1. 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 In last August at Palestine, Texas, we have successfully launched the Balloon Flight Engineering Model (BFEM), that represents one of 16 towers of the LAT on board the GLAST satellite planned to be launched in 2006. One of the main purpose of this experiment is to collect particle incidences as well as validate the LAT design in a space-like environment, and utilize the obtained data as a background data base for GLAST. For this balloon experiment, we have developed a Geant4-based Monte-Carlo simulator for the BFEM and cosmic-ray generators of protons, alphas, electrons, positrons, muons, and atmospheric gammas. We have succeeded to reproduce the observed trigger rate and hit distributions in the Tracker for charged events by our simulator. Study of Data from the GLAST Balloon Prototype Based on a Cosmic-Ray and Instrument Simulator 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 TKR consists of 26 Si layers. Layers with strips along x and y directions alternate throughout the TKR to measure the direction of incident gamma-rays. 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 (S. Giani et al. 1998, CERN/LHCC 98-44). 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. 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, ApJ 545, 1134), 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. We also implemented cosmic-ray alphas, electrons and positrons, gammas and muons as shown in Fig. 3. (a)The primary alpha spectrum generated by our simulator with reference data of Sanuki et al. (2000) and Seo et al. (1991, ApJ 378, 763). Solar modulation effect and geomagnetic cutoff are taken into account in the same way as for protons. (b)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 10-100 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. (c)The same as Fig. a, but for positrons instead of electrons. The positron fraction (e+/(e- + e+)) is assumed to be 0.078 (Golden et al. 1996, ApJL 457, 103). (d)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. (e)CR muons (plus and minus), shown with references (Boezio et al. 2000, ApJ 532, 653). Instrumentation: Abstract: (a) (b) (c) (a) (b) (c) Cosmic-ray generator: solar modulation (phi~540MV) solar modulation (phi~1100MV) with magnetic cutoff (@Palestine) secondary (@Palestine) spectrum outside the solar system (c) CR positron(d) CR gamma (e) CR muon Figure 1: Instruments of the Balloon Flight Engineering Model (BFEM) (b) CR electron Figure 2: How we constructed cosmic-ray proton spectrum (a) CR alphas Figure 3: Model spectrum and reference data of other particles