AN ABSTRACT OF THE THESIS OF
Daniel C. Konnoff for the degree of Master of Science in Radiation Health Physics
presented on March 23, 2012.
Title: SSPM-Based Optical Fiber Radiation Dosimeter
Abstract approved:
Abi T. Farsoni
Current state-of-the-art environmental, clinical, and in-vivo radiation sensing systems utilizing various inorganic and tissue-equivalent plastic scintillators are not user
friendly, suffer from electron-beam-generated noise, and are difficult to deploy successfully
for real-time dosimetry. A robust, real-time detection system using different scintillating
materials coupled to solid-state detectors by optical fibers is developed. This system enables radiation monitors/clinicians to conduct meaningful real-time measurements using
different inorganic scintillators or organic, tissue-equivalent plastic scintillators in harsh
clinical and environmental environments.
Recent solid state photomultiplier (SSPM) technology has matured, reaching a performance level that is suitable for replacement of the ubiquitous photomultiplier tube in
selected applications for environmental radiation monitoring, clinical dosimetry, and medical imaging purposes. The objective of this work is laboratory and clinical testing of the
Hamamatsu MPPC (S10362-11-050C), Photonique SSPM (0810G1), and Voxtel SiPM
(SQBF-EKAA/SQBF-EIOA) SSPMs coupled to different inorganic scintillator crystals
(Prelude 420, BGO), inorganic doped glass scintillator material SiO2 : Cu2+ , and organic
BCF-12 plastic scintillating fibers, used as detector elements. Both polymer optical fibers
(POFs) and glass optical fibers (GOFs) are used as signal conduits for laboratory and
clinical testing. Further, reduction of electron-beam-generated Cerenkov light in optical
fibers is facilitated by the inclusion of metalized air-core capillary tubing between the
BCF-12 plastic scintillating fiber and the POF.
Dose linearity, percent depth dose, and angular measurements for 6 MV/18 MV
photon beams and 9 MeV electron beams are compared using the Hamamatsu MPPC
with-and without the use of the metalized air-core capillary tubing for BCF-12 plastic
scintillating fiber. These same measurements are repeated for SiO2 : Cu2+ scintillator
material without air-core capillary tubing.
c
Copyright by Daniel C. Konnoff
March 23, 2012
All Rights Reserved
SSPM-Based Optical Fiber Radiation Dosimeter
by
Daniel C. Konnoff
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Presented March 23, 2012
Commencement June 2012
Master of Science thesis of Daniel C. Konnoff presented on March 23, 2012
APPROVED:
Major Professor, representing Radiation Health Physics
Head of the Department of Nuclear Engineering & Radiation Health Physics
Dean of the Graduate School
I understand that my thesis will become part of the permanent collection of Oregon State
University libraries. My signature below authorizes release of my thesis to any reader
upon request.
Daniel C. Konnoff, Author
ACKNOWLEDGEMENTS
First and foremost, I would like to thank Dr. Thomas Plant from the School of
Electrical Engineering and Computer Science for acting as my co-advisor. Dr. Plant has
provided me with excellent advice and has worked hard on my behalf, supporting my
research efforts and kindly making his own laboratory available to me. Further, Dr. Plant
read all of the manuscript and made constructive criticisms. Any remaining mistakes are,
of course, my responsibility alone.
I would also like to thank Dr. Abbi Farsoni for acting as my advisor. I would like to
thank Dr. Kathryn Higley for giving me the opportunity to become an RHP student, and
for helping me transition to the on-campus program, and for many thoughtful discussions.
I would like to thank Dr. David M. Hamby. I am extremely grateful for the initial research opportunity you extended to me and for making your own laboratory and
equipment available.
I would like to thank several members of my “unofficial” thesis committee. Dr.
Michael Lerner from the Department of Chemistry engaged in many valuable discussions.
Manfred Dittrich made all the optical jig-assemblies needed to complete this project in
a timely manner. Without Manfred’s excellent machine work none of the laboratory
or clinical experiments would have been possible. Elizabeth Shiner at Good Samaritan
Hospital in Corvallis Oregon who gave generously of her time and provided LINAC beam
time for testing.
I would like to thank my friends and those staff and faculty in various Oregon State
University departments who assisted me in many ways while I was there. This was not
always an easy thing to do. I would also like to thank my two fathers for instilling within
me the importance of education at an early age.
Moreover, I would like to thank Mr. Roshan Patel from Hamamatsu Corp., San
Jose CA., for providing MPPC samples, Dr. David McNally from Photonique SA, Geneva
Switzerland. for SSPM samples, Dr. Vinit Dhulla from Voxtel Corp., Beaverton OR. for
SiPM samples, Mr. Don Doize from PolyMicro Technologies for samples of their metalized air-core capillary tubing, Dr. Alan Huston from the US Navy Research Laboratory
for samples of SiO2 : Cu2+ scintillating fiber material, Dr. David Akselrod from the University of Oklahoma (now Landauer Inc.) for samples of Al2 O3 : C OSL material, and
Mr. Michael Mayhugh from Saint-Gobain Crystals for samples of their inorganic/organic
scintillator material.
This research was sponsored by a seed seed grant ESE151 from the OSU College of Engineering and by employment in the Colleges of Engineering and Business, also by Peet’s
Coffee and Tea.
TABLE OF CONTENTS
Page
1. INTRODUCTION AND PROJECT OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.2.
Goals of this project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
1.3.
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
2. OVERVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
2.1.
Overview of Photon Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
2.2.
Overview of Electron Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
Collisional Stopping Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Radiative Stopping Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
10
2.3.
Cerenkov Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4.
Specific Ionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.5.
Solid State Photomultipliers (SSPMs) as Photomultiplier Tube (PMT)
Replacements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.5.1 Avalanche Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.2 Spectral Response and Quantum Efficiency . . . . . . . . . . . . . . . . . . . . . .
2.5.3 SSPM Biasing, Equivalent Circuit, and Speed of Response . . . . . . .
2.5.4 Dynamic Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.5 Dark Count - Causes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Temperature Dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Afterpulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Optical Crosstalk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.6 SSPM S/N Ratio and Noise Considerations . . . . . . . . . . . . . . . . . . . . . .
