The first all-sky survey of cosmic gamma rays of energies above 50 MeV was carried out by OSO-3 in 1967-1968. The directional scintillator/Cerenkov detector recorded 621 "sky events" in 16 months of operations (Fig. 4). The observations can be attributed to three components: (1) a galactic component, concentrated along the galactic plane and well-correlated with the column density as deduced from 21-cm radio measurements; (2) a galactic center component; and (3) an isotropic, extragalactic component with a steep power-law spectrum. Subsequent measurements made using an entirely different technique - spark chamber detectors - confirmed the general picture. These included the SAS-2 (Fichtel et al. 1975) mission and several balloon flights (Table 1). SAS-2 measured a diffuse GRB component with a very steep differential power law of energy index ~ 1.7 (photon index = + 1 ~ 2.7) between 35 MeV and 200 MeV (Fichtel et al. 1978) (Fig. 5). The extrapolated intensity of this component to lower energies agreed well with measurements at 10 MeV. Above several hundred MeV, the flux of the extragalactic GRB falls below the galactic high latitude background and the determination of its value is highly dependent on data analysis techniques.
Figure 4. OSO-3 sky events plotted in galactic coordinates (Kraushaar, Clark & Garmire 1972).
Figure 5. Differential photon spectrum of the SAS-2 diffuse gamma-ray background (Fichtel et al. 1978).