Fixed Pattern Signal fluctuations in Si pixel detectors

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Fixed Pattern Signal fluctuations in Si pixel
detectors

• L. Tlustos, D. Davidson, M. Campbell, E.
  Heijne, B. Mikulec

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Outline

• Flatfield studies with Medipix1 chip and
  underdepleted and depleted standard high
  resistivity Si p on n sensors
• Charge sharing

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Flatfield Correction

• Additive correction

• Multiplicative correction

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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

SNR
theory corrected uncorrected
Normalized acquisition time
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Flatfield Measurements

• Large number of flatfield acquisitions (150-1000)
• Count rate 10kHz/pixel
• accumulated total counts 2.5 106 counts/pixel
• with
• 3 Medipix1 assemblies
• Detector bias ranging from 4V-128V
• Using
• Mo tube, 30um Mo filter, 20keV, 28kV
• W tube, 125um Al filter, 1cm PMMA, 20kV, 30keV

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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
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Number of counts - Vbias

• Plateau of count rate is a measure for depletion
  voltage
• Normalized number of counts increase with energy
  content of the beam ? charge sharing

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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

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Results 10C3_G8
Single detached pixel
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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

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Results 11B6_5J
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10C3_G8 map 4V corrected
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10C3_G8 map 8V corrected
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10C3_G8 map 12V corrected
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10C3_G8 map 16V corrected
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10C3_G8 map 24V corrected
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10C3_G8 map 32V corrected
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11B6_5J map 12V corrected
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11B6_5J map 16V corrected
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11B6_5J map 24V corrected
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11B6_5J map 32V corrected
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11B6_5J map 48V corrected
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11B6_5J map 64V corrected
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10C3_G8 map profile 4-16V
3x3 mean filter
Raw data
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10C3_G8 map profile 4-32V
3x3 mean filter
Raw data
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10C3_G8 map profile 32-128V
3x3 mean filter
Raw data
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11B6_5J map profile 4-32V
3x3 mean filter
Raw data
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11B6_5J map profile 32-100V
3x3 mean filter
Raw data
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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

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Detached Pixel 10C3_G8
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Detached Pixel 10C3_G8

• Count decreases with detector bias
• Generated charge collected by adjacent pixels

Normalized count
Vbias V
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Detached Pixel 10C3_G8

• Sum of map weights of the 4 direct neighboring
  pixels lt 5
• Field not strong enough to redistribute all the
  chare generated

weight
Vbias V
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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