PILATUS 2M a detector for small angle X-ray scattering

86 Pages·2010·7.28 MB·English

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PILATUS 2M
a detector for small angle X-ray scattering
Doctoral Thesis
Author(s):
Kraft, Philipp
Publication date:
2010
Permanent link:
https://doi.org/10.3929/ethz-a-006023165
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Diss. ETH N◦ 18466
PILATUS 2M
A Detector for Small Angle X-ray
Scattering
A dissertation submitted to
ETH ZURICH
for the degree of
Doctor of Sciences
presented by
PHILIPP KRAFT
Dipl. phys. ETH
born Januray 31st, 1978
citizen of G¨achlingen (SH)
accepted on the recommendation of
Prof. Dr. R. Eichler, examiner
Prof. Dr. F. Pfeiffer, co-examiner
Dr. Ch. Br¨onnimann, co-examiner
2010
ii
Abstract
For the (coherent) small-angle X-ray scattering (SAXS) beamline X12SA of the Swiss
Light Source at the Paul Scherrer Institut, a novel X-ray detector is built, the PILA-
TUS 2M. Contributions in module fabrication, detector assembly, characterization and
calibration are covered within the framework of this thesis. Furthermore, data analysis
of a particular SAXS experiment demonstrating the performance of the beamline and
the detector are accomplished.
PILATUS is a silicon hybrid pixel detector system for synchrotron applications. Its core
unit is a module consisting of 16 readout chips which are flip-chip bump bonded to a
320 µm thick silicon sensor. The readout electronics of a single pixel of 172×172 µm2
size comprise an amplifier, comparator and a digital counter featuring single-photon
counting. A module contains 487×195 pixels which can be read out in within 2.85 ms.
In the frame of detector calibration and characterization the intrinsic threshold disper-
sion of a PILATUS module is reduced to a minimum value of 36 eV root mean square
(RMS). The minimum selectable threshold is 2 keV with all pixels of the module reliably
counting photons above the electronic noise. Furthermore, a minimum overall energy
resolution of 863 eV full width at half maximum (FWHM) and a minimum dead time
of 101 ns are measured.
The PILATUS 2M is a large area detector consisting of an array of 8 × 3 PILATUS
modules. Atotalof1475×1679 = 2,476,525pixelscoveranactiveareaof254×289mm2.
The maximum frame rate of the full detector is 31.4 Hz. The PILATUS 2M is custom
built for SAXS experiments.
One-dimensional tomographic reconstruction of SAXS images is demonstrated on the
example of alumina dip-coated polyamide 6 monofilaments. The sample is scanned with
a focussed synchrotron beam with a spot size at sample level of 5×20 µm2 (FWHM).
The reconstructed sequence of SAXS images reflects the local nanostructure variation
along the sample radius.
iii
Zusammenfassung
Fu¨rdie(koh¨arente)Klein-Winkel-R¨ontgen-Streu-Strahllinie(KWRS-Strahllinie)X12SA
derSynchrotronLichtquelleSchweizamPaulScherrerInstitutwirdeinneuartigerR¨ont-
gendetektor gebaut, der PILATUS 2M. Beitr¨age in der Modulfabrikation, dem Detek-
torzusammenbau, der Charakterisierung und Kalibration werden im Rahmen dieser
Arbeit geleistet. Des weiteren wird die Datenanalyse von einem speziellen KWRS-
Experiment durchgefu¨hrt, die die Leistungsfhigkeit der Strahlinie and des Detektors
demonstriert.
PILATUS ist ein Silizium-Hybrid-Pixel-Detektorsystem fu¨r Synchrotronanwendungen.
SeineGrundeinheitisteinModul,dasaus16Auslesechipsbesteht,dieu¨berkugelf¨ormige
Kontakte mit einem 320 µm dicken Silizium Sensor verbunden sind. Die Ausleseelek-
tronik von einem einzigen Pixel der Gr¨osse 172×172 µm2 beinhaltet einen Verst¨arker,
einen Komparator und einen digitalen Z¨ahler, der das Z¨ahlen von einzelnen Photonen
erm¨oglicht. Ein Modul enth¨alt 487×195 Pixel, die innerhalb von 2.85 ms ausgelesen
werden k¨onnen.
ImRahmenderDetektorkalibrationund-charakterisierungwirddieintrinsischeSchwellen-
dispersion eines PILATUS Moduls auf einen minimalen Wert von 36 eV Quadratmittel
(RMS) reduziert. Die minimal w¨ahlbare Schwelle ist 2 keV wobei alle Pixel des Moduls
zuverl¨assig u¨ber dem elektronischen Rauschen Photonen z¨ahlen. Des weiteren wurden
eine pauschale Energieau߬osung von 863 eV Halbwertsbreite (FWHM) und eine mini-
male Totzeit von 101 ns gemessen.