2.5.7 Comparison of SSPM vs. Vacuum Tube (PMT) Technology . . . . .
2.6.
14
16
18
21
22
24
25
26
26
29
Important Characteristics of Optical Fibers - Glass, Plastic . . . . . . . . . . . . . 30
2.6.1 Light Propagation in OFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.2 Metalized Air-Core Capillary Tube Cerenkov Light Removal . . . . .
Other Cerenkov Removal Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.3 Other Optical Fiber Radiation Effects . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
37
38
39
TABLE OF CONTENTS (Continued)
Page
2.7.
SiO2 : Cu2+ Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3. EXPERIMENTAL METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.1.
SSPMs and Device Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.1.1
3.1.2
3.1.3
3.1.4
3.1.5
3.1.6
3.2.
Photon Counting Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I-V and CV Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SSPM Gain Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SSPM DCR Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SSPM PDE Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
GRIN Lens Design for the Voxtel SQBF-EKAA . . . . . . . . . . . . . . . . .
43
46
50
50
52
53
Optical Coupling using Adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.2.1 Characteristics of Optical Fibers used in this work . . . . . . . . . . . . . . .
58
3.3.
Scintillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.4.
Optical Fiber System Efficiency Considerations . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.4.1 Estimated Detected Light from Scintillators . . . . . . . . . . . . . . . . . . . . .
3.4.2 Efficiency: Overall Light Coupling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.3 Optical Signal-To-Noise-Ratio (S/N) Considerations . . . . . . . . . . . . .
63
64
67
3.5.
SiO2 : Cu2+ Fiber Optic Probe Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.6.
BCF-12 Fiber Optic Probe Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
3.6.1 Angular Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
3.7.
Dose Linearity Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
3.8.
Percent Depth Dose Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4. RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.1.
Photonique SSPM Comparisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.2.
Photonique, MPPC, and Voxtel SSPM Comparisons . . . . . . . . . . . . . . . . . . . . 91
TABLE OF CONTENTS (Continued)
Page
4.3.
Clinical Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
4.3.1 Mitsubishi Eska GH4001 POF Cable Clinical Background Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.2 Angular Dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.3 Dose Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.4 Depth Dose Measurements: Photon and Electron Beam . . . . . . . . .
102
102
102
107
4.4.
Summary of Clinical Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
4.5.
Uncertainty Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
5. CONCLUSIONS AND RECOMMENDATION FOR FUTURE WORK . . . . . . . 116
5.1.
Optical and Electrical Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.2.
Recommendations for Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
APPENDICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
A
APPENDIX A TEC Cooler Schematic for VOXTEL SQBF-EKAA . . . . 133
B
APPENDIX B Fiber Efficiency Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
C
APPENDIX C Laboratory Dose Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . 137
D
APPENDIX D Decay of Laboratory Sources, Number of Photons Emitted, and Prelude 420 Crystal Activity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
D1
D2
D3
Decay of Laboratory Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
Number of Photons Emitted by Decayed Sources . . . . . . . . . . . . . . . . 138
Prelude 420 (Lu1.8 Y.2 SiO5 : Ce) Crystal Activity . . . . . . . . . . . . . . . . . 139
LIST OF FIGURES
Figure
Page
1.1
Detector system block diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
2.1
Three principle photon interactions. (source: Knoll) . . . . . . . . . . . . . . . . . . . . .
5
2.2
Compton scattering kinematics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.3
Avalanche diagram for SSPM. (source Kasap) . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
2.4
Representative structural diagram for Si SSPMs. (source Photonique) . . .
15
2.5
Optical absorption coefficients for different photodetector materials.(source:
Sze) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.6
SSPM pixel array and signal shape, bias circuit, SPICE model, and Vov
definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
(a)
SSPM array structure and signal shape. . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
(b)
SSPM bias circuit, SPICE model, and Vov . . . . . . . . . . . . . . . . . . . . . . . . .
19
2.7
SSPM Dynamic range. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
2.8
Photodetection process in SSPM. (source: Sze). . . . . . . . . . . . . . . . . . . . . . . . . .
27
2.9
Optical fiber waveguide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
2.10 Index and light ray profiles for step index optical fiber. . . . . . . . . . . . . . . . . . .
31
2.11 Snell’s Law of reflection and refraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
2.12 Critical angle for fiber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
2.13 TIR for fiber when incident angle exceeds the critical angle. . . . . . . . . . . . . .
34
2.14 Important angle relationships for an OF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
2.15 Illustration of fiber coupling issues: light source coupled to fiber. . . . . . . . .
37
2.16 Illustration of metalized air-core glass capillary tube. . . . . . . . . . . . . . . . . . . . .
39
(a)
Cross section of metalized air-core capillary tube. (Source: Polymicro Inc.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
LIST OF FIGURES (Continued)
Figure
(b)
Page
Interfaces between metalized air-core glass capillary tube and scintillator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
3.1
Block diagram of laboratory and clinical measurement system. . . . . . . . . . .
42
3.2
Photon counter operation. (source: Hamamatsu) . . . . . . . . . . . . . . . . . . . . . . . .
44
3.3
Dark counts showing p.e. levels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
3.4
Typical output from photon counter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
3.5
Circuit diagram for battery operated SSPM power supply. . . . . . . . . . . . . . . .
45
3.6
Circuit diagram for photon counter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
3.7
Power supply circuit diagram for photon counter. . . . . . . . . . . . . . . . . . . . . . . .
48
3.8
ADC output charge frequency distribution. (source SensL) . . . . . . . . . . . . . .
