Monte Carlo simulation of the imaging properties of a scintillator- coated X-ray pixel detector M....

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