Transcript Fixed Pattern Signal fluctuations in Si pixel detectors E. Heijne, B. Mikulec

Fixed Pattern Signal fluctuations
in Si pixel detectors
L. Tlustos, D. Davidson, M. Campbell,
E. Heijne, B. Mikulec
September 2002
Lukas Tlustos, IWORID2002
Outline
• Flatfield studies with Medipix1 chip and
underdepleted and depleted standard high
resistivity Si p+ on n sensors
• “Charge sharing”
September 2002
Lukas Tlustos, IWORID2002
Flatfield Correction
• Additive correction
img corr  img  img dark
img dark  0 ...due to discrimina tor in photon counting
• Multiplicative correction
img corr  img  corr
corr (i, j ) 
September 2002
mean counts
 flood field
Lukas Tlustos, IWORID2002
Motivation
S
N
SNR  

N
SNR
• 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
September 2002
Lukas Tlustos, IWORID2002
Flatfield Measurements
Large number of flatfield acquisitions (150-1000)
o Count rate ~10kHz/pixel
o accumulated total counts ~ 2.5 106 counts/pixel
with
o 3 Medipix1 assemblies
o Detector bias ranging from 4V-128V
Using
o Mo tube, 30um Mo filter, 20keV, 28kV
o W tube, 125um Al filter, 1cm PMMA, 20kV, 30keV
September 2002
Lukas 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
September 2002
Lukas Tlustos, IWORID2002
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
September 2002
Lukas 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
September 2002
Lukas Tlustos, IWORID2002
Results 10C3_G8
Single detached pixel
September 2002
Lukas 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
September 2002
Lukas Tlustos, IWORID2002
Results 11B6_5J
September 2002
Lukas Tlustos, IWORID2002
10C3_G8 map 4V corrected
September 2002
Lukas Tlustos, IWORID2002
10C3_G8 map 8V corrected
September 2002
Lukas Tlustos, IWORID2002
10C3_G8 map 12V corrected
September 2002
Lukas Tlustos, IWORID2002
10C3_G8 map 16V corrected
September 2002
Lukas Tlustos, IWORID2002
10C3_G8 map 24V corrected
September 2002
Lukas Tlustos, IWORID2002
10C3_G8 map 32V corrected
September 2002
Lukas Tlustos, IWORID2002
September 2002
Lukas Tlustos, IWORID2002
11B6_5J map 12V corrected
September 2002
Lukas Tlustos, IWORID2002
11B6_5J map 16V corrected
September 2002
Lukas Tlustos, IWORID2002
11B6_5J map 24V corrected
September 2002
Lukas Tlustos, IWORID2002
11B6_5J map 32V corrected
September 2002
Lukas Tlustos, IWORID2002
11B6_5J map 48V corrected
September 2002
Lukas Tlustos, IWORID2002
11B6_5J map 64V corrected
September 2002
Lukas Tlustos, IWORID2002
September 2002
Lukas Tlustos, IWORID2002
10C3_G8 map profile 4-16V
3x3 mean filter
Raw data
September 2002
Lukas Tlustos, IWORID2002
10C3_G8 map profile 4-32V
3x3 mean filter
Raw data
September 2002
Lukas Tlustos, IWORID2002
10C3_G8 map profile 32-128V
3x3 mean filter
Raw data
September 2002
Lukas Tlustos, IWORID2002
11B6_5J map profile 4-32V
3x3 mean filter
Raw data
September 2002
Lukas Tlustos, IWORID2002
11B6_5J map profile 32-100V
3x3 mean filter
Raw data
September 2002
Lukas Tlustos, IWORID2002
September 2002
Lukas 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
September 2002
Lukas Tlustos, IWORID2002
Detached Pixel 10C3_G8
September 2002
Lukas Tlustos, IWORID2002
Detached Pixel 10C3_G8
count disconnected pixel - 10C3_G8 - Mo 20kV
6
• Count decreases with
detector bias
• Generated charge
collected by adjacent
pixels
count [%]
Normalized
normalized count [%]
5
4
3
2
1
0
0
20
40
60
80
V bias [V]
100
120
140
Vbias [V]
September 2002
Lukas Tlustos, IWORID2002
Detached Pixel 10C3_G8
weight
• Sum of map weights
of the 4 direct
neighboring pixels < 5
• Field not strong
enough to redistribute
all the chare generated
Vbias [V]
September 2002
Lukas 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
September 2002
Lukas Tlustos, IWORID2002