GLAST The GLAST Balloon Flight experiment was performed with the collaboration of NASA Goddard Space Flight Center, Stanford Linear Accelerator Center, Stanford University, Hiroshima University, Naval Research Laboratory, University of California Santa Cruz, and INFN-Pisa and University of Pisa 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 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 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 detailed comparisons between the data and simulation is underway. Geant4 Based Cosmic-Ray Background Simulator for Balloon Experiment -- Gamma ray Large Area Space Telescope Balloon Flight Engineering Model: Geant4 Simulation -- a.The BFEM tower consists of a pair-conversion type gamma-ray Tracker (TKR) using silicon strip detector, a Calorimeter (CAL) made of arrayed CsI crystals and an Anti-Coincidence Detector (ACD) made of plastic scintillators. The detectors had been originally utilized for BTEM, Beam Test Engineering Model (E. do Couto e Silva et al. 2001, NIMA 474, 19), and were employed for BFEM after some modifications. A set of plastic scintillators, called eXternal Gamma-ray Target (XGT), were newly mounted above the ACD to get tagged gamma-ray events. b.BFEM instruments were mounted in a Pressure Vessel (PV), since not all of the laboratory engineering versions of the support components were designed to operate in a vacuum. c.The detectors, as well as the PV and support structures are implemented in a Geant4-based Monte-Carlo simulator. We constructed Cosmic-Ray models referring to previous measurements and taking into account the solar modulation effect (Gleeson and Axford 1968, ApJ 154, 1011) and geomagnetic cutoff. Three figures above show how we constructed the proton models. (a)The primary spectrum outside the solar system is expressed as a power-law function of particle rigidity (black line). Low energy protons are modulated by solar activity, as shown in red line (solar potential phi=540 MV, solar minimum) and blue line (phi=1100 MV, solar maximum). The former shows good agreement with the BESS data obtained at polar region (Sanuki et al. 2000, astro- ph/ ), indicating that our model formula is appropriate. (b)Low energy charged particles cannot penetrate the air due to the Lorentz force of the geomagnetic field, hence the spectrum suffers cutoff in low energy region. At Palestine, Texas, the cutoff rigidity (COR) is about 4.46 GV. (c)Particles with lower energy are generated via the interaction between primary cosmic-rays and molecules of the air. They are called the secondary component, and their energy spectrum depends on COR. We modeled secondary protons referring to the AMS data (Alcaraz et al. 2000, Physics Letters B 490, 27). We do not have reliable data below 100 MeV, and so we extrapolated the spectrum down to 10 MeV with E^-1. (a)The generated CR electron spectrum with reference data points. Primary component refers to Komori et al. (1999 Proceeding of Dai-Kikyu Symposium, p33), where they compiled measurements in GeV region. Solar modulation and geomagnetic cutoff effects are taken into account as applied to the proton. We modeled the secondary component referring to the AMS data (Alcaraz et al. 2000, Physics Letters B 484, 10) and extrapolated the spectrum down to 10 MeV with E^-1. (b)The same as Fig. a, but for positrons instead of electrons. The positron fraction (e+/(e- + e+)) is assumed to be (Golden et al. 1996, ApJL 457, 103). (c)The secondary (atmospheric) gamma ray spec- trum generated by our simulator, with Schonfelder et al. (1980, ApJ 240, 350) and Daniel et al. (1974, Rev. of Geophys. and Space Phys. 12, 233). The data referred to are scaled to 3.8 g/cm^2, atmospheric depth of our level flight. We also constructed a primary (cosmic origin) gamma-ray generator, but these particles do not contribute to the trigger rate significantly. (d)CR muons (plus and minus), shown with references (Boezio et al. 2000, ApJ 532, 653). The flux of primary muons is negligible, hence we modeled only secondaries. Instrumentation: Abstract: (a) (b) (c) (a) (b) (c) Cosmic-ray generator: solar modulation (phi~540MV) solar modulation (phi~1100MV) with magnetic cutoff secondary spectrum outside the solar system (a) CR electron(b) CR positron (c) CR gamma (c) CR muon