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

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200

0.5

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1.5

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2.5

x 103

y mmm

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1015

20 0

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0.5

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1.5

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2.5

x 103

y mmm

0100

200300

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308

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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)

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

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

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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

SNR, CsI - diode detector

0 1 2 3 4 5 6 7 8 90

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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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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

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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