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Development of Small-Pixel CZT and CdTe
Detectors with Hybrid Pixel-Waveform
Readout System
L.-J. Meng1,2,3
1Department of Nuclear Plasma and Radiological Engineering,
2Department of Bioengineering, and
3Beckman Institute for Advance Science and Technology,
University of Illinois at Urbana-Champaign.
We would like to thank NCI (R21/R33CA004940, R21CA135736-01A1), DOE, Office of Biological and
Environmental Research (DE-FG2-08ER6481) for their generous support for our work.
Table of Contents
• The need for a compact and high resolution gamma ray detector for
nuclear imaging applications.
• Previous efforts on the ERPC detectors.
• Development of small pixel CdTe and CZT detectors for nuclear
medicine applications.
• Motivations for developing the hybrid pixel-waveform (HPWF)
readout system
• Preliminary Results I: waveform-based timing estimation.
• Future outlook.
L. J. Meng, TIPP 2011
Motivation of Our Detector Development Effort
One CZT/CdTe detector architecture that fits manygamma ray applications – such as SPECT and PET for
medical imaging, Coded aperture and Compton cameras for
security and astrophysics applications?
We would like to have a detector that has
Excellent spatial resolution in 3-D (a 100-250μm),
Excellent timing resolution (a few ns),
Excellent energy resolution (a few percent),
Adequate count rate capability (250k per cm2),
Being able to deal with multiple interaction events.
All across a wide energy range 30 keV - several MeV
L. J. Meng, TIPP 2011
A High-Resolution MRI-Compatible SPECT System
Readout PCB
MRI Scanner Bore
Rotary motor
RF coil
CdTe Detector
Collimator
Non-magnetic SPECT
system chassis
Patient bed
1.5 m long arm to allow a
remotely mounted motor
We are developing a MRI-compatible SPECT system with four
heads installed on a rotational gantry.
Two key objectives: (a) demonstrate the capability of achieving
an sub-500 μm SPECT resolution inside MRI scanner (b)
provide a flexible platform for testing different detector and
system designs Left: A 3-D whole-body image of a rat acquired with the 3
T Allegra scanner. Right: T2* relaxation of the tissues.
The images were obtained with a multiecho fast low
angle shot (FLASH) sequence written by Professor Brad
Sutton of UIUC. It resulted in 0.5 mm isotropic resolution
from an 8 minutes whole body scan.
The Siemens Allegra 3 T MRI scanner at BIC
that will be used in the combined SPECT/MRI
System.
NCI, R21/R33CA004940, R21CA135736-01A1.
L. J. Meng, TIPP 2011
A Sub-500 m Resolution PET Insert
A (A) Geometry of a potential 4-panel VP-PET insert
device inside an animal PET scanner.
(B) A potential implementation of the detector
technology proposed in this work.
(C) A prototype PET detector developed for the PET
application.
B
VP-PET insert
Animal
PET scanner
animal bed
B
C
DOE, Office of Biological and Environmental Research
(DE-FG2-08ER6481). PI Y. C. Tai, WashU
L. J. Meng, TIPP 2011
Focal Plane Detector for the EXIST Mission
IRT (1.1m, -30˚C),
• 0.3-1 μm (CCD),
• 0.9-2.2 μm (NIRSPEC).
Launch: Altas V-401 into LEO.
SXI (0.1-10 keV):
• 950 cm2 @ 1.5 keV.
HET (5-600 keV):
• Tungsten mask (7.7 m2),
• CZT detectors (4.5 m2),
• BGO rear shield (4.5 m2).
Supported by NASA, Grant #
08-APRA08-0122
L. J. Meng, TIPP 2011
Energy-Resolved Photon Counting Detector –
Design Concept and Considerations
Basic design target: generic, high performance and flexible.
Hybrid photon detector concept with highly pixelated (pixel size: 350 m) CdTe or
CZT bump-bonded to 2-D readout ASIC – compact, high resolution.
ADC on each channel – all digital output, amplitude, time stamp, pixel address for
each hit.
Flexible sparse logic – allowing signals from adjacent pixels to be summed together.
