New halide scintillators for γ-ray detection Mikhail S. Alekhin

143 Pages·2013·5.54 MB·English

New halide scintillators for γ-ray detection

Mikhail S. Alekhin

New halide scintillators for gamma ray detection

Proefschrift

ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen op 26 november 2013 om 15:00 uur
door Mikhail Sergeevich ALEKHIN
Master of Science in Physics,
Lomonosov Moscow State University
geboren te Podolsk, USSR
Dit proefschrift is goedgekeurd door de promotor:
Prof. dr. P. Dorenbos

Samenstelling promotiecommissie:

Rector Magnificus, voorzitter
Prof. dr. P. Dorenbos Technische Universiteit Delft, promotor
Prof. dr. E. H. Brück Technische Universiteit Delft
Prof. dr. A. Meijerink Universiteit Utrecht
Prof. dr. C. R. Ronda Universiteit Utrecht
Prof. dr. L. D. A. Siebbeles Technische Universiteit Delft
Prof. dr. C. W. E. van Eijk Technische Universiteit Delft
Dr. K. W. Krämer Universität Bern, Zwitserland
Prof. dr. H. T. Wolterbeek Technische Universiteit Delft, reservelid

This research was financially supported by the Dutch Technology Foundation STW
(project 07644 “The ultimate scintillator”).

Cover design: Dmitry Kileynikov
Printed by: Proefschriftmaken.nl | | Uitgeverij BOXPress

ISBN: 978-90-8891-738-7
Table of contents

List of acronyms and abbreviations iv

Chapter 1 Introduction 1

Chapter 2 Scintillation materials 3
2.1 Scintillation mechanisms 3
2.1.1 Interaction of γ-rays with matter 3
2.1.2 Interaction of electrons with matter, thermalization and transport 4
2.1.3 Emission 5
2.1.4 Self-absorption of emission 6
2.2 Scintillator requirements 6
2.2.1 Light yield 7
2.2.2 Non-proportionality 7
2.2.3 Energy resolution 8
2.2.4 Decay time 9
2.2.5 Other requirements 9
2.3 Scintillation materials overview 10
2.4 Applications 11
2.4.1 High energy physics 11
2.4.2 Medical diagnostics 12
2.4.3 Space explorations 13
2.4.4 Scientific investigations in nuclear physics 13
2.5 Selection of the studied compounds 14

Chapter 3 Experimental Techniques 17
3.1 X-ray excited luminescence and thermoluminescence 17
3.2 VUV excitation and emission spectroscopy 18
3.3 Diffuse reflectance 19
3.4 Pulse-height measurements 19
3.4.1 Light yield 19
3.4.2 Non-proportionality of energy response 21
3.5 Scintillation time profiles 22

i
Chapter 4 Improvement of LaBr :5%Ce scintillation properties by Li+,
3
Na+, Mg2+, Ca2+, Sr2+, and Ba2+ co-doping 25
4.1 Introduction 26
4.2 Crystal growth 26
4.3 Results and discussion 27
4.3.1 Proportionality improvement of LaBr :Ce and CeBr 27
3 3
4.3.2 Light yield and energy resolution of LaBr :Ce,Sr 29
3
4.3.3 Scintillation properties of LaBr :Ce co-doped with Li, Na, Mg,
3
Ca, Sr, and Ba 34
4.4 Summary and conclusion 40

Chapter 5 Optical properties and defect structure of Sr2+ co-doped
LaBr :5%Ce scintillation crystals 43
3
5.1 Introduction 43
5.2 Crystal growth 44
5.3 Results 45
5.3.1 Optical properties 45
5.3.2 Time response 49
5.4 Discussion 52
5.4.1 Three different Ce3+ sites 52
5.4.2 Re-absorption of Ce3+ emission and its decay time 54
5.4.3 Point defects in LaBr :Ce,Sr 55
3
5.5 Conclusion 58

Chapter 6 Scintillation properties of and self-absorption in SrI :Eu2+ 61
2
6.1 Introduction 61
6.2 Crystal growth 62
6.3 Results 62
6.3.1 Excitation and emission spectra 62
6.3.2 Photoelectron yield 70
6.3.3 Decay time measurements 74
6.4 Discussion 76
6.5 Conclusion 81

Chapter 7 Self-absorption in SrI :2%Eu2+ between 78 K and 600 K 83
2
7.1 Introduction 83
7.2 Experimental methods 84
7.3 Results and discussions 85
7.3.1 Photoelectron yield studies 85
ii
7.3.2 Decay time studies 87
7.3.3 Probability of self-absorption 88

Chapter 8 Non-proportional response and energy resolution of pure
SrI and SrI :5%Eu scintillators 91
2 2
8.1 Introduction 91
8.2 Experiment 92
8.3 Results 93
8.3.1 Scintillation non-proportionality to X-rays 94
8.3.2 Energy resolution of X-ray total absorption peaks 95
8.3.3 Scintillation non-proportionality to K-shell photoelectrons 98
8.4 Discussion 100
8.5 Conclusion 103

Chapter 9 Optical and scintillation properties of CsBa I :Eu2+ 105
2 5
9.1 Introduction 105
9.2 Crystal growth 106
9.3 Results 106
9.3.1 Excitation and emission spectra 106
9.3.2 Light yield and energy resolution 109
9.3.3 Decay time 113
9.4 Discussion 114
9.4.1 Optical properties 114
9.4.2 Light yield and energy resolution 116
9.4.3 Decay time and probability of self-absorption 117
9.5 Summary and conclusion 119

