Detection of High Energy Gamma-rays from the Galactic Disk at Balloon Altitudes

IN recent years there have been many attempts to detect extraterrestrial gamma radiation of energy greater than 50 MeV (refs. 1–14), but the only positive evidence of the presence of this radiation was obtained recently by the OSO III satellite experiment of Clark et al.15. Their measurements reveal a significant flux of gamma-rays, associated with the galactic plane and an enhancement in the direction of the galactic centre. At present it is not-known whether this flux comes from a number of unresolved point sources, collisions between cosmic-ray protons and interstellar hydrogen, or Compton collisions between cosmic-ray electrons and the high intensity infrared radiation background16,17. Attempts to detect the radiation by balloon experiments have suffered from a high level of locally generated background, which made the identification of the weak galactic source of gamma-rays a difficult task. So far all balloon experiments have had an extrapolated primary flux at the top of the atmosphere greater than 1 × 10−3 photons cm−2 s−1 sr−1, irrespective of the type of detectors used (usually nuclear emulsions, scintillation Cerenkov detectors and spark chambers). This flux is an order of magnitude higher than the value of the isotropic primary flux of (1.2 ± 0.2) × 10−4 photons cm−2 s−1 sr−1 reported by the OSO III experiment.