51
3.9
SSPM DCR measurement setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
52
3.10 SSPM PDE measurement setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
52
3.11 Voxtel SQBF-EKAA mounted on 3-stage Peltier-cooler showing recessed
die. Can diameter is approximately 0.5 inch. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
53
3.12 GRIN lens system showing important dimensions. . . . . . . . . . . . . . . . . . . . . . . .
54
3.13 GRIN lens design parameters for the Voxtel SQBF-EKAA. . . . . . . . . . . . . . .
57
(a)
SLW-1.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
(b)
SLW-1.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
3.14 BCF-12, BC430 scintillator emission spectrum . . . . . . . . . . . . . . . . . . . . . . . . . .
61
3.15 Light collection efficiencies for components in the optical signal chain. . . .
65
3.16 Cerenkov threshold energy vs. material refractive index.. . . . . . . . . . . . . . . . .
67
3.17 Predicted Cerenkov angle vs. incident energy. . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
3.18 Number of Cerenkov photons produced in visible light spectrum. . . . . . . . .
71
LIST OF FIGURES (Continued)
Figure
Page
(a)
1 mm material depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
71
(b)
1 cm material depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
71
3.19 Cone of Cerenkov Radiation: electron beam ⊥ to fiber. . . . . . . . . . . . . . . . . . .
72
3.20 Cerenkov cone approaching θcritical for the optical fiber. . . . . . . . . . . . . . . . . .
72
3.21 Schematic of BCF-12 and SiO2 : Cu2+ dosimeter cables. . . . . . . . . . . . . . . . . .
73
3.22 Eska GH4001 POF and BCF-12 scintillator dosimeter cable. . . . . . . . . . . . .
74
3.23 BCF-12 Air-Core Capillary Tube Dosimeter. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
3.24 Photo of electron beam angular measurement setup. . . . . . . . . . . . . . . . . . . . .
76
3.25 Photo of dose linearity measurement setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
78
3.26 Photo of electron beam percent depth dose measurement setup. . . . . . . . . .
80
4.1
SSPM signal shape capacitance SPICE simulation. . . . . . . . . . . . . . . . . . . . . . .
81
4.2
Die capacitance for five Photonique SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
83
4.3
Dark count rates for Photonique SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
84
4.4
4.5
(a)
Measured DCR for Photonique SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . .
84
(b)
Calculated DCR for Photonique SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . . .
84
Gain dependence for two Photonique SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . .
85
(a)
Gain dependence for 050701GR.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
85
(b)
Gain dependence for 0611B1 and CLNS. . . . . . . . . . . . . . . . . . . . . . . . . . .
85
Gain dependence for two Photonique SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . .
86
(a)
Gain dependence for 0701BG. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
86
(b)
Gain dependence for 0810G1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
86
4.6
IV-characteristics for MPPC, Photonique, and Voxtel SSPMS. . . . . . . . . . .
91
4.7
Die capacitance for MPPC, Photonique, and Voxtel SSPMs. . . . . . . . . . . . . .
92
LIST OF FIGURES (Continued)
Figure
4.8
4.9
Page
Dark count rates for Photonique, Voxtel and MPPC SSPMs. . . . . . . . . . . . .
93
(a)
Measured DCR for (4) compared SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . .
93
(b)
Calculated DCRs for (4) compared SSPMs.. . . . . . . . . . . . . . . . . . . . . . . .
93
Gain dependence for Voxtel and MPPC SSPMs. . . . . . . . . . . . . . . . . . . . . . . . .
94
(a)
Gain dependence for Voxtel SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
94
(b)
Gain dependence for MPPC SSPM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
94
4.10 Optical fiber and plastic scintillator junction ends. . . . . . . . . . . . . . . . . . . . . . . 101
(a)
Optical fiber junctions resulting in signal loss. . . . . . . . . . . . . . . . . . . . . .
(b)
BCF-12 scintillator tip under 10x magnification: tapered end is
485.5 µm diameter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
101
101
4.11 GH4001 POF cable background response. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
(a)
GH4001 POF photon background response. . . . . . . . . . . . . . . . . . . . . . . .
103
(b)
GH4001 POF electron background response.. . . . . . . . . . . . . . . . . . . . . . .
103
4.12 Angular dependence of standard and capillary tube POF dosimeters:
BCF-12 scintillator: measured and theoretical results . . . . . . . . . . . . . . . . . . . 104
4.13 6 MV photon dose linearity: Prelude 420 scintillator. . . . . . . . . . . . . . . . . . . . . 105
4.14 Photon dose linearity: standard and capillary tube dosimeters . . . . . . . . . . . 106
(a)
BCF-12 and SiO2 : Cu2+ Dosimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
106
(b)
BCF-12 Capillary Tube Dosimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
106
4.15 Electron dose linearity: BCF-12 and SiO2 : Cu2+ Dosimeters. . . . . . . . . . . . . 107
4.16 Electron dose linearity: capillary tube dosimeters. . . . . . . . . . . . . . . . . . . . . . . . 108
(a)
BCF-12 Capillary Tube Dosimeter: 6 MeV, 9 MeV. . . . . . . . . . . . . . . .
108
(b)
BCF-12 Capillary Tube Dosimeter: 12 MeV, 16 MeV, 20 MeV. . . . .
108
4.17 10 cm x 10 cm 6 MV photon depth dose profile in water tank . . . . . . . . . . . 110
LIST OF FIGURES (Continued)
Figure
Page
4.18 10 cm x 10 cm 18 MV photon depth dose profile in water tank . . . . . . . . . . 111
4.19 10 cm x 10 cm 9 MeV electron depth dose profile in water tank . . . . . . . . . 112
5.1
0.2
Proposed light guide for small area SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
(a)
Light guide schematic drawing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
121
(b)
Light guide ray trace diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
121
Circuit diagram for TEC cooler controller. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
LIST OF TABLES
Table
Page
2.1
w for some common radiation detection materials. . . . . . . . . . . . . . . . . . . . . . .
12
2.2
G, PDE, and Bias variations as a function of temperature. . . . . . . . . . . . . . .
26
2.3
Optical photon sensing: vacuum vs. solid-state technology. . . . . . . . . . . . . . .
30
2.4
The three types of optical fiber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
3.1
c
SLW GRIN lenses at λ = 440 nm and
Calculated values for SELFOC
580 nm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
3.2
Characteristics of GH4001 cable and SiO2 optical fiber. . . . . . . . . . . . . . . . . .
59
3.3
Summary of published scintillator emission characteristics. . . . . . . . . . . . . . .
62
3.4
Estimated Scintillator Outputs: number of photons and SSPM current I.