Summary 121

Samenvatting 125

Acknowledgements 129

Curriculum Vitae 131

List of publications 133

iii
List of acronyms and abbreviations

a Probability of Self-absorption
η Quantum Efficiency
APD Avalanche Photo Diode
Bq Becquerel
CB Conduction Band
CT Computed Tomography
e Electron
FWHM Full Width at Half Maximum
h Hole
HPGe High Purity Germanium Detector
LHC Large Hadron Collider
LY Light Yield
NPMT Number of photoelectrons produced in photo multiplier tube
phe
nPR Non-proportional Response
ph Photon
phe Photoelectron
PMT Photo Multiplier Tube
R Energy Resolution
SiPM Silicon Photo Multiplier
SLYNCI Scintillator Light Yield Non-proportionality Characterization Instrument
SPECT Single-Photon Emission Computed Tomography
SSL Steady-State X-ray Excited Luminescence
STE Self Trapped Exciton
TE Trapped Exciton
ToF PET Time-of-Flight Positron Emission Tomography
UV Ultraviolet
VUV Vacuum Ultraviolet
VB Valence Band
XRD X-ray Diffraction

iv
Chapter 1

Introduction

A scintillator is a material that converts the energy of an ionizing particle into a
flash of light. Scintillation materials are used for the detection of ionizing particles and
measurement of their types and energies. The position of an interaction between an
ionizing particle and a scintillator and the moment of this interaction can also be
determined.
ee
ee
ee
Conduction band
ee ee
γ
photon
Forbidden gap phonon
hh
hh hh Valence band
hh
hh
Fig. 1. Schematic of the scintillation mechanism.

A typical inorganic scintillator is an insulator with 4-12 eV gap between the top of
the valence band (VB) and the bottom of the conduction band (CB), the so called
forbidden gap, see Fig. 1. In the VB, all electrons (e) are bound to atoms. When excited
into the CB, electrons leave holes (h) in the VB. Both electrons in the CB and holes in
the VB can move freely within the crystal. The general idea of a scintillation material
is to find an effective way to recombine all the excited electrons in the CB with the
holes in the VB via a photon emission. For this, one requires efficient e-h transport to
efficient luminescence centers.
In 1895, Wilhelm Röntgen discovered X-ray radiation by observing a faint glow
from barium platinocyanide. Visual scintillation counting was introduced by Crookes
and Regener in 1908 [1]. The photomultiplier was applied to scintillation counting by
Curran and Baker in 1944 [1, 2]. An intensive search for scintillation materials started
after the discovery of NaI:Tl by Robert Hofstadter in late 1940s [3]. Tens of
compounds have been discovered and successfully applied since then.
New halide scintillators for gamma ray detection
2013 - M. S. Alekhin
Chapter 1
Today, the search for new scintillation materials is a separate scientific branch with
numerous institutes and industries involved. Scintillator research is driven by the ever-
increasing requirements for more advanced applications. Among them are medical
diagnostics, high energy physics, space exploration, oil well logging, and different
scientific investigations. There are various types of scintillation materials: inorganic
single crystals, ceramic, plastic, liquid, and organic scintillators. Single crystal
inorganic scintillators are the most efficient and wide-spread ones, and are the subject
of this thesis.
Thesis outline: chapter 2 of this thesis reviews all the steps of scintillation
mechanisms, from interaction of γ-rays and high energy electrons with matter to
emission from luminescence centers and its self-absorption. Several well-known
compounds, applications, and application requirements are discussed. The choice of the
studied materials is motivated. Chapter 3 explains the principles of the experimental
techniques used. Chapters 4-9 are devoted to the studied compounds. In chapter 4, a
new outstanding LaBr :5%Ce,Sr scintillator is presented. Its energy resolution, γ-ray
3
and electron response proportionality are considerably improved as compared to
standard LaBr :5%Ce. The effects of Li+, Na+, Mg2+, Ca2+, Sr2+, and Ba2+ co-dopants
3
on light yield, energy resolution, proportionality, decay time, and charge carrier trap
creation of LaBr :Ce are also discussed. In chapter 5, attention is paid to the optical
3
properties and lattice point defects of LaBr :Ce,Sr. Three different Ce3+ sites are
3
revealed and their possible origins are discussed. Chapter 6 deals with scintillation
properties and self-absorption in SrI :Eu. Temperature, sample size and Eu
2
concentration appear to strongly affect the emission spectrum, decay time and light
yield of SrI :Eu. A model of self-absorption is introduced to explain this behavior. In
2
chapter 7, the scintillation properties and probability of self-absorption in SrI :2%Eu
2
are compared to those of SrI :5%Eu. Studies of non-proportionality of pure SrI and
2 2
SrI :5%Eu at temperatures between 80 K and 600 K gain attention in Chapter 8.
2
Chapter 9 is devoted to the optical and scintillation properties of CsBa I :Eu, which
2 5
appears to have similar amount of self-absorption as SrI :Eu. Finally, the thesis is
2
summarized.

References:

[1] J. B. Birks, The theory and practice of scintillation counting, Macmillan, New York, 1964.
[2] S. C. Curran and W. R. Baker, Rev Sci Instrum 19 (1948) 116.
[3] R. Hofstadter, Phys Rev 75 (1949) 796.

2