The Gas Electron Multiplier (GEM):

A New Detector for Scanned Projection

Radiography

by

Mei Li

B. Sc. (Shandong University. 199 1 )

M. Eng. (China Institute of Atornic Energy. 1998)

A thesis submitted to

the Faculty of Graduate Studies and Research

in partial fulfillment of

the requirements for the degree of

Master of Science

Ottawa-Carleton Institute for Physics

Department of Physics

Carleton University

Ottawa, Ontario

July 20,2000

O copyright 2000, Mei Li

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Abstract

A prototype scmned projection radiography (SPR) system has been constmcted

using ti high-pressure photon counting detector filled with a Xe:CH4 güs mixture. The

dctcctor combines a Gas Electron Multiplier (GEM) and a Gas Microstrip Detrctor

(GMDi. The enrrgy deposited by each incident photon can be rneasured when photon

counting is donc. A GMD imager using Xe:CH4 w u previously buiit in our Iab. and

showcd very promising spatial resolution and energy resolution. In order to irnprovr the

clcctriçal stability of the GMD imager. the GEM is coupled to the GMD as a pre-

ümplification element. The GEM is büsically a foi1 perforated by many high precision

holcs. Electron amplification happens at the crnters of these holes. With the gain

contribution from the GEiM. the GMD gain and operating voltage ciin be decreased.

rhcrcby improving its reliability.

The stable operütion of the GEWGIMD hybrid detector from 1-4 atm hris hem

denionstrateci. The cathode voltage on the GMD is significantly lower than that which

was applied in the GMD imager üt a given gas gain. The modulation transfrr function

wns meiisurrd at I-4 atm for 10-50 kV x rays. Compared with the GMD imager. the

limiting resolution at 4 atm increases from 14.0 to 17.8 lp mm-' for 13 kV. increûlies from

6.1 to 7.0 lp mm" for 30 kV. and increases from 7.0 to 11.9 ip mm" for 50 kV. At 4 atm.

thc energy resolution is 10% for 17.7 keV and 7.9% for 59.6 keV. The systrm is more

robust thün the GMD imager without degndation of the spatial resolution.

Acknowledgements

As an international student. I gained really appreciable experiencr for the past two

ycars' study in Carleton. Ianguage. physics and life. Without lots of kind help from those

people who would be kept in my minci forever. none of thcse would be possible. H m 1

ulould like to take this chance to give them my sincere appreciation:

1 woiild like to thank my personal sponsor. Prof. Kenneth A. Rahn. With his

pcncroiis financial aid. 1 could get this opponunity to experience a totiilly ncw life. to get

to know myself and to improve myself.

I would like to thlink my two supervisors. Prof. Paul Johns and Dr. ~Madhu Dixit.

Paul met me at the airport and Irt me live with his family whcn I got Ottawa from China.

whiçh helped me a lot to stÿrt rny ncw lire here. His excellent lectures. patient

esplnnntion to the questions. broad and bright knowlrdge in the medical physics activate

my intrrest in the mrdical imaging. Madhu shares with me his valuablr enperience in the

nucleür instrumentation and kindly helped me survive when 1 had financial problem.

which allows me to finish the reseiirch work for my master study.

1 would like to thank Jacqur Dubeau. who helprd me get familiar with the lab and

the experiment. shared with me his idcas. experiences and his humor: Ernie Neuhrirner.

who helpcd troubleshoot the amplifier board and solve the oscillation problem: Philippe

Grnvellr. who not only gave me lots of trchnical support. but also taught me English and

Canadian life: Bill Jack, Wade Hong. and Jim Carleton. who helped me maintain rny

compiiting facilities.

Finally. 1 would like to thmk my husband. Zhanrong. thank for his love. support.

patience. and encouragement: T m sure you'll make it!"

Table of Contents

Abstract

Acknowled, ~ements

Table of Contents

List of Abbreviritions

List of Tables

List of Figures

Chapter 1: Introduction

1 . 1 Conventionai X-ray Imaging

1 .L Digital Rridiog'aphy

1 2 . 1 Energy tntegration Mode Detectors

1.2.1.1 Direct Conversion Detectors

1 2 . 1 2 indirect Conversion Detectors

1 2 . 2 Photon-Counting Mode Detectors

1.3 Scannrd Projection Radiogr~phy (SPR)

1.4 Previous Work: Gas Microstrip Detector (GMD) Imager

1 -5 Motivation for This Project

1.6 Thesis Ovemiew

Chapter 2: Theory

2.1 GEM Geometry

2.3. Oprr~ting Principle of the GEM

2.2.1 Electric Field in GEM Holes

2.2.2 Ga Avalanche

2.2.3 Signal Development in the GEMIGMD Hybnd

7.2.4 Opt icallElectricd Transparency of the GEM

7.23 Collection Efficiency of the GEM

2.2.6 GEM Gain

2.3 Optimization of the GEM

7.1 Charge Drift and Diffusion

2.5 Spatial Resolution

2.6 Enerzy Rrsolution

7.7 Dçtector Efficiency

Chapter 3: Prototype Imaging System with GEWGMD

3.1 The Imaging System with GEMIGMD Hybrid Detector

3.1.1 Geometry

3.1.2 GEM Foil Fabrication

3.1.3 GEM Testing Procedure

3.1.4 Assrmbly

3 - 1 3 High-Voltage Application

3.2 Appusitus

3.3 Electronics

3.4 Fil1 Gas

Chapter 4: Experiments and Results

4.1 Noise Reduction

4.2 Gain Measurement

4.7.1 S ystem Crilibration

4-22 Method for Gain Measurement

4.1.3 Resuits 54

4.2.3.1 Detector Gain vs GEM Voltage and GMD Cathode Voltage 54

4.2.3.2 Detector Gain vs Drift Voltage

4.3 Energ Spectra and Energy Resolution

4.3.1 Crilibration Sources

-1.3 2 Energy Resolution

4.3.3 S pectra used for Spatial Resolution Measurement

4.3.4 Sprctra Measured with QPAO2

1.4 Spatial Resolution

4.4.1 Modulation Transfer Function ~Me~surement

4.4.2 Rssults

4.4.2.1 Modulation Transfer Function

4.4.2.2 Test Pattern Images

4 Imügrs of Biologicül Specirnen

Chapter 5: Conclusions and Future Work

5.1 Conclusions

5.2 Summary of the Operational Felttures of the GEM

5.2.1 Advrintages

5 2 . 2 Disadvantiiges

5.3 Suggestions For the Shon Term Future

5.4 Some Options for the Long Term Future

5.5 Applications

Appendix A NIodeling the GEWGMD Hybrid detector

Appendix B Gas Gain Fluctuation

References

vii

List of Ab breviations

ADC

CCD

CERN

DQE

ES F

FFT

FOV

m'HM

GEM

G bI D

LBNL

Ip m n i '

LS F

kl c A

bl DC

MSGC

kt 1c b1 ICROMEGAS

MTF

MWPC

Opamp

PCB

PM MA

SDRD

S S R

SPR

TFT

analog-to-digitai converter

charge coupled device

European Organization for Nuclear Research

detrctive quantum rfficiency

Edge Spread Function

Fast Fourier Trmsform

field of view

full-width half maximum

o u eirctron multiplier C

sas microstrip detector

Lawrence Berkeley National Laboratory

linc pairs prr millimeter

line sprrad function

mu 1 t i-c hanne l anal yzer

Micro-Dot rtvrilrinche Chamber

Microstrip Gas Chamber

~Multistrip Ionization Chamber

MICRO-MEsh GAseous Stnicture

modulation transkr tùnction

MultiWire Proportional Chember

operational amplifier

printed circuit board

polymethy 1 mcthacrylate

Siberim Digital Radiographic Device

signal to noisc ratio

scanned projection radiography

thin-film transistor

List of Tables

Conversion field and collection field (at the highest gas gain) applied

for the sain measurement

Ernission Energies (keV) and absolute intensities of ' J ' ~ r n

Enersy Rcsolution of the GEM/GiMD hybrid for Ar ( 1 atm) and Xe

( i-4 atm)

Average Energy (keV) depositcd by x-ray beam in the GEiM/GMD

hybrid using Xr:CH4 iit 2-1 atm

Gas gains uscd Tor the iMTF measuremrnt and imaging

Spatial tkquençy (lp mm*') ai which MTFV) crosscs 0.05

List of Figures

1 . 1 Concept diagram of the GMD in the GMD imager

1.2 Schematic of one GEM amplification channel

1.3 Schematic of one micro-dot ce11

Schematic of the MICROMEGAS detector

A close view of ü GEM foi!

The orientation of the GEM/GMD hybrid detector

The range of the electrons with energies 10 - 50 keV in xenon

The cffect of the different relative position of the GEM holes to the

GMD t.lectrodes on the bias of the centroid calculation. when the

ccntroid of the charge cloud is aligned with one GMD anode

The eftect of the different relative position of the GEM holes to the

GMD electrodes on the b iu of the centroid calculation. when the

centroid of the cliargr: cloud is aligned with one GMD cathode

The SPR imaging system built with the GEM/GiMD hybrid detector

Diügrüm for the grornetry of the GEWGMD hybrid detector

Diagram of the connrçton rnounted on the vesse1 cover and thrir

functions

Sçhemiitic of the two methods to supply voltage to the GEM: (a )

use individual HV supplies: (b) use resistor divider

Schematic of the high voltage application in the GEiWGMD

hybrid drtrctor

Diügrüm of the alignment setup

Block dicigram of the imaging electronics

Block diagram of the simple pulse height analysis system used in

the gain and spectrum measurernent

Block diagram of the trigger circuit and the red-time/live-time

scalers

A calibration plot of the anaiyzer with a test pulser

4.2 The setup for the gain measurement

4.3 Gas gain for the GEMfGMD hybrid vs. AV,,, and V, at 1 atm in

Xe:CH4

4.4 Güs gain for the GEM/GMD hybrid vs. AV,, - and V, iit 2 atm in

Xe:CH4

4 . Güs gain for the GEMIGMD hybrid vs. AVEC, and V, at 3 atm in

Xc:CH4

1.6 Güs sain for the GEWGMD hybrid vs. AV,,, and V, at 4 atm in

Xt.:CH4

4.7 Güs gain for the GEiWCMD hybrid vs. AVEC") - and V, at 1 atm in

Ar:CH4

-1.8 Detector gain vs. the drift voltage at 4 atm in Xt.:CHI

4.9 " ~ e spectrum obtained with Ar:CH4 at I atm

4.10 in spectnim obtained with Xe:CH4 at 1 atm

-1.1 1 %rn spcctrum obtained with Xe:CH4 at 2 atm 241 -1.12 Am spectnim obtained with Xe:CH4 at 3 atm

4-13 '".Am spectnim obtained with Xr:CH4 üt 4 atm

4.11 Energ spectrri for x rüys used for spatial resolution meiisurement

and iniaging at 2 atm

-1.15 Energy sprctra for n rays used for spatial resolution measurement

and imriging at 3 atm

4.16 Energy sprctra for x rays used for spatial resolution merisurement

and imiiging at J atm

4.17 Energy spectrum showing the bearn hardening effect of ridding a

72.2 mm PMMA (polymethy I methacrylate) to 50 kV at 4 atm

4.1 S A cathode spectrum obtained by QPAO?

4.19 Speçtrs frorn individual 16 anodes obtained by QPAOl

4.20 MTF curves for increasing x-ray energies at 1 atm

4.2 I MTF curvrs for increasing x-ray energies at 2 atm

4.22 MTF cumrs for increasing x-ray energies at 3 atm

-1.23 MTF curves for increasing x-rciy energies üt 4 atm

4.21 MTF curves for increasing gas pressures at 13 kV

4.25 MTF curves for increasing gas pressures at 30 kV

4.16 MTF curves for increasing ;as pressures ai 40 kV

4.27 MTF curves for increasing gas pressures at 50 kV

1.28 MTF curves of 13 kV at 1 atm and 10 kV at 2 atm in Ar:C&, and

of I3 kV at 1 atm in Xe:C&

1.29 The spatial resolution is improved by adding a 72.2 mm PMMA

i polymethy 1 msthacrylate)

-1.30 The d k c t of the cnergy window on the spatial resolution

4.3 1 The paral tel bar pattern and the star pattern

4.37 The test pattern images

4.33 Fihh spine image ( 13 kV iit 3 atm)

4.24 Rat phalange image (30 kV at 1 atm)

5.1 Oricntrition of the keystone GEM detector

h.i Dingrarn of one eeometry for charge distribution simulation. Thc

centroid of the charge cloud is ; k t GiMD anode 8

A 2 Diügrüm of one geometry for charge distribution simulation. The

centroid of the charge cloud is at GMD cathode 7

A Simulüted charse distribution over the 16 anodes of the GMD in

the GEMIGMD hybrid detector. where the charge cloud crntroid is

at GiMD anode 8

X.4 Sirnulated charge distribution over the 16 anodes of the GLMD in

the GEM/GMD hybrid detector. where the charge cloud centroid is

rit GMD cathode 7

xii

Chapter 1

Introduction

Since the discovery of x rays in 1895 by Wilhrlm C. Roenrgen. the realm of

dinyostic rncdical imaging has witnessed continua1 dramütic changes in the capabilitics

and cxtcnt of visutilizing the humm body. This chapter reviews the conventional n-ray

i m q i n s technology and the recent advances in digital radiography. The previous work.

thc Ga.; Microstrip Detector (GMDI imager. is described. An overview is piven of the

thcsib.

1.1 Conventional X-ray Imaging

Conventional n-ray imagin: uses photographie film in combinat ion wit h a

tluorescent screen as the imaging recrptor [ I I . In responsr to x-rüy absorption. the

phosphor scrern crnits light by which the film is exposrd to Form a latent image. This

image is rendered visible through chernical processing of the film. and the image is

usually viewed with the aid of a view box or illuminaior [ I I .

The film operates on the principle of enrrgy integration. i.e. the value of each pixel

is (i function of the energy deposited in the detector volume corresponding to that pixel.

The primary disadvantage of scrern/film radiography is that the image cannot br adjusted

iitrer i t has been developed. Since the film is used as the x-ray dctector. the image display

and for the storcige medium. there is a limitation on the imase quality and dose elficicncy.

1.2 Digital Radiography

The term "digital radiography" means that the analog data collrcted from the

patient are convertrd into digital form so that the diagnostic information can be recorded.

processed. displayed and stored by a digital computer [2]. The speed. accuracy. and

vcrsatiliiy of the cornputer have greatly broadened the selection and presentation of

inforrniition which are subsets of the primary images [1].

Disita1 radiogrüphy has many advantapes over conventional radiogrnphy. The

primxy advantrige is the levcl and window interactive display. The image contrat in

nunierous segments of one image cm be expanded with selectrd base lcvels and window

widihs. Therrfore the user c m see details which otherwise would require several

cspoïurcs. evcn for digiiized film data. On the other hand. sincc digital images can be

siored. sortrd. retrieved. and duplicated with no degradation. it is easier to compare x-ray

iniayes with those obtaincd from other irnaging rnodalities: prrmits image networking

within the hospital for rernotc access and archiving: md perrnits telrrüdiology - wherein

highly q~ialiticd personnel could service remote or poorly populatcd regions h m a

ccntnil facility [-Il.

Reccnt advances in digital rlectronics and miinuf~cturing mrthods have led to many

~ipproaches for the design and construction of digital x-ray drtectors. Little by litile. the

tï l ni- büsed x-ray imaging technology is griidudly bring replaceci by digital radiograph y.

Electroniç a-ray detectors crin be divided into two classes liccordinp to thrir working

niodc: rnerpy i n t e p i o n mode and photon-counting mode.

1.1.1 Energy Integration Mode Detectors

Fur this type of drtector. images are generated by integrrithg the total enrrgy

Jepositrd in the drtector during the whole radiation exposurr. Most of the available

Itirparea. elrctronicdly readable drtectors currently on the market work in the energy

intqration mode. including photostirnulable phosphors. Charge-Couplrd Device (CCD)

sensors couplrd to conventional phosphors. and solid-state detectors made of semi-

çonductor mate rials or photo-conductor materials such as silicon. limorphous selenium.

etc. These detectors can be classitied into two categories: direct conversion and indirect

conversion. Direct conversion means that the x rays deposit their energies in a layer of

photoconductor and the charge signal is read out to fonn the image. In indirect

conversion. a phosphor screen is used to absorb x rays and the consequent light photons

are captureci by a photo-detector to generate the image charse signal.

Rrcr nt advances in photolithography and electronic micro-fabrication techniques

have enabled the development of large-area r-ray detectors with integrated readout

mec hanisms basrd on arrays of thin-film transistors (TFT) [j 1. Unlike oldrr. CCD-bued

deteçtors that require optical coupling and image drmiignification. TFT based systems are

çonsiructed such thiit the pixel charge collection and readout electronics for each pixel

are immediately adjacent to the site of the x-ray interactions.

TFT cirrays are used as the active electronic switches in both indirect and direct

çonvcrsion dçtcctors [ 5 ] . These arrays are typically deposited ont0 a g l s s substrate in

miiltiplc Inyers. The rradout rlectronics are üt the bottom of the multiple Iriyers. In the

rniddli: Iayers are the charge collecter mays. which include the pixel clcctrodes and the

intcgrated htortigc clipxitors that are connected with Tm switches. These TFT swiiches

arc controllcd by ü sçünning control circuit by which the charge signals stored in the

cxpnçitors arc read out row by row. Then. depending on the type of detrctor being

constructcd. x-ray rlements. light-sensitive rlçments. or both. are drpositrd to form the

top laycr. The mi r e usembly is encased in a protective enclosure with extemnl cübling

for computer connection.

