Cosmic ray electron and gamma-ray telescope in a lake – LACTEL

The idea of the project is to re-use optical modules of ANTARES neutrino telescope, that has recently finished data taking, in a novel type Water-Cherenkov Detector Array (WCDA) for detection of cosmic ray electrons and gamma-rays. The spectrum of cosmic ray electrons has been previously measured up to 10 TeV energy by space-based detectors and by HESS Cherenkov telescope. At higher energies, measurements were hampered by the challenge of suppression of proton and nuclei cosmic ray background. WCDAs use the Earth atmosphere and an array of light sensors immersed in water (tanks, pond, lake) to detect high-energy particle Extensive Air Showers (EAS) produced by protons, atomic nuclei, electrons/positrons and gamma-rays with multi-TeV energies. Existing WCDAs have never measured the electron spectrum, because of the same problem as HESS: presence of background of proton and nuclei induced EAS background. It is possible to suppress this background by adding a second photosensor layer at several meter depth. Such a layer, optically isolated from the top layer, will be able to detect muons inevitably present in the proton and nuclei EAS, but rarely found in the electron and gamma-ray induced EAS. This “muon tagging” background rejection technique can be efficient already at 10 TeV energy if the muon detection layer is continuous and large (>100 m scale). Such a large detector needs to be assembled from smaller optically isolated Elementary Cells (ECs). We plan to design, build and test the EC building blocks of the large two-layer WCDA. Each EC will have two tarpaulins one on top of the other, enclosing ANTARES optical modules. The ECs will be operating autonomously, powered by solar panels. We plan to deploy and test a small WCDA (30-60 m scale) assembled of such ECs in Geneva Lake, to validate our design choices in realistic environmental conditions, to characterize performance of the WCDA made of such ECs, and to prepare larger WCDA deployment in a higher altitude lake.