Optimising the design of water-Cherenkov detectors for gamma-ray astronomy

• Applicants: Professor Dr. Stefan Funk; Dr. Harm Schoorlemmer
• Subject Area: Astrophysics and Astronomy ; Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 438123869

• Final Report Year: 2024

Final Report Abstract

The objective of this project was an optimisation of the design of a single water Cherenkov detection unit, the fundamental building block of a future observatory to be built in South America at a high-altitude site. Additionally, we intended to develop a framework for the reconstruction and event classification of the array, an essential ingredient in any optimization of array parameters. This project was carried out in cooperation between the Max-Planck-Institute for Nuclear Physics in Heidelberg (MPIK) and the Erlangen Centre for Astroparticle Physics (ECAP) at the Universität Erlangen- Nürnberg. In recent years, it has been shown by observatories, such as LHAASO and HAWC, that a telescope system consisting of arrays of water tanks instrumented with fast electronics can detect the Cherenkov light of charged shower particles that are created by the interaction of a gamma ray with the Earth atmosphere. LHAASO and HAWC are both located in the northern hemisphere, no such facility exists in the south. Water Cherenkov gamma-ray observatories are characterised by their wide field-of-view and deep high-energy exposure, making it ideal for observing the Galactic centre and large emission regions such as the Fermi bubbles. However, parameters of the array of Water tanks, such as the design of the building blocks, the spatial distribution, and the triggering of the array have to be chosen to maximise performance. In 2019, an international collaboration formed to perform R&D towards a Southern Gamma-ray observatory, the so-called SWGO collaboration. More than 90 research institutions from 15 countries have signed the agreement for this collaboration for a future wide fieldof-view gamma-ray observatory in the southern hemisphere. The collaboration aims to develop a detailed proposal for the implementation of such an observatory, including site selection and technology choices. Our project ties into these efforts and has provided essential elements towards the final design of SWGO. The project was divided into three modules for each of which the central goals were achieved. First in O1, we aimed at characterising the components of the single detector unit, in particular the photon sensor, and the material of the inner surface of the tanks. Second, we performed dedicated simulations and compared them to laboratory measurements to characterize the unit response. Third, we developed the shower reconstruction pipeline and performed detailed characterisation studies to optimise the layout of the array.

Publications

• Estimating the potential of SWGO to measure the composition-dependent behaviour of the CR anisotropy. PoS(ICRC2023)486. 2023
  R. G. Lang et al.
  
• Status of the SWGO air shower reconstruction using a template-based likelihood method. PoS(ICRC2023)593. 2023
  F. Leitl, V. Joshi & S. Funk
  
• "The Southern Wide-field Gamma-ray Observatory: The Lake Approach and the Pulsar Wind Nebula HESS J1825-137" (PhD thesis)
  H. Goksu
  
• Application of graph networks to a wide-field water-Cherenkov-based Gamma-ray Observatory. Journal of Cosmology and Astroparticle Physics, 2025(02), 066.
  Glombitza, J.; Schneider, M.; Leitl, F.; Funk, S. & van, Eldik C.
  
• Detectability of Supernova remnants with the Southern Wide-field Gamma-ray Observatory. Journal of Cosmology and Astroparticle Physics, 2025(05), 096.
  Scharrer, N.; Spencer, S.T.; Joshi, V. & Mitchell, A.M.W.