[en] A balloon-borne germanium γ-ray telescope was flown over Alice Springs, Australia, in an attempt to detect spectral features from the galactic center (GC) direction. A 511 keV positron annihilation line was detected at a flux level of (1.22 +- 0.22) x 10^-3 photons s^-1 cm^-2. Suggestive evidence for the detection of the three-photon positronium (ps) continuum is presented. A brief discussion of the possible origin of the positrons is given

No abstract available

[en] Our balloon-borne germanium γ-ray telescope has been reflown over Alice Springs, Australia. The detection of a sharp 511 keV electron-positron annihilation line from the galactic center (GC) direction has been confirmed at a flux level compatible with our previous result. No evidence for several other candidate lines was found, indicating that they were statistical fluctuations in the previous data. Evidence for a significant change in the GC inverse power-law continuum has been obtained, suggesting that its origin lies in the many unresolved and highly variable pointlike X-ray sources in the field of view

[en] Our balloon-borne germanium γ-ray telescope has been flown for a third time over Alice Springs, Australia to further study the galactic center source 511 keV annihilation-line radiation. The source was found to be in an ''off'' or ''low'' state. Our best fit experimental value for the line flux at Earth is zero, with a ''1 sigma'' upper bound of 3.8 x 10^-4 photons cm^-2 s^-1. This null result is discussed in the context of other measurements and available theoretical models, the most promising of which involve a galactic supernova or central compact object like a massive black hole

[en] A coded aperture imaging system has been designed for the Gamma-Ray Imaging Spectrometer (GRIS). The system is optimized for imaging 511-keV positron-annihilation photons. For a galactic center 511-keV source strength of 10^-3 cm^-2s^-1, the source location accuracy is expected to be +-0.2⁰

[en] High resolution gamma-ray spectroscopy is a relatively new field that holds great promise for further understanding of high energy astrophysical processes. Preliminary results such as the annihilation radiation from the galactic center, the ²⁶Al line from the galactic plane and cyclotron lines from neutron stars may well be just the initial discoveries of a rich and as yet undeveloped field. When the high resolution gamma-ray spectrometer (GRSE) was removed from the GRO payload NASA decided to initiate a balloon program to permit continued development and improvement of instrumentation in this field, as well as continued scientific observations. The Gamma-Ray Imaging Spectrometer (GRIS) is one of the experiments selected as part of this program. The instrument contains a number of new and innovative features that are expected to produce a significant improvement in source location accuracy and sensitivity over previous balloon and satellite experiments. 6 refs., 2 figs

[en] A coded aperture Imaging System was designed for the Gamma-Ray Imaging Spectrometer (GRIS). The system is optimized for imaging 511 keV positron-annihilation photons. For a galactic center 511-keV source strength of 0.001 sq/s, the source location accuracy is expected to be + or - 0.2 deg

[en] High resolution gamma-ray spectroscopy is a relatively new field that holds great promise for further understanding of high energy astrophysical processes. When the high resolution gamma-ray spectrometer (GRSE) was removed from the GRO payload, a balloon program was initiated to permit continued development and improvement of instrumentation in this field, as well as continued scientific observations. The Gamma-Ray Imaging Spectrometer (GRIS) is one of the experiments selected as part of this program. The instrument contains a number of new and innovative features that are expected to produce a significant improvement in source location accuracy and sensitivity over previous balloon and satellite experiments