DerPILATUS2Misteingrossfl¨achigerDetektorbestehendaus8×3PILATUSModulen.
Imganzen1475×1679 = 2,476,525PixelbedeckeneineaktiveFl¨achevon254×289mm2.
Die maximale Anzahl ausgelesener Bilder pro Sekunde ist 31.4 Hz. Der PILATUS 2M
wurde speziell fu¨r KWRS-Experimente angefertigt.
Eindimensionale tomographische Rekonstruktion von KWRS-Bildern wird am Beispiel
vonAluminiumoxid-tauchbeschichtetenPolyamid6Filamentendemonstriert. DieProbe
wirdmittelseinemfokussiertenSynchrotronstrahlvon5×20µm2(FWHM)aufProbenh¨ohe
abgetastet. DierekonstruierteSequenzderKWRS-BilderwiederspiegeltdielokaleA¨nderung
der Nanostruktur entlang dem Radius der Probe.
iv
Contents
1. Introduction 1
2. X-ray 3
2.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2.1. X-ray tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2.2. Synchrotron light source . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.3. Brilliance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Interaction with matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.1. Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.2. Absorption and Fluorescence . . . . . . . . . . . . . . . . . . . . . 10
2.3.3. Refraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3.4. Intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4. Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4.1. Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4.2. Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3. PILATUS 17
3.1. Semiconductor hybrid pixel detectors . . . . . . . . . . . . . . . . . . . . . 17
3.1.1. Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1.2. Bump bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1.3. Readout Chip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2. The PILATUS 2M . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.1. Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.2. Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.3. Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.2.4. Readout modes and frame rates. . . . . . . . . . . . . . . . . . . . 23
3.2.5. Calibration and corrections . . . . . . . . . . . . . . . . . . . . . . 23
3.2.6. Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2.7. Detector comparison . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4. Scientific Application 29
4.1. Polymer filaments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.1.1. Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.1.2. Melt spinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.1.3. Filament structure . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
v
Contents
4.2. Small-angle X-ray scattering . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.2.1. Magic square of scattering . . . . . . . . . . . . . . . . . . . . . . . 31
4.2.2. Polydisperse two-phase system . . . . . . . . . . . . . . . . . . . . 32
4.2.3. Isotropic scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.2.4. Anisotropic scattering . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.2.5. Peaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.2.6. Data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.3. SAXS with the PILATUS 2M . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.4. One-dimensional tomography of SAXS images . . . . . . . . . . . . . . . . 35
4.5. cSAXS beamline X12SA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.6. Article I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5. Outlook 47
A. PILATUS publications 49
A.1. Article II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
A.2. Article III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
List of Tables 67
List of Figures 69
Bibliography 71
Curriculum vitae and list of publications 75
Acknowledgements 79
vi
1. Introduction
Semiconductor detectors for nuclear radiation and particles have experienced a rather
rapid development in the last few years. In particular, the development of position-
sensitive detectors was initiated by experimental particle physics, which required detec-
tors capable of measuring particle tracks with approximately 10 µm precision that at
the same time could cope with high rates.
The development of detectors with these properties was made possible by the adapta-
tion of technologies used in microelectronics for the fabrication of silicon detectors. The
introduction of silicon strip detectors marked the start to a revolution of experimental
techniques of particle physics including the development of low-noise–low-power analog
microelectronics for the readout of semiconductor detectors. It was soon realized that
this technology could be transfered to other experimental fields involving detection of
ionizing radiation. The here presented PILATUS detector system for synchrotron ap-
plications is a spin-off of the silicon hybrid pixel detector development at Paul Scherrer
Institut (PSI) for the compact muon solenoid (CMS) experiment at the large hadron
collider (LHC) at Cern [1].
PILATUS stands for pixel apparatus for the SLS1. It is a single-photon counting silicon
hybrid pixel detector which has been developed by the SLS Detectors Group at PSI
since 1998 for protein crystallography. After testing the feasibility of the concept by
means of test structures, the PILATUS I chip was designed in a 0.8 µm DMILL process
[2]. Despite some design flaws the chip was functional. Several single-module detectors,
consisting of 16 readout chips and a sensor, and a 1 megapixel detector (PILATUS 1M)
forproteincrystallography,consistingof6×3modules,werebuiltandcalibratedstarting
in 2003 [3, 4].
