Photosensors for DarkSide-20k

Author: Andrea Maransciulli

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.

Each SiPM is a collection of pixels with a size of som e tens of microns, called Single Photon Avalanche Diodes (SPADs) and they are the smallest autonomous unit of the SiPM. Every SPAD is capable to detect even a single photon providing a well-defined electrical signal whose shape depends on the engineering of the p-n junction itself. The output of a SiPM is the sum of the output currents of all its SPADs. Since the latter are able to detect only one photon at a time, SiPMs are very useful in low-light environments, where the average light occupancy is less than 1 photon per SPAD and we need to catch more photons as possible employing the high quantum efficiency of the SiPM, like in direct dark matter searches.
When more photons hit the same SiPM on different SPADs, the output signal has an amplitude proportional to the number of photons: for this reason SiPMs are also known as counting devices.

The main drawback of the SiPMs is the dark count rate (DCR): thermal quivering can ionize an electron in the silicon junction in the same way as a photon does, generating noises that are identical to the expected signals. At room temperature it follows a detection of photons in a complete dark environment, resulting in a strong pile-up of the detector. This problem can be overcome by using SiPMs at cryogenic temperatures, such as in liquid argon, where the thermal noise is heavily suppressed. DarkSide-20k SiPMs were customly designed by Fondazione Bruno Kessler (FBK) in Trento to have bigger size (about 1 cm²), an high quantum efficiency (40% in LAr) and a low DCR (order of 1 Hz/cm² in LAr). The mass production of SiPM wafers have been done by the company LFoundry in Avezzano (AQ). Our SiPM are assembled in arrays of gradually increasing sizes to tessellate the 21 m² of the two optical planes of DS-20k’s TPC. As shown by the so-called “fingerplot”, the histogram of the signal amplitude of our SiPM arrays, the peaks corresponding to 1,2,3 photons and so on are clearly separated between each other. The peak corresponding to one photons is also clearly separated by the 0-photons peak, corresponding to the baseline noise.