1.2.1.1 Direct Conversion Detectors

In (i direct conversion dctector. an x-ray photoconductor is usrd to conven x-ray

photons into an electric charge directly. Typicdly. amorphous selenium is used as the

photoconductor material because of its excellent x-ray detection propenies [6. 7. 81 and

ex trrmel y high intrinsic spatial resolution [9 ] .

Sol id-s tatr detectors made of morphous selenium layered on an aluminum support

were introducrd in the mid 1950's as nerondiogriphy that used the powder readout

method. Attçmpts at using a sensitive electrostatic probe to sxtnct the latent charge

distri but ion were not cornmerciall y successful until a charged selenium dnirn/electrostatic

reüdout systrm for chest radiology wa5 introduced in 1993 by Philips Medicd Systems

C to1. Amorphous selenium is weli developed iechnologically as it has been used as a

photoconductor in photocopiers for decades. It has two dvantages as an x-ray imûging

detcctor. First. as selenium is used in its cimorphous form. selenium plates can be made

by means of rvaporation and thus can be made large in ivea relatively eiüily and

inexpensivcly. Second. i t has low divk current.

Soiid-stiite scmiconductor detectors with TFT a m y s Iayered dircctiy on the surface

to collcct the signal and read i t out are currently in research and devclopment for

iclenium and for silicon [-Il. During n-ray exposure. rnergy is absorbed by the

aniorphous selenium layer and the charge creatrd is drawn to the surtàces by the interna1

clcctric field. The ima_oe charge is collected by the pixel electrode and accurnulated onto

thc pincl storüpe capacitor. The pixel electrodr and storage capacitor are connected to the

TFT witçh of cach pixel. During readout. the sciinning control circuit generares pulses to

turn on al1 the TFT switchrs on the first row of the ÿrriiy and transfrrs charge frorn the

pixel cnp;icitora to the reüdout r d s (columns). The charge is thsn collrcted and rimplitird

by an iirnplifitx on cach rail and the data for the entire row are multiplexed out. This

srqucnce is repeated for rach subsequent row until the entire arny is readout.

Tcchnolop for solid-state drtrctors is üdvancing rapidly and will likrly provide

m a y hizes of 4.000 x 4.000 elrmrnts in the near future to match the capabilities of

scrccnlfilrn systems for both static and dynamic projection imaging [ 1 O].

1.2.1.2 Indirect Conversion Detectors

Indircct conversion detectors have a two-strp process for x-ray detection: a

phosphor scrrrn is used to absorb the x rays and converts the 'r-ray rnergy into visible

light. The light is thrn convenrd into an electric charge by CCDs. or by means of plioto-

detectors such as amorphous silicon photodiode m a y s followed by a Tm array.

CCD sensors are typically comprised of an m a y of electron "wells" etched in

silicon or p-manium thiit collect eiectrons released via the photoelectric rffect by

impinging light. Charge transfer of the stored electron charge occurs over vertical

registers line by line: charge detection is accomplished by sequential readout of the

horizontal register on a pixel-by-pixel basis with charge conversion to a voltage ai

vürious readout rates. Pixel location is accomplished using synchronization signds often

identical to standard video format. Common array sizes for CCD cameras are 1.000 x

L.000 and smaller. CCD cameras are presently being used in digitd photo spot

applications with the image-intensifier television system that is usrd in tluoroscopy.

Dircçt coupling of CCD m ü y carneraï to fiber optic plastic and glus scintillators will

likely be wailable in the ne= future [lO].

Xnother indirect dctector is the photostirnulable phosphor thnt is used in computed

radiography [[O]. In the eÿrly 1980's. Fuji Film Company of Japan introduced the

photostirnulable phosphor plate and plate readers. An "electron" latent image is formed

whcn the phosphor electrons are rxcited by ihe radiation. then the latent image is

dcwloped in the reader by scanning the rxposed plate with a focusrd laser beam of low

encrgy to release those excited electrons back to ground state.

1.22 Photon-Counting Mode Detectors

Compüred with energy integration. the photon-counting mode h a several

üdvontüges. First. the drtective quantum rfficirncy (DQE) (the definition of DQE is

given in section 1.7) of a photon-counting system can be nrariy equril to the quantum

rffiçirncy. So it dues not introduce extra noise besides the statisticül noise. Second. i t can

provide the energy of each photon thai is stopped inside the drtector and contributes to

the images. Third. because each photon produces a signal on multiple detection elemrnts.

the çrntroid of the charge distribution c m be calculated and the resolution c m therefore

be bstter thnn the elrment pitch.

Generally. photon counting is usrd in nuclear medicine where the photon tluence

rate is low. but it is not used in radiography because of the technical difficulties involved

in count in= photons at radiographie tluence rates and of providing adequate spatial

resolution. With the advent of new high resolution. high-mte radiation counters

drveloped for pluticle physics experiments. photon-counting radiography haï become

feasible.

The recently introduced micro-pattern detectors for particle physics. such as the Gas

Microstrip Detector (GMD). Micro-Dot Avalanche Chamber (MDC), MICROMEGAS.

and the Gas Electron Multiplier (GEM) etc., broaden the wsy for digital radiography

pho ton-counting detector development.

Cas Microstrip Detector (0)

The GMD. iriventsd in 1988 by Oed. waï the first of the microstructure _pas

detectors [ 1 1 1. Its anodes and cathodes are metül strips printed ont0 a tlat glass substrate,

as s h o w in Figure 1 . 1 . Wi th the help of high-ciccuracy photolithographic technolo_gy. the

distiincs betwern modes and cathodes is an order of magnitude smallrr thün rhat in the

conventional multi wirr proportional chamber ( MWPC). which bnngs adwntages in

trirms of rate capability and spatial resolution. High voltage is applird on the cathodes so

that a high clcctriç field is produced üround the anodes and provides the proportional

niilltiplic;\tion of the electrons. The GMD hüs beçn found to agr and dso subject to

duniügc dur to slectriçal disc harges resulting in an unreliuble operation. whrn it is

exposed to high-rate radiation.

Cas Electron bIultiplier (GEM)

The Gas Electron Multiplier (GEM) was introduced by F. Sauli in 1997 as ii gain

boostrr for rhr GMD [El. It consists of a thin metal-clad polyrnrr foi1 chrmically

perforatcd by a high density of holes. Figure 1 .1 shows the geometry of one GEM

çhannel. On application of a difference of potential across the GEM rnesh. it acts as an

amplifier for eltxtrons released by radiation in the overlying gaï volume. With the gain

contributrd from the GEiM. the GMD clin bc operated iit lower and safer voltage resulting

in more reliablr operation of the device.

Conversion Gap -- (Depth) /'-

Cathode Anode

GMD X-Ray Photon 1 ;

- .

Drift Plane

Figure 1 . 1 Concept diagram of the GMD in the GMD imager.

Cross-section Top view

Figure 1.2: Schcmatic of one GEM amplification channri (D: the copper diameter: d: the Kripton dilimeter: S: the thickness of the copper elrctrode: T: the thickness of the Kapton).

Micro-Dot Avalanche Chamber (MDC)

The Micro-Dot avalanche Chamber (IMDC) was introduced by Biagi et. al. [ 131. It

is a 2-D gas detector. Figure 1.3 shows the geometry of one micro-dot cell. It is reülized

on a silicon substrate. The device consists of metallic anode dots surrounded by circula

field and cathode rings acting as individual proportional counters. The structure has becn

implementcd with an underlying backplane bus enabling the anode dots to be read out in

strings which have been produced with both 100 and 200 pm readout pitch. The other

dimension is provided by the cathodes which are connected togethrr in lines too. The

anode dots are typically 13 to 30 pm in diameter and the cathodes are brtween 20 and J O

p i wide. The anode cathode gap is of the order of 75 pm for a 200 yrn pitch and 35 ym

for a 100 pm pitch structure. Compared with the GEM. the fabrication is more

cornplicated and expensive.

MICRO-MEsh GAseous Structure (MICROMEGAS)

WCROMEGAS w ~ s inventeci at Saclay by Y. Giomataris. Ph. Rrbourgrard and

J.P. Robert. in collaboration with G . Charpalr [14. 151. The geometry of the

411CROiMEGAS detector is shown in Figure 1 A. It consists of a conversion Sap and a

thin ümplification gap. The two regions are separiited by a thin (3 or 4 Pm) grid which

rcsts on \mil11 insulating pillus of 100 microns diamrtrr. The incident radiations liberate

ionization rlectrons in the conversion gap. and then these rlectrons drift into the

amplification gap. A high field is applied across the amplificütion sap. typically above 60

kVlcm [ M l . Herc the rlectrons are amplifird and finally collrcted by printrd rlectrodrs

of üny shape.

The major practical inconvenience is the necessity of stretching and maintaining

piiraIlel meshes with very good accuracy [17]. This requires a hravy support frame. The

presrnçe of sirons electrostatic attraction forces makes the mesh sag. especially for l w ~ e

sizes. The introduction of the insulating pillars makes the assrmbly of a MICROMEGAS

detector complicated.

Buneù &de réadout but

Figure 1.3: Schematic of one micro-dot cell.

O r f i Plane /

Quartz Fibres

Figure 1 A: Schernatic of the MICROMEGAS drtector.

1.3 Scanned Projection Radiography (SPR)

When only one dimension of the field of view (FOV) of a detector is as large as the

image and the other dimension is much smaller. scanning is used to generate a two-

dimensional image. In a scanned projection radiography system, the x rays are collimated

into a fan-beam before and after the object to inadiate the FOV of the detector. The two

dimensional image is grneniteci by scmning the object with the fan-beam.

Compared with direct two-dimensional projection systems. e.g. CCDs or Selrnium

hystcrn. scünned projection radiography h a two advantages if designed properly: it

reduces xcütter background and the nurnber of rcadout channrls. The reduction of the

sclittcred photons results in better detection of low contrrist lesions. The drriwbacks of

SPR are high x-rliy tube k a t loading because of poor x-ray utilization with a f ~ n bcarn.

and difficulty in implemrnting real time imaging (tluoroscopy) becüuse the tirne rrquired

to produce one image is quite long.

A digital n-ray imagine systcm. the Sibrriün Digital Radiographic Deviçe t SDRD).

has bccn developed by a group from the Budker Institute of Nucleu Physics in

Novosibirsk [ 18. I9) and is in clinical use at Iiospitals in Novosibirsk and Mossow. The

SDRD is a slit scannrd projection radiography system baseci on the multi-wire

proportional çhümber (MWPC). The MWPC works in photon-counting mode with the

keystone geornetry. i.r.. the anode wires are stretchrd alon; the direction of the bram and

dirccted to the source of rdiation (focus of the a-rüy tube) [20]. The advantages of the

SDRD over film-screen have been demonstrated [ 2 1 1.

A nrw derector. the Multistrip Ionization Chamber (MIC). is bring developed by

the sarnr group [XI . The signal rlrctrode consists of 1024 strips with a pitch 0.4 mm.

which are made by printed circuit technology. Each strip is about 60 mm long and is

directed to the focal spot of the x-ray tube. There are 16 groups of 64 strips each. The gap

between the drift plane and the readout electrode is 4 mm. and is filled with pure Xe at 10

atm. The detector operates in the ionization regirne. It was demonstrated that the MIC h a

brttrr spatial rrsolution. higher quantum cfficiency and much sirnpier design compared

with the LMWPC.

1.4 Previous Work: Gas Microstrip Detector (GMD) Imager

A Gas Microstnp Detector Imager filled with high pressure Xe:CH4 has been built

in our [ab. The dit-scan mode of scanned projection radiography (SPR) was used in the

GMD imaging system. Good performance of the GMD imager was demonstrated [ 2 3 ] .

The GMD used in the imager has 7 pm wide anodes. 90 Pm wide cathodes, and a

pitch of 700 Pm. The orientation of the detector is s h o w in Figure 1.1. Any electrons

1ibcrntr.d by the interaction of an x-ray photon with the xenon brtween the GMD and the

drift plane 3.9 mm away are driven towards the anodes. The GMD mountin_o geometry.

whiçh was carried ovrr kom particle physics tracking applications. restriçted the

scnsitivc xcnon thickness to 3.9 mm.

1 .j Motivation for This Project

A slow dcgradation of performance and a zubstantiiil increase with rime of the

disc hürges was observed during the operation of the GMD imager [XI. The dischargr

contributcs io ihe spurious signal and damages the GMD anodes and cathodes which

co~ild rcsult in dead channels after a period of operntion.

Discharge formation

The formation of the discharges has been resrarched by many groups [Zj. 76. 17.

ZS] . Many parameters like the quality of GMD strips or the contaminants in the gas can

provoke discharges. Besides, two physicd processes have been proposed to be

responsible for discharges. which would produce permanent damage to the GMD

electrodcs. One is field rmission at the cathode strip edgcs. and the othrr is the formation

of strearners [29]. Both of them are related with the Raether condition. i.e. the number of

electrons in the avalanche rexhes 10'- 10'.

First. considrr the field emission. Due to the high field üround the cathodes. one or

more clccrrons are emitted at the cathode strip rdses. This field ernission would be

cnhançrd by the presrnce of ions produced in avalanches around anodes. Once ejected.

the electrons are multiplied in the high field around the cathode e d p s before reaching the

anode strips. This process c m easily become divergent. Then the discharge happens when

the avalanche size approaches the Raether condition.

An alternative mechanism involves the transition from avalanche to streamer at the

Rririther condition. When the total charge in the avalanche exceeds Raether's limit. the

enhancement of the rlectric field in front of and behind the primary avalanche is such as

to induce thc Lst growth of secondary avalanches. and the appearance of a long.

filament-iike torward and backwud charge propagation named a streamer. In a uniform.

strong drctric field. the streamer propagates al1 the way through the gap. The outcome of

the proccss i \ the creiition of a densely ionized. low-resistivity channel between anode

and cathode. inevitiibly ieading to dischargr [)O!.

Solution

A discharge initiateci by iiny of the mrchanisrns cm lcad to spürk. which would

darnnge the elcctrodes and further rrsult in the failurr of the drtector. The GMD is riither

drliçiitr to use and prone to failures caused by discharges at the operüting voltage

required hr hl1 efficicncy. muc ch research work has becn done to look for solutions to

this problcrn [XI. One of thrm would be coupling the rrcrntly introduccd Gris Electron

M~iltiplier ( G E M ) to the GMD.

The GEM is a nrw concept for electron amplification in ;as detrctors [17]. An

intrresting usc of the GEM mesh is as gain booster for the GMD. It acts as a powerful

prcamplifier for electrons released in the conversion volume. thereby the "primary"

çhiirgr is incrcased Crom the point of view of the second amplification stage [3 11. In this

wiiy. the drmand of a high GMD gain for an appropriate signal-to-noise ratio is relued.

so that thc detector ciin be operated at lower and safer anode-cathode voltages. Thus the

wholt. system is more reliable and robusr.

The purpose of our research work is to determine:

1. If the GEWGMD hybrid is more robust:

1. If the addition of the GEM degrades the spatial resolution and energy resolution

of the GMD:

3. If the GEM is suitable for high pressure digital radiography.

1.5 Thesis Overview

Chapter 1 introduces the GEM and its operating principles. The signal development

in the GEM/GMD hybrid is described. Chapter 3 describes the geometry of our imagine

system with the GEWGMD hybrid detector. Some concepts used for the GEM are

introduced. Thc apparatus and electronics used in the imaging system are described.

Chripter 4 introduçes the experirnental rnethods usrd to rneitsure the performance of our

hybrid system. The experimental results are presented and compared with the

performance of thc GMD imager. Some images obtained from the hybrid system are

bhown. The conclusions from the merisurement. and future work for this project are

cliscussed in Chapter 5.

Chapter 2

Theory

Thc Gas Elrctron Multiplier (GEM) was introduced by F. Sauli in 1997 (121. Its

principal distinction from the MWPC and the GMD is that the gas amplitïcation occurs in

micro-holcs [ 3 4 . This chapter introduces the geometry of the GEiM and its

ni;inut'r~cturing technology: the principle of the GEM operation. including the signal

dcvclopinent in the GEM/GMD hybrid and the piirümetcrs affecting the gi~s gain of the

GE41: the optimization of the GEM: and finally describes the spatial resolution and

c n q y rcsolution in the GEM/GMD hybrid.

Figure 2.1 : A close view of a GEM foil.

2.1 GESI Geometry

-A GEM consists of a Kapton mesh typically 50 ym thick coated with 5 pm of

çoppcrr on both sides of the Kapton. and perforated regularly by a high density matrin of

holes. The cross-section and top view of the GEM holes are shown in Figure 1.2. The

approximate dimensions of a standard GEM are: Kapton thickness (T) 50 Pm. copper

thickness ( S ) about 5 Pm. copper hole diameter (O) about 70 - 90 Pm. Kapton hole

diametrr (d) about 50 - 60 Pm. and the pitch 140 Pm. A close view of the GEM foi1 is

shown in Figure 2.1.

2.2 Operating Principle of the GEM

The GEM works in the proportional regimr. which means thrit the amplitude of the

output signnl is proponionül to the initial number of ion pairs produced by the incident

photon.

In our hybrid GEWGMD detector. the GEM is usrd as an rivalanche preamplifier

for the GMD. Figure 2.1 rives the orientation of the hybrid detector. The GEM is placed

bctwccn the plinille1 drift plant: and the Gkf D substrate. The gap between the drift plane

;ind thc GE91 top is called the drift region or conversion gap. and the lower spnce. whrre

oion or aniplified electrons are collccted by the GMD anodes. is nrirned the induction re,

collection güp [33].

