optimization of planar pixel detector. t. habermann planar pixel detectors l w h ground u
DESCRIPTION
T. Habermann Distortion effects a)Surface leakage current (10 – 20 pA) E.L. Hull, R.H.Pell, …NIMA 364(1995) b) Effects of the enclosureTRANSCRIPT
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Optimization of planar pixel detector
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T. Habermann
Planar pixel detectorsPlanar pixel detectors
L
W
H
ground
U
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T. Habermann
Distortion effectsDistortion effects
a) Surface leakage current(10 – 20 pA)
E.L. Hull, R.H.Pell, …NIMA 364(1995) 488-495
b) Effects of the enclosure
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T. Habermann
poissons equation for the potential :
electric field :
boundary conditions :
Electric field calculation in germaniumElectric field calculation in germanium
grad
2
6.16
10854.8
:constant dielectric
120
0
r
r
VmC
316
19
10
106.1
:density charge
mN
Ce
eN
A
A
. )2(0 )1(
constEn
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T. Habermann
applied potential :
U = 2000 - 3000V
20mm
80mm
d
Detector geometry Detector geometry 2-dimensional2-dimensional
pixel (φ = U)
ground (φ = 0)
pixel width :
d = 4 - 16mm
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T. Habermann
Electric field calculationElectric field calculation2-dimensional2-dimensional
2
2
2
2
yx
0)0,( x
2-dimensional poisson equation :
x
y
H
L
),( UHx
0),0( yx 0),(
yLx
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T. Habermann
Finite-Volume-MethodFinite-Volume-Method
The region is divided into N rectangular controlvolumes (CV). The potential isapproximated in the center of this CV’s
→
For every CV we get an equation of the form :aPΦP - awΦW - aNΦN - aEΦE - aSΦS = bP
→
linear system of equations with N variables : AΦ=b
2-d grid
3-d grid
(“Computational Methods for Fluid Dynamics”,J.H.Ferziger,M.Peric)
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T. Habermann
Potential in germaniumPotential in germanium
12x12 grid pointsU0 = 3000VH = 0.02 mL = 0.02 m
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T. Habermann
Potential in germaniumPotential in germanium
50x50 grid pointsU0 = 3000VH = 0.02 mL = 0.02 m
2 pixelwidth : 0.005 mdistance to the edge : 0.0025 m
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T. Habermann
ConvergencyConvergency
L
dxHxy0
i ),( Flux
1-i
1-ii
FluxFluxFlux
ire
Flux in y-direction at the top surfacevs number of grid points in x-direction
(= number of grid points in y-direction)
relative errorvs # of grid points in x-direction
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T. Habermann
outlookoutlook
• further examination of convergency behaviour(→ adjustment of the numerical method for better convergency)
• 3d model • examination of the electric field and reconstruction of the expected
effects :• distortion caused by the enclosure• distortion caused by surface leakage current
• How do the free parameters influence these effects ? (distance between pixels, pixelsize, size of the capsule, ...)
• GOAL : Minimize the electric field distortion by choosing the right parameter values
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T. Habermann
Model of the coupled systemModel of the coupled system
0)( )1( Gv EEnContact conditions :
sGGVV
GV
EEnDDn
)( )( )2(
00
Parameters :distance from the left chamber wallLV= [5 10 15] mm
contact surface charge densityρS= [0.04 ... 10.0] μC/m^3
pixel sizeLP = [2 4 ... 18 20] mm
applied voltage U = [2500 3000 4000 5000] V
(→648 different parameter sets)
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T. Habermann
Finite volumes for the coupled systemFinite volumes for the coupled system
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T. Habermann
Results : Field distortion caused by Results : Field distortion caused by surface charge densitysurface charge density
0s 310 mC
s
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T. Habermann
Results : depth of affected region in Results : depth of affected region in dependence of surface charge densitydependence of surface charge density
pixelsize = 16mmL1 = 15mm
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T. Habermann
Results : different pixel sizesResults : different pixel sizes
LP = 2mm LP = 10mm
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T. Habermann
Results : depth of affected region in Results : depth of affected region in dependence of pixel sizedependence of pixel size
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T. Habermann
Results : depth of affected region in Results : depth of affected region in dependence of applied voltagedependence of applied voltage
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T. Habermann
Results : depletion voltageResults : depletion voltage
electric field propagation for small (2mm) pixelsize and 3000V applied voltage :
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T. Habermann
Results : depletion voltage in Results : depletion voltage in dependence of pixel sizedependence of pixel size
pixel size = 20mm → planar detector
according to Glenn KnollVd = 2177,22 VsimulatedVd = 2176,32 V
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T. Habermann
Results : depletion voltage in Results : depletion voltage in dependence crystal thicknessdependence crystal thickness
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T. Habermann
outlookoutlook
• investigation of the electric field strength inside the germanium in dependence of pixel size, ...
• investigation of the electric field in dependence of the crystal thickness
• 3d model• another solver could be used to decrease memory usage and
calculation time (essential for 3d calculations)• ...