Geant4 and GLAST

1
Geant4 and GLAST

• Description of the mission and instrument
• Simulation requirements
• How we are a little different
• Experience with G4
• Recovery
• Validation strategy

Toby Burnett University of Washington
2
The GLAST Mission

• GLAST measures the direction, energy and arrival
  time of celestial gamma rays
• LAT measures gamma-rays in the energy range 20
  MeV - gt300 GeV
• There is no instrument now covering this range!!
• - GBM provides correlative observations of
  transient events in the energy range 20 keV 20
  MeV

Launch December 2006 Florida Orbit
550 km, 28.5o inclination Lifetime 5 years
(minimum)
3
detection pair conversion telescope
Pair production is the dominant photon
interaction above 10 MeV

• Characteristics
• Low profile for wide f.o.v.
• Segmented anti-shield to minimize self-veto at
  high E.
• Finely segmented calorimeter for enhanced
  background rejection and shower leakage
  correction.
• High-efficiency, precise track detectors located
  close to the conversions foils to minimize
  multiple-scattering errors.
• Modular, redundant design.
• No consumables.
• Low power consumption (580 W)

4
GLAST Large Area Telescope (LAT)
?
Si Tracker Tower pitch 228 µm 5.52 104
channels 12 layers 3 X0 4 layers 18 X0
2 layers
Grid ( Thermal Radiators)
e
e
3000 kg, 650 W (allocation) 1.8 m ? 1.8 m ? 1.0
m 20 MeV 300 GeV
CsI Calorimeter Hodoscopic array 8.4 X0 8
12 bars 2.0 2.7 33.6 cm

• cosmic-ray rejection
• shower leakage
• correction

16 identical towers 30 Hz average downlink
5
Why we need Simulation

• Calibration, assessment of pattern recognition,
  track fitting strategies
• Predict the following figures of merit, depend on
  incoming photon energy E? and angle ?
• Effective area Aeff depends on geometric area,
  conversion probability, reconstruction efficiency
• Point Spread Function (PSF) depends on multiple
  scattering, detector resolution, pattern
  recognition accuracy
• Develop a strategy and assess its effectiveness
  in suppressing the background from hadronic
  interactions. (Average trigger rate 4 kHz
  science rate lt10 Hz)

6
Requirements EM processes

• Tracking region small scale
• individual processes pair conversion,
  bremsstrahlung picture of 1 GeV shower, in
  tracking region
• Multiple scattering zoom to conversion in W
• Calorimeter large scale
• Basic shower properties for leakage correction
  pan to calorimeter region maybe cycle thru
  showing charged only, hit cell segments

7
Requirements Hadronic Processes

• proton-nucleus cross sections, multiplicities
  picture of a side or bottom-entering event
• Same for He, C, N, O, etc. (components of cosmic
  rays)
• E loss for min-I heavy nuclei (used to calibrate
  calorimeter)

8
How we are (not) using G4

• Geometry database is XML based, independent
• We have our own graphics, GUI.
• The event loop is controlled by our framework,
  Gaudi (G4 is invoked event-by-event from an
  Algorithm)

9
What we do really differently

• Most of our code was developed using Microsoft
  Developer Studio (VS6, now VS.NET)
• We support development under both Windows (not
  cygwin) and Linux
• In order to build our G4 with the new MS
  compiler, we developed our own build procedure.
• This allows comparison of both results and
  timing the following table is used to estimate
  the CPU time needed for a major data challenge,
  using Linux machines at SLAC.

Operating system/compiler rh7.2/gcc 2.95 Window 2000 server/ vcc 7.0
Total CPU time (sec) 708 185
10
The wake-up call, our reaction

• We switched from G4 3.2 to 5.0 on 2 Feb 03, and
  to 5.1 on 15 May 03.
• Motivation it is better, and wanted to set step
  size by region
• Each involved non-backwards compatible
  modifications to the run manager
• Discovered that a significant change, 20 had
  happened from 3.2 to 5.1 in the multiple
  scattering widths (by getting too good
  answers!)
• We could not easily revert to 3.2, and decided to
  try to graft the MCS code used by 3.2 into our
  5.1.
• One little technical problem a new multiple
  scattering object must be attached to every
  charged particle, 20 for us
• Our solution
• 1. Replace the wired-in new with a factory call,
  giving control over the objects to create
• 2. Adapt the code from 3.2 to work in the 5.1
  environment
• 3. Provide a switch to allow us to use either
  G4s default or our patch

G4ProcessManager pM G4ElectronElectron()-gt
GetProcessManager() G4VContinuousDiscreteProcess
electronMS new G4MultipleScattering()pM-gtAd
dProcess(electronMS)
11
Comparison 3.2 vs 5.2but which is correct? Why
did it change?
100 MeV e- on 105 microns of W
G4 5.2
G4 3.2
actually using 5.1 with a correction from G4 5.2
for MCS
12
Measure it Multiple Scattering validation

• Electron test beam at Frascati for AGILE, (2003)
  F. Longo
• Geometry
• 6 planes with 300 ?m of W
• Inter-plane distance 1.6 cm
• Analysis
• Require single cluster on the 1st and 6th plane
• plot x/z

Energy (MeV) Data x/z distribuition Fit sigma deflection (mrad) Fit sigma deflection (mrad) Fit sigma deflection (mrad) Fit sigma deflection (mrad)
Energy (MeV) Data x/z distribuition Expt G3 G4 5.2 G4 3.2
79 109 104 81 101
650 14.6 13.3 8.4 14.2
13
Our response trust, but verify, by setting up
systematic monitoring of

• Photon processes Photoelectric, Compton
  Scattering and Pair Production
• Cross Section, Angular and Energy Distribution
• Charged particles processes
• Ionisation
• Landau and Bethe Bloch
• Range, Straggling, Stopping Power
• Multiple Scattering
• Angular distribution, Energy Dependence
• Bremsstrahlung
• Cross Section, Angular and Energy Distribution
• Delta Ray production
• Energy distribution, Multiplicity
• Positron Annihilation
• EM shower development

• Muon-nucleus interactions
• Neutron interactions
• HE hadron-nucleus interactions
• Nucleus-nucleus elastic scattering
• Hadronic showers in CsI
• Radioactive decay

14
Conclusions, I

• We are satisfied with the general design and
  support of Geant4, and do not regret switching to
  it from Gismo (EGS4Gheisha)

15
Conclusions, II

• We will NEVER again use a new version of G4
  without careful analysis and testing to detect
  changes unsupported by experimental verification