64
3.5
v for a few energies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
69
4.1
Summary of published SSPM parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
82
4.2
PDE measurements for Photonique SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
4.3
Summary of measured breakdown voltages and device currents for Photonique SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
4.4
Summary of measured Photonique SSPM characteristics. . . . . . . . . . . . . . . . .
89
4.5
Photonique laboratory photon count rates†(cpm) 60 Co: direct scintillatorSSPM attachment and using POF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
90
4.6
PDE measurements for PH0810G1, Voxtel, and MPPC SSPMs. . . . . . . . . .
95
4.7
Summary of measured breakdown voltages and device currents. . . . . . . . . .
95
4.8
Summary of measured characteristics for Photonique, Hamamatsu, and
Voxtel SSPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
96
Laboratory photon count rates†(cpm) 60 Co: direct scintillator-SSPM attachment and using POF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
96
4.9
4.10 Photon and Electron Dose Linearity Differences (range extrema) from
Reference Ion Chamber (in %). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
LIST OF TABLES (Continued)
Table
Page
4.11 Photon and Electron Percent Depth Dose Differences (range extrema)
from Reference Ion Chamber (in %). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
0.1
GH4001 plastic fiber cable attenuation characteristics.. . . . . . . . . . . . . . . . . . . 136
DEDICATION
This thesis is dedicated to the memory of my two fathers: Morris Nickoliavich Konnoff
and Leo Miles Clausen. Together you both made me the man I am today.
This thesis is also dedicated to my wife and best friend who endured years of separation
while I was thousands of kilometers away at OSU.
SSPM-BASED OPTICAL FIBER RADIATION DOSIMETER
1.
1.1.
INTRODUCTION AND PROJECT OBJECTIVES
Introduction
Optical fiber (OF) dosimetry using plastic or glass fibers coupled to a wide variety
of scintillators has been reported for clinical, diagnostic radiography, and environmental
radiation sensing purposes.[Akselrod et al., 2007, Anderson et al., 2009, Beddar, 1994,
Beddar et al., 2001, Hyer et al., 2009]
Copper-doped glass (SiO2 : Cu2+ ) may be used as an Optically Stimulated Luminescence (OSL) material or as a real-time Radio Luminescence (RL) scintillator [Huston
et al., 2001, Justus et al., 2004] while BCF-12 plastic scintillator may be used in RL mode
(photomultiplier tube as photosensor).[Beierholm et al., 2008] Water equivalence and energy independence of plastic scintillators as a detector material are well documented, and
considerations of detector size and signal-to-noise (S/N) ratio have been analyzed.[Beddar
et al., 2005, Clift et al., 2000, Archambault et al., 2005] BCF-12 plastic scintillator represents a good choice for a dosimeter as variants of this material have produced good results
in clinical studies.[Bartesaghi et al., 2007]
Recent work by groups in Japan and Russia have advanced solid state photomultiplier (SSPMs, SiPMs, MPPCs) technology [Golovin and Saveliev, 2004, Gomi et al., 2007]
making it a viable alternative to photomultiplier tubes (PMTs) for laboratory, environ-
2
mental, clinical dosimetry, medical imaging applications (PET, CT, SPECT)[Heckathorne
et al., 2006], dosimetry in diagnostic radiology, and in-vivo applications (catheters and
brachytherapy[Anderson et al., 2009, Suchowerska et al., 2007]). Their small size, high
gain, low bias voltage, and non-magnetic characteristics are distinct advantages when
compared with the PMT.[Saveliev and Golovin, 2000] Previous clinical studies using optical fibers and different inorganic/organic scintillators have used standard photodiodes
or PMTs as optical photon detectors. The large signals generated by Linac (linear accelerator) beams coupled with external amplification proved sufficient for analyzable signals.[Beddar, 1994, Lee et al., 2006] However the added advantages of built-in gain, low
voltage operation, greater photon detection efficiency (PDE) and ruggedness, together
with low noise, make SSPMs more suitable photon detectors than standard photodiodes
in a wider variety of applications.
1.2.
Goals of this project
This project has two goals. The first is to investigate the design and characterization of SSPM-optical fiber-coupled systems (both OSL and RL) as radiation sensors for
laboratory, environmental, diagnostic radiology, clinical radiotherapy, and military applications. The second is to characterize and compare the performance of different sampled
SSPMs from Hamamatsu, Photonique, and Voxtel when coupled with different scintillator
material types and scintillator coupled optical fibers. Note that SensL, currently the other
major SSPM manufacturer, refused to supply samples for use in this work.[SensL, 2006]
Figure 1.1 shows a block diagram of the detector system.
current
voltage
3
Ionizing radiation:
x−ray,
gamma ray,
beta particle
t
Scintillator
SSPM
t
Interface
and
Readout
Electronics
Host
Computer
FIGURE 1.1: Detector system block diagram.
1.3.
Applications
SSPM-based radiation sensing systems can solve a number of problems that plague
PMT-based systems, among them:
• 800-1200 V PMT bias voltages and glass envelopes are hazardous and fragile.
• High sensitivity to vibration and magnetically unstable.
• Large size, not easily portable.
• More expensive compared to SSPM-based systems.
Advantages of SSPMs are:
• Extremely small size, compact.
• Low bias voltage, battery operable (< 80 V).
• Magnetically insensitive.
• High PDE.
• High gain.
• Fast time resolution.
• Not damaged when saturated by ambient light.
4
• Low cost potential.
However there are disadvantages, among them:
• High room temperature noise rate ( ≥ 100 kHz/mm2 ) for 1 mm2 active areas.
• PDE is smaller than QE.
• PDE is a function of overvoltage Vov and sensitive to small changes.
• Gain and noise are dependent on temperature.
• Limited dynamic range.
• Optical cross talk issue.
• High initial cost currently.
This work explores these issues. Many of these disadvantages can be corrected by:
using a temperature and voltage stabilized SSPM bias source, and ongoing technology
improvements.
5
2.
2.1.
