GLAST

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
Geant4 Based Cosmic-Ray Background Simulator for
Balloon Experiment -- Gamma ray Large Area Space
Telescope Balloon Flight Engineering Model
Geant4 Simulation --
Abstract
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
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.
Instrumentation

• 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.
• 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.
• The detectors, as well as the PV and support
  structures are implemented in a Geant4-based
  Monte-Carlo simulator.

(c)
(a)
(b)
Cosmic-ray generator
solar modulation (phi540MV)
with magnetic cutoff (_at_Palestine)
spectrum outside the solar system
solar modulation (phi1100MV)
secondary (_at_Palestine)
(a)
(c)
(b)

• 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.
• 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 phi540 MV, solar minimum) and
  blue line (phi1100 MV, solar maximum). The
  former shows good agreement with the BESS data
  obtained at polar region (Sanuki et al. 2000,
  astro-ph/0002481), indicating that our model
  formula is appropriate.
• 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.
• 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.

• 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.
• 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).
• 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/cm2,
  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.
• 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.

(a) CR electron
(b) CR positron
(c) CR muon
(c) CR gamma