Energy dependent charge spread function in a dedicated synchrotron beam pnCCD detector [Elektronische Ressource] / Hazem Yousef

Hazem Yousef

Sujets

• Physik

Informations

• Publié par: universitat_siegen
• Publié le: 01 janvier 2011
• Nombre de lectures: 24
• Langue: Deutsch
• Poids de l'ouvrage: 2 Mo

Extrait

ENERGY DEPENDENT
CHARGE SPREAD FUNCTION
IN A DEDICATED SYNCHROTRON BEAM
pnCCD DETECTOR



DISSERTATION
Zur Erlangung des Grades eines Doktors
Der Naturwissenschaften





Vorgelegt von
Hazem Yousef





Eingereicht beim Fachbereich Physik
Der Universität Siegen
Siegen 2011


Gutachter der Dissertation : Prof. Dr. A.H. Walenta

thDatum der Disputation: 20 Mai 2011
Prüfer: Prof. Dr. U. Pietsch
Prof. Dr. H.D. Dahmen





















Zusammenfassung

Um die Größe der Elektronenwolke in einem pnCCD-Röntgendetektor auflösen zu können, wurden an
der EDR-Beamline des Synchrotrons BESSY II in Berlin mit einem Nadelstrahl die Pixelkanten des
Detektors abgetastet.
Der Radius der Elektronenwolke wird in Abhängigkeit von der Photonenenergie und des Einstrahlwinkels
des Röntgenstrahls zur Oberfläche des Detektorchips analysiert. Die Messungen werden durch
entsprechende numerische Modelle in einer Simulation bestätigt.
Für verschiedene Einstrahlrichtungen ergeben sich aus der Spur der Röntgenstrahlung im Volumen des
Detektorchips unterschiedliche Verteilungen der Elektronenwolke über mehrere Pixel. Dazu wurde ein
kollimierter Röntgenstrahl der Energie 12,4 keV unter den Eingangswinkeln 30° und 40° benutzt.
Es wird gezeigt, dass die zwei Effekte zur Verbreiterung der Elektronenwolke, Diffusion und
elektrostatische Abstoßung, aus den Messdaten separiert werden können. Desweiteren wird beobachtet,
dass die elektrostatische Abstoßung die Verbreiterung der Elektronenwolke während der Drift der
Elektronen dominiert.
Aus den Daten bei senkrechter Bestrahlung wird der Radius der Elektronenwolke in Abhängigkeit von
der Photonenenergie bestimmt. Die Ergebnisse zeigen, dass im Energiebereich von (5,0 - 21,6) keV die
2Elektronenwolke kleiner als die Pixelgröße von (75 * 75) µm ist.












I
Abstract

A scan on the pixel edges is the method which is used to resolve the electron cloud size in the pixel array
of the pnCCD detector. The EDR synchrotron radiation in BESSY is the source of the X-ray photons
which are used in the scans. The radius of the electron cloud as a function of the impinging photon energy
is analyzed. The angle of incidence of the X-ray beam is employed in the measurements. The
measurements are validated by the numerical simulation models.
The inclined X-ray track leads to distribute the electron clouds in a certain number of pixels according to
the incident angle of the X-ray beam. The pixels detect different electron clouds according to their
generation position in the detector bulk. A collimated X-ray beam of 12.14 keV is used in the
o omeasurements with 30 and 40 entrance angles. It is shown that the two factors that leads to expand the
electron clouds namely the diffusion and the mutual electrostatic repulsion can be separated from the
measured electron clouds. It is noticed as well that the influence of the mutual electrostatic repulsion
dominates the cloud expansion over the diffusion process in the collection time of the detector.
The perpendicular X-ray track leads to determine the average radius of the electron cloud per photon
energy. The results show that the size of the electron clouds (RMS) in the energy range of [5.0-21.6] keV
is smaller than the pixel size.












II
Contents

Zusammenfassung .................................................................................................................................... I
Abstract ................................................................................................................................................... II
Contents ................................................................................................................................................. III
Introduction ............................................................................................................................................ VI
1. The pnCCD Structure: Device and Readout Design ............................................................................. 1
A. The pnCCD Architecture ................................................................................................................ 2
A.1- The Design of the Register Side ............................................................................................... 3
A.2- The Metal Insulator Semiconductor structure (MIS) ................................................................. 4
A.3- The back side and the substrate of the detector ......................................................................... 5
A.4- Single Register Storage ............................................................................................................ 6
A.5 Frame-Store Scheme of the pnCCD Array ................................................................................. 7
B. The Readout Process and the associated electronics ........................................................................ 7
B.1- Three Phase Charge Transfer Mechanism ................................................................................. 9
B.2- The Front- End Electronics ..................................................................................................... 10
B.3- The CAMEX Chip ................................................................................................................. 11
C. Related Semiconductor Physics Relations .................................................................................... 12
C.1 The Reverse Bias PN Junction and the Depletion Voltage ........................................................ 13
C.2- The Metal-Oxide Semiconductor Capacitor ............................................................................ 17
D. The Potential Profile in the Silicon Substrate ................................................................................. 21
2. Characterization and Operational Parameters of the pnCCD ............................................................... 23
A. Attenuation of Photon Beam in Silicon......................................................................................... 23
B. The Position Resolution and Charge Sharing ................................................................................. 24
C. Energy Resolution ......................................................................................................................... 29
D. The Quantum Efficiency ............................................................................................................... 29
E. Leakage Current and Readout Noise .............................................................................................. 30
F. Common Mode Noise .................................................................................................................... 30

III
G. Charge Transfer Noise .................................................................................................................. 31
H. Pile-up Events ............................................................................................................................... 31
I. Parallax Effect ................................................................................................................................ 32
3. Theoretical Computation and Expectations ....................................................................................... 34
A. The Detection of the X-rays .......................................................................................................... 34
B. The Computation Process .............................................................................................................. 36
C. Charge Collection Time ................................................................................................................ 37
D. Charge Cloud Expansion ............................................................................................................... 40
D.1-Primary Charge Cloud Size ..................................................................................................... 40
D.2- The Diffusion Process as a Random Walk .............................................................................. 40
D.3- Mutual Electrostatic Repulsion............................................................................................... 42
E. Numerical Model of the Charge Spread ......................................................................................... 43
F. Expectations from the Numerical Model ........................................................................................ 45
F.1- The Scan Profile for Perpendicular Incoming Beam ................................................................ 48
F.1- Statistical Fluctuations in the Calculated R