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Monte Carlo simulation of the imaging properties of a scintillator-
coated X-ray pixel detector
M. Hjelm*
B. Norlin
H-E. Nilsson
C. Fröjdh
X. Badel
Department of Information Technology and MediaMid-Sweden UniversitySundsvall, Sweden
Department of Microelectronics and Information TechnologyKTH, Stockholm, Sweden
*Also affiliated to KTH
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Outline
• Simulated devices
• Simulation method
• Results
• Conclusions
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Detector top view
Division 45 mWall thickness 6 mScintillator
SiO
Si
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CCD detector, side view
• No transmission of X-rays into Si detector is assumed
• Wall: 2 x (1 m SiO2 +2 m Si)
• Poly-Si layer thickness:0.6m => large damping
Real device also includes a fiber plate in order to avoid direct absorption in the CCD
Scintillator
SiO
Si
Poly-Si
SiO
Si
Si
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Diode detector, side view• Two wall designs
simulated:– 2 x (1 m SiO2 + 2 m Si)– 2 x (2 m SiO2 + 1 m Si)
• Two layouts of diodes simulated:– On sides and bottom– On bottom
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Simulation method
Based on 3 MC simulations:
1. X-ray absorption– MCNP
2. Light transport– In-house ray-tracing code
3. Complete detector– Small special program for
each detector type
Pixel detector MC25x25pixels
X-raysMCNP simulation3x3 pixels
LightRay-tracing MC3x3 pixels
Energy ab-sorption data
Light trans-mission data
Flood expo-sure image
Post-processingwith Matlab
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Example of X-ray energyabsorption data
CsI Si
Absorbed energy in 15-20 keV range238 m deep pore, walls: 2 x (1 m SiO2 + 2 m Si)
05
1015
20 0
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0.5
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x 103
y mmm
05
1015
20 0
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0.5
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2.5
x 103
y mmm
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DepthmmDiag. distancemm
Example of light transport data
Light absorbed in 2 m bottom diodeCsI pore, 238 m deep pore, walls: 2 x (1 m SiO2 + 2 m Si)
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0 1 2 3 4 5 6 7 8 90
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Sim. depth=238mm with pore defectsSim. depth=50mmSim. depth=100mmSim. depth=238mmExper. Badel et al.
SNR, CsI - CCD light detector
16*N defects with a damping of 5 % each are randomly distributed in the scintillator pores
N=number of pixels=625
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Fixed pattern image due to pore defects
Defects as in previous picture
Compensated with fixed-pattern noise correction, which is considered in SNR calculation
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0 1 2 3 4 5 6 7 8 90
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Sim. depth=238mm with pore defectsSim. depth=50mmSim. depth=100mmSim. depth=238mmExper. Badel et al. CsI
SNR, Gadox - CCD light detector
Defects as in previous two pictures
Gadox compares well to CsI due to longer wave length of light, which better passes the poly-Si layer
This is very much dependent on the charac-teristics of the poly-Si layer
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SNR, CsI - diode detector
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Diode on 5 surfaces, 2mm thickDiode on 5 surfaces, 1mm thickDiode on bottom surface, 2mm thickDiode on bottom surface, 1mm thick
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Diode on 5 surfaces, 2mm thickDiode on 5 surfaces, 1mm thickDiode on bottom surface, 2mm thickDiode on bottom surface, 1mm thick
SNR, Gadox - diode detector
Gadox is poor for diode on 5 surfaces due to relatively low light emission
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50 100 2380
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Scintillator thickness
a)
Si CsIContribution
b)
Thickness and contribution
a) SNR for different thicknesses of CsI diode detector
b) SNR for signal from X-ray direct absorption in Si diode and indirect CsI – light – light absorption in diode, 238 m thickness
a) b)
X-ray dose=25 mR diodes on 5 surfaces
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• The walls should not be completely depleted to permit collection of charge– Depletion controlled with bias
• Charge collection from walls can be switched off with high bias
• To suppress direct absorption from bottom:– Important to select suitable diffusion length
• Limiting lifetime and/or mobility in substrate
– Alternatively: thick scintillator with high X-ray absorption
Charge transport issues fordiode detector
L
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Conclusions
• To get high SNR, the signal from direct absorption of X-rays has to be minimized compared to the signal generated from scintillator light absorptionHigh light emission from the scintillator material is
very important for designs with diodes on side surfaces
• From the point of view of SNR:– The designs based on diode light detectors at pore
surfaces are not better than the CCD design
• Diode solutions have other advantages:– higher signal, less damage by high radiation dose
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Conclusions
• For designs with diodes on side surfaces:– Increasing the SiO2 layer thickness leads to less high-
energy electrons emitted from scintillator into silicon
– Should be balanced with less X-ray absorption in a smaller scintillator pore