The design of the readout chip was improved and instead of a DMILL, a 0.25 µm UMC
process was used for the PILATUS II chip which was available by the end of 2004. The
PILATUS II readout chip is superior compared to its predecessor amongst others in
stability, pixel size, total number of pixels per chip and counter depth. The readout chip
was tested and characterized [5]. Single-module detectors (PILATUS 100K) were built
for a wide range of applications. Two large area prototype-detectors were fabricated
for beamlines at the SLS. First, the PILATUS 6M with an array of 6×5 modules was
accomplished and commissioned for the protein crystallography beamline X06SA where
it is successfully operated [6]. The second area detector was the PILATUS 2M, which
was custom built for the cSAXS beamline X12SA as further described in this thesis.
1The Swiss light source (SLS) is a third generation synchrotron light source at PSI.
1
1. Introduction
Besides the SLS Detectors Group other groups have developed semiconductor hybrid
pixel detectors for X-ray applications [7]. The most prominent ones among them are
Medipix [8] and XPAD [9]. Nevertheless, PILATUS have been the only semiconductor
hybrid pixel detectors comprising large module arrays so far.
Small-angle X-ray scattering (SAXS) studies date from the classical works of A. Guinier
in 1938. During further development of the theoretical and experimental fundamentals
of the method, the potential was shown for applying it for determination of the general
structural characteristics of various types of highly dispersed systems. The develop-
ment of synchrotron sources, position-sensitive detectors and the use of computers made
further refinement and new progress of the method feasible.
In particular since the development of highly brilliant synchrotron light sources, a lim-
iting factor in SAXS experiments has frequently been the insufficient performance of
the employed detector. Small dynamic range, dark and readout noise are essentially
compromising the obtainable resolution in reciprocal space. Low detection efficiency
inhibits measurements of weakly scattering samples which suffer from radiation damage.
Long readout times prolongate scanning experiments considerably and complicate time
resolved measurements.
Depending on the experimental setup and the wave length of the applied X-rays, SAXS
exhibits information about the samples nanostructure between 1 nm and 1 µm. For
example liquids, dispersions of alloys, powders and glasses, features of aggregated poly-
mer chains or biological macromolecules are subjected to its analysis. SAXS is not a
common imaging technique. There is not even a direct way to reconstruct the structure
from SAXS data. These data must be interpreted or analyzed with the proper analysis
strategy.
This thesis covers two major topics in three chapters. Chapter 2 gives a brief introduc-
tion into generation of X-rays, their interaction with matter and X-ray detectors. The
PILATUS detector system is adressed in chapter 3. The PILATUS 2M detector, which
has been produced in the frame of this work, and results regarding detector calibration
and characterization are presented. Chapter 4 is dedicated to a scanning microbeam
SAXS experiment on polymer filaments and the corresponding data analysis. It also
contains a reprint of a submitted article describing the experiment, data processing and
analysis, and the obtained results. Reprints of two published articles regarding system
description,characterization,calibrationandperformanceofthePILATUSdetectorsare
added in appendix A.
2
2. X-ray
This chapter is an introduction to the field of structure analysis by the means of X-
rays. It is not meant to cover all aspects of the subject in detail but to be a basis
for the subsequent chapters. The discovery of X-rays and common X-ray sources are
presented in §2.1 and §2.2. The introduction to X-ray interaction with matter in §2.3
is focussed on the basics required for chapter 3 and 4. Detailed treatment of the topic
and derivation of the presented formulae are found in e.g. [10, 11, 12, 13, 14, 15, 16]. A
short introduction to X-ray detectors is presented in §2.4. Therein the most important
detectorcharacteristicsandaselectionofthemostcommonintensitymeasuringdetectors
are given.
2.1. Discovery
Wilhelm Conrad R¨ontgen was the first scientist to observe and record
X-rays, first finding them on November 8, 1895. He had been experi-
menting with a set of gas discharge tubes and was surprised to find a
flickering image, cast by his instruments and separated from them by
some distance. He knew that the image he saw was not created by the
fast electrons as they could not penetrate air for any significant dis-
tance. After some considerable investigation, he named the new rays
”X” to indicate they were unknown.
2.2. Production
Figure 2.1.: W.C.Ro¨ntgen
Until the visual observation of synchrotron radiation in 1947, X-rays were generated
with X-ray tubes similar to the models used by R¨ontgen.
2.2.1. X-ray tube
The most widely used model is the hot cathode tube by William Coolidge in 1913. In a
vacuum glass tube (cf. Fig. 2.2) electrons are emitted by the thermionic effect from a
tungsten filament which is heated by an electric current (I). The filament is the cathode
of the tube. A high voltage potential (U) between cathode and anode accelerates the
electrons (e−) towards the anode. The electrons hit the anode which emits X-rays (γ).
TheemissionspectrumofanX-raytubeisgivenbybremsstrahlung, characteristic radia-
tion and absorption (cf. Fig. 2.3). Bremsstrahlung arises from the decelerated electrons
3