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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 Media Mid-Sweden University Sundsvall, Sweden.

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monte carlo simulation of the imaging properties of a scintillator coated x ray pixel detector
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
Outline
  • Simulated devices
  • Simulation method
  • Results
  • Conclusions
detector top view
Detector top view

Division 45 mWall thickness 6 m

ccd detector side view
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:0.6m => large damping

Real device also includes a fiber plate in order to avoid direct absorption in the CCD

diode detector side view
Diode detector, side view
  • Two wall designssimulated:
    • 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
Simulation method

Based on 3 MC simulations:

  • X-ray absorption
    • MCNP
  • Light transport
    • In-house ray-tracing code
  • Complete detector
    • Small special program for each detector type
example of x ray energy absorption data
Example of X-ray energyabsorption data

Absorbed energy in 15-20 keV range238 m deep pore, walls: 2 x (1 m SiO2 + 2 m Si)

CsI

Si

example of light transport data
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)

snr csi ccd light detector
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
Fixed pattern image due to pore defects

Defects as in previous picture

Compensated with fixed-pattern noise correction, which is considered in SNR calculation

snr gadox ccd light detector
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 gadox diode detector
SNR, Gadox - diode detector

Gadox is poor for diode on 5 surfaces due to relatively low light emission

thickness and contribution
Thickness and contribution

b)

a)

X-ray dose=25 mR diodes on 5 surfaces

  • SNR for different thicknesses of CsI diode detector
  • SNR for signal from X-ray direct absorption in Si diode and indirect CsI – light – light absorption in diode, 238 m thickness
charge transport issues for diode detector
Charge transport issues fordiode detector
  • 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
slide16

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 absorption
    • High 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
slide17

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