monte carlo simulation of the imaging properties of a scintillator- coated x-ray pixel detector m....
TRANSCRIPT
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
Outline
• Simulated devices
• Simulation method
• Results
• Conclusions
Detector top view
Division 45 mWall thickness 6 mScintillator
SiO
Si
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
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
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
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
10
200
0.5
1
1.5
2
2.5
x 103
y mmm
05
1015
20 0
10
200
0.5
1
1.5
2
2.5
x 103
y mmm
0100
200300
0
10
20
308
10
12
14
16
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)
0 1 2 3 4 5 6 7 8 90
20
40
60
80
100
120
1/2
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
Fixed pattern image due to pore defects
Defects as in previous picture
Compensated with fixed-pattern noise correction, which is considered in SNR calculation
0 1 2 3 4 5 6 7 8 90
20
40
60
80
100
120
1/2
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
SNR, CsI - diode detector
0 1 2 3 4 5 6 7 8 90
20
40
60
80
100
120
1/2
Diode on 5 surfaces, 2mm thickDiode on 5 surfaces, 1mm thickDiode on bottom surface, 2mm thickDiode on bottom surface, 1mm thick
0 1 2 3 4 5 6 7 8 90
20
40
60
80
100
120
1/2
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
50 100 2380
10
20
30
40
50
60
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
• 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
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
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