High-energy gamma-astronomy with large underground detectors

Large underground (underwater) detectors (such as ‘Baikal’ and DUMAND) are discussed for their possible use as gamma-ray telescopes. The signal is caused by high-energy muons (E _μ ? 30–100 TeV) produced by the primary gamma-radiation in the Earth's atmosphere. The production of gamma-rays in the source through the reaction p + p → π ⁰ + X is calculated for a low density target of arbitrary thickness taking the electromagnetic cascade into account. The muon production by gamma-rays in the Earth's atmosphere is calculated using three processes: (i) photoproduction of π- and K-mesons followed by decay to muons, (ii) the direct production of μ⁺ μ^?-pairs: γ + Z → Z + μ ⁺ + μ^?, and (iii) photoproduction of charmed particles. It is shown that for thin sources with a flat spectrum (integral exponent γ = 1.1) a large (S = 0.1 km²) underground detector can detect both neutrinos and gamma-quanta generated by the source. Finally we compare the performances of underground detectors with S = 0.1 km² for the search of gamma sources at E = 100 TeV with those of the previously proposed 1 km² EAS array.