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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 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

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  1. 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

  2. Outline • Simulated devices • Simulation method • Results • Conclusions

  3. Detector top view Division 45 mWall thickness 6 m

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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)

  9. 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

  10. Fixed pattern image due to pore defects Defects as in previous picture Compensated with fixed-pattern noise correction, which is considered in SNR calculation

  11. 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

  12. SNR, CsI - diode detector

  13. SNR, Gadox - diode detector Gadox is poor for diode on 5 surfaces due to relatively low light emission

  14. 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

  15. 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

  16. 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

  17. 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

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