The positron annihilation line at 511 keV in the upper atmosphere

We present the results of a series of balloon flights made at two latitudes over Aire sur l'Adour, France (4.5 GV), and Sao José dos Campos, Brazil (12.5 GV). We used two types of omnidirectionnal detectors: a 4.5 × 5.1 cm NaI crystal and a 23 cm³ Ge‐Li high‐energy resolution detector. The two were surrounded by a 0.8‐cm thick plastic anticoincidence. These measurements allowed us to determine precisely the characteristics of the 0.511‐MeV line versus altitude and latitude. We have also determined from the analysis of the growth curves at the two latitudes an extraterrestrial component value of (4.3±1.5) × 10 −2 photons/cm² s and an upper limit of 4 × 10 −3 photons/cm² s for the flux from the galactic center region.