The proposed ERPC detector. (1) CZT crystals of 4.4cm 4.5 cm 2-4
mm in size, (2) ERPC ASICs, (3) Readout PCBs, (4) indium bump-
bonding between CZT detector to the ASIC, (5) wire-bonds between the
ASIC and the PCBs and (6) Cathode signal out.Z. He et al, NIM A380 (1996) 228, NIM A388 (1997) 180.
L. J. Meng, TIPP 2011
Pixel Circuitry
P/H
Threshold
Stop 1
Common Start
To
Re
ad
ou
t Bu
s
Pixel circuitry
Start 1
Cathode
Triggering
Amp
Comparator
=
DAC
Start 2
TDC
Stop 2
Clock in
On periphery board
Anode
pixel
Pixel layout: 350 m x 350 m,
containing 2682 electrical components
Pixel circuitry
On-chip DAC/TDC
Common Start
TDC
Stop
TDC Reading Sig. Amp.
Pe
ak/H
old
Ou
tpu
t
On-chip DAC
Output
L. J. Meng, TIPP 2011
Pixelated CdTe Detectors
A pixelated CdTe detector of 11mm 22mm 1 or 2 mm in size
and having 3264 350 m 350 m pixels.
ERPC detectors with 2 mm thick CdTe detectors will be used in
the prototype system.
Other pixel sizes – 515 um, 700 um read out with the same ASIC?
L. J. Meng, TIPP 2011
The Prototype ERPC Detectors
Detector hybrids
1.1 cm 2.2 cm
Wire-bonding to the
readout PCB
FPGA for controlling the
readout sequence
Copper substrate for
supporting the hybrids
2 mm CdTe detector
A Compact CdTe Detector
(Picture courtesy, Dr. K. Spartiotis, Oy Ajat)
(Picture courtesy, Dr. K. Spartiotis)
L. J. Meng, TIPP 2011
1.1
cm
< 1
cm
2.2 cm
CdZnTe Detectors
We are exploring the use of CZT detectors of 2
mm and 5 mm thicknesses with the ERPC
ASIC (fabricated by Creative Electron Ltd.).
Two different CZT-ASIC bonding techniques
(SnBI bump-bonding and Ag/Cu conductive
epoxy bonding) are under evaluation.
1.9cm1.9cm5mm CZT
detector, cathode side Anode side
2mm thick CZT detector,
cathode side
Anode side
L. J. Meng, TIPP 2011
Next Generation Ultrahigh Resolution CZT/CdTe Detectors
High-speed readout; eSATA interface,
1000fps;
Anode and cathode readout (ERPC
ASIC for anode and NCI ASIC for
cathode),
Relatively compact, width of the
readout PCB is equal to the width of the
CZT/CdTe detectors (4.5 cm), allowing
a compact ring geometry.
A New Digital Readout System for the SPECT/MRI Project
Left: The proposed MRI-compatible ERPC CdTe detector. Right: Schematic of using
the cathode-to-anode ratio to derive the depth-of-interaction information.
CdTe detector,
2.2cm 2.2 cm 2 mm
Digital readout PCB
ERPC
ASICs
4.5 cm4.5 cm
L. J. Meng, TIPP 2011
Energy Resolution
(Upper left) Energy spectrum with events
acquired on all 16384 pixels after correcting
the channel-by-channel variation of gain and
offset.
(Upper right) Experimental setup for
illuminating the detector with a fine pencil-
beam.
Lower right: energy spectra measured on a
single pixel with events at different depths-of-
interaction.
E. R.: ~3 keV
Non-magnetic x-y
linear stages
Support
structure
Tilting stage for
controlling the
angle of incidence
of the gamma ray
beam
Collimator to create
a pencil beam of
100 m width
Detector entrance window
L. J. Meng, TIPP 2011
Problems of Current Small Pixel CZT Detectors
The key problem for further improvement is the degraded charge
collection efficiency associated with CZT or CdTe detectors relying on
small-pixel effect for single polarity charge sensing.
Energy information collected on the anode pixels is not reliable.
DOI information acquired with C/A Ratio will not be accurate.
Timing resolution obtained at anode pixels will be subject to
systematic error – therefore limited to several tens of ns –
questionable for PET applications.
DOI information measured by electron drifting time (as currently
used in the Michigan system) will be limited by the poorer timing
resolution.
Increased system complexity in readout electronics.