2.2.1 Eleçtric Field in GEM holes

Lrpon application of ü potential difference between the GEM rlrctrodes (copper

Iiiyrrs). a high electric field devrlops in the holes focusing the field linrs betwern the

drih electrode and the GMD anodes. thereby providing iui effective amplification path for

elcctrons relritsed by ionization in the gas and drifting in the high tield through the open

c hrtnnt.1.

The field density in the amplifying channrl c m be varied rithrr by increasing the

potential difference between the upper and the lower rlrctrode of GEM. or by reducing

the dicimeter of the GEM holes [-Il]. The lrngth of the amplifying channrl. for a single

GEM prid. is fixed by the thickness of the insulating foi1 [34]. So Car most GEM foils

have an insulator thickness of 50 Fm. So the electric field in the GEM hole is about 100

kVlcm whrn the poteniial difference across the GEM is 500 V.

Anode Cathode

Collection

Conversion Gap ( D ~ P ~ I -L-=

Y A

Drift Plane Fill Gas . -

Z k - X

Figure 2.2: The orientation of the GEMIGMD hybrid detrctor.

Mons directions perpendicular to the axis of the holes, the field strength is almost

uniform in the center and increases shürply near the sides. particularly close to the

copper-Kiipton interface. This determines how high a voltage could be applied to the

GEM. i.ç. the point of electrical breakdown (331. The charge avalanche transverses the

GEM holç mostly in the center. but a hction of i t could üpprorich this rxtrernely high

field rcgion on the sides of the hole due to diffusion and trigger ii discharge. Also electron

cniission from the GEM electrodes in this area is possible [34].

2.2.2 Cas Avalanche

The mtllin free path for ioniziition is defined üs the average distance an clectron bas

to travcl More it sets involved in an inelastic collision. whrre ionization occurs. The

inverse of the mean frer path for ionization is called the first Townsend coefficient. a.

and is the niimbrr of ion pairs produced per unit length of drift [3JJ.

.-\v;ilançhr. miiltipliciition is büsrd on a process of ionization hy collision. Consider

onc t'rcr. elecrron producing one more electron lifter a mean frtx path. Both continue

;tlons thc drift field. and a k r one more meiin free path. two funhcr ion pairs are created.

;ind r o on. If iV is the nurnber of electrons üt a given position. aftrr a displacernent d x .

the incrcrisc. in the nurnber wil1 be:

B y intcgrating Eq. (2-1):

This expression. whrre M 1s the g u gain. is deduced for the case of a uniform rlectric

field. I f the elrctric field is non-uniFomi. a depends on the distance r . so the giis gain is:

The lower integration Iimit x, is the coordinüte m which the charge multiplication starts

dur. to hish rlectric field. and x, is the coordinate where the multiplication process stops.

II a single primary rirctron enters a region of sufficiently high field. it will

e'rpericnce tùrther ionizing collisions. The different drift velocities of slectrons and ions

(swcral thousand timrs slower) cause the avalanche to have a characteristic drop shapc.

with electrons situated in the front of the drop and the ions in the taii.

2.2.3 Signal Development in the GEWGMD Hybrid

Thc interaction of photons with müiter is describrd by four fundumental processes:

t hc phoroclcctric e ffcct. the Compton effect. coherent scattering and pair production. For

thc photon znergies ussd in diagnostic radiolosy ( 15 - 150 keV). only two interactions

arc irnportnnt. the photorlectric effect and Compton scattering. Coherent scattering is

nuiiiericülly unimportant. since it does not deposit energy in the detector. The probability

of Compton scattering depends on the total number of elsctrons in the absorber. and it is

more important for highcr diagnostic energiçs [35] . The photoelectric rffect is the

prcdominant interaction with low rnergy photons and with high iitomic numbcr absorbers

[Xi]. The law govemiii~ the photoelcctric effect is [ 3 7 ] :

c whçrc - ( o>i2 / ,y ) is the mus coefficient. r (cm") is the linear attenuation coefficient. p

P

ig/c/ori-? is the density of the absorber. Z the atomic number of the absorber. and E, the

photon energy. The powers n and m are photon energy and Z dependent. For biological

materials at al1 x-ray energies. or for high Z materials at photon energies lower than that

0.3 .MeV. ti = 3 . For low Z materiais. nr = 3.8 : and for high Z matenais. rri = 3 . In our

case. the x-ray energy is lower than 50 keV and xenon gas (2 = 51) is used. so the

photorlectric e ffect is the main mechanism.

As shown in Figure 2.2. n-ray photons are incident on the detector normally. i.e..

the x-ray beam is perpendicular to the drift plane, and interact with gas atoms in the

conversion güp. Any primuy electrons liberated by the interaction of photons with the

gcis in the conversion gap experirnce two stages of amplification. The first stage is in the

intense electric field in the GEiM holes and second is in the semi-dipole field fomed by

the anodes and cathodes of the GMD. The conversion and collection fields are almost

uni ti~rm: the field distonions due to the hole or strip structure penetrate only about 700 to

400 pm (two times the pitch) into the conversion or collection gap.

The electrons grnerateci during the ionization drift toward the GEM holes following

ihc electric tield lines. while the ions drift toward the drift plane. When the electrons

cipproxh the GEM holes. they enter a region of increiising field strength. and are

aççclcr;itcd into the hole. Within the chrinncls the electron will reach a region of very

hish cleçtriç field. Staning frorn a critical value of field. slectrons begin to multiply in an

ilvalanche. incrcasing exponentially dong the channrl (Eq. (2-3)) [33].

The tïcld density bccomes weükrr at the bottom side of the hole. The electrons drift

out of the chünnel. largely amplifird by the avalanche proccss. The multiplication factor

deprnds on the field drnsity in the central region of the channel and its irngth [3J].

Positive ions generritcd in an avalanche are collrcted by eithrr the drift rlectrode or

the top GEM electrode. whcreüs riectrons are trapprd üt the GEM bottom or movr

toward the GMD anodes. the fraction depending upon the ratio brtwcen drift md

collection fields for a given potentiiil across the GEM.

The second avalanche happens around the GMD anodes. After the second

avri/ancht., ions move toward the GlMD cathode or toward the bottorn GEM electrode.

and electrons arc collrcted by the anodes. The charge signal detected by the GMD anodes

i.4 2ivt.n by:

whrrr. Q is the charge signal detected:

n,, is the initial number of electrons produced by the incident photon:

r is the charge of the electron:

E , , is the average rnergy deposited in the gas per interaction by the incident

photon:

LV is the average energy lost by the incident photon per ion pair formrd. In

principlr. I.V is a funciion of the specirs of gas involved. the type of radiation.

and its ensrgy. But empirical observations show thnt i t is not a strong function

of encrsy but is remarkably constant for different types gases and radiation. It

is about 26.2 eV for Ar and 21.5 eV for Xe (381:

M is the total gas gain from the hybrid. and is given by:

where M :n,, is the absolute gain or the GEM. M :,, is from the GMD. and T is

the charge transition çoelficient that drpends on the trünsparency and

collection efficiency of the GECI:

The trmsparency and collection efficiency for the GEM are discussrd in the next

two sections.

2.2.4 OpticdElectricrl Transparency of the GEM

The transmission probability of the primary electrons through ü GEiM holr is

determincd by the optical transparency and elrctncal transparency. The optical

trünsparency is determinrd by the ratio of the open areü to the total GEM area. Assurning

a GEM mesh with cyiindncd holes of diameter D üt spacing P. its opticd transpareccy is

oiven by the expression [33j: C

For a GEM with P = 140 Pm. D = 70 Pm, the optical transparency is about r= 0.70.

The elrctrical trinsparency is defined by the proportion of field lines passing

through the GEM holes. in contrast to those which terminate on the upprr GEM

slectrodr. aiid is controlled by the strength of the field in the conversion field relative to

that in the holes [39]. For a high conversion firld. some electric field lines trrrninate on

the top GEM elrctrodcs. so that some slrctrons are lost by stopping at the GEM top. On

rhe other hand. the electrical transparency is higher for higher GEM voltages. and for

elcctrodrs with Inrger optical transparency. Bellazzini cf . (ri. [39] measured the elrctriciil

triinsparcncy. The results showed that elcctrical transparency reilched the maximum

( 100% ) w hen using a conversion field of around 4 kVIcrn. The GEM foi1 they used hüd a

pitçh 120 pin. Kapton thickness 50 Pm. copper hole diameter 65 Pm. Kapton hole

dianieter 30 Pm. The collection field was fixed at 4.5 kVIcm. The potentid across the

GEL1 foil was 500 V. The filled gas was Nc/DME (5050).

2.2.5 Collection Efficiency of the GEM

The collection cfficiency is the ratio of the number of ricctrons that enter the

rniiltiplicütion region of the GMD (but have not yri been amplifiçd by the GkID) to the

total avalanche electrons producrd in the GEiM holrs. It depends on the ratio betwern

conwrsion and collection tields.

For a hirher conversion tirld than a given collection field. some conversion field

lines rire dirccted towrird the bottom GEM electrode instead of to the GMD. Cho rr. [ i l .

[401 did n simulation on a GEM foil with Kapton diametrr 40 Pm and pitch 140 Pm. The

collection field WLS t k d cit 5 k W m . The results showed thnt about 60% of conversion

field linrs were prrdicted to be drawn to the bottom GEM elrctrode at a conversion tïrld

of 15 kVIcm. In the experiments conducted by Bellazzini et. [il. [39]. the collection

efficiency reachrd 80% with collection field 10 kVIcm. The geometry of the GEiM.

potentiül across the GEM and the fil1 gas were the same as that used in the experirnents

for trsting the electrical tnnsparency that I mentioncd in the 1 s t section. Therefore. the

conversion field should be no larger than the collection field for maximal signai

detection.

2.2.6 GEM Gain

The gain in the GEM has two categories. One is called the "absolute gain" ( M , ; ,

in the equation (2-6)). and is defined as the ratio of number of avdanche rlectrons to

initial clectrons. The other is the "effective gain". and is the ratio of the number of those

clcçtrons thiit reüch the multiplication region of the GMD to initial charge. which

includes the cffect of both the transparency and collection efticiency of the GEM. In Eq.

i 2-6 1. the effective GEM p i n is the fiictor of M ,,,T .

From thc last two sections (2.24 and 2-25) . we can srç thüt the effective p i n is

urii:illy l e s than the absolute gain by an amount thüt drpends on the field values. The

eifcct i vc gain increüsrs rxponentinlly with the voltage applied ricross GEM. and nlmost

lincarly with the strength of the collection tield. For a 3ivt.n voltage. it is largcr for

n;irroucr holrs. but independent of the pitch [331.

1.3 Optimization of the GEPI

The ümplificütion of the GEM mesh depends on grometry and operating conditions.

Holc diümetcr. pitch and geometry of the hole are detemineci by the manuhcturing

p m C h S . The thickness of the mesh is at present limitsd by the availnbility of high-quality

rnct~il-coatcd pol ymers that can be processed chemically [4 I 1.

Ar CERN. a range of hole diameters brtwern 40 and 140 Pm. and a pitch between

90 and 200 pm have been explorcd [33]. In order to obtain good manufacturing qualiry.

r.?. avoid under-ctching. the maximum hole diamrter is usudly limitrd to about two

more thirds of rhr pitch. Recently. it is found thüt producing GEMs on glass would brin,

benefits [3 11.

Influence of hole diameter

As mentioned before. reducing the diameter of the GEM holes c m increase the

electric field in the GEiM holes. Bachmann rr. al. found that the gain increrises

considerably with drcreaîing diameter until uound 70 Pm. and reaches a plateau for

lower values [33]. It was explained that. when the hole diarneier is reduced brlow ri value

close to the GEM thickness, there are more avalanche electrons lost to the bottom GEM

electrodt..

Influeiice of pitch

The pitch plays no role in the gain characteristics. but combinrd with the holr

diiimctcr efkcts the ~lectricül transpÿrency. for electrons released in the drift volume. as

wcll as the distribution of ions produced in the avalanches [4 1 1.

2.1 Charge Drift and Diffusion

Drift Velocity

I f lin extemiil elcctric field is applied to the region in which ions or elet rtrons exist

in the gas. electrostlitiç forces will tend to move the charges awriy from their point of

ilrigin. The net motion consists of a superposition of a nindorn thermal velocity together

with a net drift velocity in a given direction. The drift velocity is givrn by [38]:

u-hert. Y is the drift velocity. E is electric field strength. p is sas pressure. p is the

charse mobility. For a siven gas pressure. the hizher the field is. the faster the drift

vslocity will be. So in order to collect the charge in a shoner time. a higher drift field is

required.

Diffusion

When the electrons drift toward the GEM holes or the GMD. they are scattered in

random directions. causing deviations from the mean trajectory. This leads to a

brocidening of the electron cloud in both the transverse and longitudind directions. which

is cdlrd diffusion. Elsctrons. staning to drift as a point-like cloud. show after a drift

distance r ri Gaussian distribution in the transverse and longitudinal direction wi th width

dç fined by the standard devititions a, and a, respectively:

Hcrc. D: (D: ) is the transverse i longitudinal component of the diffusion rrnsor.

Cornmonly iised are the transverse and longitudinal diffusion coefficients j , and ;, . with

diniension (k>rLSdi)"' .

2.5 Spatial Resoiution

The intrinsic spatial resolution of the GEWGMD detector

The spatial rrsolution of the GEM/GMD detector is intrinsically determinrd by the

physiçal absorption procrss of the .u-ray photon in the detector.

The absorption of an I-ray photon in xenon results in the ejection of a fast

photorlrctron from a xcnon atom followed by one or more fluorescent x rays ( KaU. Lmu.

etc.). The photorlectron then ionizes xenon gas dong its path so that a charge cloud is

formed bcfore it stops. The distribution of this charge çloud over the GMD anodes is used

to ioçatr the coordinates of the incident photon. The size of the charge cloud dives an

intrinsic limit to the spatial resolution that the detector c m achieve.

There are mainly two caregories of electrons generated in Our detector after a-ray

absorption: photoelrctrons and Auger electrons. The range of these electrons controls the

sizr of the charge cloud formed after the photoelectric eftect. One measure of the electron

ran_oe is the continuous slowing down approximation (CSDA) range. It is an integration

dong the actual path followed by the rlectron. and is given by [QI:

whcre E,, is the initial encrgy of the electron. and S ( E ) is the stopping power for the

clcctrun in xcnon.

Figure 7.3: The range of the electrons with energies 10 - 50 keV in nenon. Ciilculntcd with Eq. ( 2 - 12).

The CSDA range gives the total track Iength but not the mcan radial distance tLie

eleçtron movçs from its point of origin. which is what we want to know. Bateman es. d.

sive a formula for the range of electron with energy E (10 - 50 keV) in their Monte

Cu10 simulated p hysical mode1 of the x-ray absorption processes (431:

*' This formula uses units of MeV for rnegy and g cm - for distance. and it c m be used for

h i s h Z materrils such as xenon. It takes account of the multiple scatterin; rxperienced by

the clcctron. so it givrs a brtter estimation than the CSDA range. The range for the

clrcirons with rnergirs 10 - 50 keV in xcnon is plottrd in Fisure 1.3. it is observcd that

for the range of energirs mentionrd above ( 10 - 50 hV). the average distance from the

origin point of ihe rlcctron to the centrr of the charge cloud is about 604 of the range

siyen by Eq. ( 2 - 11) [43 1. which grnrrally limits the spatial resolution of our detector.

hccording to Eq. (2-12). the electron range increases rapidly with its energy. An

elcctron's range increases by more than a factor of thrce whcn its enrrgy doubles. So the

spatial rcsolution degrades with electron cnergy. which is controlled by the rneqy of the

incident x-rüy photon and the rnrr_eies of the shell electrons in xenon.

Other absorption processes in the gas simply cause degrridation of the image. First.

the rc-absorption O C tluorescrnt x-rays contributes to the spread of the linc spread

function and point spread function of the detrctor. Since thry grnerate multi-prab rvcnts

ivhich rire rejected during the rvent analysis. their contribution is negligible in our case.

Sccond. photon scattering of the GEiM loi1 would generatr a uniform background.

The effect of the GEM hole structure

Thc charge ccntroid algorithum applied in the GMD to find the location of cach

photon interaction makes the spatial resolution of the GMD better than the anode pitch

(200 Pm). With the GEkI coupled to the GMD. the main concern is that. whrn the

primary elrctrons drift towlud the GEM holes. they rire "wrnpled" by the discrete holes at

1-10 prn pitch. Is this going to introduce significrint distortion in finding the centroid of

thc dtlctron cloud'? The answer should be no. The diffusion of eiectrons in the conversion

and collection gaps would minimise the effect of the GEM hole structure. For a electric

field of 2 kV/cm. the G of the charge cloud after 2 mm drift is > 150 pm for Xe:CH4 (9: 1 )

at 1 atm. The average o of the charge cloud in the conversion gap (3.9 mm) was

calculated to be about 200 pm by using the simulation model described in Appendix A.

This means that the charse cloud produced in the conversion gap would cover at l e s t

two GEM holrs. The effect of the discrete s a m p h g of the GEM holes will be lrss when

more GEM holrs rire covered. Funher diffusion of those rivalanche electrons in the

collection field would help to minimise the effect further.

The calculated centroid vcrsus the GEM hole position with respect to the position of

thc GMD elcctrodes was studird using the simulation model described in Appendix A.

Figurc 2.4 and 1.5 givti two results of the simulation at 1 arm in xcnon. In the simulation.

the çentroid of the charge cloud produced in the conversion grip was known and fixrd (it

ont: point. and the GMD position was fixed. The GEM was moved dong the GMD

rcxiout direction to change the relative position between the GEM holrs and the GMD

clcctrodcs. The p rne t r i e s are shown in Figurc A. 1 mi A.?.

