The heart of the veto system is a device called a vTile, a five-centimetre-square printed circuit board carrying 24 silicon photo-multipliers, or SiPMs. These sensors are remarkable pieces of engineering: each one packs 94,904 single photon avalanche diodes into an area of just 11.7 by 7.9 millimetres, and each can register the arrival of a single photon even when chilled to cryogenic temperatures. Unlike the vacuum-tube photomultipliers used in earlier generations of detectors, SiPMs are solid-state devices that can be manufactured with extremely low levels of natural radioactivity, a crucial property when the faintest gamma ray or neutron can masquerade as a dark matter signal.

The scale of the production effort is staggering. Around 1,400 eight-inch silicon wafers, each containing 268 SiPMs, were fabricated by LFoundry and tested in an ISO-6 cleanroom at the Nuova Officina Assergi facility at Italy’s Gran Sasso National Laboratory. Every accessible sensor on every wafer was probed at liquid nitrogen temperature, with strict acceptance criteria on breakdown voltage, quenching resistance and leakage current. More than 93 percent of the sensors passed. The accepted devices were then diced from their wafers and shipped to assembly sites in the United Kingdom, where they were mounted onto circuit boards and connected with aluminium wire bonds just 25 micrometres thick.

The electrical design of each vTile reflects a careful trade-off between performance and simplicity. The 24 SiPMs are arranged in four quadrants, each containing three parallel branches of two sensors wired in series, a configuration chosen to keep the overall capacitance low enough to preserve the ability to resolve single photons. A precision network of resistors divides the input bias voltage so that every sensor receives an even share, and a custom application-specific integrated circuit amplifies each quadrant’s signal before the outputs are summed. Sixteen vTiles are then mounted on a larger motherboard to form a Veto Photo-Detector Unit, whose four output channels each sum 96 SiPMs covering roughly 100 square centimetres. This aggressive summing slashes the number of data channels and, just as importantly, reduces the mass of radioactive cabling that would otherwise sit inside the detector.

Every single tile was subjected to a punishing battery of tests at both room temperature and, in a bath of liquid nitrogen, at around 77 kelvin. Physicists at the University of Oxford and STFC’s Interconnect facility built identical test stands that lower four tiles at a time into dewar flasks, illuminating them with pulses from a 405-nanometre laser. The team measured how the sensors respond to individual photons, recording the voltage of the single and double photoelectron peaks and comparing them to the electronic noise floor. The requirement was a signal-to-noise ratio above 8 for each tile, a threshold that guarantees the assembled detector units can reliably distinguish genuine light signals from background fluctuations.

Not everything survived the ordeal, and the failure modes proved scientifically interesting in their own right. Some tiles displayed a puzzling double breakdown in their current-voltage curves, a signature that could indicate mechanical damage to a sensor or a deformed wire bond shorting against a silicon surface. Rather than discarding these units, the collaboration developed a diagnostic procedure that biases each of the 24 SiPMs individually to identify the single faulty device, then removes it, cleans the solder and bonds in a replacement. This rework rescued more than 86 percent of the affected tiles, saving precious silicon and keeping the production on schedule.

Radioactive cleanliness demanded almost obsessive attention throughout. Every component, from the capacitors to the circuit boards themselves, was assayed using three independent techniques: mass spectrometry, high-purity germanium gamma spectroscopy and polonium extraction. The dominant neutron background traces to the lower uranium chain in the capacitors, followed by the boards, whose sheer mass makes them significant despite modest activity levels. Because ordinary dust carries radioactive contaminants and radon daughters can plate onto surfaces, all assembly took place in cleanrooms with radon levels around 2 becquerels per cubic metre, and parts were triple-bagged in vacuum-sealed barriers for transport. Automated scanners photographed each tile at 6400 dots per inch before and after every production stage, using image analysis algorithms to count and characterise every particle of dust on the silicon surfaces.

The results validate the entire production line. A fully populated tile was radio-assayed directly and compared with the sum of its individual components, and the two agreed, confirming that the assembly process itself added negligible radioactivity. Based on these measurements, the 120 detector units destined for the Inner Veto will contribute only about 4 percent of the total neutron background and a gamma rate of 25 hertz, comfortably within the specifications needed to keep the experiment essentially free of instrumental backgrounds for a decade of operation. Dust levels held steady at under 200 counts per square centimetre across all testing and integration phases.

Perhaps the most striking number in the paper is the production yield. The collaboration needed an 80 percent cumulative yield across all manufacturing steps to have enough working tiles, and it achieved 87 percent, corresponding to 1,920 qualified vTiles filling 120 detector units, with a further 6 percent held as spares. For a technology being deployed at this scale for the first time, that figure represents a genuine industrial achievement, comparable to the yields of mature commercial semiconductor production lines rather than a first-of-its-kind scientific instrument.

When DarkSide-20k begins taking data around 2029, it will operate the largest SiPM-based detector ever built, with a projected sensitivity to dark matter interactions down to cross-sections of 10 to the minus 48 square centimetres for a 100 gigaelectronvolt particle. The Inner Veto will surround the central time projection chamber with nearly complete four-pi coverage, tagging neutrons through their capture on hydrogen and argon nuclei and rejecting any event with coincident light. If a dark matter particle finally reveals itself in the argon below, it will be thanks in no small part to these meticulously tested squares of silicon, each one a tiny sentinel standing guard against the universe’s most persistent background noise.

Subject of Research: Production and cryogenic characterisation of silicon photo-multiplier veto tiles for the DarkSide-20k dark matter experiment

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

Article References: DarkSide-20k Collaboration, Acerbi, F., Adhikari, P., Agnes, P., Ahmad, I., Albergo, S., Albuquerque, I. F. M., Alexander, T., Alton, A. K., Amaudruz, P.-A., Anastasi, G. A., Angiolilli, M., Aprile, E., Auty, D. J., Pernas, M. A., Azzolini, O., Back, H. O., Balmforth, Z., Olmedo, A. I. B., … Zuzel, G. (2026). Construction and characterisation of the DarkSide-20k veto silicon photo-multiplier tiles. The European Physical Journal C, 86(9), Article 1089. https://doi.org/10.1140/epjc/s10052-026-16217-7

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16217-7

Keywords: DarkSide-20k, dark matter, silicon photo-multipliers, liquid argon, neutron veto, cryogenic detectors, Gran Sasso, radiopurity, particle physics, detector instrumentation, wire bonding, quality control