Simulation of HEAO 3 background

A Monte Carlo technique for modelling background in space-based gamma-ray telescopes has been developed. The major background components included in this modeling technique are the diffuse cosmic gamma-ray flux, the Earth´s atmospheric flux, the decay of nuclei produced by spallation of cosmic rays, trapped protons and their secondaries, the decay of nuclei produced by neutron capture, and the de-excitation of excited states produced by inelastic scattering of neutrons. The method for calculating the nuclear activation and decay component of the background combines the low Earth orbit proton and neutron spectra, the spallation cross sections from Alice91, nuclear decay data from the National Nuclear Data Center´s (NNDC) Evaluated Nuclear Structure Data File (ENSDF) database, and three-dimensional gamma-ray and beta transport with Electron Gamma-ray Shower version 4 (EGS4). This Monte Carlo code handles the following decay types: electron capture, β ^- β⁺, metastable isotopes and short lived intermediate states, and isotopes that have branchings to both β ^- and β⁺. Actual background from the HEAO 3 space instrument is used to validate the code