Photosensors
Silicon PhotoMultipliers (SiPMs) are extremely sensitive detectors for visible photons based on a p-n silicon junction. They are usually little devices with an area of some mm2 and a thickness of some hundreds of microns, with the active layer implanted in the first tens of microns.
SiPMS
Silicon Photomultipliers (SiPMs) are highly sensitive light detectors composed of thousands of tiny photon-counting pixels called Single Photon Avalanche Diodes (SPADs). They can count individual photons and produce signals proportional to the number of photons detected, making them ideal for the extremely low-light conditions of dark matter experiments. While thermal noise can generate false signals at room temperature, operating SiPMs in liquid argon greatly suppresses this effect.
Because each SPAD can detect a single photon, SiPMs excel in extremely low-light environments, making them ideal for dark matter experiments, where only a handful of photons may be generated during a particle interaction. When multiple photons are detected at the same time, different SPADs respond independently, producing a signal that is proportional to the total number of photons. This photon-counting capability, combined with their high efficiency, makes SiPMs a key technology for the future.
The SiPM team with their product.
The SiPM team
The SiPM team
diagram of a SiPMS
DarkSide-20k's SiPMS
tile substrate
Die attach onto tile substrates in NOA
NOA assembly cleanroom in action
NOA assembly cleanroom in action
Completed photon detector unit
One challenge of silicon photomultipliers (SiPMs) is dark count rate (DCR)—background signals caused by thermal energy rather than light. At room temperature, heat can trigger false signals that are indistinguishable from those produced by real photons, creating unwanted noise. Fortunately, operating SiPMs at the cryogenic temperatures of liquid argon dramatically suppresses this thermal noise, making them exceptionally sensitive detectors for dark matter experiments.
For DarkSide-20k, researchers worked with the Fondazione Bruno Kessler (FBK) to develop custom SiPMs with a large active area, high light-detection efficiency, and an extremely low dark count rate in liquid argon. The sensors are manufactured by LFoundry and assembled into increasingly larger arrays that will cover the detector's 21-square-meter optical system.
The performance of these SiPM arrays is remarkable: they can clearly distinguish between signals produced by one, two, three, or more photons (as shown in below, left), while also separating true photon signals from background electronic pedestal noise (as shown in below, right). This level of precision is essential for identifying the extremely rare interactions that could reveal the presence of dark matter.
DarkSide-20k SiPM array measured voltage (mv) vs. time (microseconds) waveforms from laser calibration data showing 1, 2 and 3 photoelectron signals in blue, orange and green respectively.
Histogram of number of events as a function of pulse amplitude (V) measured in laser calibration data by DarkSide-20k photon detector units (PDUs), showing clear separation between 0, 1, 2, 3, … N photoelectron peaks.
The DarkSide Collaboration aims to unveil the nature of dark matter through liquid argon detector technology and innovative underground experiments. It is located at the Laboratori Nazional del Gran Sasso in L’Aquila, Italy.