OVERVIEW
Overview of Photon Interactions
There are six primary radiation interactions with matter: photoelectric effect, compton scattering, pair production, Raleigh scattering, photodisintegration, and triplet production. Of these six, photodisintegration and triplet production have low probabilities
for the energy range typically encountered in radiation monitoring or oncology treatments
(0 to 20 MeV) while Rayleigh scattering is a coherent, elastic process which does not
contribute more than a few percent to dose.[Podgorsak, 2005, Martin, 2006] Figure 2.1
shows the three most important photon interaction mechanisms: the photoelectric effect,
compton scattering, and pair production.[Turner, 2007, Attix, 1986, Tsoulfanidis, 1995,
Knoll, 2000]
FIGURE 2.1: Three principle photon interactions. (source: Knoll)
1. The photoelectric effect refers to the interaction of an incident photon with a tightly
6
bound atomic electron. The incident energy of the photon Eγ is absorbed by the
bound electron which is then ejected from the atom leaving a vacancy in one of the
inner orbitals. For a photon incident on the atom with energy hv, the kinetic energy
(KEγ ) of the ejected photoelectron is:
KEγ = hv − Ebinding
(2.1)
where Ebinding is the binding energy of the ejected electron in the atom. The probability of a photoelectric interaction (a function of τ the photoelectric cross section)
varies approximately as Z 4−5 /Eγ3.5 (Z is atomic number) at low photon energies and
with the atomic number of the target.[Knoll, 2000, Turner, 2007]
2. Scattering by a loosely bound or free electron is known as Compton scattering or
the Compton effect; it is the predominant mode of photon interaction especially for
low Z absorbing material (see Figure 2.1). Here the incident photon is scattered by
an electron with some of the energy transferred to the same electron (know as the
recoil electron). Figure 2.2 shows the kinematics of this interaction.
recoil electron
Ee
Eγ
φ
θ
incoming γ −ray
E’γ
scattered γ−ray
FIGURE 2.2: Compton scattering kinematics.
The recoil kinetic energy of the Compton electron is:
7
′
Ekinetic = Ehv − Escattered = Eγ − Eγ
′
(2.2)
′
where (Escattered = Eγ = Ehv ) is the energy of the scattered photon. This energy
depends upon the incident photon energy and the scattering angle which is given by
′
′
Eγ = Ehv =
1+
Eγ
Eγ
(1
me c2
(2.3)
− cosθ)
′
′
where Eγ = Ehv is the energy of the incoming gamma ray photon, Eγ = Ehv is
the energy of the scattered x-ray photon, and me c2 is the rest mass energy of an
′
electron (511 keV). Note that Eγ = Eγ at θ = 0 (scattered photon maximum energy)
′
and at θ = π (scattered photon minimum energy) Eγ = Eγ /1 +
2Eγ
511keV
. Compton
scattering probability (σ = the Klein-Nishina cross section) generally decreases with
increasing incident photon energy, varying approximately as Z/hv and is directly
proportional to the number of electrons per gram of material. [Knoll, 2000, Turner,
2007, Tsoulfanidis, 1995, Attix, 1986]
3. When incident photon energy is greater than 1.022 MeV pair production predominates. The photon interacts with the nucleus electromagnetic field, is absorbed, and
is replaced by an electron-positron pair. Rest mass energies for both the electron and
positron are me c2 (511 keV); thus the minimum incoming photon threshold energy
needed is 1.022 MeV (2me c2 ). Excess photon energy is transferred to the electron
and positron as kinetic energy:
Epp−transf erred = Ehv − 2me c2
(2.4)
Transferred KE is equally shared between the electron and positron. Probability
of pair production (κ = the pair production cross section) varies approximately as
Z 2 ln(Ehv − 2me c2 ).[Knoll, 2000, Turner, 2007, Tsoulfanidis, 1995, Attix, 1986]
8
2.2.
Overview of Electron Interactions
The primary photon interactions all transfer energy to electrons in the material
in the form of KE, setting electrons in motion. Propagation of these electrons causes
the downstream deposition of radiation effects or dose. The process of energy transfer
between the electrons set in motion in the material can be described four ways:[Turner,
2007, Tsoulfanidis, 1995]
1. Coulumbic interactions with orbital electrons and other nuclei,
2. Emission of electromagnetic radiation (Bremsstrahlung radiation),
3. Nuclear interactions,
4. Emission of Cerenkov radiation.
Energy transfer is primarily through Coulombic interactions between the electric
field of moving electrons and the electric field of either electrons or atomic nuclei in the
material. If the particle is an electron or positron, it may collide with an atomic electron
and lose all of its energy in a single collision because the collision involves particles of
the same mass, or it may be scattered in a zig-zag pattern with large angles.[Tsoulfanidis,
1995] Contrast this with heavier charged particles (alpha, proton, deuteron, etc.) that lose
much smaller amounts of energy with each collision with atomic electrons and experience
little deflection.
Interactions involving electrons in the material result in ionizations along the initial
electron’s trajectory and are called ionizational losses. The rate of energy loss per unit
length of material as a result of these interactions is characterized by the linear stopping
power dE/dx, where E is the kinetic energy of the initial energetic electron and x is
the length traveled by the particle. The total mass ionization stopping power S(E) =
− 1ρ (dE/dx) for electrons is given by dividing dE/dx by ρ, the density of the material
9
(minus sign makes S positive). S(E) is separated into two components: the collisional
stopping power and the radiative stopping power shown in equation 2.5.[Turner, 2007]
1
S=−
ρ
dE
dx
total
1
=−
ρ
dE
dx
1
+ −
ρ
col
dE
dx
[
rad
M eV cm2
]
g
(2.5)
Collisional Stopping Power
Collisional stopping power for electrons/positrons is given by equation 2.6. [Turner,
2007, Attix, 1986]
1
−
ρ
dE
dx
±
col
√
4πk02 e4 n
mc2 τ τ + 2
±
√
=
+ F (β)
ln
mc2 β 2
2I
(2.6)
gives the energy loss per unit thickness of material where:
k0 = 8.99 × 109 [N m2 /C 2 ],
e is the electronic charge = 1.6 · 10−19 C,
n is the number of electrons per unit volume in the material,
m is the mass of an electron,
β is the electron relativistic speed v/c,
τ = E/me c2 is the particle energy divided by electron rest mass energy (511 keV),
I is the mean atomic ionization potential [eV] (found in tables, varies with Z)[Turner,
2007, Podgorsak, 2006],
F ± is defined differently for electrons and positrons.