L. J. Meng, TIPP 2011
Limitation on CdZnTe Detector Fabrication
Photos of the pixels on the 2 mm, 5 mm CZT detectors and the 1mm CdTe detectors
The anode side of the detectors has square pixels of 350 um pitch and the actual
pixel contacts are 250 um 250 um in size (fabricated by Creative Electron Ltd.)..
Each anode pixel has a thin layer of gold (50 nm thickness) in direct contact with the
CZT crystal and a second layer of nickel of 100 nm on top of the gold layer.
L. J. Meng, TIPP 2011
Problem I: Poor Energy Resolution due to
Charge Sharing Between Small Pixels
50 100 1500
200
400
600
800
1000
1200
Energy reso.:
3.60 kev
50 100 1500
100
200
300
400
500
600
3.51 kev
50 100 1500
50
100
150
200
250
4.86 kev
50 100 1500
50
100
150
200
250
4.0205kev
50 100 1500
100
200
300
400
500
600
700
4.62 kev
50 100 1500
100
200
300
400
500
600
4.91 kev
50 100 1500
20
40
60
80
100
120
6.03 kev
50 100 1500
100
200
300
400
4.94 kev
Center of a pixel
50 100 1500
100
200
300
5.74 kev
50 100 1500
50
100
150
200
6.50 kev
50 100 1500
50
100
150
200
6.50 kev
50 100 1500
20
40
60
80
100
120
8.30 kev
50 um from the
center
100 um from
the center
Center of the gap
Center of a pixel 50 um from the
center
100 um from
the center
Center of the gap
Center of a pixel 50 um from the
center
100 um from
the center
Center of the gap
CZ
T, 2
mm
Cd
te, 2
mm
Cd
te, 1
mm
L. J. Meng, TIPP 2011
A simulated pulse-height spectra with different
pixel size. The detector is 0.75mm CdTe
irradiated by 60keV gamma rays. (Konstantinos
Spartiotis et al, NIM A550, 2005)
Photo of a CdTe detector used with Medipix2
readout chip. Pixel size: 45µm. (Pellegrini at al,
NIM A53, pp361, 2005).
Right: Measured charge-collection efficiency
on a given pixel.
Problem I: Poor Energy Resolution due to
Charge Sharing Between Small Pixels
L. J. Meng, TIPP 2011
0 500 1000 1500-10
-5
0
x 10-3
0 500 1000 1500
0
5
10x 10
-3
CA
R:
0.2
,
Clo
se
to
Ca
tho
de
CA
R:
1.0
,
Clo
se
to
An
od
e
Measured mean waveform from an anode pixel – charge collection time depends on DOI
Comparing events from the same depth and
having the same energy deposition
CAR
An
od
e S
ign
al
Am
plit
ude
(V
)
0 500 1000 1500-8
-6
-4
-2
0
x 10-3
0 500 1000 1500
0
5
10x 10
-3
Box 5
Box 4
Box 3
Box 2
Box 10 500 1000 1500-8
-6
-4
-2
0
x 10-3
0 500 1000 1500
0
5
10x 10
-3
Box 5
Box 4
Box 3
Box 2
Box 1
0 500 1000 1500-8
-6
-4
-2
0
x 10-3
0 500 1000 1500
0
5
10x 10
-3
Box 5
Box 4
Box 3
Box 2
Box 1
Cathode WF
Anode WF
A closer look reveals
that even for events
with the same DOI,
charge collection times
could vary …
Problem II: Poor Timing Resolution
Analogue Triggering on Anode Signals
L. J. Meng, TIPP 2011
Problem III: Difficulties in Getting DOI
Signal from anode pixel No. of electrons collected by the pixel (N)
and its lateral position (x, y).
C/A Interaction depth (z).
N, x, y, z Energy deposition E0 and interaction location.
Z. He et al, NIM A380 (1996) 228, NIM A388 (1997) 180.
L. J. Meng, TIPP 2011
3-D Position Sensitive Detectors Developed by Prof. Zhong He
Time
t2 z2
C a1 a2
t1 z1Triggers
• For multiple interactions, C/A
ratio is no longer sufficient for
determining interaction sites.
• Extra information is provided by
drifting times for each electron
cloud.
