Geant4 based cosmic-ray background simulator for balloon experiments

The Gamma-ray Large Area Space Telescope (GLAST) Balloon Flight Engineering Model (BFEM) represents one of the 16 towers that compose the Large Area Telescope (LAT) instrument to be launched in March 2006. In low earth orbit and at balloon altitude, background generated by cosmic-ray interactions is known to dominate over the astronomical gamma ray signal in its rate. A Geant4 based simulation program has been developed to study this cosmic-ray background. Although the program is intended primarily for the BFEM, the cosmic-ray generator producing primary and secondary cosmic-ray fluxes is adaptable to most balloon and satellite experiments at any geomagnetic latitude and solar modulation cycle. The balloon was successfully launched on August 4, 2001 at Palestine, Texas. We well reproduced the observed trigger rate in our simulator, and a detailed comparison between the data and simulation is underway.