Depth dependent background measurements with NCT

The Nuclear Compton Telescope (NCT) is a balloon- borne soft gamma-ray (0.2 MeV to 10 MeV) Germanium Compton Telescope (GCT) designed to study astrophysical sources of nuclear line emission and polarization. A prototype instrument was successfully launched from Ft. Sumner, NM on June 1, 2005. The NCT prototype consists of two 3D position sensitive High- Purity-Germanium (HPGe) strip detectors fabricated with amorphous Ge contacts. The compact design and new technologies allow NCT to achieve high efficiencies with excellent spectral resolution and background reduction. Here we present a study of approximately 8 hours of background measurements made from a ground altitude of 1.2 km to an average float altitude of 40 km. The expected contributions to the background component are discussed, especially in light of detailed Monte Carlo simulations incorporating complete depth dependent environmental inputs, including cosmic and atmospheric photon distributions, primary cosmic rays (protons), cosmic ray secondaries (protons and neutrons), and activation induced components. The contributions of the leptonic component to the total observed background is also discussed. Special emphasis is placed on the 511 keV annihilation line, and a first attempt to image this background component is presented.