• The accuracy of determining
interaction depth is <0.5mm
L. J. Meng, TIPP 2011
Problem III: Difficulties in Getting DOI
Small Pixel CZT Detectors with a Hybrid Pixel-Waveform
Readout System
Amp 2
High-speed
digitizer to
sample the
cathode WF
Pixel address
Energy I
Read
ou
t P
CB
s
• Energy II
• Timing
• DOI
• Photoelectric
or Compton?(1) CZT or CdTe crystals of 4.4cm 4.5 cm 1-5
mm in size, (2) ERPC ASICs, (3) Readout PCBs
for both reading out ERPC ASICs and digitizing
cathode waveforms, (4) indium bump-bonding
between CZT detector to the ASIC, (5) wire-bonds
between the ASIC and the PCBs and (6) Cathode
signal out.
Fig. 1: Gen-II ERPC detectors with HPWF readout system. Left: Design
schematic; Right: 3D rendering of the HPWF-ERPC detector design.
L. J. Meng, TIPP 2011
DOI Measurements with 2 mm Thickness CZT Detectors
53.8ns 54.2ns
855.1ns
1293.1n
s
DOI (from cathode)
1.9125mm
DOI (from cathode)
1.2794mm
852.1ns
39.4ns
DOI 1.26mm
268.6ns
72.2ns
DOI 0.48mm
1099.2n
s
32.8ns
94.8ns
DOI 1.61mm
1384.1n
s
DOI ~0 mm DOI ~2 mm
time (ns) time (ns) time (ns)
time (ns) time (ns) time (ns)
L. J. Meng, TIPP 2011
Theoretical Waveform Models
(Meng, NIM, 2005).
L. J. Meng, TIPP 2011
Results II: Measured Timing Resolution
Measured timing resolution with full energy
events close to the cathode (CAR of ~0.9).
Time (s)
Cou
nts
-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.080
5
10
15
20
25
30
35
FWHM:
~7ns
Timing resolution as a function of CAR with
full energy events and events having
energy deposition >250keV (circles).
CAR
Tim
ing re
so
lutio
n (
s)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
0.022
e > 250keV, measured
e > 250keV, fitted
full-energy, measured
full-energy, fitted
Measured timing resolution, full energy
events and CAR [0.1, 1.0].
Time (s)
Cou
nts
-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.080
50
100
150
200
250full energy evt, all depth, 9.5ns
FWHM:
~9.5ns
-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.080
100
200
300
400
500
600all evt (e>250keV), all depth, 11.6ns
Time (s)
Cou
nts FWHM:
~11.6ns
Timing resolution, energy deposition
greater than 250keV and CAR [0.1, 1.0].
L. J. Meng, TIPP 2011
Results IV: Predicted Timing Resolution
Timing ResolutionCZT, 10 mm
thicknessCZT, 5 mm CZT, 2 mm
Trig. on cathode
(2.25cm2, VBias: 140V/mm)30 ns 1 ~20 ns 2 8-10 ns
Trig. on anode
(11mm2, VBias: 140V/mm)40 ns
WF fitting
(VBias: 140V/mm)10 ns
WF fitting,
(VBias : 500V/mm)~ 7 ns 3 ~ 3 ns ~ 2 ns
WF fitting,
(VBias : 500V/mm)~ 4 ns ~ 2 ns Sub-ns (?)
Measured and estimated timing resolution
1. Experimentally measured.
2. Simply scaled with increasing electric field strength.
3. Simulated using the analytical waveform model.
L. J. Meng, TIPP 2011
Summary
We have developed a relatively generic detector architecture for small pixel
CZT or CdTe detectors.
We have produced and experimentally evaluated CdTe and CZT detectors
of 1 mm to 5 mm thicknesses readout with the ERPC readout system. Both
1 mm and 2 mm thickness detectors have offered reasonable imaging
performance for SPECT applications.
An improved detector pixelation process is needed for CZT detectors to
improve the charge-collection process and therefore their spectroscopy
performance.
To further improve the performance for future small-pixel CZT or CdTe
detectors, we are currently developing a readout circuitry that utilizes both the
anode signals and cathode signal waveforms.
L. J. Meng, TIPP 2011
Many thanks and questions?
L. J. Meng, TIPP 2011