Exploration of a 100 TeV gamma-ray northern sky using the Tibet air-shower array combined with an underground water-Cherenkov muon-detector array

Aiming to observe cosmic gamma rays in the 10 - 1000 TeV energy region, we propose a 10000 m^2 underground water-Cherenkov muon-detector (MD) array that operates in conjunction with the Tibet air-shower (AS) array. Significant improvement is expected in the sensitivity of the Tibet AS array towards celestial gamma-ray signals above 10 TeV by utilizing the fact that gamma-ray-induced air showers contain far fewer muons compared with cosmic-ray-induced ones. We carried out detailed Monte Carlo simulations to assess the attainable sensitivity of the Tibet AS+MD array towards celestial TeV gamma-ray signals. Based on the simulation results, the Tibet AS+MD array will be able to reject 99.99% of background events at 100 TeV, with 83% of gamma-ray events remaining. The sensitivity of the Tibet AS+MD array will be ~20 times better than that of the present Tibet AS array around 20 - 100 TeV. The Tibet AS+MD array will measure the directions of the celestial TeV gamma-ray sources and the cutoffs of their energy spectra. Furthermore, the Tibet AS+MD array, along with imaging atmospheric Cherenkov telescopes as well as the Fermi Gamma-ray Space Telescope and X-ray satellites such as Suzaku and MAXI, will make multiwavelength observations and conduct morphological studies on sources in the quest for evidence of the hadronic nature of the cosmic-ray acceleration mechanism.

💡 Research Summary

The paper proposes a hybrid detector system that combines the existing Tibet air‑shower (AS) array with a new underground water‑Cherenkov muon‑detector (MD) array covering 10 000 m². The motivation is to extend the sensitivity of ground‑based gamma‑ray astronomy into the ultra‑high‑energy regime (10 TeV–1 PeV), where conventional air‑shower arrays are limited by the overwhelming background of cosmic‑ray‑induced showers. Gamma‑ray‑initiated air showers contain far fewer muons than hadronic showers; by measuring the muon content underground, one can discriminate gamma‑ray events with high efficiency.

The MD array consists of cylindrical water tanks (≈7.2 m diameter, 1.5 m height) placed 2.5 m beneath the surface, each instrumented with high‑gain photomultipliers that record Cherenkov light from traversing muons. The tanks are spaced ≈15 m apart, providing a uniform coverage of the AS footprint. Monte‑Carlo simulations were performed with CORSIKA (for shower development) and GEANT4 (for detector response). Over 10⁶ primary particles (gamma rays, protons, helium, heavier nuclei) were generated across the 10 TeV–1 PeV range. Key performance metrics—muon detection efficiency, background rejection, gamma‑ray retention, angular and energy resolution—were extracted.

Results show that at 100 TeV the MD array can reject 99.99 % of the hadronic background while retaining 83 % of gamma‑ray events. The combined AS+MD system improves the point‑source sensitivity by roughly a factor of 20 relative to the AS array alone in the 20–100 TeV band. Angular reconstruction benefits from the joint trigger, reaching ≤0.2° resolution, which enables morphological studies of extended sources such as supernova‑remnant shells or pulsar‑wind nebulae. Energy reconstruction, using the correlation between shower size at the surface and muon content underground, achieves a relative uncertainty better than 30 %, sufficient to identify spectral cut‑offs that signal the maximum acceleration energy of the source.

The authors emphasize the scientific impact of this capability. With a reliable measurement of gamma‑ray spectra up to several hundred TeV, the Tibet AS+MD array can test whether the observed high‑energy emission is of hadronic origin (π⁰‑decay) or leptonic (inverse‑Compton). This complements observations by space‑based instruments (Fermi‑LAT, Suzaku, MAXI) at GeV energies and by imaging atmospheric Cherenkov telescopes (CTA, H.E.S.S., MAGIC) in the sub‑TeV regime. The northern‑hemisphere location fills a geographic gap left by southern‑hemisphere facilities such as HAWC and LHAASO, allowing continuous monitoring of sources that transit at high declinations.

In conclusion, the proposed underground water‑Cherenkov muon detector dramatically enhances the Tibet AS array’s ability to conduct ultra‑high‑energy gamma‑ray astronomy. The simulation study validates the design concept, demonstrating unprecedented background suppression and robust gamma‑ray retention. The paper outlines a roadmap for construction, optimization of tank depth and photomultiplier layout, and integration of real‑time data pipelines that will enable rapid multi‑wavelength alerts. If realized, the Tibet AS+MD observatory will become a premier instrument for probing the acceleration mechanisms of Galactic cosmic rays and for uncovering the most energetic particle accelerators in the Milky Way.