[Gate-users] Low electron energies (<1MeV)

Sat Aug 25 01:21:35 CEST 2012

Hello,

I have been trying to simulate a (non-medical) detector in GATE, where I
want to detect the electrons that are scattered out of a phosphor screen
(after hit by gammas). The results I am getting are not what I expect
however, which prompted me to look deeper into how electrons are treated by
the code. I set the default value for vacuum (d=0.000000001 g/cm3, or 1
ng/cm3), and placed an electron pencil beam about 10 mm from my detector.
The detector is a silicon detector (a CCD), with 10.5-um size pixels. Even
though I am only 10 mm away from my detector, only ~70% of the electrons
make it to the detector. If I place the beam closer -- 1 mm -- this ratio
does not improve. In fact, if I place the beam 1 km away, I still detect
roughly 70% of the electrons.

The second problem I noticed is that even for a very thin detector, the
electron scattering angle is very wide. (The angular deviation is measured
with a second silicon CCD, located 1 mm behind the first CCD, and the
scatter angle is measured as the difference between the electron beam's
initial direction and the angle the electrons create between their
interactions between the first and second CCD.). I tried increasing the
electron energy from 400keV up to several 10's of MeV, and notice a very
sudden change at 20MeV, where the electron scattering angle goes from
uniformly distributed to a very narrow angle. I made my detector very thin
(1 um) and expect to see a small angular deviation of the electrons, but
even for this unphysical thin detector, the angular distribution is very
wide (uniform?). These simulations were performed with 400keV and 1.5MeV
electrons.

I am using Gate v6.1, with Geant4.9.4.p04 on a Mac (3.4GHz Intel Core i7,
OS 10.7.4).

What is a good value to chose for "vacuum"?
Is there an better physical model than the "Penelope" model for this kind
of simulation?

My code is listed below, and I will be happy to send my figures to anyone
interested in trying to figure out what is wrong. Thanks,

Martin

##  V I S U A L I Z A T I O N
#/vis/open OGLSX
#/vis/viewer/reset
#/vis/viewer/set/viewpointThetaPhi 60 30
#/vis/viewer/zoom 50
#/vis/viewer/set/style surface
#/vis/drawVolume
#/tracking/storeTrajectory 1
#/vis/scene/endOfEventAction accumulate 10
#/vis/viewer/update
#/gate/output/verbose 2

# M A T E R I A L   D A T A B A S E
/gate/geometry/setMaterialDatabase ../GateMaterials.db

# W O R L D
/gate/world/geometry/setXLength 0.50 m
/gate/world/geometry/setYLength 0.50 m
/gate/world/geometry/setZLength 10.00 m
/gate/world/setMaterial Vacuum
/gate/world/vis/forceWireframe
/gate/world/vis/setColor white

# S C A N N E R
/gate/world/daughters/name scanner
/gate/world/daughters/insert box
/gate/scanner/geometry/setXLength 5.25 mm
/gate/scanner/geometry/setYLength 5.25 mm
/gate/scanner/geometry/setZLength 2.0 cm
/gate/scanner/setMaterial Vacuum
/gate/scanner/vis/forceWireframe
/gate/scanner/vis/setColor white
/gate/scanner/placement/setTranslation 0.00525 0.00525 0.0 mm
#/gate/scanner/placement/setRotationAxis 0 0 1
#/gate/scanner/placement/setRotationAngle 0 deg

# C C D
/gate/scanner/daughters/name CCD
/gate/scanner/daughters/insert box
/gate/CCD/geometry/setXLength 5.25 mm
/gate/CCD/geometry/setYLength 5.25 mm
/gate/CCD/geometry/setZLength 0.001 mm
/gate/CCD/setMaterial Silicon
/gate/CCD/vis/forceWireframe
/gate/CCD/vis/setColor yellow

# C C D   P I X E L
/gate/CCD/daughters/name pixel
/gate/CCD/daughters/insert box
/gate/pixel/geometry/setXLength 0.010499 mm
/gate/pixel/geometry/setYLength 0.010499 mm
/gate/pixel/geometry/setZLength 0.001 mm
/gate/pixel/setMaterial Silicon
/gate/pixel/vis/setColor red