For electrons F − (β) is:
1 − β2
τ2
F β) =
− (2τ + 1)ln2
1+
2
8
−
(2.7)
and for positrons F + (β) is:
10
4
14
β2
+
+
.
23 +
F (β) = ln2 −
24
τ + 2 (τ + 2)2 (τ + 2)3
+
(2.8)
10
Substituting in the constants and rearranging equation 2.6 gives,
−
1
ρ
dE
dx
±
=
col
with G± (β) given by
5.08 × 10−31 n ±
G
(β)
−
ln
I
β2
√
G± (β) = ln 3.6 × 105 τ τ + 2 + F ± (β).
(2.9)
(2.10)
Radiative Stopping Power
Acceleration interactions involving atomic nuclei in a material result in radiative
loss of energy through bremsstrahlung processes (when an electron/positron is deflected
by a nuclear electric field) that in turn result in the emission of photons. Bremsstrahlung
production varies approximately with Z 2 ; losses are greater in high-Z materials.
The mass radiative stopping power for electrons is given by equation 2.11. [Turner,
2007, Attix, 1986]
1
−
ρ
dE
dx
rad
ZE 1
×
≈
800 ρ
dE
dx
(2.11)
col
where the individual terms are the same as above. [Podgorsak, 2006] gives complete
expressions for radiation stopping power for various ranges of electron kinetic energies.
2.3.
Cerenkov Radiation
Another process through which energetic electrons can lose energy is through the
emission of Cerenkov radiation. Cerenkov emission occurs when a charged particle passes
through any medium in which the phase velocity of light is less than the particle velocity
(i.e. βn > 1, n being the refractive index of the medium). These conditions occur
when high-speed charged particles pass into a transparent dielectric material. While the
velocity of the particle is unaltered, the electric field associated with the particle’s charge
11
and the magnetic field associated with the motion of this charge are propagated with
a phase-velocity of c/n. As the particle moves ahead from a slower-moving portion of
its own electromagnetic field, an electromagnetic wave front is formed. The number of
quanta emitted is inversely proportional to the square of the wavelength, thus shorter
wavelengths are favored; Cerenkov radiation appears as bluish-white light. The threshold
particle energy required to generate Cerenkov radiation is given by equation 2.12. [Jelly,
1958]
ECerenkov−threshold = mo c2
r
!
1
−1
1+ 2
n −1
(2.12)
Photons are emitted anisotropically with the direction of emission characterized by a cone
spreading out at the Cerenkov angle from the direction of the interacting particle, given
by equation 2.13. [Jelly, 1958]
κ = cos−1 (
1
) = Cerenkov angle
nβ
(2.13)
where β is is the ratio of velocity in the material to the speed of light in a vacuum. This is
an important noise source in several types of specialized radiation sensing devices which
use optical fibers for light signal transmission.
The angle at which this light is predicted to be a maximum is given by[Jelly, 1958]
θCerenkov−M ax = cos−1
1
(2.14)
ncore
where ncore is the refractive index of the optical fiber core.[Jelly, 1958] The approximate
theoretical intensity curve for the captured Cerenkov radiation in optical fiber is given by
Icapture
ncore 2 − 1
≈ρ
cos−1
πncore 2 − 1
3
ncore − ∆n − cos γ
√
sin γ ncore 2 − 1
(2.15)
where ρ is the OF core radius, ∆n is the difference between the OF core and cladding
indices of refraction and γ is the angle between the electron beam and the OF axis in the
12
direction of the detector.[Law et al., 2007]
2.4.
Specific Ionization
An important concept when working with different materials is the number of ionpairs formed per unit distance traveled in a material. The specific ionization is given by
equation 2.16. [Tsoulfanidis, 1995]
SI =
dE/dx [eV /cm]
w
[eV /ip]
(2.16)
where w is the energy needed to create an electron-hole (e-h) pair or an ion, which is
specific to a material. w for some common radiation detecting materials is shown in
Table 2.1.[Turner, 2007, Attix, 1986, Martin, 2006]
TABLE 2.1: w for some common radiation detection materials.
Material
Air
Plastic
SiO2
Si
Ge
GaAs
(w) e-h pair
34 eV
60 eV
17 eV
3.6 eV
2.8 eV
4.8 eV
generation energy
2.5.
Solid State Photomultipliers (SSPMs) as Photomultiplier Tube (PMT)
Replacements
Various commercial manufacturers have recently released SSPM devices that can
replace PMTs in selected applications. [Gomi et al., 2007, Swain et al., 2005, Jackson,
2007, Zecotek, 2008] These devices are small, low voltage, non-magnetic (important for
active MRI and military uses), rugged and relatively low cost (PMTs of comparable or
greater performance are currently more expensive). Their principle drawbacks at present
13
are higher dark current (2-3 or more orders of magnitude greater than low noise PMTs)
and, until recently, small active areas. Each potential application must be evaluated
individually, as one cannot simply drop SSPMs in as a PMT replacement.
SSPMs are similar to PIN diodes in function having a more complex structure (an
array of diodes connected in parallel), with the main exception being built-in gain from
operating in the geiger-mode avalanche breakdown region of the I-V curve (also known as
geiger breakdown).[Sadygov et al., 1996, Saveliev and Golovin, 2000, Golovin and Saveliev,
2004, Sadygov et al., 2006] Avalanche breakdown is caused by impact ionization where
an electron or a hole gains sufficient energy from the applied electric field such that the
energy gained initiates a transition of an electron from the valance band to the conduction
band, thus creating a new electron-hole (e-h) pair.[Sze, 2006] Newly created e-h pairs are
also accelerated by the applied electric field, generating further new e-h pairs by process
repetition. If the bias voltage Vbias is high (hence a high E-field) an uncontrolled rise in
the current is the result. Left unchecked this process leads either to device destruction
or current limiting by an external load resistor, Rquenching , for individual SSPM pixels.
