1. T. BurnettGeant4 2003 Workshop user presentation 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. T. BurnettGeant4 2003 Workshop user presentation 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 - >300 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, 28.5 o inclination Lifetime: 5 years (minimum)

3. T. BurnettGeant4 2003 Workshop user presentation 3   detection – pair conversion “telescope” Csi Calorimeter (energy measurement) Si strip detectors Conversion foils (W) tiled scintillation detectors  e+e- 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) Pair production is the dominant photon interaction above 10 MeV

4. T. BurnettGeant4 2003 Workshop user presentation 4 GLAST Large Area Telescope (LAT) GLAST Large Area Telescope (LAT) e+e+ e–e–  Si Tracker Tower pitch = 228 µm 5.52 10 4 channels 12 layers × 3% X 0 + 4 layers × 18% X 0 + 2 layers Grid (& Thermal Radiators) 3000 kg, 650 W (allocation) 1.8 m  1.8 m  1.0 m 20 MeV – 300 GeV CsI Calorimeter Hodoscopic array 8.4 X 0 8 × 12 bars 2.0 × 2.7 × 33.6 cm  cosmic-ray rejection  shower leakage correction ACD Segmented scintillator tiles 0.9997 efficiency  minimize self-veto Data acquisition 16 identical towers 30 Hz average downlink

5. T. BurnettGeant4 2003 Workshop user presentation 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 A eff : 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: <10 Hz)

6. T. BurnettGeant4 2003 Workshop user presentation 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. T. BurnettGeant4 2003 Workshop user presentation 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. T. BurnettGeant4 2003 Workshop user presentation 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. T. BurnettGeant4 2003 Workshop user presentation 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/compi ler rh7.2/gcc 2.95Window 2000 server/ vcc 7.0 Total CPU time (sec) 708185

10. T. BurnettGeant4 2003 Workshop user presentation 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 G4’s default or our patch G4ProcessManager * pM = G4Electron::Electron()->GetProcessManager(); G4VContinuousDiscreteProcess* electronMS = new G4MultipleScattering(); pM->AddProcess(electronMS);

11. T. BurnettGeant4 2003 Workshop user presentation 11 Comparison: 3.2 vs 5.2*…but which is correct? Why did it change? G4 5.2G4 3.2 100 MeV e- on 105 microns of W *actually using 5.1 with a correction from G4 5.2 for MCS

12. T. BurnettGeant4 2003 Workshop user presentation 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 1 st and 6 th plane –plot x/z Beam Direction x z e-e- Energy (MeV) Data: x/z distribuitionFit sigma deflection (mrad) ExptG3G4 5.2G4 3.2 79 10910481101 65014.613.38.414.2

13. T. BurnettGeant4 2003 Workshop user presentation 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. T. BurnettGeant4 2003 Workshop user presentation 14 Conclusions, I We are satisfied with the general design and support of Geant4, and do not regret switching to it from Gismo (EGS4+Gheisha)

15. T. BurnettGeant4 2003 Workshop user presentation 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