Cryogenic characterisation of the FBK NUV-HD-cryo Silicon Photomultipliers for the DarkSide-20k dark matter experiment

Deep beneath the Gran Sasso mountain in Italy, a massive experiment is being built to listen for the faintest whisper of the universe's most elusive inhabitants: dark matter. Scientists believe this invisible substance makes up most of the matter in the cosmos, yet it rarely interacts with the ordinary stuff we can see and touch. To catch a glimpse of a dark matter particle, researchers use a tank filled with tons of liquid argon, a super-cold fluid that acts as both a target and a detector. When a dark matter particle strikes an argon atom, it creates a tiny flash of light. The challenge is that these flashes are incredibly dim, often consisting of just a few photons, and they happen in a sea of background noise. To see them, the experiment needs eyes that are not only sensitive enough to catch a single photon but also quiet enough not to generate their own false signals in the freezing dark.

The DarkSide-20k experiment, currently under construction, relies on a new type of light sensor called a silicon photomultiplier. Unlike the traditional vacuum tubes used in older experiments, these sensors are solid-state devices made of silicon, similar to the chips in a computer but designed to detect light. They are chosen for their ability to operate at extremely low temperatures and their lack of natural radioactivity, which is crucial because any background radiation from the sensor itself could mimic a dark matter signal. However, these sensors had to be perfected for the job. Early versions struggled in the cold, generating too much internal noise and creating false signals that would drown out the faint dark matter whispers. The researchers needed to prove that the latest generation of these sensors, developed by the Fondazione Bruno Kessler, could perform flawlessly in the harsh environment of liquid argon.

To ensure the sensors were ready for the deep underground, a team of scientists conducted a rigorous series of tests at two different laboratories, one in Canada and one in Italy. They cooled the sensors down to 77 Kelvin, a temperature just 77 degrees above absolute zero, which is the same temperature as liquid nitrogen. In this freezing state, they measured how the sensors behaved when they were in total darkness. They counted the random electronic pulses that occur even without light, known as dark noise, and found that the new sensors were remarkably quiet. The rate of these false signals was low enough that the experiment could distinguish a real event from background noise. They also studied how the sensors reacted when a single photon hit them, measuring the electrical charge produced. This allowed them to calculate the sensor's gain, or how much it amplifies a tiny signal, and confirmed that this amplification increases predictably as the voltage is raised.

A critical part of the testing involved understanding how the sensors might accidentally trigger each other. When a photon hits a sensor and creates a signal, it can sometimes release a secondary photon that flies over and hits a neighboring sensor, causing a chain reaction of false signals. The researchers measured this "crosstalk" and found that while it does happen, the new sensors have been engineered to suppress it significantly. They also looked at "afterpulsing," a phenomenon where a sensor fires again shortly after the initial hit due to trapped electrical charges. The new design reduced this effect so much that it is now practically negligible for the experiment's needs. These findings confirmed that the sensors would not create a confusing mess of false alarms in the final detector.

The team then turned their attention to how well the sensors could actually see the light they were designed to detect. In the DarkSide-20k experiment, the liquid argon emits light in the ultraviolet range, which is invisible to the sensors. To make this light visible, the tank is coated with a special chemical that absorbs the ultraviolet light and re-emits it as blue light. The researchers measured the sensors' efficiency in detecting this blue light across a wide range of colors. They found that the sensors are exceptionally good at catching these photons, with an efficiency that peaks at nearly 50 percent. This means that for every two photons of the right color that hit the sensor, one is successfully detected and recorded. They also mapped out how this efficiency changes with different wavelengths, confirming that the sensors are perfectly tuned to the specific blue light produced by the experiment's coating.

Finally, the scientists investigated the secondary photons emitted by the sensors themselves during an avalanche of electrical charge. These stray photons could travel through the liquid argon and trigger other sensors, creating a form of external crosstalk that the experiment's computer models need to account for. By shining a laser on the sensors and capturing the faint light they emitted with a sensitive camera, the team measured the spectrum and intensity of this secondary light. They used these measurements to predict how many of these stray photons would escape the sensor and travel through the liquid argon in the final detector. This data is vital for building accurate computer simulations of the experiment, allowing physicists to distinguish between real dark matter signals and the complex web of interactions caused by the sensors themselves.

The comprehensive testing described in this paper confirms that the new silicon sensors are ready for the challenge. They combine high sensitivity with low noise and minimal interference, making them ideal for the DarkSide-20k experiment. By proving that these devices can operate reliably in the extreme cold and accurately detect the faint flashes of light from potential dark matter interactions, the researchers have laid a solid foundation for one of the most sensitive searches for dark matter ever attempted. The work ensures that when the detector is fully operational, it will be listening with the clearest possible ears, ready to hear the universe's deepest secrets.