The ground-based large-area wide-angle gamma-ray and cosmic-ray experiment HiSCORE

The question of the origin of cosmic rays and other questions of astroparticle and particle physics can be addressed with indirect air-shower observations above 10 TeV primary energy. We propose to explore the cosmic ray and gamma-ray sky (accelerator sky) in the energy range from 10 TeV to 1 EeV with the new ground-based large-area wide angle (~0.85 sterad) air-shower detector HiSCORE (Hundred*i Square-km Cosmic ORigin Explorer). The HiSCORE detector is based on non-imaging air-shower Cherenkov light-front sampling using an array of light-collecting stations. A full detector simulation and basic reconstruction algorithms have been used to assess the performance of HiSCORE. First prototype studies for different hardware components of the detector array have been carried out. The resulting sensitivity of HiSCORE to gamma-rays will be comparable to CTA at 50 TeV and will extend the sensitive energy range for gamma-rays up to the PeV regime. HiSCORE will also be sensitive to charged cosmic rays between 100 TeV and 1 EeV.

💡 Research Summary

The paper presents HiSCORE (Hundred Square‑km Cosmic ORigin Explorer), a novel ground‑based, wide‑field, non‑imaging air‑shower detector designed to explore the sky in the ultra‑high‑energy (UHE) regime from 10 TeV to 1 EeV. The authors motivate the need for such an instrument by highlighting the unresolved origin of cosmic rays, especially in the “knee” (∼10¹⁵ eV) to “ankle” (∼10¹⁸ eV) region, and the scarcity of γ‑ray observations above 10 TeV. Existing facilities (e.g., CTA, HAWC, LHAASO) are optimized for lower energies and cannot provide the several‑km² instrumented area required for statistically significant UHE measurements.

HiSCORE addresses this gap by employing a non‑imaging Cherenkov technique. Each detector station consists of four 8‑inch photomultiplier tubes (PMTs) equipped with Winston cones (30° half‑opening angle) that together provide a light‑collecting area of about 0.5 m². Stations are arranged on a regular grid with 150 m spacing, forming a 22 × 22 array that covers roughly 10 km². A local trigger is generated when the summed signal from the four PMTs exceeds a threshold of ~100 photo‑electrons, suppressing night‑sky background (NSB) via a local coincidence requirement. The read‑out uses GHz‑rate digitizers (DRS4 chips) to capture the full arrival‑time distribution of Cherenkov photons, enabling reconstruction of the shower front and the depth of maximum development (Xmax).

Monte‑Carlo simulations were performed with CORSIKA‑v6.75 (GHEISHA hadronic model) for primary γ‑rays, protons, helium, nitrogen, and iron nuclei spanning 10 TeV–10 PeV (spectral index –1). Cherenkov photons were stored in 1 m spheres representing detector stations, and a detailed detector response model (including atmospheric absorption via MODTRAN, Winston‑cone ray‑tracing, PMT quantum efficiency, after‑pulses, and electronic clipping) was applied. Effective trigger areas were derived as a function of primary energy. For γ‑rays, the effective area reaches ∼10⁶ m²·sr at 10 TeV and exceeds 10⁷ m²·sr above 100 TeV; similar or slightly lower values are obtained for hadronic primaries, confirming that the array can trigger efficiently on both γ‑ray and cosmic‑ray showers in the target energy range.

The timing resolution afforded by the GHz digitizers allows Xmax reconstruction with an accuracy better than 30 g cm⁻², providing a robust γ/hadron separation tool and enabling composition studies of cosmic rays from 100 TeV up to 1 EeV. The wide field of view (≈0.85 sr) ensures continuous monitoring of a large fraction of the sky, making HiSCORE sensitive to transient phenomena as well as to diffuse emission from the Galactic plane, super‑nova remnants, and potential PeVatrons.

Scientifically, HiSCORE aims to (1) identify Galactic PeVatrons by detecting hard γ‑ray spectra extending beyond 100 TeV, (2) map the transition from Galactic to extragalactic cosmic rays by measuring spectral and composition changes across 10¹⁵–10¹⁷ eV, and (3) probe new physics such as photon‑axion conversion, Lorentz‑invariance violation, or hidden‑photon oscillations through precise measurements of γ‑ray attenuation on the cosmic microwave background and interstellar radiation fields.

Technical challenges include suppression of NSB, reliable power and data transmission over a sparse, large‑area network, and autonomous station operation (lid control, temperature monitoring, remote firmware updates). Prototype stations integrating the four‑PMT module, Winston cones, high‑voltage supply, and DRS4 read‑out have been built and tested, demonstrating the expected trigger rates and noise performance.

The authors conclude that scaling the array to ∼100 km² would provide a sensitivity comparable to CTA at 50 TeV for γ‑rays while extending the observable energy range into the PeV domain, and would deliver unprecedented statistics for cosmic‑ray composition studies up to the ankle. HiSCORE thus promises to fill a critical observational gap in astroparticle physics and to open a new window on the most energetic processes in the Universe.