Construction and characterisation of the DarkSide-20k veto silicon photo-multiplier tiles

Abstract

Silicon photo-multipliers (SiPMs) are state-of-the-art sensors capable of detecting a single photoelectron under cryogenic conditions, with potentially lower radioactivity than widely used photomultiplier tubes. The DarkSide-20k experiment, designed to perform direct dark matter searches using liquid argon as the target material, employs SiPM technology to detect interactions in the active detector volumes, including the central dual-phase Time Projection Chamber and the Inner and Outer Veto volumes. The vetoes are designed to discriminate against radiogenic neutron and cosmic muon backgrounds associated with the dark matter search. This paper describes the completed production and test protocols for the "Veto Tiles" (called vTiles, arrays of 24 SiPMs integrated on a printed circuit board providing the power distribution and signal amplification); 16 vTiles are grouped into "Veto Photo-Detector Units" to instrument the Inner Veto volume. Each vTile underwent detailed testing at room and cryogenic temperatures, confirming stable operation, high signal-to-noise ratio, and low radioactive contamination, demonstrating the robustness of the proposed design for cryogenic conditions. The final production yield exceeded 87%, surpassing the 80% requirement and corresponding to 1920 Veto Tiles to populate 120 Veto Photo-Detector Units, plus an additional 6% as spares.

Figures (18)

• Figure 1: (\ref{['fig:ds20k-a']}) Drawings of the DarkSide-20k experiment, showing the cryostat (red and yellow structures), the Outer Veto volume and the Inner Detector inside the stainless steel vessel. (\ref{['fig:ds20k-b']}) The Inner Detector, including the upper TPC optical plane, the calibration pipes (in red) and the Inner Veto Photo-Detector Units (in yellow) attached to the PMMA walls (in blue) and at the rear of the optical plane (in green).
• Figure 2: Schematic of the vTile electronics: it consists of 24 SiPMs arranged in a "2s-3p" configuration, with 3 parallel branches of 2 SiPMs in series (purple shaded region). A precision bias network (orange shaded region) divides the input bias voltage (VBIAS) by two to provide the mid-bias to all SiPMs; the divider is composed of two 10 M$\Omega$ resistors (R10 and R2). The overall divider chain across all 12 bias nodes (V12 to V2324) has 10 M$\Omega$ steps (R12 to R17 and R4 to R9) providing evenly distributed bias taps for each SiPM pair. Upstream of the divider, a 10 k$\Omega$ resistor (R1) in series with VBIAS forms an RC low-pass filter (green shaded region) together with capacitors C1--C3 (100 nF each, to ground); this filter decouples high-frequency noise from the bias supply. Each SiPM branch includes a 61.9 $\Omega$ resistor (R31--R42) in series, providing frequency compensation and damping for the ASIC input. The combined output from each 2s-3p group (6 SiPMs) is routed to the corresponding ASIC channel input (IN1--IN4).
• Figure 3: (\ref{['fig:vtile-sipms']}) 24 SiPMs mounted on a vTile. (\ref{['fig:vtile-circuit']}) Populated PCB for the vTile front-end electronics. The packaged ASIC amplifier is located at the bottom. The copper pillars used for the vPDU integration are present at the four corners. (\ref{['fig:Bham_tray']}) The holding fixture used throughout the vTile readout electronics population process.
• Figure 4: Fully assembled vPDU housed within the handler fixture and covered with an acrylic protection sheet.
• Figure 5: Production and QA/QC of vTiles and vPDUs are distributed across institutes and universities in the UK and Poland. The population of the back side of the vTile PCB with electronic components is followed by the die attach and wire bonding of 24 SiPMs on the PCB front side. The vTiles' performance is evaluated under room temperature and cryogenic conditions for compliance with the production specifications (Table \ref{['table:specs']}). Subsequently, vTiles are shipped for integration, and the final step is the testing of the completed vPDUs. The qualified vPDUs are delivered to LNGS to be installed in the DarkSide-20k detector. Inspection activities are indicated in green, production in blue, testing in orange, and delivery to DarkSide-20k in grey.