The result shown in Figure 2.1 is for the gcometry shown in Figure M. the

centroid of the charge cloud was at GMD anode 8 (at 1.4 mm. the anode I was at the

«rigin of the coordinates). The initial positon of one of the GEM holes was at the samç

position cis the charge cloud centroid and thus of the anode 8. The calculated centroid for

rach GEM position was plottrd vrrsus the movins distance of the GEiM From its initial

position to the right. i.e. the relative position between the anode 8 and the GEM hole

which \vas cit the anode 8 at the beginning. As shown in Figure 1.4. the calculated

ccntroid varies periodically when the GEM holrs shift respect to the GiMD anodes.

Figure 2.5 shows the same periodic variation for the bias of the calculated crntroid

with the geometry shown in Figure A.2. In this case. the centroid of the charge cloud wxs

Snrd üt GMD cathode 7 ( 1.3 mm) and the GEM was moved with respect to this cathode

to the right. The initial position of one of the GEM holes was aligned with the cathode 7.

From the two simulations shown in Figure 1.4 and 2.5. ive can see that the variation

prriod of the calculated crntroid is the GEM pitch 140 Pm. The maximum bias of the

ccilculütcd centroid is lrss than 1 prn The smallest bias happens when the GEM holes

distribute symmrtrically with respect to the centroid of the charge cloud. So the

introduction of the GEM foi1 does bring m o r s on the centroid calculrition and the enor is

Figurc 2.4: The effect of the different relative position of the GEM holrs to the GMD ~.lcctrodcs on the b i ~ s of the centroid crilculrition. The centroid of the charge çloud is aligncd with one GMD anode.

O 20 40 60 80 100 120 140 Relative Position bm)

Figure 2.5: The effect of the different relative position of the GEM holes to the GMD electrodcs on the bias of the centroid calcullition. The centroid of the charge cloud is nligned with one GMD cathode.

GEM hole structure dependent. Cornpared with the electronic noise (9.5 pm) and

statisticiil rrror of the charge cloud (7.4 Pm) which were calculated by D. Gobbi [X]. the

bias introduced by the GEM can be negligiblr.

2.6 Energy Resolution

The encrgy resolution of a detector is the ability to distinguish individuül linrs in

the spectrum. Assume that the formation of rrich electron is a Poisson process. i f this

wcrc thc only source of fluctuation in the signal. the response function of the dctrctor for

a mono-rncrsetic linr emitted by the source would have a Gaussian shape. The

convent ional expression for the energy resolution uses the full width at hülf maximum

i W H M ) of the peak:

- -

E p e d positio~r prok posiriort perlk pu.sirim

In ii p s dctrictor. the tluctuation in the output signal for a tixed gamma energy origintites

lrom the stütisticd tluctuations in the following phases in the signal dçvelopmctnt: the

number of primary rlçctrons. the interaction of the primary rlrctrons in the conversion

rcgion. the üviilanche. the noise of the prearnplifier and pedrstal shifts (pedestal shifts are

shiRs in the DC-offset of the sigui. which c m occur whcn the outputs from several

ch~inncls cire multiplexrd).

Neslrcting contributions that originate from the rexi out clectronics. the intrinsic

cnergy resolution of a proportional counter is dctermined by tluctuations in the charge

Q = M!r,,r. and thus from variations in the gain M and the number of primary rlrctrons

I I , , . Assuming that thesr two distributions are independent. then the intrinsic energy

rcsolution is givrn by:

with b the variance of the Polya distribution (typical value of 0.4-0.7 [38]). that describes

the singie electron gain distribution (sec Appendix B). The parmeter F is the Fano

hctor. which expresses the variation in the number of primary electrons at the

conversion. Dias et al. [U] did a Monte Carlo crilculation for F in xenon. which showed

that i t is snergy dependent. It ranges from 0.17 to 0.32 for x rnys in the energy range of

0.1 to 25 keV and has shnrp increase near the xenon absorption edgçs. Since the Fano

fiictor is relatively small. the main factor influencing the rnergy resolution is the gain

v;irilition in the avalanche. which depends not only on the gas but also on the operational

conditions of the detector [45j.

In the case of thc GEiM/GMD configuration. it is the fluctuation in the combined

y i n of the GElLIiSMD that determines the cnrrgy rrsolution. For incident x rays

dcpositing encrgy Et,,, . the number of the primary electrons is: u,, = E,,, / I V ( W is the

mcan tnr rgy IO producc an ion pair). Substitute this expression and Eq. (2-14) into Eq.

i 2- 1 3 ). to y ield the expression for the intrinsic enersy resolution R,,, of the GEWGMD:

R,,, = 2.35 ~100% ,

1 whcrc J = - + h . I t is a Fictor that is detrrminsd by the variation of the combined gain

IV

of the GEWGMD ( ib1 ).

For a sas detector. the best energy resolution that can bc: achirved is limited by the

\t'-value and the Fano factor F [46]:

in order to achieve energy resolutions chat approach this lirnit. care must be tÿken to

minirnizr potentially hamful effrcts of electronic noise. grometric non-uniformitirs in

the drtrçtor. and variations in the oprrating pÿrariieters of the drtector. Signiticant

operating parameters that can affect energy resolution are gas purity. gas pressure. and

stability of the high voltage applied to the detector [38].

In practice. factors other than r i , . F and b add to the spreüd in the output

amplitude (311. These include inhomogeneities in the electnc field at the edge of the

detrctor. miilfunctioning areas on a non-perkct substrate. or non-complete charge

collection due to the absorption of the photoelectron in one of the walls of the detector.

2.7 Detector Efficiency

Due to the window attenuation and finitr conversion gap. not al1 of the incident

photons interact with xenon in the detector. The fraction of the incident photons that

intcrücts is the quantum çfficiency ( q ) of the detector:

where the N,,, is the number of incident photons. and N,,,,,,,r the number of photons chat

interxt with xenon. The q is drtermined by the product of xrnon detector depth and

pressure. Sincc the conversion gap and window matrrial arc krpt the same as bcfore. the

q of the hybrid detector should be the samç as that of the GMD examined previously.

One of the advüntages for a photon-counting detrctor is that idrally its detectivr

qiicinium efficiency (DQE) is equal to q . The DQE of a drtector tells us how the signal-

to-noise (SNR) ratio varies through the system. It is detined by:

where SLVR~NZ) is determinrd only by the mean photon lunval rate and the time period of

observation. and S N R ( i t ~ g e ) is the SNR rneasured on the finai image.

The DQE of an energy integrating detector is degraded by the Swmk factor (q,,,ni )

Dl :

Typiçnlly. the Swmk factor is about 0.8 - 0.9 (11. It is determined by the distribution of

ihc cncrgy dcposited in the detector.

Eq. (2-19) doss not apply to a photon counting detector. sinçe the signal hrrr is the

n~tiiiber of photons. which follows a Poisson distribution. For a photon countin? drtccior.

ihc SiVRfirwge) is detemined by the following equation:

whcrc A',,,,,, ... is the nümber of photons thüi contribute [O the image. Similarly.

t i tu t ing Eq. (1-20) and (2 -1 1 ) into Eq. (2-18). wr get the DQE for a pho

driector in the ideal case where rvery photon which intrracts is used for the image:

Typically. sincr there is a set of cvrnt

milysis. the DQE of a photon counting detector

rejection criteria applicd in the rvrnt

s less thiin the quantum çfficiency:

where r is the numbrr of events rejected in the event anrilysis. In our detector it is about

5% - 15%. so only 8 5 8 - 95% of the detected photons are includcd in the image.

Chapter 3

Prototype Imaging System with GEMJGMD

The GEM is couplrd to the GMD aï a prr-ampliticlition structure to form a hybrid

dctcctor. The whole imüging system works on the principle of scanned projection

radiography (SPR). In this chapter. the GEM/GMD hybrid imaging system is described.

The techniques spplicd to the GEkl. including the test. rissrmbly and the high voltage

application to the GEM are introduced. The apparatus of the bçnch-top system. its

elrctronic rquipment and the fil1 gases are drscribed.

3.1 The Imagine System with GEiCVGMD Hybrid Detector

3.1.1 Ceometry

Scanned Projection Radiography System

Figiire 3.1 shows the grometry of the scanned projection radiography (SPR) systrm

built with the GEM/GiMD hybrid drtector. A stainlrss steel pressure vessel with a 1.5 mm

thick Leun window is used to house the GEMfGMD hybrid and the drift plane. Thrre is

ü vertical stase between the x-ray tube and the vessel on which the object is fixrd. A

cornputer-controllrd rnotorized micrometer on the stage can bring the object up and

down. In this way. the object is scanned by a Fm beam that is defined by a horizontal fiin-

berirn slit placed between the x-ray tube and the vertical stage. So the imaging system con

gcnerüte 2D images -one image coordincite is provided by the GMD modes. and the

7

Anti-Scatter Slit t-- Preampli fiers

Vertical Stage for Object

kV X-Ray Tube Fan Beam Slit

Figiire 3.1: The SPR imliging system built with the GEMIGMD hybrid detrctor. Mrnsurrincnts arc to the local spot of the x-ray tube and to the GEM top.

Conversion Gap 3.9 mm

Collection Gap 1.8 mm

X R a y Photon

, . . .

Detector Window

Drift Plane (-2800 V)

- -. - - ,_- GEM Top E lechode (-2070 V)

- -, - - +- Kapton lnsulcrtor x - "-- GEM Bottom €lemode I- 1 240 V)

Figure 3.1: D iag rm for the geometry of the GEWGMD hybrid detector. The HVs presentrd in the diagram are for Xe:CH4 at 4 atm. and the gris gin is L35.

second one by scanning. An anti-scatter slit is placed right before the drift plane to

remove the x rays scattered out of the fan bem.

The geornetry of the imaging system is c-ed over from that of the previous GMD

imager. eacept that the GEM is coupled to the GMD and an alignment frame is added to

the systcm. The dimensions labeled in Figure 3.1 are a bit different h m the previous

system. but the geornetry magnification factor is still about 1.1.

Geometry of the GEWGMD Hybrid Detector

.A.\ \hown in Figure 3.1. the GEM is pliiced brtwren the GiMD and the drift plane.

an rilumini~ed Myiar sheet. They are housed in ti pressure vessel. with the früme of the

drift plane toiiching the Lexan window of the vessel. The sensitive area of the GEM foi1

is 20 mm x 70 mm. There is a copper strip rxtending oui frorn the GE,M elecrrode on

each side of the foil. to which the HV is ripplied. One of the advantages of the GEM

dcvicw is that the y are alrnost sr If-supporting. thrreforr permitting the realization of light

dctcctors with minimum kame thickness. In our case, the frame of the GEM is made of

G 10. and i t is 1 mm thick. The drift plane. GEM. and GMD are individually glued on thin

trames. cind supported with insularing screws and nuts. These nuts also serve as spücers to

cldinc the gaps. The conversion gap betwrrn the GEM top and the drift plane is 3.9 mm.

where the ionization happens. The conversion gap is krpt the same as in the pervious

GMD iniagcr so thnt we can compare the performance between the GEMfGMD and the

GMD only. The collection Cap between the GEM bottom and the GMD is I.8 mm. where

the electrons tire entracted from the GEM holes cuid collected by the GMD anodes.

The sensitive volume in Our detector is 3.2 mm x 50 mm x 3.9 mm (width ( x ) x

tieisht ( y ) x drpth (2). Figure 2.1). The height is siven by the GMD. Since we only use

the ccntral 16 anodes of the GMD and they have 200 pm pitch. the width is 3.1 mm.

3.1.1 GEM foi! Fabrication

The hbrication technology for the GEM was developed by the CERii Surface

Treittment Service. where our GEMs were manufactured. Since the nurnber of holes on

one foi1 is very large (about 46.000 on 3x3crn2). techniques which allow simultaneous

liole production are required. So far. the available technologies include: wet or dry

etching. ultrasonic drilling or grinding, and laser drilling.

Typically. n GEM mesh is produced by conventional photolithographic mrthods

followcd by chemical etching (39 1. Fint. the pattern of holes is engraved by conventional

photolithography on the copper on both sidcs of the foil. Then the hole pattern is used as

the mask for the etching. and the channels are opened with a Kapton-specific solvent

tloni both sidrs of the loil. Due to the chemicül process employed. the GEM holes have a

doublc-conical shape with the diameter in the center of the Küpton slightly smclllrr thün

thnt iit the copper surface.

3.1.3 GEX1 Testing Procedure

We tesrcd the GEM in our lab in two steps before mounting. First. in the clean

roorn. the resistance in air between the two sides of the GEM foil is merisureci with an

ohninirtcr. and i t is iicccpted if the value exceeds 1 GR. Second. since the HV power

siipply is not available insidr the clean room built in Our lab. the GEM is mounted and

sealed in thc vessel with air in it. and is brought out of the clean room. The high voltage

crossing the GEM is fed into the vessel through the high voitagr connectors mounted on

the coïer of the vcssrl. and thrre is a high value protection resistor connectcd in seriss

with the GEM. The lrakrige current is measured as the voltage is slowly incrcased. It is

accepted i f the Icakage current is below -5 nA at 500 V between the electrodes. This

corresponds to an inter-elrctrode resistance above 100 GR.

The GEM foi[ is always handled in the clem room wirh glovrs. Contact with din or

hnrd surfxrs was iivoided ris this could generite shorts.

Our GEM is very small (70 x 70 mm). It is glued on a supporting GIO frarne. In

ordrr to get uniform gain over the GEM. the foil has to be stretched well when it is jlued

on the final frarne. First. we tension-taped the foil on ii temporary frame which was the

same as the final frame. then overlaid it on the final frarne previously coated with epoxy.

Thrrc wrre two screws on the temporary frame at opposite corners. which helped to align

the sensitive area of the srna11 GEM foil with the final frame. So the GEM foil was

sandwichrd by the rwo frames. and they were compressed with clamps. They were left in

the oven for about Z hours at 80°C. This guürantees good mechanical tension of the GEM

at room temperature.

Before the GEM foil was assembled in the final detector. it was grntly blown with

nitrogcn in the clrün room. Thrn the üssernblrd detector waï left for several hours on

open gas tlow. Bcfore gas tïlling. the vessel wüs svacurited with o mechriniciil pump. The

asscrnbled dettxtor was rxposed to vacuum for a kw minutes. which was tound to

improvc its operation in terms of the maximum attainable gain [32]. It might be brcause

water absorbed in the Kapton was rernoved.

HV for the GMD Cathode

HV for the GEM . . HV for Drift plane b - - &

- BNC - - Connectors

8 anode signais - 8 anode signais + cathode signal --- - - + -- readout

readout - -- -.-

BENDIX- 1 0 PIN BENDIX-8 PIN

+ - - Vessel cover

Figure 3.3: Diagrarn of the connectors mounted on the vessel covcr and t heir îünctions.

3.1.5 High-Voltage Application

In our hybrid drtector. two Bertan HV power supplies. rnodcls 380 and 1720

(Benan Associates Inc.). eüch with two independent HV outputs. are used to supply the

hish voltage to the drift plane, the GEM elcctrodes and the GMD cathode.

Wr nred three feed-through connectors to feed three high voltages into the pressure

vessel to the drift plane. the GEM and the GMD cathode. We have three BNC connectors

and two Bendix connectors (PT07H-12-10P and PT07H-12-8P) on the cover of the

pressure vessel. The orientation of these connectors on the vessel cover is shown in

Figure 3.3. One of the BNC connrctors was used to read out the GMD cathode signal

beforc. In order to get an extra pin to read out the cathode signal and save one BNC

çonnector for the HV application. we chansed one of the two Bendix connectors on the

vessel cover from right pins to ten pins. One of the two extra pins is usrd to read out the

cathode signal. the other one is lcft floiiting. The remaining eight pins are still used to

rcüd out signals from eight modes. The other Bendix connector is left unçhiingcd. Its

cisht pins cire usrd to read out the signals from the other eight anodes.

There are two methods for powcring GEiMs: 1 ) Direct application of the rrquircd

wltages to ecich of the two slectrodrs through independent. current limited HV power

supplies with a high-value protection rcsistor (typically 10 iMR). as s h o w in Fisure 3.4

iü): 3 h single power supply with a resistivr divider network to provide the potrntial

Licross thc GEiM. ris shown in Figure 3.4 (b).

Method one is convrnient for systematic studies. sincr it pcrmits one to vary the

poicntial differencr betwrrn any pair of elrctrodes and keep al1 others constant. In ordrr

to avoid enceeding maximum local differences. the voltages have to be ramped very

slowiy. In the case of discharges. the current frorn one unit can feed into the other with

unprediciable results. A solution to this problem is to connect a Ixge value resistor ro

gound. through which the leakage current from each unit will go [33]. Wr did not use

this method because this method cm destroy the GEM if one of the two supplies trips

Ircivino the othcr HV supply on. This would increase the voltage across the GEM

suddeniy and drstroy the GEM.

Ws chose method two. Le. the oprrating voltages are iipplied to the GEM ttirough

resistive dividers. The value of each resistor is shown in Figure 3.5. The Benan 1792

powcr supply is used to supply the high voltage. The ratio of the voltage ciifference across

the GEM to the voltage at the GEM top is fixed at 2 5 .

