Technical Article

Bridging the measurement gap between MeV and GeV particles in space: Design and first results of the CHerenkov Atmospheric Observation System (CHAOS)

Christian-Albrechts-Universität zu Kiel, Leibnizstraße 11, Kiel D-24118 Schleswig-Holstein, Germany

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 January 2026
Accepted: 17 April 2026

Abstract

Solar energetic particles are of major importance for understanding space weather. While numerous spaceborne instruments measure the particles with energies below several MeV and ground-based measurements address the particles with energies above approximately 1 GeV, the intermediate energies are sparsely covered. The CHerenkov Atmospheric Observation System (CHAOS) is a particle detector for solar energetic particles and galactic cosmic rays that bridges this gap in energy coverage. CHAOS uses multiple solid-state detectors and a bismuth germanium oxide scintillator to enable energy-resolved measurements of different particle species using the dE/dx − dE/dx method. Furthermore, an aerogel Cherenkov detector serves as a velocity threshold detector. The Cherenkov detector allows a clean measurement of protons with kinetic energies of tens of MeV up to 2.1 GeV, free from electron contamination. In this paper, we describe the design of CHAOS and show the first results to demonstrate the instrument’s capabilities. For this, we use measurements obtained during a stratospheric balloon flight as part of the BEXUS 35 campaign as well as measurements conducted at the CERN-EU high-energy Reference Field and compare them with GEANT4 Monte Carlo simulations.