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The background spectrum in balloon and satellite Ge spectrometers consists of a continuum with discrete nuclear gamma-ray lines superimposed. Although many background lines can be distinguished in a Ge background spectrum, only a few are at energies of astrophysical interest. In this work, tools for estimating these background lines in different spectrometer configurations are provided. The 511 keV background line is studied in detail since it is one of the most intense background features. This line can be described as the sum of three components: (1) atmospheric 511 keV photons entering through the aperture of the instrument; (2) 511 keV photons produced in the (beta) + decays of unstable nuclei in the passive material inside the shield; and (3) 511 keV photons coming from outside that pass through the shield without scattering and are completely absorbed in the detector. The variation of the different components as a function of the passive material and shield characteristics is studied to provide techniques to determine the instrument configuration that minimizes the background line intensity. The mechanisms producing the 1809 keV Al background line are not as numerous. The background line can be described as the sum of a delayed component originating inside the shield and a prompt component from outside the shield. The use of a thick shield seems to be an appropriate way to reduce this background line. However, the lack of information about relevant cross sections has not allowed us to perform accurate modeling. Finally, the mechanisms that cause other narrow background lines at energies of astrophysical interest (i.e., the 476 keV Be line, 844 keV Al line, 847 Fe line, 1157 keV Ti line, 4.439 MeV C line, and 6.129 MeV O line) are briefly described.
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