Mrthod two is safer than rnethod one because any sudden HV supply trip prociuces

O V across the GEM. However. the resistor chain rnethod to supply GEM high voltages

c m be usrd only if the current induced by x ray absorption and avalanches is srnall. This

was the case for our meaîurements. If the x-ray flux were very high (such as was used by

D. Gobbi [14] to demonstrate high count rate capability of the GMD). the radiation

induced current in the detector could be signiticant. The increased current tlowing

through the ~~oltage divider resistors would be accompanied by a voltage drop across the

GEM leading to a lower gain. Method two cannot be used for hi$ rate operation.

However. it was acceptable for our work and we used it because it minimizes the

possibility of destroying the GEM as stated earlier.

Drift Plane, GMD Anodes and Cathodes

The drift plane and the GMD cathodes are at high voltage. As shown in Figure 3.5.

thc high voltqe for both the cathodes and drift plane is filtered through an RC circuit

(RC = 1.2 ms) to filter the tluctuütion from the power supplies. The central 16 cathodes

arc sroiiprd together and hcld at high voltage. The central 16 anodes are grounded

inclividiililly.

In ordrr to provide a uniform collection field for the central 16 anodes. a group of

eight cathodes to either side of the central sixteen cathodes is held rit high voltage. and a

group ot'eight anodes to either side of the central 16 anodes is grounded.

The circuits used to supply the high voltage to the drift plane and the GMD cnthodr.

and the ones used CO read out signals from the GMD modes and cathodes were ;~ssernbled

before I staned my work on this projrct. Thc values of these resistors and capacitors wrre

not chanseci [21].

The Bertan 380 is used to supply HV to the GMD cathode and to the drift plane.

The voltage must be ramped gradually. Meanwhile. the output of the cathode is

monitored on the scopr. If breakdown appem. the HV has to be decreasçd.

- - - HV - GEM Bottom &)

s ,-, GEM Bottom b)

Figure 3.4: Schematic of the two methods to supply voltage to the GEM: ( a ) use individual HV supplies: (b) use resistor divider.

g 2 2 M n ---- - G E M Bot to rn (Lg)

Figure 3.5: Schrmatic of the high voltage application in the GEWGMD hybrid drtector. The part within the dotted line is housed in the pressure vessel.

3.2 Apparatus

Since the geometry of the imaging system is carried over from the GMD imager,

most of the components used in the system are maintained. As shown in Figure 3.1. the

apparatus of the system consists mainly of the x-ray tube. fan-beam slit. object stage.

ünt i-scatter sli t. the pressure vessel, and the alignment frarne. 1 designed the alignment

Iranic. The rest of the apparatus wu built before I started to work in the lab. Plense refer

to D. Gobbi's thcsis [24] for details.

30 kV X-Ray Tube

Thc x-my tube is an Oxford XTFjOl 1 5-50 kV, 0- 1 mA tungsten-target K-ray tube

(Oxford Instruments Inc.. 176 Tçchnology Circle. Scotts Valley. CA 95066 USA) with a

77 um x 196 ym focal spot.

In niy experiments. the power of the tube w u kept 2 75% of full power due to the

h ~ i t çnpacity of the tube. The tube current WLS chosen according to tube voltage usrd.

Fan-Beam Slit

Two pieccs of lead are tïxed on an aluminum stage to f o m the fin-beüm slit. Two

srtiril1 foils are used to derine the height of the hn-beam d i t . It is 80 yrn in our system. It

has to be checked regularly and clraned by blowing with high-pressure air.

Objeçt Stage

Thttrc are two micrometers on the objrct stage to move the object venicdly and

horizontülly within the object plane. In this way the object is scanned by the Fm beam.

The horizontal micrometer is tuned mrinuaily. and the vertical one is controlled by the

compter.

Anti-Scatter Slit

Two small pieces of lead are glued to a U-shliped sheet of steel to form a 110 pm

lead d i t . It is uscd to elirninate those x-ray photons scattered out of the fan beam. and to

define the field of view of the detector. It is placrd right in front of the drift plane. about

5.5 mm from the GEM top electrode and 7.5 mm from the GMD print.

Pressure Vessel

The GEWGMD hybrid detector is housed in a 6-liter cylindrical stiiinless-steel

prçssurc vcssel. It w ~ s tested to 9 atm in 1999 by Philippe Gravelle. the technician in Our

Irrb.

Aiignrnent Frame Detector Window

- Square Object

A Staae -----'

Pressure Vessel

Figure 3.6: Diqram of the alignment setup.

Alignment Setup

Alignmcnt is very criticai in the experirnents. especially in the LMTF mecisurement.

It was done in three steps. First. the front face of the pressure vesse1 and the object plane

have to be paralle1 to cach other. and perpendicular to the x-riiy bram: second. the fan-

beam slit and the anti-scatter slit have to be aligned: third. the sensitive area of the

detector has to be at the center of the x-ray beam. i.e. aligned with the focal spot.

In the previous setup, there was not a reference point or plane for the alignment. I

designeci and built an "alignrnent frame", which is shown in Figure 3.6. Two square

aluminum bars ( 1 10 cm long) with screws at each end are fixed at two sides of the

systrrn. dong the longitudinal direction and perpendicular to the vesse1 window. The two

bars are movable with the help of the screws at each end. and they are used as the

rcference. First. they are tunrd to be perpendicular to the front plane of the x-ray tube

with the help of a square (the x-ray tube is fixed on the optical bench). Then the objrct

plmc and the vessel window can be tuned to be perpendicular to the bar with the square.

in tliis way. we know the objrct plane and the detector window are parallel to rach other

and perpendicular to the x-ray beam.

Thc fan-bcan slit is mounted on a micrometer stage. which cari br moved

wrticrilly. The maximum count rate will be reached when the fan-beam slit and the anti-

scatter slit cire aiigned. So the count rite at diffrrent micrometer readings is plorted. and

the Lin-beam slit is fixed at the position for the maximum count rate.

Thcre is a micrometer under the pressure vessel. It can move the vesse1 Iaierally.

The rniixirnurn çount rate is obtained whrn the detector is aligned with the lowl spot. So

the count rate at different vessel positions is plotted. and the vessel is fixed at the position

iit which the maximum count rate is obtained.

Once the rili~nrnent is done. the two bars c m be tïxrd with nuts to keep the

rilignrnent unchanged during the rxprriment. which is more solid and reliabic: than before.

3.3 Electronics

A block diagrarn of the imaging electronics is given in Figure 3.7. The charge

si_onrils from 16 GMD anodes are amplified by the pre-amplifiers (QPA07) and

amplifiers. Then they are digitized in the andogue-to-digital converters (ADC) and

transfcrred to the computer to generate images. The imaging scan and data acquisition are

controlled by a 40 MHz 80386 PC through a CALMAC interface. AH the raw data are

wved onto ri 6 Gbyte spiice in the hard drive of the Unix "Bragi" machine.

16 GMD rinodcs rire r d out individually a

( grouped and uscd as

lb

Figure 3.7: Block diagram of the imüging elecrronics.

GEMIGMD -> Pre-Amplifier . 3 hybrid (QPAOî)

Amplifier Board

Multichannel Andyzer (MCA)

9' I I I

Figure 3.8: Block diagram of the simple pulse height analysis systrm uscd in the gain and spectrum meÿsurements.

9 . :

ORTEC 143 PC (pre-amplitïer)

G EiM/GiMD hybrid

ORTEC 350 (amplifier) 3

At this stage. we mainly concemed about the physical performance of the detector.

So our imaging system was built with off-shelf electronic modules. The electronics was

built before 1 came to the lab, but 1 had to rnke them work properly again.

Though the imaging electronics can provide energy information for sach image. a

commercial pre-amplifier and amplifier EG&G Onec 142PC and 450, and an Onec 916

MCA are uscd to test the energy response of the imaging system for their low noise. As

s h o w in Figure 3.8. a pütch board allows the 142PC to be connected to üny combinütion

of rhe 16 anodes or to the cathodes. The pulses from the 142PC are shaped by an Onrc

450 amplifier cind thrn sent (via 6 V positive unipolar output) to an Oncc 916 MCA card

that is housed in the PC.

The QPXO7 charge sensitive preamplifiers were developed by Tom Zimrncrman at

Rrmilnb [47]. These nmplifiers accept cither positive or negative input and provide

rnirrorcd positivc and negative output to drive a 50 R transmission line. There are 8

chnnncls per chip. and each preamplifier board (also drveloped by T. Zimmrnnan) h;is

128 chmncls. Therc are two protection diodes of opposite polÿnty on sach preamplitier

input as a prccaution üpainst high-voltage transirnts.

The QPA07 amplitiers have a nominal 6 ns rise time and 35 ns recovery tirne for an

impulse input whrn the input capxitancr is 10 pF. The nominal maximum gain is 500

mVlpA for a constant current input. and 17 rnVlfC for an impulse charge input.

Amplifier Board

The diffttrential signal from the QPAOl is cmied via a 5 rn shielded ribbon cable to

the amplifier board which houses 64 Signetics NE5539 operational amplifiers ( 17 of

which cire usrd. 16 for anodes and 1 for cathode). This amplifier board w;is drsigned by

Ernie Neuheimer. It amplifies the signals from the QPAOl into a negative unipolar signal

in the r a n s of 0-0.5 V. The quoted bandwidth for these amplifiers is > 20 MHz.

The amplifier board used in the GMD imager was not available when I started my

work on this project. The one that was available did not work ai the beginning. 1 did the

troublcshooting of the board with the help of Emie Neuheimer. This board was isolated

from the imaging electronics and tested independently. Test pulses were injected from a

pulser. and the whole circuit on this board was traced. Oscillation problem showed up

when this board was mounted on the rack and connecred with the QPAO2. It w u found

that thcsc wcre mainly due to the following reasons.

First. there are digital and analog circuits on the samr board. The ünalog part is the

operationof amplifiers, and the digital part is the discriminators wbere the analog signds

froni the Opümps are digitized to binary signal (O and 1 ). Since the digital ground and

analos ground werc not isolated well. tluctuations on the digital circuit entered the malog

circuit and triggered the oscillation. Some inductors and ciipacitors were added to the

circuit to get rid of the oscillation. Second. the ampliticlition factor of this board was too

hish. I t wcis decreased from 10 to 3. Third. to set a ncgütive unipolür signal from rach

çhonncl. o spçcific polarity for e x h input of the Opamp NE5539 is required. The

polnriiics of thc output signals from the QPAOl were not mritchrd to the requiremrnt. A

patch board was drsigned and made by myself to fcrd into the Opamp NE5539 cach

difkrcntitil signal output tiom QPAO2 (32 chünnels from the 16 anodes and 2 channels

from the cathode) with correct polarity. Finally. the threshold of thosr discriminators on

the amplifier board wcis set very high (-170 mV) which was found helpful in stopping the

uscill;ition.

Trigger Circuit

The cathode signal is used as the trigger signal for the whole imqing electronics.

The circuit of the triggrr system is shown in Figure 3.9. The cathode sisna1 is fed into a

disçriminator (Lscroy 6138) through a Linear Fan WOut module (Lecroy J18F). When

the amplitude of the cathode signal is higher than the threshold set on the discrirninator.

this rvent is regÿrdrd as a vdid event. Then the Gate Gsnentor (#2) (Lecroy 722) is

tumed on to bloçk the consequent events until the NiM driver (Lecroy ND0271 sends a

puise to the Gate Gcnerator indicating that the data acquisition cycle is completed. The

20 ns delay ensures that the tngger pulse does not veto itself.

Circuit for Dead-Time Correction

The period during which the Gate Generator is on. as mentioned above. is called the

..derid iime" of the system. Events which hüppen during this time are not registered by

the slectronics. Two scders (GEC-Elliott Sr1608) are employed in Our system to correct

thc losses.

As shown in Figure 3.9. the gate generator ( # I ) (Lrcroy 222) is used u a pulser.

Once the powcr is on. the connector has to be shaken gently to stün the self-triggering

through a delay line to provide 385 pulses per second. The total number of the pulses is

the rcal time. and the nurnber of pulses counted when the systrrn is not busy is the live

timc. The dead timr correction is realized in the event analysis software.

S t q t O Gate Generdor _ - A

# 1

Figure 3.9: Block dirigram of the trizger circuit and the red-timdlive-time scrilers.

Analog-to-Digital Converter (ADC, Lecroy 2249W)

The job to be performed by the ADC is to generate a digital number that is

proportional to the amplitude of the pulse presented at its input.

The ADC module LeCroy 7249W used in our system only accepts negative polarity

signal. The charge signal from the amplifier board is digitized to a maximum 1 1 bits of

prçcision by intrgrating the input current over a gate width which is set to 100 ns. The

sipnals from anodes and cathode are delayed 50 nu before thry are fed into the ADC to

rnsure thrit the ADC does not miss the leading rdge OC the signal pulses.

3.4 FiIl Gases

The fil1 gascs consist of a noble g a and an organic quenchsr. and thçy are mixcd

togcthcr in ii 9: 1 ratio in the vesse1 by partial pressure. Two f i l 1 gÿs mixtures were usrd:

liryn:rncthane (Xr:CH4) and xenon:mrthanr (Xe:CHa). The former mixture was fillcd

into the vt.sst.1 tïrst lifter the detector w u itsscmbled. and w u mainly used to test the

perforninnce of the new irnaging systcm.

The Xe:CH4 mixture was used for irnaging. Since it has the highrst atomic number

within thc sinblc noble gars. xrnon h a the highest photoelrctric absorption coefficient.

Bcsidcs. its K shrll photoelrctrons have lower cnrrgies and consequently lowrr ranges.

For cxnmplr. for the x-ray enrr_oy of 40 keV. the range of a photoelectron in argon at I

atm is - 14 mm. but - 0.3 mm in xenon [43].

Chapter 4

Experiments and Results

in our dctcctor development lab. the physiciil ciiariicteristics of the GEM/GMD

hybrid detector wrre investigated. including the gain. the energy resolution and the

spntiül resolution. Erich of these parameters was rneasured for Xe:CH4 9: 1 at each of 1 . 1 .

3 ;ind 4 atm. In this chapter. the experirnental mcthods are describeci and results are

presented. Finally. some imcljes obtained with the hybrid detector are shown.

4.1 'Voise Reduction

During the whole experiment. the pick up noise level on the signal hod to be kept at

a reasonübly low level. and noise reduction is not a minor tlisk. The following is a list of

thc ninin steps to reduce noise that I did in the experiment.

The 3M connector on the pre-amplifier board introduced spike-like noise. which

was due to a loose connection. Tape was used to jet it bettcr connection to reduce

this noise.

The Bcndix connectors on the cover of the pressure vessel introduced burst noise.

A plastic cable strap was used to get a better connection and the noise was gone.

The pressure vessel and everything connectrd with the vesse1 had to be insulated

h m the bench.

High frequency noise on the long signal cablr and power supply cable was

reduced by adding ferrite cores.

4.2 Gain Measurement

The gain was measured from pulse height analysis. The simple pulse height analysis

system is shown in Figure 3.8. Amplifiers EG&G Onec 142PC aiid 450, and Onec 916

MCA (embedded in the PC) were used.

4.2.1 System Calibration

Betore the gain measurements. the system. i.r. the amplifiers and MCA. was

calibrateci iising ii test input to determine the pulse height scde of the analysis system. so

that the gain vcilucs obtained from this analyzer could br compared to values taken at

othcr limplitïtx mi MCX srttings.

Ideall y. the prrümplifier and amplifier thüt are used to read out the signal from the

dctcctor would be sufficiently linear. and the MCA woiild perform a perfcct linear

conversion of pulse height to channel numbrr. Under thçsr conditions. (i plot of channrl

number versus pulse height would be ü simple striiight linr:

wherc Q is the charge signal collected by the detector and is the input signal of the

iinalyzcr. zliti is the channrl number corresponding to the s i x of charge signal Q , and

d i 0 is thc zero offset of the MCA.

To test the linecirity and detemine the zero offset of the analyzer. a pulser is able to

providr points for a cülibration plot. The pre-amplifier EG&G Onec lJlPC hÿs a test

input with a nominal 1 pF input capacitor. A series of pulses with known arnount of

chorgr Q ore injçcted into the 142PC from a pulser ( Q = VC. V is the voltage of the test

pulsc. and C is eqiinl to 1 pF). and a series of sorresponding Gaussian pulses are captured

by the MCA. From the iMCA. the channel nurnber of the peak of each Gaussian is

c:ilculüted. By applying a linear fit to thesc points (Q. c h ) . chOand A rire obtained.

Fisure 4.1 gives an example of the ciilibration plot of the analyzer with a test pulser.

Figure 4.1: A calibration plot of the anrilyzer with a test pulser.

In order to determine the energy scalr of the pulse height distribution. calibration 'iFe or "1 source. c.3. Am. 1s needed. These sources emit chrirxteristic lines with known

rnrreirs. so we can calculate the primüry charge y ciccording to:

where Etlep is the energy of eüch known spectral line. and I.V is the energy for producing

one ion pair. Drnoting the gain of the detector as W. then the charge signal dctectrd by

the detcctor is given by:

Substituting (4-3) into (4-1 ). we get the calibration line for the channel numbcr versus

rnrrgy drpositrd:

So wc cm calibrate ch0 and B by several known spectrum lines. The more iines we use.

the more accurate the calibration.

Source Container Antl-Scatter Slit

Vesse! Wall

man Wndow Drifi Plane GEM GMD

Figure -1.7: the setup for the meüsuremeni.

4.2.2 Method for the Gain Measurement

The p i n of the detector was measured with a calibration source m. When

mclisuring the p i n . the absorption spectrum w u criptured by the hybrid detector and

rccorded by the MCA. Eiich peak in the spectrum corresponds to a specific cimount of

charge q drpositçd in the hybrid detector. which is calculated by Eq. (4-1). Wr know the

rellitionship bctwrrn the MCA channel number and the charge injected into the

preamplifier. i.e. the charge collected by the detector, which is given in Eq. (4-1).