Figure 2.3 shows the e-h avalanche diagram for an SSPM.
FIGURE 2.3: Avalanche diagram for SSPM. (source Kasap)
SSPM multiplication gain is approximated by equation 2.17. [Sze, 2006]
14
M=
1
1
= IM /Ip =
RL
1 − α(x)Wd
1 − 0 α(x)dx
(2.17)
where α is the electron or hole multiplication coefficient, L is the electron space charge
region width, IM is the average value of the total multiplied output current, Ip is the
primary unmultiplied photocurrent, and Wd is the depletion region width. This is true
for equal ionization coefficients α = αn = αp .[Sze, 2006]Note that αWd = 1 corresponds
to the device breakdown voltage Vbreakdown . α(x)Wd is the number of ionization’s in
width Wd ; this is a function of electric field E and thus on reverse bias voltage Vbias ,
m with C a constant. Here m is an empirical exponent;
approximated as α(x)Wd = CVbias
a function of the material and its doping levels. Using the above C can be expressed as
−m
C = Vbreakdown
.[Bar-Lev, 1993, Sze, 2006]
Substituting into equation 2.17 gives the common form for M
M=
1
1 − (Vbias /Vbreakdown )−m
(2.18)
Guard ring structures are used to fabricate SSPM devices to minimize leakage currents caused by regions of high electric field at the junction edges. These areas of local
high E fields, known as micro-plasmas, have low breakdown voltages and the potential
for uncontrolled avalanches.[Pellion et al., 2009, Golovin and Saveliev, 2004, Renker and
Lorenz, 2009, McNally and Golovin, 2009]
Figure 2.4 shows a representative structural diagram for a Si SSPM showing trench
based optical isolation and E-field intensity by region.
2.5.1
Avalanche Noise
If m is the statistically varying SSPM gain, then < m2 > > < m >2 = M 2 where the
(<>) represent an ensemble average, and < m >= M is the average carrier multiplication
as defined in equation 2.17.[McIntyre, 1966] This noise results from the avalanche process
itself and depends upon < m2 >, the mean square gain, Thus SSPM avalanche noise can
15
FIGURE 2.4: Representative structural diagram for Si SSPMs. (source Photonique)
be high.[Fyath and O’Reilly, 1988] It has been found empirically that < m2 > can be
approximated as:
< m2 >= M 2+x
(2.19)
where x varies between 0 and 1 and is a function of SSPM crystal material (Si, InGaAs,
etc.) and device structure.[Scansen and Kasap, 1992, Kasap, 2001]
The excess noise factor, F(M), is a measure of the increase in SSPM noise resulting
from the randomness of the multiplication process.[Sze, 2006] F is the ratio of the actual
noise generated in an SSPM to the noise that would exist if all carrier pairs were multiplied
by M. It is given by F =
<m2 >
<m>2
=
<m2 >
.[Kim
M2
et al., 1997] From equation 2.19, F(M) is
approximated as:
F = Mx
(2.20)
where x = 0.3 for Si and x = 0.7 for InGaAs.[Scansen and Kasap, 1992, Kasap, 2001]
16
2.5.2
Spectral Response and Quantum Efficiency
Figure 2.5 shows optical absorption coefficients for different optical materials. From
this figure one can see that Si and modified Ga compounds are suitable materials for
SSPMs.
FIGURE 2.5: Optical absorption coefficients for different photodetector materials.(source:
Sze)
The value of the long wavelength cutoff frequency λc is given by equation 2.21:
hc
Eg
(2.21)
1.241
[µm]
Eg (eV )
(2.22)
λc =
which is commonly expressed as:
λc =
Room temperature bandgap energies are 1.124 eV for Si and 1.42 eV for GaAs; the
corresponding λc cutoffs for each material are 1.1 µm and 0.87 µm, respectively.
17
Quantum efficiency η(λ) for a SSPM is the number of e-h pairs generated per incident photon, given by equation 2.23:
η(λ) =
Iphoto
q
P
hλ
=
number of emitted electrons
number of incident photons
(2.23)
where Iphoto is the photocurrent generated by incident optical power P. Overall
responsivity is defined by equation 2.24:
R=
Iphoto
η(λ)q
η(λ)λ
=
M = Ro M =
M [A/W ]
P
hν
1.241
(2.24)
(with λ in µm), as the ratio of the photocurrent Iphoto to the incident optical power P,
and Ro is the unity gain responsivity.[Kasap, 2001, Palais, 2005]
In order to have high η(λ), an SSPM must have enough absorption layer width,
which is determined by the light absorption coefficients as shown in Figure 2.5. Most of
the incident light on an SSPM will be absorbed when the depletion width Wd is of the same
order as 1/α. For semiconductor homojunctions, the depletion region must extend from
the material surface to avoid surface absorption. For semiconductor heterojunctions such
as modified Ga compounds, η(λ) does not depend on the junction-to-surface distance. The
larger bandgap material is used as a window for light transmission. Indeed, this is how wide
spectral range optical photon harvesting detectors are constructed. Antireflection coatings
must be used to minimize light reflection at the semiconductor-air interface.[McNally and
Golovin, 2009, Barton et al., 2009]
Overall SSPM efficiency, sometimes referred to as Photon Detection Efficiency (PDE)
is given by equation 2.25,
ǫ = η(λ) × ǫGeiger × ǫGeometry = PDE
(2.25)
Here η(λ) is the quantum efficiency as defined in equation 2.23, ǫGeiger is the probability of an avalanche breakdown, and ǫGeometry is the SSPM photosensitive area packing
18
factor.[Buzhan et al., 2006]
SSPM pixel capacitance at the breakdown voltage Vbreakdown is used to calculate
gain using[Buzhan et al., 2006]
Gain =
R
i(t)dt
= Qpixel /q = Vov Cpixel /q
q
(2.26)
where gain is proportional to the overvoltage, Vov = Vbias − Vbreakdown above the SSPM
breakdown voltage. SSPM manufacturers typically recommend an operating voltage at a
specific gain, known as Vop .
