of the viability of this technology and sets a milestone toward the design of future large-scale experiments.

Figures (9)

• Figure 1: (a) 3D design of the detector, showing how the NaI(Tl) crystal is encapsulated into the synthetic quartz shell. The copper frame allows coupling the SiPM matrix and the front-end electronics to the crystal. (b) Photo of the detector.
• Figure 2: (a) 3D design of the cryostat showing the cryogenic chamber, where the detector is hosted in a helium atmosphere (1 bar), surrounded by a cryogenic bath. A sequence of discs allows for the stratification of the inner gas and minimizes convection cycles along the height of the chimney, which is directly exposed to room temperature at its top. The inset details the section of the thermal bridge, responsible for cooling the inner volume. (b) Photograph of the inner crystal support structure, hanging from the main chimney flange and featuring the anti-convection disks, ready to be inserted into the cryogenic chamber.
• Figure 3: Block schematic of the electronics front-end. The cold section sums the signals of the 64 SiPMs over two stages of amplification and converts them into four differential pairs, one for each quadrant of the matrix. Signals are then routed out of the cryostat on an Ethernet cable and converted back to single-ended by the warm section; finally they are sent to data acquisition.
• Figure 4: Schematic diagram of the calibration setup; details are described in the text.
• Figure 5: 1000.0 baseline-subtracted waveforms acquired while illuminating the SiPMs with a low intensity laser. The pulse amplitude linearly corresponds to an increasing number of detected photoelectrons (phe). The dashed lines show the charge integration window, which was selected to integrate at least the 95 % of the charge as described in the text.