1. September 2002Lukas Tlustos, IWORID2002 Fixed Pattern Signal fluctuations in Si pixel detectors L. Tlustos, D. Davidson, M. Campbell, E. Heijne, B. Mikulec

2. September 2002Lukas Tlustos, IWORID2002 Outline Flatfield studies with Medipix1 chip and underdepleted and depleted standard high resistivity Si p+ on n sensors “Charge sharing”

3. September 2002Lukas Tlustos, IWORID2002 Flatfield Correction Additive correction Multiplicative correction

4. September 2002Lukas Tlustos, IWORID2002 Motivation Signal to noise ratio SNR is a crucial parameter for image quality and resolution Poisson statistics of the incident beam marks the upper limit of the achievable SNR SNR of Medipix1 approaches Poisson limit when flatfield correction is applied Trying to understand the sources of inhomogeneities in the raw data in order to improve image quality theory corrected uncorrected Normalized acquisition time SNR

5. September 2002Lukas Tlustos, IWORID2002 Flatfield Measurements Large number of flatfield acquisitions (150-1000) oCount rate ~10kHz/pixel oaccumulated total counts ~ 2.5 10 6 counts/pixel with o3 Medipix1 assemblies oDetector bias ranging from 4V-128V Using oMo tube, 30um Mo filter, 20keV, 28kV oW tube, 125um Al filter, 1cm PMMA, 20kV, 30keV

6. September 2002Lukas Tlustos, IWORID2002 Flatfield Measurements Spectra calculated following J M Boone, A E Chavez; Med Phys 23, 1997 J M Boone, T R Fewell, R J Jennings; Med Phys 24, 1997

7. September 2002Lukas Tlustos, IWORID2002 Number of counts - V bias Plateau of count rate is a measure for depletion voltage Normalized number of counts increase with energy content of the beam → charge sharing

8. September 2002Lukas Tlustos, IWORID2002 Results 10C3_G8 Detector bias voltage: 3,4,8,12,16,24,32,48,64,80,100,128V Correction map calculated using 128V detector bias data

9. September 2002Lukas Tlustos, IWORID2002 Results 10C3_G8 Single detached pixel

10. September 2002Lukas Tlustos, IWORID2002 Results 11B6_5J Detector bias voltage: 3,4,8,12,16,24,32,48,64,80,100V Correction map calculated using 100V detector bias data

11. September 2002Lukas Tlustos, IWORID2002 Results 11B6_5J

12. September 2002Lukas Tlustos, IWORID2002 10C3_G8 map 4V corrected

13. September 2002Lukas Tlustos, IWORID2002 10C3_G8 map 8V corrected

14. September 2002Lukas Tlustos, IWORID2002 10C3_G8 map 12V corrected

15. September 2002Lukas Tlustos, IWORID2002 10C3_G8 map 16V corrected

16. September 2002Lukas Tlustos, IWORID2002 10C3_G8 map 24V corrected

17. September 2002Lukas Tlustos, IWORID2002 10C3_G8 map 32V corrected

18. September 2002Lukas Tlustos, IWORID2002

19. September 2002Lukas Tlustos, IWORID2002 11B6_5J map 12V corrected

20. September 2002Lukas Tlustos, IWORID2002 11B6_5J map 16V corrected

21. September 2002Lukas Tlustos, IWORID2002 11B6_5J map 24V corrected

22. September 2002Lukas Tlustos, IWORID2002 11B6_5J map 32V corrected

23. September 2002Lukas Tlustos, IWORID2002 11B6_5J map 48V corrected

24. September 2002Lukas Tlustos, IWORID2002 11B6_5J map 64V corrected

25. September 2002Lukas Tlustos, IWORID2002

26. September 2002Lukas Tlustos, IWORID2002 10C3_G8 map profile 4-16V Raw data 3x3 mean filter

27. September 2002Lukas Tlustos, IWORID2002 10C3_G8 map profile 4-32V Raw data 3x3 mean filter

28. September 2002Lukas Tlustos, IWORID2002 10C3_G8 map profile 32-128V Raw data 3x3 mean filter

29. September 2002Lukas Tlustos, IWORID2002 11B6_5J map profile 4-32V Raw data 3x3 mean filter

30. September 2002Lukas Tlustos, IWORID2002 11B6_5J map profile 32-100V Raw data 3x3 mean filter

31. September 2002Lukas Tlustos, IWORID2002

32. September 2002Lukas Tlustos, IWORID2002 Conclusions (1) Low detector bias: wave pattern clearly visible, ascribed to bulk doping inhomogeneities causing fluctuations of the width of the depletion layer Magnitude of fluctuations decreases with detector bias But: small variations remain even at high bias voltages, may be due to local variations of the pixel geometry

33. September 2002Lukas Tlustos, IWORID2002 Detached Pixel 10C3_G8

34. September 2002Lukas Tlustos, IWORID2002 Detached Pixel 10C3_G8 Count decreases with detector bias Generated charge collected by adjacent pixels Normalized count [%] V bias [V]

35. September 2002Lukas Tlustos, IWORID2002 Detached Pixel 10C3_G8 Sum of map weights of the 4 direct neighboring pixels < 5 Field not strong enough to redistribute all the chare generated V bias [V] weight

36. September 2002Lukas Tlustos, IWORID2002 Conclusions Fixed pattern fluctuations are consistent with bulk doping inhomogeneities originating in Float Zone technique of crystal growth Inhomogeneities can influence the charge collection properties of sensor Fixed pattern fluctuations remain even in overdepletion and affect the obtained resolution in imaging applications → correction is necessary Implications also to optimization of resolution in particle tracking Effects could also be used to study material properties