INSTRUMENT DESCRIPTION AND PERFORMANCE OF THE IMAGING GAMMA-RAY TELESCOPE COMPTEL ABOARD THE COMPTON GAMMA-RAY OBSERVATORY

The imaging Compton telescope COMPTEL is one of the four instruments on board the Compton Gamma-Ray Observatory (GRO), which was launched on 1991 April 5 by the space shuttle Atlantis into an Earth orbit of 450 km altitude. COMPTEL is exploring the 1-30 MeV energy range with an angular resolution (1 sigma) between 1-degrees and 2-degrees within a large field of view of about 1 steradian. Its energy resolution (8.8% FWHM at 1.27 MeV) makes it a powerful gamma-ray line spectrometer. Its effective area (for on-axis incidence) varies between 10 and 50 cm2 depending on energy and event selections. Within a 14 day observation period COMPTEL is able to detect sources which are about 20 times weaker than the Crab. The measurement principle of COMPTEL also allows the measurements of solar neutrons. COMPTEL's neutron sensitivity above 15 MeV is 5 neutrons cm-2 for a 30 minute observation time. In a so-called single-detector mode, COMPTEL uses two of its D2 modules to study the temporal spectral evolution of gamma-ray bursts or other transients. In its telescope mode COMPTEL is performing the first complete sky survey at MeV energies during the first 18 months of the mission. Targets of special interest are Galactic gamma-ray sources (such as radio pulsars, X-ray binaries, the Galactic center, the unidentified COS B sources, supernova remnants, and molecular clouds), external galaxies (especially the nuclei of active galaxies), gamma-ray line sources (e.g., the distribution of the 1.8 MeV line emissivity throughout the Galaxy), the diffuse gamma-ray emission from interstellar space, the cosmic gamma-ray background, cosmic gamma-ray bursts, and gamma-ray and neutron emission during solar flares. In later phases of the mission, the objects discovered in the survey will be studied in more detail. This paper provides a detailed description of the instrument: its principle of measurement, design, and performance. The telescope characteristics were derived from calibration measurements and simulations. First flight data are being used to demonstrate its in-flight performance and to derive the achieved sensitivity of the telescope.