As part of the AugerPrime upgrade of the Pierre Auger Observatory, scintillator surface detectors (SSD) have been added to the existing water-Cherenkov surface detector stations. An account of SSD design, production, quality control and deployment has been published in Journal of Instrumentation.
The mechanical structure of the detectors is made to guarantee light tightness and robustness for more than ten years of operation in the harsh environment of the pampa. The SSD module contains two scintillator panels composed of organic plastic scintillator bars, with total area of the scintillators being almost 4 m². Optical fibers collect light from the scintillator bars and guide it to a photomultiplier housed between the scintillator panels (see Figure 1). The photomultipliers used are able to register signals of both single minimum-ionizing particle and large signals of up to 20 000 particles, with high linearity.
Figure 1: An open SSD during production, with the scintillator bars secured by aluminium bars. The polystyrene routers, the optical fibers and the photomultiplier housing are also visible. © Pierre Auger Collaboration
Figure 2: Uniformity test of a detector, in which the bundle of the optical fibers is also visible between the two scintillator panels. The colour scale indicates the average decimal-logarithm of the deposited charge in pC. Note that the external detectors extend further out from the active parts of the SSD. Non-zero charges scattering outside of the active parts are actually induced by coincidental multiple muons events.
Each SSD module was examined at a test bench equipped with a data acquisition system. The light-tightness was checked, and the detector response and efficiency were studied using through-going cosmic-ray muons. Using triggers from external detectors, SSD signals were studied, including their size, shape and uniformity of response over the active area of the detector. In particular, the ratio of signal size from a minimum-ionizing particle to that from a single photoelectron can be used as an estimate of the overall quality of the tested SSD. This ratio reflects the efficiency of the key detector components at generating, collecting, transmitting and recording the scintillation light. Its mean value exceeds the design requirements by a factor of about 2. An example of result of the uniformity test of a single module is shown in Figure 2.
Figure 3: Deployment of the SSD units in the field. © Pierre Auger Collaboration
The task of producing more than 1500 SSD units exceeds the capabilities on any single laboratory available within the collaboration, so that the work was distributed among several assembly facilities. At all these sites, uniform production procedures and quality control requirements were followed. All SSD modules were subsequently shipped to the Observatory. The deployment in the field (see Figure 3) took place between January 2019 and November 2021. The SSD photomultipliers were installed at a later stage, along with installation of new electronics. This task was completed in June 2023. The upgraded Observatory will play a fundamental role in the field of ultra-high energy cosmic rays for the next decade.
Related paper:
The Scintillator Surface Detector of the Pierre Auger Observatory
The Pierre Auger Collaboration, JINST 20 (2025) P08002
[arXiv: 2507.07762] [doi: 10.1088/1748-0221/20/08/P08002]
