Development of Small-Pixel CZT and CdTe Detectors with Hybrid … · 2018. 11. 13. · Development...

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


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.


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


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


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.


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


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?


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)


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


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


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


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.


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.


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


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


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


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.


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


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.


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)


Theoretical Waveform Models

(Meng, NIM, 2005).


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].


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.


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.


Many thanks and questions?


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Development of Small-Pixel CZT and CdTe Detectors with Hybrid … · 2018. 11. 13. · Development...

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