aa GLAST Particle Astrophysics Collaboration Instrument Managed and Integrated at SLAC/Stanford University The Gamma-ray Large Area Space Telescope (GLAST) Balloon Flight Engineering Model (BFEM) represents one of the 16 towers which comprise the Large Area Telescope (LAT) instrument to be launched in March In the 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 successfully 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 a.The BFEM tower is consists of a pair-conversion type gamma-ray Tracker (TKR) using silicon strip detector, Calorimeter (CAL) made of arrayed CsI crystals, Anti-Coincidence Detector (ACD) made of plastic scintillators. The detectors had been originally utilized for BTEM, Beam Test Engineering Model (Eduardo et al. 2001, NIMA 474, 19), and were employed for BFEM after some modifications. A sets of plastic scintillators, called eXternal Gamma-ray Target (XGT), were newly mounted above the ACD to get a tagged gamma- ray events. b.BFEM instruments were mounted in Pressure Vessel (PV), since not all of them are designed to operate in a vacuum. c.Detectors, as well as a 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 proton models. (a)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 (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, indicating that our model formula is appropriate. (b)Low energy charged particles cannot penetrate the air due to the Lorentz force of geomagnetic field, hence the spectrum suffers cutoff in low energy region. At Palestine, Texas, 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 secondary component, and their energy spectrum depends on COR. We modeled secondary protons referring to the AMS data. We do not have reliable data below 100 MeV, and extrapolated spectrum down to 10 MeV with E^-1. (a)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 proton. We modeled the secondary component referring to the AMS data and extrapolated down to 10 MeV with E^-1. (b)The same as figure a, but for positron instead of electron. Positron fraction (e+/(e- + e+)) is assumed to be (Golden et al. 1996, ApJL 457, 103). (c)Secondary (atmospheric) gamma-ray spectrum generated by our simulator, with Schonfelder et al. (1980, ApJ 240, 350) and Daniel et al. (1974, Rev. of Geophys. And Space Phys. 22, 233). The referred data are scaled to 3.8 g/cm^2, atmospheric depth of our level flight. We also implemented the primary (cosmic origin) gammas, but they do not contribute the trigger rate significantly. (d)CR muons (plus and minus), shown with references. Flux of primary muons are negligible, hence we modeled only secondary. 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