Image quality and spectroscopic characteristics of different silicon pixel imaging systems

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Image quality and spectroscopic characteristics
of different silicon pixel imaging systems

• M. G. Bisogni, D.Bulajic, M. Boscardin, G. F.
  Dalla Betta, P. Delogu, M. E. Fantacci, M.
  Novelli, C. Piemonte, M. Quattrocchi, V. Rosso,
  A. Stefanini and N. Zorzi

Universita degli Studi e Sezione I.N.F.N.,
Pisa, Italy

• valeria.rosso_at_pi.infn.it

14TH INTERNATIONAL WORKSHOP ON ROOM-TEMPERATURE
SEMICONDUCTOR X-RAY AND GAMMA-RAY DETECTORS ROME
18-21 OCTOBER 2004
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Outline

• 3 imaging systems based on different thickness
  Si pixel detectors were compared
• Spectroscopic characteristics
• 109Cd source
• Imaging quality
• contrast
• signal to noise ratio (SNR)
• Spatial resolution
• modulation transfer function (MTF)

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Imaging system
Photon Counting Chip Medipix collaboration

• ITC-Irst Detector
• Si lt111gt
• 300-800 mm thick
• pixel 170 x 170 mm2
• p side 150x150 mm2
• 64 x 64 chs
• 1.2 cm2 area

http//medipix.web.cern.ch/MEDIPIX/
SACMOS 1 mm technology pixel 170 x 170 mm2 64
x 64 channels area 1.7 cm2 threshold adjust
3-bit 15-bit counter
VTT Bump-bonding
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Spectroscopic capability
109Cd Integral spectra
109Cd Differential spectra
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109Cd Integral spectra
Detection efficiencies ratio Vth11keV
Thickness ratio Calculated ratio Experimental ratio
525/300 1.62 1.61
800/525 1.39 1.38
800/300 2.25 2.23
Wafers characteristics   Wafers characteristics   Wafers characteristics   Wafers characteristics   Wafers characteristics  
Thickness (mm) resistivity J (nA/cm2) VDEP (V) V OVER-DEP (V)
30015 gt 6 KW cm 0.5 30 60
52520 gt 5 KW cm 1.2 90 120
80025 12 30 KW cm 26 80 200
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Contrast measurements
X-ray (W-anode) settings 40 kV, 25 mA, 500 ms
Al thickness 75 mm
Air
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Contrast
mAl(E) and mair(E) are the absorption
coefficients at the energy E h(E) is the
detector efficiency at the energy E S(E) is the
incident spectrum
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Signal to Noise Ratio
75 mm Al air
Thickness ratio Calculated ratio Experimental SNR ratio
525/300 1.24 1.25
800/300 1.41 1.42
800/525 1.14 1.14
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Spatial resolution
800 mm Vth11keV Settings 40 kV, 20 mA,
4000 ms
W Slit width 101?m length 5.50.1mm thikness
1.5mm
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Counts/row pixel number experimental finely
sampled LSF 2 Boltzman functions   LSFfitted
fft MTF
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MTF
Nyquist Freq. (2.94 lp/mm) MTF 64 Evaluated
aperture 168 mm Detector pitch 170 mm
Vth (keV) 800 mm aperture (mm)
11 168
15 161
19 155
23 146
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Image of different Al thickness
C4,8
100 micron
75 micron
C3,7
50 micron
C2,4
25 micron
C1,2
300 mm
525 mm
800 mm
Exposure condition W anode, 40 kV, 40 mA, 630 ms
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Conclusions

• Increasing the detector thickness
• increases the detection efficiency
• SNR increases
• contrast decrease as expected
• spatial resolution unchanges
• More information on a PCC based digital
  mammographic system poster session R11-51
  Thursday 1100-1230

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Electric Field and Potential
width
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Photos of some details
Pixel 150 mm x 150 mm
multiguardrings
Medipix 1
guardring
multiguardrings
Pixel 45 mm x 45 mm
guardring
Medipix 2
Snake pads
multiguardrings
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Diffusione di carica Con la diffusione, una
carica che impiega un tempo t per raggiungere
lelettrodo avrà una fluttuazione sulla posizione
cor rms sdiff s2diff Dt D? alla mobilita
dei portatori di carica.
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