Combining these two equations provides an expression for the detector gain M:

Q (clin - ch0)l A M = - = 4 eEIW

In Eq. (4-3. ~110 and A are determined by cdibrating the (prearnplifier + amplifier + M C A ) using a pulser (as shown in Figure 4.1). and ch,t is the channel number into whiçh

thc ccntroid of the spcctrum line with chürücteristic enrrgy E fd!s.

During the gain measuremrnt. the rinti-scattrr slit was in place. The ' J ' ~ m source is

s w e d in a cylindrical container. A dit collimütor wris placed in front of the source

contiiiner to detlne the beam. The distance between the source and the collimator is about

46.4 mm. and therc arc about 87.5 mm brtwt.cn the source and the drift plane. as shown

in Figure 4.2.

4.2.3 Results

4.2.3.1 Detector Gain vs. GEM Voltage and GMD Cathode Voltage

The gas gain in the GEM~GIMD is very compiicated. The GEiM gain is not only

reliited io the tield in the holes but is also affrcted by the outside field. Le. the conversion

tïcld and the collection t\cld. In order to get maximum gain. thrse fields have to be

optimized. i.e. the collection field should be rnuch highrr than the conversion field. In our

sptern . since the GEiM voltage is supplied by ii resistor network. the highest collection

field in trrms of safe oprration for the GEM is limited by the voltage across the GEM:

where V, is the voltase at the GEM bottom: AVcc, is the potential across the GEM. the

I'cictor 1.5 is determinrd by the resistor network: d is the thicknsss of the collection gnp

( 1.8 mm): is the average voltage on the GMD print. it is determined by the GMD

cathode voltage V,.. the width of the GMD anode and cathode. and the gap between the

GMD anode and cathode. For AV,, = 500 V. the collection field ( E , , , ) could reach 4.2

kV/cm at maximum (assume V, = O). For a given AV, . the collection field would be

reduced hy increasing V,. to get gain From the GMD. Bellazzini et c d . [39] round that an

cfficiency of 808 can be reached with Et,,,= 10 kVkm in NelDME (50/50). Therefore

the gas p i n in our system h a not been exploited completely.

The dctcctor sain versus the GEM voltage (dl/,,,,,) and the GMD cathode voltage

( V, i w s rncrisured in Xe:CH4 korn 1 to 1 atm and in Ar:CH4 rtt 1 atm. Triblc 4.1 lists the

viilut. of the conversion field for erich meiisurement, and the collection field at the highest

gas zain rcxhed for each mrasurernent. When cülculating the collection field. was

1 iissumcd to bt: about TV' -

T;iblc 4.1 Conversion field and collection field (at the highest gas gain) applicd for the gain meristirement

Figure 4.3 to 4.6 show the combinrd gas gain curves for the GEiM couplcd to the

GMD in Xr:CHJ at 1-4 atm. and Figure 4.7 shows the gain curves in Ar:CH4 at 1 atm.

Al1 tlic gain cuwes for GMD alone are reproduced from D. Gobbi's thesis. Compared

with the GMD alone. the hybrid drtector permits one to operate the GMD at considerably

louer and safer voltages at given gains.

l

I i i tn i Xé:CH4 2 atm Xè:CHJ 3 atm Xt-:CH4 4 atm k C H 4 1 atm Ar:CHJ

Conversion field (kV/crn)

L .8 1.8 1.9 1.9 1.8

Collection field (kV/cm)

3.0 (LIV,,~, = 450 V. V , = 190 V ) 3.7 (LJV,,, = 576 V. V,. = 380 V ) 4.4 (LIV,~,, = 688 V. V,. = 480 V) 5.5 (LW,, = 830 V. V,. = 500 V) 3.1 (AV,,, = 445 V. Vt. = 700 VI

O AVgem = 430 V

c 300 - Xe:Ci-i4 Q 1 atm

200 -

100 150 200 250 300 350 400 450 500 Cathode Voltage (V)

Figure 1.3: Giis gain for the GEMIGMD hybrid vs. AV,,,, and V, at 1 atm in Xc:CHJ. The gris gain from the GMD for each pressure w u reproduced from D. Gohhi's thesis [?-Il.

300 -

Xe:CH4 @ 2 atm

Cathode Voltage (V) 200 250 300 350 400 450 500 550 600

Figure 4.4: G u gain for the GEAWGMD hybrid vs. AV,, and V, at 1 atm in Xs:CHI. The gas gain from the GMD for each pressure was reproduced from D. Gobbi's thesis [XI.

GMD only + / 50 1 350 400 450 500 550 600 650

Cathode Voltage (V)

Figure 4.5: Gas gain for the GEMIGMD hybrid vs. AVgcm and V, at 3 atm in Xe:CHJ. The güs p i n trom the GMD for cach pressure was reproducrd from 0. Gobbi's thesis [X I .

Xe:CH4 8 4 atm

350 400 450 500 550 600 650 700 Cathode Voltage (V)

Figure 1.6: Gas gain for the GEWGMD hybnd vs. AV,, and V, nt 4 atm in Xc:CHJ. The gas gain from the GMD for each pressure was reproduced from D. Gobbi's thesis [24].

O O 1 O0 200 300 400 500

Cathode Voltage (V)

+ AVgem = M 5 V + AVgem = 440 V

Figure 1.7: Gu gain for the GEiM/GMD hybrid vs. AV,, and V, ai 1 atm in Xr:CH4. The gas gain from the GMD for each pressure [vas reproduced from D. Gobbi's thrsis [NI.

Ar:CH4 @ 1 atm

1 50 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

Conversion Fieid (kVtcm)

O AVgem = 435 V

300 -

Figure 4.8: Detector gain vs. the conversion field at 4 atm in Xe:CH+

4.2.3.2 Detector Gain vs. Drift Voltage

Figure -1.8 shows the derector gain versus the conversion field at 4 atm with the

GEM potentials and the GMD cathode voltage fixed. There is an increase of gain with the

conversion tkld. Notice the electron loss when the conversion field is lower than 1.5

kV/cm.

1.3 Energy Spectra and Energy Resolution

The energy responsr of the hybrid drtector çan bc rnrasured either by the QPAO2

prc-amplifiers and their amplifier board. or by the commercial pre-amplifier Ortec 14ZPC

and amplifier Ortrc 450. But the Ortec 147PC and 450 have better linearity and lowrr

noise compared with QPA03. whiçh was designed to provide a fast signal rathrr than to

hc uscd for spectroscopy. So the Ortec IJ2PC and 450 were used in the meuuremcnt of

c n e r g speçtra from the radioactive sources and frorn x rays for the iMTF measurement.

Thc süme pulsc height anrilysis system LS s h o w in Figrire 3.8 was usrd. Somr x-tiiy

rpcçtrii collecteci by using the QPAO3 are presented here too.

When collecting the x-ray spectra. the fan-benm slit and mti-scatter slit were in

place. Thc fan-bearn slit was removed to increasr the photon tluence whrn collrcting the

spcctra from the calibration sources. and the srtup for the gain mrasurement shown in

Fisure 4.2 w u used. Only the sensitive areii on the GEWGiMD that is used for imaging

wtis irradiatcd during the spectmm mcüsurement. Somr of the electrons produced near

the boundary of the sensitive area wiIl be lost. However. that gives the energy

information usrd for the imaging. The reason thüt the full area of the detector is not usrd

for the spsctnim measurement is because the gain at different areas of the GEM and the

GMD is not rilways the same.

For al1 the rnergy spectrum rneasurements. the Ortec 450 w u set to positive

unipolÿr output. I ps shaping time. 75 - 50 gain. The central 16 anodes of the GMD were

grouped tosrther to collect the spectmm.

w 1 2 3 4 S 6 7 8 9 1 0 Absorbed Energy [keV]

Figure 4.9: " ~ e spectrurn obtained with Ar:CH4 at 1 atm.

4.3.1 Calibration Sources

5 5 For the rxperiments with argon. --Fr was used as the calibration source. The

sbsorbed snçrgy spectrum from the GEWGMD at 1 atm is shown in Figure 4.9. The full

cnerpy y peak iit 5.90 keV and the escape peak et 2.92 keV are shown in the spectrum.

After the y is ribsorbed in the argon. a 2.98 keV K tluorescrnce photon is produced for

IO? of the iimc. When this fluorescence photon escapes from the active volume of our

detector. the escape peak is obtainrd.

24 I 24 I Am spectra were measured for xenon p. Am produces srveral well-separated

lines uvcr the rmgc of 11-60 keV. Table 4.2 gives the emission enrrgics of %rn [-1S1.

@ 1 atm 1

Absorbed Energy [keV]

Figure 4.10: "',Am sprctrum obtained with Xe:CH4 ai 1 atm (0.1 1 keV per chlinne1 1.

Figure 4.1 1: " ' ~ m spectmm obtained with Xr:CH4 iit 2 atm (0.09 keV per

2 4 1 ~ m @ 3 atm

O 10 20 30 40 50 60 70 Absorbed Energy [keV]

Figure 4.12: "hm speçtrum obtained with Xe:CH4 at 3 atm (0.13 kcV per ch:inncl).

2 4 ' ~ m 8 3 atm

10 20 30 40 50 60 70 Absorbed Energy [keV]

Fisure 4.13: ' " ~ m spectrum obtained with Xç:CHa at 4 atm (O. 15 keV per chrinnel).

Table 4.2 Emission Energies (keV) and absolute intensities of ""~m

Figure 4.10 to 4.13 show the spectra prodiicrd by '" ~ r n in xrnon at 1-4 atm. The

full-energy peaks at 13.8 krV. 17.7 keV and 59.6 keV are cleluly seen. The two escape

peaks K{, and KI, are prodiiced after the Xe tluorescence photons Ku ( -33.8 L V ) and KI,

(-29.7 krV) escape tiom the sensitive volume in the detrctor. The y rmission at 26.3 keV

is hiddcn under the Kg escape peaks. The little peak betwern the 17.7 keV and K1, escape

pcük is the L escape pcak. which is about 4.3 keV below its piirent peak 26.3 keV. The

pcak ai 7 - S keV might be the copper tluorescence photons Ka (8.03 keV) which are

produccd when the incident x-ray photons or the Xe tluorescencr photons with energies

;ibovr. the coppcr K edse (8.98 keV) are absorbed in the çopper Iüyer of the GEM foil.

In the speçtnim tit 1 atm. the peak at 47 keV is due to the saturation of tbc Onec

450. The gain of . l jO was too high.

4.3.2 Energy Resolution

Np x ray

Thc ençrsy resolution was measured under two conditions: &:CH4 nt 1 atm and

Xr:CH, from I to 4 atm. T h energy resolution wiis determined by meauring the full-

width üt half-maximum (FWHM) of the 5.9 keV ' 5 ~ e peak for Ar:CH4. and of the 17.7

and 59.6 keV %m peaks for Xe:CH4.

The computing method for cdculating the FWHM of each peak from the obtained

\peçtr;i is described as follows. First. locate a peak. Since the two prAs concrmrd in the 'J i Am spectrum are resolvable, and the position of each peak. i.r. the energy of each peak

E,!. is known. this stçp is very straightfoward. Second. the data in the local area of the

pecik cire fitted with a Gaussian function. Third. the standard deviation CF of this Gaussim

distribution is ctilculatrd. Then we c m ciilculate the energy resolution by using Eq.

(2.13).

Y raY

26.3 (2.4%)

4 20.8

(1.93%)

LI 11.9

(0.8 5 % ) 59.6

(35.7%)

La 13.9

( 13.3%)

LP 17.7

( 19.3%)

When the effects of background continuum and peak tailing are significant. they

have to be subtracted lrom the peak area. The data in the channels on either side of the

peak are normally used to define the continuum, and an assumed linear curve is fitted to

produce an estimated continuum in the region under the peak. Channel-by-channel

hubtraction of the continuum then produces correcteci data for the subsequent Gaussiiin

f i t .

The rnerzy resolutions of the hybrid detector in Ar:CHJ at 1 atm. and in Xr:CHI at

1-4 atm are listed in Table 1.3. As shown in Table 1.3. the energy resolution varies from

1 to 4 atm in Xe:CH4. tt is mainly due to the variation of the operating condition of our

dritcctor. D. Gobbi [24] rnriisured the rnrrgy resolution of the GMD in Xe:CH4 at 1 atm

to bt. 10.9% for 17.7 keV. which is about 1.6 cimes worse thm these results with the

GEWGMD. The resolution for 5.90 keV in Ar:CH4 ( 7 0 4 ) obtiiined frorn our hybrid

deteçtor is a bit worse than D. Gobbi's result (18%). As rnentioned in Chapter 2. the

rnersy resolution is mainly affected by the variation of the _pas gain. Since our dctector

nin lit very low gain. the limiting energy resolution was not determinrd.

Tablc 1.3: Encrgy Resolution of the GEMIGMD hybrid for Ar ( 1 atm) and Xe ( 1-4 atm)

4.3.3 Spectra Used for Spatial Resolution Measurernent

Fisure 4-14 to 4.16 are the energy spectra for x rays usrd for spatial resolution

mrasurements and imqing rtt 2-4 atm. Each spectrum is normalized by uea in order to

compare thrm. Al1 the spectra at each pressure were collected tit the s m e time. and the

SICA was ccilibrated with '41~m. The average energies for these sprctra at different

pressures are listrd in Table 4.4. more high-energy photons are absorbrd in the detector.

Absorbed Energy [keV]

Figure 4.14: Energy spectra for x rays used for spatial resolution measurement and imaging ai 2 atm (0.09 krV per channsl).

Fi_oure 4.15: Energy spectra for x rays used for spatial resolution

0.05

0.04

measurement and imaging at 3 atm (0.13 keV per channel).

+

E.?

ZJ 1. Xe:CH4 Q 3 atm - !

0J 1 i

3 't ? ;':+13 kv

A 0.03 - '

O 10 20 30 40 50 Absorbed Energy [keV]

Xe:CH4 Q 4 atm 9.1

O 1 O 20 30 40 50 Absorbed Energy [keV]

Figure 4.16: Energy spectra for x rays used for spatial resolution -

meusurement and imaging at 4 atm (0.15 krV per chünnrl).

0.08 - - 50 kV + 2.4 mm Al

0.07 - - 50 kV + 2.4 mm Al + 72.2 mm PMMA

II

Xe:CH4 @ 4 atm

"0 1 O 20 30 40 50 Absorbed Energy [kew

Figure 4-17: Energy spectrum shows the beam hardening effect by adding a 77.2 mm PMMA (polymethy 1 methacrylate) filtration to 50 kV at 4 atm (0.15 keV per channel).

when the pressure goes higher. So as shown in Table 4.4, the average energies for each x-

ray tube voltage increase with the pressure.

Table 4.4 Average Energy (keV) deposited by x-ray beam in the GEMIGMD hybrid using Xe:CHJ at 2-4 atm.

For the spectra iit 10 kV and 50 kV. al1 the escape peûks are suprrimposcd and are

hidden under the continuous bremsstrahlung spectrum. which is due <O the broad

continuous incident x-ray spcctrum (the output of x-ray tube). So thrre are no escape

peaks visible in the bprctrü rven though almost al1 the K, and Ku tluorescence photons

escape from the Jetcctor.

The Y keV pçak that shows in the %m spectra üppears in al1 x-ray spectrü (20 -

50 kV) except the 13 kV spectrum. From Figures 4.14 to 4.16. the areü under this 8 keV

pcak decreaïes whrn the x - n y energy decreases. This is because there are less x-ray

photons above the Cu K edge (8.98 keV) iit lowrr tube voltage. The proportion of these

x-rüy photons sets so low at 13 kV thüt rhis 8 keV prak does not show in the 13 kV

spectrum. It mighr be hidden undrr the bremsstrahlung spectnim. This peak did not show

in the spectrü collrctrd by the GiMD alone [U].

Figure 4.17 shows the beam hardened by üdding 72.2 mm PMMA (polymethy l

methücrylate) to 50 kV x ray at 4 atm. The portion of a-rays with energy higher than the

senon K-rdge is increased by the extra filtration of PMMA.

50 kV + 2.4 mm Al 40 kV + 1.7 mm Al 30 kV + 1.3 mm Al 20 kV + 0.5 mm Al 13 kV + 0.25 mm Be

4.3.4 Spectra measured with QPAOZ

The QPAO2 preamplifiers were usrd for imaging. The absorbed energy information

for those photons that generate vdid signais in the detector during imaging cün be

2 atm 15.6 16.5 15.7 12.8 9.56

3 atm 18.1 18.9 17.6

10.1

4 atm 19.8 30.1 18.3

10.0

obtained when analyzing the raw imaging data. The software for image generation

provides this option. Figure 4.18 ;ives an exarnple of the cathode spectrum. It has lower

resolution compared with spectra collected with Ortec 1 J2 PC and 450 shown in Figure

-1.14 to 4.17. The cathode spectrum could in principle be used to üpply an energy window

to the imaging data for interactive image display, or be used for dual energy imaging with

one éxposure.

Figure 4.19 shows the spectra collected from 16 anodes individually. Thesr anode

hpeçtra are generated after the relative gain between the 16 anodes [24] are calibrated.

Itledly we should gct same spectrum from each anode. but due to the electron ioss at the

boundüry of the detector's sensitive volume. the spectra collected on those anodes chat

are close to the boundmy. i.r. anodes 1 . 1 and 15. 16. are distorted.

