Image quality and spectroscopic characteristics of different silicon pixel imaging systems M. G....
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Image quality and spectroscopic Image quality and spectroscopic characteristics of different silicon characteristics of different silicon
pixel imaging systemspixel 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 eSezione I.N.F.N., Pisa, Italy
14TH INTERNATIONAL WORKSHOP ON ROOM-TEMPERATURE SEMICONDUCTOR X-RAY AND GAMMA-RAY
DETECTORS ROME 18-21 OCTOBER 2004
OutlineOutline
3 imaging systems based on different thickness Si pixel detectors were compared:
Spectroscopic characteristics109Cd source
Imaging qualitycontrastsignal to noise ratio (SNR)
Spatial resolutionmodulation transfer function (MTF)
Si detector
p+ side
Imaging systemImaging system
ITC-Irst DetectorSi <111>300-800 m thickpixel 170 x 170 m2
p+ side 150x150 m2
64 x 64 chs1.2 cm2 area
0 200 400 600 800 1000 1200 14000
20
40
60
80
100
with multiguardrings without multiguardrings
elec
tric
pot
entia
l (V
olt)
width (m)
Photon Counting ChipMedipix collaboration
SACMOS 1 m technology
pixel: 170 x 170 m2
64 x 64 channelsarea 1.7 cm2
threshold adjust 3-bit15-bit counter
VTT Bump-bonding:
http://medipix.w
eb.cern.ch/ME
DIP
IX/
http://medipix.w
eb.cern.ch/ME
DIP
IX/
http://medipix.w
eb.cern.ch/ME
DIP
IX/
http://medipix.w
eb.cern.ch/ME
DIP
IX/
Spectroscopic capabilitySpectroscopic capability
109Cd Integral spectra 109Cd Differential spectra
Thickness ratio
Calculated ratio
Experimental ratio
525/300 1.62 1.61
800/525 1.39 1.38
800/300 2.25 2.23
109Cd Integral spectra
Wafers characteristics
Thickness (m) resistivity J (nA/cm2) VDEP (V) V OVER-DEP (V)
300+15 >= 6 K cm 0.5 30 60
525+20 >= 5 K cm 1.2 90 120
800+25 12 ÷ 30 K cm 26 80 200
Detection efficiencies ratioDetection efficiencies ratioVVthth=11keV=11keV
Contrast measurementsContrast measurements
Al thickness 75 m
Air
X-ray (W-anode) settings : 40 kV, 25 mA, 500 ms
air
Alair
N
NNC
Si detector
140
cm
Collimator
X-ray focus
1.5
cm
Al
8 10 12 14 16 18 20 22 242,4
2,6
2,8
3,0
3,2
3,4
3,6
3,8 800m detector
Con
tras
t (%
)
Energy threshold (keV)
ContrastContrast
8 1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4
2 , 8
3 , 0
3 , 2
3 , 4
3 , 6
3 , 8
4 , 0
4 , 2
C (
%)
E n e r g y t h r e s h o l d ( k e V )
0 5 10 15 20 25 30 35 40 450
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
12000
13000
14000
15000 40kV W seen by 800 m seen by 525 m seen by 300 m
Ph
oto
ns
per
(m
A s
mm
2 ) at
750
mm
Energy (keV)
xdEESE
dEESEEE airAl
eC
)()(
)()())()((
1
Al(E) and air(E) are the absorption
coefficients at the energy E (E) is the detector efficiency at the energy E S(E) is the incident spectrum
Signal to Noise RatioSignal to Noise Ratio
22Alair
Alair NNSNR
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
75 m Al
air
5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 50 , 0
0 , 2
0 , 4
0 , 6
0 , 8
1 , 0
No
rm
ali
ze
d c
ou
nts
P i x e l n u m b e r
Spatial resolutionSpatial resolution800 m Vth=11keVSettings : 40 kV, 20 mA, 4000 ms
W Slit :width: 10±1mlength: 5.5±0.1mmthikness: 1.5mm
4 , 5 5 , 0 5 , 5 6 , 0 6 , 50
1 0 0 0
2 0 0 0
3 0 0 0
4 0 0 0
5 0 0 0
6 0 0 0
7 0 0 0
LS
F
p o s i t i o n
5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 50 , 0
0 , 2
0 , 4
0 , 6
0 , 8
1 , 0
No
rm
ali
ze
d c
ou
nts
P i x e l n u m b e r
Counts/row pixel number
experimental finely sampled LSF
2 Boltzman functions
LSFfitted fft MTF
xdxxdxxx ee
AAAy /)(/)(212 00 1
1
1
1)(
MTFMTFNyquist Freq. (2.94 lp/mm)
MTF: 64 %Evaluated aperture 168 m
Detector pitch 170 m
Vth (keV)
800 m aperture
(m)
11 168
15 161
19 155
23 146
Exposure condition: W anode, 40 kV, 40 mA, 630 ms
300 m 525 m 800 m
C=4,8%
C=1,2%
C=2,4%
C=3,7%
25 micron
50 micron
75 micron
100 micron
Image of different Al thickness
ConclusionsConclusions
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 11:00-12:30
width
Electric Field and PotentialElectric Field and Potential
0 200 400 600 800 1000 1200 14000
20
40
60
80
100
with multiguardrings without multiguardrings
elec
tric
po
tent
ial (
Vol
t)
width (m)0 200 400 600 800 1000 1200 1400
0
5000
10000
15000
20000
25000
30000
with multiguardrings without multiguardrings
ele
ctri
c fie
ld (
Vo
lt/cm
)width (m)
Photos of some detailsPhotos of some details
Medipix 1
Medipix 2
Pixel 150 m x 150 m
Pixel 45 m x 45 m
guardring
guardring
multiguardrings
multiguardrings
multiguardrings
Snake pads
Diffusione di caricaCon la diffusione, una carica che impiega untempo t per raggiungere l’elettrodo avrà una fluttuazione sulla posizione cor rms diff
2diff = D*t D alla mobilita’ dei portatori di carica.
Ediff
2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 02
3
4
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
d
iff
us
ion
(
m)
d e t e c t o r t h i c k n e s s ( m )
0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5
0 , 0
0 , 2
0 , 4
0 , 6
0 , 8
1 , 0
4 0 k e V s i m u l a t e d 5 2 5 m
No
rm
ali
ze
d c
ou
nts
E n e r g y [ k e V ]