The neutron spectrum inside the shielding of balloon-borne Ge spectrometers

Understanding the background of balloon-borne germanium detectors used in high energy astrophysics is crucial for the design of future space missions. In this paper we present a method which is able to estimate the poorly understood beta(-) and beta(+) background components inside the shield of an balloon borne instrument. The method consists of an experimental estimate of the neutron flux by determining the activation of isomers with measured gamma-ray line strengths and known cross sections. The gamma-ray line intensities of typical isomeric transitions in germanium detectors are calculated for a neutron spectrum model. After comparing the calculated line fluxes with the measurements of balloon experiments the model neutron spectrum is changed appropriately. The procedure is repeated iteratively until an acceptable fit is achieved. Applying this method to spectra of balloon-borne experiments results in neutron spectra that are harder than those generally used in standard background models. The neutron spectra that we calculate for various balloon-borne Ge spectrometers have made possible a reestimation of the continuum background components due to beta decays. It is shown that the enriched Ge-70 produces more beta(-) than expected. The model spectra also explains the poorly understood spectral features of Ge-70 detectors; e.g.; the behavior of the neutron activation line at 198 keV and the increase of the background in the 1.5-4 MeV range, which is due to an enhanced production of isotopes that disintegrate via beta(+) decays. Finally, the neutron spectrum of shielded balloon-borne germanium detectors is studied as a function of shield thickness. By combining the resulting beta decay components with shield leakage spectra, we optimize the shield thickness for specific energy ranges.