3500

3000

2500

cn 2000 C

C 3

s 1500 1 O00

500

O. O 400 800 1200 1600 2000

ADC value

Figure 4.18: A cathode spectrum obtained by QP402.

r

- Xe:CH4

-

-

-

-

-

L

50 kV + 2.4 mm Al @ 2 atm

A 1

Figure 1.19: Spectra from individual 16 modes obtained by QPAOI.

4.4 Spatial Resolution

The spiitid resolution of the GEM/GMD hybrid is determined by the Modulation

Trrinsfer Function (MTF) mesurement. The MTF characterizes how well the detector

cnn transfrr the contrat information at any spatial frequency. It is defined by [-El:

where M. the modulation (contrast) of signai. is drfined by:

herc ty is the amplitude of the signal.

4.4. I Modulation Transfer Function Measurement

The MTF of the drtcctor is obtained by rneiisuring the Line Spread Function (LSF)

of the detector. There rire two dits used for the LSF rnsrisurernents. The tïrst one is a Ièad

slit with width of 40 + 5 Pm. and was used at x-ray encrgies from 30 to 50 kV. The

second is a 13 k 3 pm wide steel slit. It w u only usrd at 10 and 13 kV. During the

mensurement. the d i t wüï mounted on the object stage with its orientation the same as

that of the GiMD strips.

The GEiM hole and the GMD strip structure introduce spatial phase shift to the

image ohtained from the hybrid. in order to rrduce this effect and ensure a consistent

result. ri standard proccdure was applird to the LSF merisurement:

First. for each LSF measurement. four slit acquisitions were taken 50 pm apart on

the image plane. The time for each LSF meiisurement depends on the x-ray energy used.

Second. in the position space. the four LSF acquisitions are shifted to be coincident

with each other. and then are summed and averaged. The average background WLS

substracted. and al1 pixels outside of the LSF peak were set to zero.

Finally. a Fast Fourier Transfonn (F'FT) is applied to the average LSF. Then the

absolute vaiiie of the transform was taken, and the result w s rescaled to 1 at zero to

provide the MTF:

whcre F ( Jdcnotrs the Fourier Trmsfonn.J'is the spatial Frequency (usually in units of Ip

mm" 1. Everything iifter the first zero was set to zero for each MTF curve.

4.4.2 Resul ts

Thc spatial resolution of the hybrid detector w u meiisured in Ar:CHa iit 1-2 atm.

and in Xe:CHJ at 1-4 atm. Al1 quoted measurements reflrct distances in the plane of the

GMD (the same distances apply at the GEM foil. since those rlcctrons amplifieci by the

GEM are brought pcrpendicularly to the GMD by the collection field. although the x-ray

beani itsclf is diversent). For the mçasuremrnt in Xe:CH4 ut 2-4 atm. the potential across

the GEM was incrrasrd as high ÿs possible. The output signal was monitored by a digital

scope to make sure the voltage across the GEM was not excessive. Once the GEM

potential wüs fixrd. the GMD cathode voltage wwüs chosen according to the I-ray energy

to maintain the cathode signal output from the amplifier board new its maximum of

-500 mV. For the measurement in Xr:CH4 at I atm. both dVV,,, and V,- were chanpd for

each x-rüy energy.

Table 4.5 G i l ~ gains used for the MTF measurement and imaging

l Xe:CE& 1 atm 2 atm 3 atm 1 atm

The gas gains used for the MTF measurement and imaging are listed in Table 4.5

for rekrence.

4.4.2.1 blodulation Transfer Function

The MTF curves for the GEWGMD hybnd detector are demonstrated in two

dilfrrrnt ways. In Figure 4.10 to 4.23. the curves are presented for increasing x-ray

energiss at eüch gas pressure. In Figure 4.24 to 4.77. the curves are demonstrated for

increasing gas pressures rit e x h x-ray energy.

When the x-rriy enrrgy is below the xenon K-cdge (34.6 krV). x rÿys intèrcict

prirnarily in the L-shrll (L-cdge is 5.32 keV). producing L-shell photoelectrons. Thrir

range increasrs with the incident x-ray energy. so the resolution of the detector decreüsrs

when the x-ray rnergy increrises from 13 kV to 30 kV. Once the x-rüy rnergy is over the

usnon K-edge, K-shell photoelrctrons are produced. Since they have shorter range than

thc L-shell photoelcctrons. the resolution gets better at 40 kV and 50 kV than that at 30

kV. On the other hiind. sincc there are more x-rays above the xenon K-edse when the x-

ray tube voltage is 50 kV. the resolution lit 40 kV is worse than that at 50 kV. But if the

tubc voltage could be increued funher. the MTF curvr would fdl since the K-

phoroeiectron range would increase with the energy.

The photoclectron's range is reducrd by the increased gas densiry. so the resolution

increases with the gas pressure. This is dernonstrated in Figure 4.24 and 4.75 for 13 kV

and 30 kV rrsprctivrly. The resolution for 40 kV and 50 kV increases from 1 atm to 3

ritm. but drueases From 3 atm to 4 atm (Figures 4.26.4.27). The reason for this is that the

meün cncrgy of the photons absorbed by the drtector increases as the pressure x depth

product of thc detcctor incrrases. so the range of the K photoelcctrons increues.

Fisure 4-33 shows the results at 13 kV of 1 atm. and 10 kV of 2 atm in Ar:CH4. and

13 kV of 1 atm in Xr:CH4. The K-edje of Ar is 3.70 keV. and the L-rdge of Xe is 5 -22

krV. So the K photorlrctrons in Ar have higher energy than the L photoelrctrons in Xe

with the süme incident x-ray energy. which resuits in longer range (Eq. (2-12)) and

therefore the worse resolution in Ar.

30 kV + 1.2 mm Al

Xe:CH4 @ 1 atm

0 1 2 3 4 5 6 7 8 9 1 0 Spatial Frequency [Ip mm'l]

Figure 4.20: iMTF curvrs for increiising x-ray rnergies at 1 atm.

40 kV + 1.7 mm Al 30 kV + 1.2 mm Al

Xe:CH4 8 2 atm

0.1 - \ O * . -. 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6

Spatial Frequency [Ip mm'']

Figure 4.7 1 : MTF curves for increasing x-ray energirs at I atm.

40 kV + 1.7 mm Al

Xe:CH4 Q 3 atm

0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 Spatial Frequency [Ip rnm''j

Figure 4.17: MTF curves for increasing x-rüy energirs tit 3 atm.

atm

Spatial Frequency [Ip mm-']

Figure 4.13: MTF curves for increasing n-riiy energirs at 4 atm.

Spatial Frequency [Ip mm-']

Figure 4.24: MTF cuwes for incrrasing j a s pressures at 13 W .

Spatial Frequency [Ip mm-']

Figure 4.25: MTF curves for increasing gas pressures at 30 kV.

Spatial Frequency [Ip mm-']

Figure -1.26: MTF curves for increasins $as pressures at 40 kV.

0.9

0.8

0.7

Spatial Frequency [lp mm-']

l . o \ ~ l l . v ~ l l r ~ - T 1 \

- \ \

\\

- \ \

(-;aiml 4 atm

I\ - \ Xe:CH4 @ 40 kV ' \

Figure 4.27: MTF curves for increasing gru: pressures at 50 kV.

Ar:CH4 10 kV 8 2 atm

Spatial Frequency flp mm7]

Figure 4.28: MTF curves of 13 kV at 1 atm and 10 kV üt 1 atm in Ar:CHJ. and of 13 kV at 1 atm in Xe:CH4.

- 50 kV + 2.4 mm Al 50 kV + 2.4 mm AI + 72.2 mm PMMA

Xe:CH4 @ 4 atm

0.2 -

0.1 - . 0 .O

O t 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 Spatial Frequency [Ip mm-']

Figure 4.29: The spatial resolution is improved by adding 77.1 mm PMMA ( polyrnethy l methacrylate) filtration.

Figure 4.29 demonstrates an irnprovement of the resolution when 72.1 mm PMMA

tpolymethy 1 methacrylate) is in the beam. This is because the proportion of x rays above

the K edge increases after the soft part of the x -ny beam is filtered out by the PMMA.

Table 4.6 lists the spatial frequency. f. at which MTFV) crosses 0.05 for each

mrasurement. Those numbers quoted in brackets in the table are the results from the

GMD alonr. Cornpared with the spatial resolution obtained from the GMD alonr [NI.

the resolution increases for almost ail the measurernents. as shown in the Table 4.6. One

possible rrrison is that. with the gas gain from the GEM. the GMD is operatrd with much

lowcr voltage. so the noise in the channel signais used to calculate the event crntroid is

lower. Addiiionally. the readout electronics were improved.

Figure 4.30 shows the effect of the enrrgy window on the spatial resolution. which

ih an intriguing advantage of the photon-counting radiogriiphy.

Tnblc 4.6 Spatial irtiqurncy (lp mm') at which MTFLî) crosses 0.05 (the numbrrs quotçd in the brückrts <ire the results from the GMD imager [241)

1 atm 2 atm 3 atm 3 atm 1 atm

r

- Full spectrum

50 kV + 2.4 mm Al @ 2 atm

\

\ - ,- - 1 - -,

1 2 3 4 5 6 7 8 9 1 0 Spatial Çrequency [Ip mm-']

Figure 4.30: The effect of the rnergy window on the spatial resolution.

Figure 4.3 1 : The pûrallel bar pattern and the star pattern.

4.1.2.2 Test Pattern Images

Therr are two test patterns that were imapd: one parallel bar pattem and one star

pattem (sçe Figure -1.3 1). When the bar pattem was used, one set of three bars was placed

in the field of virw of the detector, and 0.5 - 1.0 mm of the pattern was scanned in 100

pm steps.

For the star pattern. rach of the wedges provides a range of line densities. About

7-3 mm of one of the wedpes was scanned. The line density in the image can be

determineci by the following method. The angle and radius of one wedge were rneasured.

and the nurnber of lines was counted. then the line density in the object plane at one

location is equal to the nurnber of lines divided by the arc iength.

Bar pattern and stÿr pattem images were obtained in Xe:CH4 for some of the a - n y

hpcçtra and pas pressures at which MTF curves were measured. Figure 4.32 shows threr I bar pattern irnases and one star pattem image. The lp mm' values given in the images are

thow in the image plane. The two numbers shown to the right of rach image reprcsrnt the

rnü'timiim and minimum pixel values in the presented image. which correspond to the

white cind black respectively in the image.

Figure 4.32 (a) is the bar pattern image for 13 kV at 1 atm. Figure 4.32 (b) is the

imagc for 50 kV at 2 atm. Figure 4.32 ( c ) is for 50 kV at 3 atm.

The stÿr pattem image was obtained for 13 kV nt 3 atm (Figure 4.32 (dl). The

spatial frequency at the top end of the image is 1 1 lp mm-'. and it is 7.5 Ip mm-' at the

bottom end. Al1 the lines are clearly resolvable including 1 1 Ip mm". The line to the very

left of the pattem does not extend to the top end. It wÿs found scratched when examined

under the microscope. Pan of the first two rows in the image shows some pattern. whrre

i t was found thrit the lrad layer on the star pattem was damaged. There is no severe

artiîict present in these test pattern images.

1.5 Images of Biological Specimen

.-\ tish spine and a rat phalange were imaged with Our hybrid detector filled with

Xr:CHJ. The fish spine was obtained for 13 kV at 3 atm (Figure 1.33). The rat phalange

Figure 4.32: The test pattern images (pixel size is 25 pm).

was obtained for 30 kV at 4 atm (Figure 4.34). As in the test pattern images. the two

numbers shown to the right of each image represent the maximum and minimum pixel

values and correspond to the white and black respectively in the image. These specimens

were chosen because: first. they have both soft tissue and bone that have different

attenuation coefficients for x rays; second, both the fish spine and the joint of the rat

phalange have fine structures. In this way. the good contrast sensitivity and spatial

resolution of our hybrid detector can be shown.

As mentioned in Chapter I. the primas, advantage of the digital radiography is the

level and window interactive display. For a given gray scale. by changing the window

width and the level (the center of the window) on the input intensity. the contrast

sensitivity is changed and different information from one exposure c m be extracted. in

Figure 4.33 and 4.34. the concept of windowing and level is demonstrated. The window

width and level in this case are al1 in term of pixel value, Le. the number of photons hits

:it one pixel.

Figure 1.33 (a) shows the fish image with the full range of the pixel values [O. 2141

that is presented in the image data. The window width is 2 14 and the level is 107. In

Figure 4.33 (b) the window width is 84 ruid level is 92. We can see the fish spine

structure. the different bone density at the different part of the spine, and some tissue

attached to the spine. When the window width is changed to 44 and the level changed to

82 in Figure 4.33 (c). we c m only see the structure of the fish spine. in Figure 4.33 (d).

with the window width 134 and the level 67. both the spine structure and the tissue are

shown but the contrüst is lower cornpared with Figure 4.33 (b).

Figure 4.34 shows the effect of the application of different window width and level

on the rat phalange image. Figure 4.34 (a) is the image obtained by using the full range of

the pixel values [1069. 36551 from the image data. The minimum vaiue is not zero, since

there are ülways x rays transmitted through the object. The window width is 1586 and the

level is 2362. In Figure 4.34 (b), we c m see the three parts of the rat phalange: distal,

middle and proximal phalange (from top to bottom), the two joints and the srnooth

development of the bone density in the phalange with the window width of 1086 and the

level of 16 12. After the window width is changed to 856 and the level is changed to 2097

in Figure 4.34 (c). we can see clearly the shadow of the thin skin on the phalange. but the

bone structure is not visible. In Figure 4.34 (d), the window width and the level are

changed to 1456 and 1797 respectively. We can see the three paris of the rat phalange,

the phalange joint structure, the smooth development of the bone density in the phalange,

and the shadow of the thin skin of the phalange.

In Figure 4.34, between the nail and die first piece of phalange bone, there are two

rows sharply bright, i.e. the gradient is quite big between the dark part and bright part. It

looks like an artifact, which might be due to the line-to-line corrections applied in the

image analysis program, the pixel efficiency correction and the dead-time correction.

Figure 4.33: Fish spine image (13 kV at 3 atm, pixel ske is 25 pm).

Figure 4.34: Rat phalange image (30 kV at 4 atm, pixel size is 25 pm).

Chapter 5

Conclusions and Future Work

This chnpter summarizes al1 the cxperimental results and the operation of the GEM.

Somç options for future work. including shon term and long term. and the applications of

thc systcrn arc discussed.

5.1 Conclusions

The presence of the GEM I r d s to a decreue of the GMD voltage of more thün 100

V for the same resulting goin. aliowing o much safer operating mode for the GMD.

R e d thüt the GEM gain is not only drtermined by the field in ihr hole but is also

ciffectt.d by the outsidr fields. i.r. the conversion field and the collection field. When the

cathode voltage of the GMD is increasrd to get funher amplification. the collection field

strength is reduced. which results in a decrease of the collection efficiency when working

with a low collection field. In our system. the GEM voltage is applied with a resistor

network. The voltage at the GEM bottom is lirnitrd by the potentid across the GEM.

The gain of the GELWGMD hybnd wrü l e s than 400 for the iMTF meüsurement

and imaging. The gain was chosrn to maintain a muirnum output signal from the

amplifier board (- 500 mV).

The rnrrgy resolution varies from 1 atm to 4 atm. which is due to the different

opercitins condition of our detector rit the diffrrent pressures. The highest cnergy

resolution is obtained at 1 atm. 7% for 17.7 keV. and the worst resolution is 1 1% for 17.7

ksV (it 3 atm. At 59.6 keV. the resolution is 5% at 2 atm and 10% at 3 atm. Compxed

with the GMD imager [241, the energy resolution is improved from 10.9% to 7% for

Xe:CH4 at 1 atm.

Cornparcd with the results from the GMD imager at 4 atm [24]. the spatial

resolution of our hyhrid detector increases from 14.0 lp mm" to 17.8 lp mm" for 13 kV:

the resolution for 30 kV increases from 6.2 lp mm" to 7.0 ip mm": for 50 kV. it increases

from 7.0 lp mm" to 11.9 lp mm'!

As a conclusion. the GEM/GMD hybrid detector is suitable for high pressure digital

radiogiiphy. The whole systern is more robust than the GMD alone. The spatial

reholution is not degraded by the GEM hole structure. The image quülity looks brtter than

ihat obiained with th<: GMD imager. There are fewer cirtifacts present in the obtained

i r n l t p x

5.2 Summary of the Operational Features of the GEM

Increase of the primsry charge

The nice future of the GEM is that it can be used as a pre-amplifier and it amplifies

ihc primüry electrons insidr the detector. This property distinguishes the GEM from other

glis deteciors. From the point of view of the GMD strips. the prirnary charge increues.

Owing to the Iiirger primary charge. the GMD gain. and thus the anode-cathode voltage.

can bc lowercd. which rrduces the risk on the occurrence of discharges.

Operation at higher gas pressure

With the GEM as a pre-amplifier. the GMD gain nreds not to be turned up to the

maximum. If we grt an effective gain of 100 from the GEM and of 5 from the GMD. a

combined effective gain would be on the order of 500. In pressurized gasrs. where the

single GMD gain is drastically reduced. the addition of the GEM extends the maximum

usüblr gas pressure. improving the x-ray detection efficiency and position resolution.

5.2.2 Disadvantages

High voltage application

The use of a GEM in a GMD necessitates two more high voltase (GEM top and

hottom) connections to the detector. Considrrably high voltages have to bi: applied to the

elricirode planes to tichievr high charge collection efficirncy. For our experimrnts at 4

atm. the voltage on the drift plane was -2.8 kV. the GEM top -2.1 kV and the bottom - 1.2

kV. Since the conversion and collection gap are 3.9 and 1.8 mm respectively. application

ot' these high voltages demands special attention in the design of the detector to prevent

d ix hrirgc.

