The vertical component of 1--20 MeV gamma rays at balloon altitudes

From the gamma-ray data obtained during a balloon flight of the large-area Compton telescope of the Max-Planck-Institut, the diffuse cosmic and the vertical atmospheric component of 1--20 MeV gamma rays were determined. The basic improvement of this new measurement above previous measurements with a first version of the Compton telescope is based on the fact that for each registered event various additional parameters are determined. By means of the time-of-flight distribution of upward-scattered gamma rays and by means of the pulse shape measurement of neutron-induced events it is possible to determine most of the undesired background events from the data themselves. The resulting energy spectrum of cosmic gamma rays from high galactic latitudes supports the existence of bump in the spectrum at MeV energies above the extrapolation from hard X-ray energies. Above about 5 MeV the spectrum becomes very steep and nicely connects to the SAS 2 measurements between 35 and 200 MeV. The vertical component of downward-moving atmospheric gamma rays between 1 and 20 MeV can be fitted by one single power law: (6 +- 1) x 10-3 E/sup() -1.65plus-or-minus0.15/ photons cm-2 s-1 sr-1 MeV-1 g-1 cm2. The question about the origin of the diffuse cosmic gamma-ray component is discussed, and the results about the atmospheric gamma-ray component are compared with different models calculations