A Balloon Investigation of Galactic ?-ray Lines with a High Resolution Ge(Li) Spectrometer

Abstract

Galactic y-ray lines have been investigated with a balloon-borne Ge(Li) spectrometer. The galactic plane was surveyed over the longitudinal interval 40°_210°, but upper limits only were obtained for the intensities of y-ray lines from positron annihilation at 0·511 MeV and the nuclear decays of 2°Ne· (1· 634 MeV), 24Mg* (1·369 MeV), 28Si* (1· 779 MeV) and S6Pe· (0·847 MeV). These upper limits are of the order of 10-3 photons cm-2 S-l sct, which are consistent with measurements for other lines observed so far. A possible y-ray line feature at 400 keV, with a flux of (7 ·4± 5 ·4) x 10-3 photons cm-2 s-t, was detected when the Crab Nebula was in the field of view. This line seems to be identical with one reported by Leventhal et al. (1977), who suggested that it may originate from a gravitational redshift of the 0·511 MeV annihilation line produced near the surface of the neutron star.