T. Burnett Geant4 2003 Workshop user presentation 1

Geant4 and GLASTGeant4 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

T. Burnett Geant4 2003 Workshop user presentation 2

The GLAST MissionThe 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, 28.5o inclination

Lifetime: 5 years (minimum)

T. Burnett Geant4 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

T. Burnett Geant4 2003 Workshop user presentation 4

GLAST Large Area Telescope (LAT)GLAST Large Area Telescope (LAT)

e+ e–

Si Tracker Towerpitch = 228 µm5.52 104 channels12 layers × 3% X0

+ 4 layers × 18% X0

+ 2 layers

Grid (& Thermal Radiators)

3000 kg, 650 W (allocation)

1.8 m 1.8 m 1.0 m

20 MeV – 300 GeV

CsI CalorimeterHodoscopic array8.4 X0 8 × 12 bars

2.0 × 2.7 × 33.6 cm cosmic-ray rejection shower leakage correction

ACDSegmented scintillator tiles0.9997 efficiency

minimize self-veto

Data acquisition

16 identical towers30 Hz average downlink

T. Burnett Geant4 2003 Workshop user presentation 5

Why we need SimulationWhy 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: <10 Hz)

T. Burnett Geant4 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]

T. Burnett Geant4 2003 Workshop user presentation 7

Requirements: Hadronic ProcessesRequirements: 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)

T. Burnett Geant4 2003 Workshop user presentation 8

How we are (not) using G4How 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)

T. Burnett Geant4 2003 Workshop user presentation 9

What we do What we do reallyreally differently 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

T. Burnett Geant4 2003 Workshop user presentation 10

The wake-up call, our reactionThe 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 create2. Adapt the code from 3.2 to work in the 5.1 environment3. 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);

T. Burnett Geant4 2003 Workshop user presentation 11

Comparison: 3.2 vs 5.2*…but which Comparison: 3.2 vs 5.2*…but which is correct? Why did it change?is correct? Why did it change?

0.05 0 0.05 0.10

200

400

600

800

1000

datafit

Projected MS for 100 MeV (KE) electrons

projected angle (rad)

even

ts

:fit 15.5mrad

rms 26.8mrad

G4 5.2 G4 3.2

0.05 0 0.05 0.10

200

400

600

800

1000

datafit

Projected MS for 100 MeV (KE) electrons

projected angle (rad)ev

ents

:fit 19.9mrad

rms 27.1mrad

100 MeV e- on 105 microns of W

*actually using 5.1 with a correction from G4 5.2 for MCS

T. Burnett Geant4 2003 Workshop user presentation 12

Measure it: Multiple Scattering Measure it: Multiple Scattering validationvalidation

• 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

Beam Direction

x

z

e-

Energy (MeV)

Data: x/z distribuition Fit sigma deflection (mrad)

Expt G3 G4 5.2 G4 3.2

79 109 104 81 101

650 14.6 13.3 8.4 14.2

T. Burnett Geant4 2003 Workshop user presentation 13

Our response: “trust, but verify”, by Our response: “trust, but verify”, by setting up systematic monitoring of: 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

T. Burnett Geant4 2003 Workshop user presentation 14

Conclusions, IConclusions, I

We are satisfied with the general design and support of Geant4, and do not regret switching to it from Gismo (EGS4+Gheisha)

T. Burnett Geant4 2003 Workshop user presentation 15

Conclusions, IIConclusions, II

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