Mmolithic character of the GEM

The Cüct thiit the GEM is a monolithic device. meaning that breakdown at a single

spot renders the cornplete device uscless. is a weak point. The yirld and quülity control of

the current GEM production is however high enough to ensure good openition.

5.3 Suggestions for the Short Term Future

Continue testing the performance of the GEWGMD

Although the current GMD print showed spontaneous discharge signals (7 - 10

countslmin) during the rxperiments rit 1 atm. it is probably able to survive 5 atm with lrss

intensive irnidiation. it would be interesting to examine the spatial resolution of the

hybrid dctrctor for 40 kV and 10 kV x rays at 5 atm. Before increasing pressure. it would

be usrful to obtain the test pattern images at 4 atm.

Remove the dead xenon layer

There are 3.5 mm of dead xenon in front of the detector (between the window and

the drift plane). which is due to the thickness of the drift plane trame. The easier way to

diminate i t is to turn over the drift plane frarne, with the Mylar sheet touching the vesse1

window. Then tuni over the Mylar sheet. since only one side of the Mylar sheet is

iiluminized. rind the aluminized sidc must face the GEM to torm the conversion field.

If the next step would be to go to the keystone seornctry directly. this advicç could

he i~nored since the orientation of the Mylar sheet would not matter.

Install the PCB readout board for the GEM

The GEM can be used as an independent detector. and sipals can be read out by a

qirnple printed circuit board with passive pickup electrotles. i.e. thrre is no high voltage ;it

thc readout element. so there is no funher amplitkütion üfter the GEM foil.

In this mode of operiition. iifter the primary electrons released in the conversion gap

arc. ~uidcd to the GEM holrs and amplified in the avalanche. they will exit the holes and

hllow the tield lines to the reüdout board. and part of them will be trapped by the GEM

bottom clectrodc. Meantirne. the ions producrd in the conversion gap and in the GEM

holes will be collected by the drift plane iuid the GEiM top çlectrode. Sincr the s i p l is

cntirely produced by clectrons and no ions are produced in the lut gap. this operation

mode is intrinsically f ~ s t and d e . The moderate field in the collection region prevents

the propüption of a discharge in the GEM to the readout strips [29]. This is ü rare but

a l w q s possible event. So the possibility of damaging the amplifiers is reduced whrn the

signal from the GEM is picked up by a PCB readout elemrnt.

With the advanced technologies in PCB design and manufacture. a cheap large uea

dctrctor could be made. And the readout circuit could be pattemed in any desirrd

geomrtry ( strips. rings. pads. . . . ) de pending on the application. Non-planar. cylindrical

xometries are possible. b

Ws have a readout board with parailel passive strips at hand. The width of r x h

strip is 150 ym. and the pitch is 200 Pm. It was designed by J. Dubeau (an Associate

Researcher in the Dept. of Physics) and made at CERN. The PCB readout board c m be

installrd to replace the GMD in the pressure vessel. with maintaining the current

geornetry. The physical performance of the GEM with the PCB readout eiement can be

testeci and thus understood.

In the setup of the GEM + PCB, the signal from the GEM top or bottom could be

used as the trigger signal. Since the fraction of the electrons trapped by the GEM bottom

is dependent on the ratio of the collection tield and the field in the GEM holrs. there will

be a tradeoff betwesn the collection efficiency and the size of the trigger signal. But the

top electrode dors not have this probiem.

On the other hand. sxperiments conductrd by M. Dixit and J. Dubsau [49] showed

thiit tiirre wns a distribution of the induced charge signal on the neighboring strips when a

collimiitrd a source was used to irradiate only one strip on the PCB readout elrmrnt

(refcrrrd to as the central strip). This is dur to the change in the electrostatiç potenrial

cncrgy of the systcm caused by the motion of rlrctrons drifting in the coilection gap in

ihc detector. Thc PCB readout board used in their experiment hüd 1.65 mm pitch and

0.15 mm gnp brtween the readout strips. The collection gap was 2.5 mm. The ratio of

hignal amplitude iit the tirsi neighboring strip to thtit at the central strip w u found to br

dependent on the type of amplifier and the shüping tirne uscd. The rfkct of the

ncighboring induced signal on the centroid calculation should bt: examinrd for our

imaging systern.

Cpdüte the computer

It would be nice if the 80386 PC could be updtited. Since the data acquisition is

done by sotiwlire. the slow dock speed of the computer (40 MHz) lirnits the evenr count

rate of the imaging system. The dead time of the system is quite high due to the slow

speed of the PC and the CALMAC interface software. The dead iime was about 80% whrn

doing a blank field scan. The typical data rates were 100-300 eventds. In order to get

su ftïcien t photon stcitistics. a long imaging scan time is required. The rat image presrnted

in this thesis took two driys to obtain.

5.4 Some Options for the Long Term Future

The simplicity. ruggedness and low cost of the GEM device plus the available

üdvanced fiibrication technologies provide many options for the GEM applications.

I-- Drift Plane

Fan-Beam Slit h

Figure 5.1: Orientation of the keystone GEiM drtector.

Iieystone geometry

Our current geornetry of the drtector providrs vrry low quantum efficirncy (< 10%

at 30 LV [,-II). which is mainly dur to the short conversion gap (4 mm). One way to

incrrast: the quantum efficirncy significüntly without introducing parrillax srror is to use

thc "krystone" geornetry. Le. the readout strips follow the divergence of the x-ray beam.

in which case. the absorption length will br rqud to the total length of the sensitive ürea

of the GE41 (70 mm). or the readout board which might be a bit shorter than the GEM.

After trying the GEM with PCB readout board as discussed in previous section. we

could go to the "keystone" geometry. The concept of a SPR systrm with the keystone

GEM dctector is shown in Figure 5.1. Compared with the GMD, it would be much easier

and cheaper to realize the keystone geornetry with the GEM and its PCB readout element.

Dur to the unique operation mode. it would solve the charging-up problem showed in the

esprrirnents conductrd by Kiourkos et. Al [50] with microst trip Gas Chambers (MSGCs)

(the sanie structure as the GMD) of kcystone electrode geornetry.

One disadvanrage of the GEM detector with keystone geornetry is that it is still a

one-diniensional drtector. so it can only be usrd via SPR system to get a two-dirnensional

Cascaded GEM

I f the gain from a single GEM is not high enough for imiiging. two or three GEM

meshes could bc cascaded together. which is called Double GEM or Triple GELM. In this

wüy the operation on each GE.M mcsh is much safer. like the case in the GEWGLMD

hybrid.

For the Double GEM. the gap between the two GEMs is nrimed the transfer region

(the collection field for the tirst GEM). and the tïrst GEM serves as an clrctron injector

for the second one. The ovrrrill gain is the product of the effective gains of the two

clemcnts.

The charge arnplitkation and tr~nsfer processes in the Double GEM have been

investigated by Bachmann et. al [33]. Their experiments showed that the two multiplirrs

operated crssrntially independently, although the transfer tield for the tirst GElM is the

drift field for the second. therefore constraining the operation. Recall that the collection

field for rach GEM has to be much higher than its conversion (drift) field to get a high

rlectron collection sfficiency. Hoivever. it was shown that the effective gain of a double

GE.M drtrctor was almost invariant from the shiinng of the multiplying voltage betwern

the two GEMs. provided that their sum wris constant. Meanwhile. the alignment between

the holes on the two GELM meshes were not important. and the electron diffusion was

large rnoush to oblitrrate the hole structure.

Two-dimensional GEM detector

As mentioned in chapter 1. one of the drawbacks for the SPR system is the high x-

ray tube heat loading. Two dimensional readout electrodes for the GEM would

drtimütically reduce the tube loading.

A two-dimensional GEM detector was built by A. Bressttn et al. [ 5 11 The induced

charge si_onal on sets of adjacent strips were read out with glited ADCs. The signal picked

LI^ through a HV cüpacitor from the lower GEM rlectrodc was used XS the trigger for the

system. An image of the foot of a bat was obtained with S krV x rüys. and showed the

ioot proîïlc but not the joint structure.

The method of rnlinufacturing two-dimensional pick-up electrodes is extrnded froni

the principie of the GEiM drtector [ 5 2 ] . Two orthogonal sets of parcille1 metal strips are

cngrwcd. using convrnrionül printed circuit technology. on the two sidrs of a thin

polymcr t Kcipton) foil. M e r gluing the foil on a thin insulating support frame. the

polynicr in the interstices brtwern the upper strips is removed with a solvent. rnabling

ihc hottom layer to collcct part of the elrctron charge. In the elrctron collection mode.

hoth reiid-out elcctrodes crin be kept üt ground potrntial. which is ii substantial üdvantage

as cornpareci to othrr two-dimensional drvices.

Since the keystonc geometry cannoi be used with the two-dimensional readout

clemcnt. the siimc low quantum rfficirncy problem as in Our current dctector remüins.

GEM + TFT readout

As mentioned in Chapter 1. large active readout matrices consistiny of Thin-Film

Transistor (TFT) rirrays are ernployed in the current commercial digital radiography

sptema. r.g. the Sr detsctor. We could take üdvantage of this well-developed technology

to build ü large ÿrea two dimensional GEM detector. However. it would have the same

problrrn - low quantum et'ficiency due to the short absorption depth.

5.5 Applications

With the current available technologies. we could build a bench-top SPR GEM

system. with simple PCB readout and keystone pometry. The channel number could be

increased to enlarge the active area. of the detector. This system could be used as a

research tool to examine biological specimens or the quality of some tiny [Cs.

At some point in the Future. we could build a quantitative CT system with GEM + PCB. The quantum rfficiency would not be a problrm by applying the keystone

Scomriry. Suçh a system could find more applications in the medical and industrial

areas. The rcadout electronics that we are using now are only for investigating the

physicül performance of the novel drtector. They would have to be updated for the

specific application.

A s nnother option. we could build a two-dimensional GEiM derector with the same

gconietry thrit is applied in our current system. TFT urays could be applied to the readout

eltxtrodt. t« obtiiin srnall pinel sire. In order to reducr the pcirülltix error. both the GEM

foi1 and thc reüdout rlectrode could be built on the surfilce of a sphere ccntcred on the

tùcal p o t of the x-rüy tube. Sincr there is a lirnit on the quantum efficiency. this drirctor

would be for applictitions in the industrial or in the resriirch liib. where the dose is not a

wnct'rn.

Appendix A

Modeling the GEMIGMD Hybrid detector

The centroid calculütion algorithm applird in the GMD imager did not work

properly in the hybrid detector at the beginning. There wÿs a large bias toward the

rinodcs. I t wiis thousht to be dur to the GEM hole structure. So investigating the effect of

thc GEM hole structure on the centroid calculrition w ~ s the initial motivation to sirnulate

thc sisna1 developmtrnt in the hybrid detrctor. Later on. a minor bug was found in the

Wcighted Centroid Method codrd in the program i»r<i,qr.j?>r ( a fiictor of two w u missed).

This simple simulation is operatrd in two dimensions. where the .r direction is

dong the GMD reüdout direction. and the incident direction of photons is the : direction.

For cüch incident photon. it is assumed rhat it enters the detrctor perpendiculluly and the

incident position (A- , ) follows a uniform distribution along the x direction. Then it

menurites txponentially dong the : direction. so the ; coordinate of the photon

intcrüction i, is ü ründom number grnented from an exponential distribution. For each

photon interaction. it is assumed chat the charge cloud (initial ion pairs) is producrd

isotropicaliy around the interaction point (x,. r,). and the ceniroid of the charge cloud is iit

i h r location whrre the photoelectron effect happens. Then electrons generated in the

photoelectric effrct drift roward the GEiM mesh in the uniform electric field berween the

drift plünc and the GEM top. Due to the laterd diffusion. thrse electrons genrrrite a

Güiissian distribution with a mean value s, and width O, :

The width O, is given by (as for Eq. 2.10):

whcre - (= O.) cm - ;) is the drift distance of the electrons. O( 1 on) [%] is the lateral

diffusion of slectrons after 1 cm of drift.

The triinsparency of the GEM is assumed to be T I = 1. The Gaussian distributed

primary charge is t h r n integratrd over the GEM pitch [xh - 70 Pm. xh + 70pml ( nh is the

Iociitiiin of one GEM hole) to obtained ii simulated charge Qh thüt gors through sach

GEM hole:

A r h i i n i c . the GEM has uniform gain Mg,, ovcr e x h hols. then the charge aftcr cach

holc is: Q,,M :c,,, . Ir is assumed that when thrse electrons drift towrird the GMD anodes.

thcir lriterril diffusion follows ri Gaussian of mean value xh and w i d t h ~ , :

whcre. xh is the position of cach GEM hole center. a, is detcnnined by Eq. (A-?) and the

drift distance of thesr rlectrons is determined by the collection gap.

The Gaussiiin distributions descnbed by Eq. ( A 4 are integrated individually over

the GMD anode pitch [x, - LOO Pm. x, + 100 pm]. The c h m p signal for eüch anode is the

w m of the contribution tiorn each Gaussian:

It is assumrd that the collection efficiency (see section 2 - 2 3 of the detector is T? =

1. and the GMD gain is unilorm over each anode. So the simulated charge signal

collected by each anode is: M ,,,T2Q, .

The charse distribution over the sixteen anodes of the GMD in the hybrid detector

ai 1 atm in xrnon for x rays with average energy 15 keV was simulated. Figures A. 1 and

'4.7 give two examplcs of the simulation geometries. In the geometry shown in Figure

A. 1 . the centroid of the charge cloud is at GMD anode 8. At the beginning. thrre is one

GE>¶ hole liligncd with GMD anode 8 and thus with the charge cloud crntroid. Then the

GE'VI is mowd towiird the GMD anode 16 dong the GMD readout direction. The GEM

holc thiit is iligned with anode 8 at the beginning is now 100 pm to the right of anode 8.

The charge distributions over the sixteen anodes of the GMD for the two GEM positions

art. s h o w in Figure A.3 (a) and (b) respectively. We can sre that the charge distribution

\h«wn in Figure A.3 tb) is not ris symmetrical aï that shown in Figure A 3 (a) . which

introduccs the bias of 0.4 k 0.1 pm in the calculateci ceniroid that is s h o w in Figure 2.4.

For thc geometry s h o w in Figure A.?. the centroid of the charge cloud is over

GMD cathode 7. There is one GEAM holr aligned with this cathode at the beginning. and

thcn this GEM holr is moved 80 prn to the right of the cathode 7. The chürge

dihtrihutions for thrse two GEM positions are s h o w in Figure A.4 (a) and (bi. Sirnilarly.

aRer ihc GEM is moved. unsymmetricül charge distribution is shown ( Figure A.4 t b) 1.

Charge Cloud Centroid (fixed) I

100 pm to the nght

Initial Position -

GMD Anode 8 Cathode 8

Fipre A. 1: Diagram of one geornetry for the charge distribution simulation. The centroid of the charge cloud is at GMD anode 8. At the initial position of the GELM. there is one GEM holr nligned with GiMD anode 8.

Charge Cloud Centroid (fixed)

80 pm to the right

Initial Position - - : I

GMD Cathode 7 Anode 8

Figurc A.2: Diagram of one geometry for the charge distribution simulation. The crntroid of the charge cloud is at GMD cathode 7. At the initial position of the GEM. thrre is one GEM hole aligned with GMD cathode 7.

GMD Anode Number

6 7 8 9 10 11 12 13 14 15 16 GMD Anode Number

Figure A.3: Simulated charge distribution over the 16 anodes of the GMD in the GEWGMD hybrid detector. The charge cloud centroid is at the GMD anode 8: ( a ) a GELM hole is digned with GMD anode 8: (b) the GEiM hole is 100 prn ro the right of anode 8.

4 5 6 7 8 9 10 11 12 1 3 14 15 16 GMD Anode Number

GMD Anode Number

cbi

Figure A.4: Simulatrd charge distribution over the 16 anodes of the GMD in the GEWGMD hybrid detector. The charge cloud centroid is digned with GMD cathode 7: (a) a GEM hole is digned with the cathode 7: (b) the GEiM holc ir 80 ym to the right of the cathode 7.

Appendix B

Gas Gain Fluctuation

Gns multiplication is a stochastic process. The single-electron avalanche

distribution in a unifom elcctric field can br dcscribed by the Furry distribution [38].

whiçh stcitcs thüt i f d is the average gain. the probability of having a gain A in a single

;iviilanchc obeys:

This expression holds under the condition that the probability of ionization is independent

( if ils previoiis history. but dependent only on the electric field strength. When A is

rcusonühly Irirgs (greatrr than 50 or 100). the Furry distribution reduces to a simple

cxponcn tial form:

In strong clectric fields. Iike ones in the GEM and GMD. the probability of

ionization by an rlectron can no longer br considered totally indrpendrnt of its past

history. The simple sxponentid distribution of Eq. (8-2) is replaced by a Polya

distribution [38 1:

So the single-electron avalanche distribution in the GEM and GMD can be described by

Eq. ( 8-3). The relative variance prcdicted by the Polya distribution is:

For large values of the multiplication factor A.

The width of thc gain distribution in proportionid counters is mainly determineci by

the ueaker field iit the brginning of the avalanche. Consequently. a sharp transition from

drift to rivalanche rcgion like in the GMD is favorable for the energy resolution.

The average gain is what wt: usulilly cal1 the detector gain M:

wherr no is the initial number of ion pairs creatrd by the incident x-ray. Because each

avalanche is usumrd to be independent. we can apply the error propagation formula to

( B-6) to obtain:

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