# R E P E A T   P I X E L
/gate/pixel/repeaters/insert cubicArray
/gate/pixel/cubicArray/setRepeatNumberX 500
/gate/pixel/cubicArray/setRepeatNumberY 500
/gate/pixel/cubicArray/setRepeatNumberZ   1
/gate/pixel/cubicArray/setRepeatVector 0.0105 0.0105 0. mm
/gate/pixel/cubicArray/autoCenter true

# R E P E A T   C C D
/gate/CCD/repeaters/insert genericRepeater
/gate/CCD/genericRepeater/setPlacementsFilename myEvolume.placement
/gate/CCD/genericRepeater/useRelativeTranslation 0

# ATTACH SYSTEM
/gate/systems/scanner/level1/attach CCD
/gate/systems/scanner/level2/attach pixel

# ATTACH LAYER
/gate/pixel/attachCrystalSD

# D I G I T I Z E R
/gate/digitizer/Singles/insert adder
/gate/digitizer/Singles/insert readout

# P H Y S I C S
/gate/physics/addProcess Compton gamma
/gate/physics/processes/Compton/setModel PenelopeModel
/gate/physics/addProcess PhotoElectric gamma
/gate/physics/processes/PhotoElectric/setModel PenelopeModel
/gate/physics/addProcess GammaConversion
/gate/physics/processes/GammaConversion/setModel PenelopeModel
/gate/physics/addProcess ElectronIonisation e-
/gate/physics/processes/ElectronIonisation/setModel PenelopeModel e-
/gate/physics/addProcess ElectronIonisation e+
/gate/physics/processes/ElectronIonisation/setModel PenelopeModel e+
/gate/physics/addProcess PositronAnnihilation  e+
# No need for model selection
/gate/physics/addProcess Bremsstrahlung
/gate/physics/processes/Bremsstrahlung/setModel PenelopeModel e-
/gate/physics/processes/Bremsstrahlung/setModel PenelopeModel e+
#/gate/physics/addProcess SingleScattering e-
/gate/physics/addProcess eMultipleScattering e-
/gate/physics/addProcess eMultipleScattering e+
/gate/physics/Gamma/SetCutInRegion      world 1.0 mm
/gate/physics/Electron/SetCutInRegion   world 0.01 mm
/gate/physics/Positron/SetCutInRegion   world 0.01 mm
#/gate/physics/SetMaxStepSizeInRegion CCD 10 nm

#    I N I T I A L I Z E
/gate/run/initialize
#/geometry/test/recursive_test

# E L E C T R O N   B E A M
/gate/source/addSource electron_beam PencilBeam
/gate/source/electron_beam/setParticleType e-
/gate/source/electron_beam/setEnergy 20 MeV
/gate/source/electron_beam/setSigmaEnergy 0.001 MeV
/gate/source/electron_beam/setPosition 0. 0. -1. mm
/gate/source/electron_beam/setSigmaX 0.000001 mm
/gate/source/electron_beam/setSigmaY 0.000001 mm
# Angular distribution:
/gate/source/electron_beam/setSigmaTheta 0.000001 mrad
/gate/source/electron_beam/setSigmaPhi 0.000001 mrad
/gate/source/electron_beam/setEllipseXThetaEmittance 0.00000000000063
mm*mrad
/gate/source/electron_beam/setEllipseXThetaRotationNorm negative
/gate/source/electron_beam/setEllipseYPhiEmittance 0.00000000000063 mm*mrad
/gate/source/electron_beam/setEllipseYPhiRotationNorm negative
#/gate/source/electron_beam/visualize 10 red 3 #[count] [color] [size]
#/gate/source/electron_beam/setRotationAxis 1 0 0
#/gate/source/electron_beam/setRotationAngle 90 deg
/gate/application/setTotalNumberOfPrimaries 20000

# DATA OUTPUT - ASCII
/gate/output/ascii/enable
/gate/output/ascii/setFileName E_2L_dot_400keV_1um_
/gate/output/ascii/setOutFileHitsFlag 1
/gate/output/ascii/setOutFileSinglesFlag 1
#/gate/output/ascii/setOutFileCoincidencesFlag 0

# E X P E R I M E N T
#/gate/application/setTimeSlice    1.  s
#/gate/application/setTimeStart    0.  s
#/gate/application/setTimeStop    1.  s
/gate/application/startDAQ