For our laboratory and clinical measurements PDE at the peak scintillator wavelength was the best indicator of overall SSPM system performance, followed by SSPMscintillator/SSPM-OF coupling considerations, SSPM dark count rate (DCR), then by
SSPM gain. For low level environmental radiation sensing or when used together with
OFs, SSPM DCR must remain small to detect signals attenuated by long POFs (10s of
meters), where a high system S/N ratio is necessary for detection of low level scintillator
signals.[Pavlov et al., 2005, Beddar, 2007]
2.5.3
SSPM Biasing, Equivalent Circuit, and Speed of Response
Figure 2.6 shows the SSPM pixel array structure, gross output signal shape, circuit
model, SPICE model for a typical SSPM, and the definition of overvoltage Vov on the
I-V curve. Figure 2.6(a) illustrates the diode pixel elements connected in an array with a
common output together with the gross signal shape from those same elements. The overall
magnitude of the SSPM output signal is the sum of each diode pixel. The SSPM spice
model is divided into three zones: an Active zone which represents a diode pixel about to
avalanche, during avalanche, or recovering from avalanche, a Passive zone that represents
the remaining non-firing diode pixels, and a Parasitic zone which represents other stray
capacitance’s on the SPPM die.[Corsi et al., 2006, Seifert et al., 2009] External circuit
resistance is the sum of Rs + RL , the shunt resistance of any subsequent amplifier stage,
19
(a) SSPM array structure and signal shape.
+Vbias
Bias Circuit
RL
A
hv
SSPM
C shape
R shunt
Definition of Vov
reverse
I
Spice Model
Active
Passive
Parasitic
Vbreakdown
Vbias
Rquench
Cquench
R
______
quench
(N−1)
forward
Cparasitic
hv
Ipulse
V
Cquench (N−1)
Cdiode
Cdiode (N−1)
Overvoltage(Vov)
(b) SSPM bias circuit, SPICE model, and Vov .
FIGURE 2.6: SSPM pixel array and signal shape, bias circuit, SPICE model, and Vov
definition.
20
and the load resistance, while external circuit capacitance is the sum of CD + Cshape , the
pixel junction capacitance and other parasitic capacitance’s. For a typical SSPM the CD
and RL terms dominate. Note that CD is a strong function of pixel area and reverse bias
voltage.
Each diode in the pixel array is in series with a quenching resistor, Rquench , that
is typically 1 MΩ or more. Reducing the bias on each pixel element below Vbreakdown
stops the impact ionization process. Approximately, the RC time constant Rquench Cdiode
determines the time for this to occur. Rquench limits the current recharging the SSPM
diode pixel. A loop equation for each pixel element gives:
Vbias = Vdiode + IRquench + IRL
(2.27)
where the resulting pixel element signal (ideal) is given by the following steps.
• The diode pixel is in the dark and no current flows (I = 0), and Vdiode = Vbias .
Assume a photon impinges on the pixel at some time t0 .
• Diode pixel current increases. The diode pixel is charged with τr = Cdiode Rquench
time constant. The diode pixel bias will now decrease until it reaches breakdown
voltage.
• At some time t1 , Vdiode = Vbreakdown , and uncontrolled geiger mode avalanche multiplication is stopped (quenching).
• At times > t1 , the diode pixel capacitance discharges with a time constant τf =
Cdiode (Rquench + Rs ).
• After a finite diode pixel recovery time between approximately 4τf to 9τf , current
is reduced to zero and the diode pixel element is reset. This pixel recovery time is
strongly dependent on SSPM manufacturer and technology used.
21
The rise time and fall time of an SSPM are defined as the times for the signal to rise
or fall from 10% to 90% or 90% to 10% of the final value, respectively. This parameter
can be also expressed as frequency response, which is the frequency at which the SSPM
output decreases by 3 dB, and is given by equation 2.28:[Kasap, 2001, Palais, 2005]
trise =
0.35
f3db
(2.28)
For an SSPM there are four principle factors which determine the speed of response:
• The RC quenching time for each individual pixel. This is the product of the individual pixel capacitance and on-chip quenching resistor.
• The RC time constant (tRC = 2.2RC) of the pixel (diode) array and external circuit.
• The diffusion (charge) collection time of carriers in the undepleted region.
• The drift charge collection time of carriers in the depletion region of each pixel.
2.5.4
Dynamic Range
The total number of pixels firing in an SSPM determines the dynamic range. The re-
lationship between the number of pixels fired (Nf ired ) and the number of incident photons
(Nphotons ) is given by equation 2.29;
Nf ired = Ntotal 1 − exp
−Nphotons
Ntotal
(2.29)
Ntotal is the total number of SSPM pixels in the array.[Renker, 2006] The signal output is
proportional to the number of fired cells. When Nphotons reaches the same order of magnitude as Ntotal , the probability that multiple photons hit the same pixel increases.[Renker,
2006] As a result the output signal will saturate independent of the number of photoelectrons. An SSPM signal can saturate at high light levels without any resulting physical
22
damage. Contrast with a PMT that suffers physical damage at high or ambient light levels. Equation 2.29 is plotted in Figure 2.7, showing Nf ired vs. Nphotons for representative
Hamamatsu, Photonique, SensL, and Voxtel SSPMs.
SSPM Dynamic Range
10000
100 pixels
516 pixels
556 pixels
Number of Fired Pixels
400 pixels
1000
1024 pixels
1600 pixels
3900 pixels
8100 pixels
14560 pixels
100
10
1
1
10
100
1000
10000
Number of Photoelectrons
FIGURE 2.7: SSPM Dynamic range.
2.5.5
Dark Count - Causes
The three contributors to SSPM dark counts are thermal e-h carrier generation,
afterpulses, and optical crosstalk.
SSPMs, being made of semiconductor material, suffer from thermal generation of eh pairs in addition to those caused by incoming photons.[Quimby, 2006] These thermally
generated e-h pairs typically lead to a dark count rate (DCR) from 105 − 107 counts
s−1 /mm2 at room temperature with a threshold at half of one photo-electron amplitude
(p.e) approximately 0.5 · 106 .[Quimby, 2006, Renker and Lorenz, 2009] The dark count
23
rate decreases with temperature and decreasing Vov . The density of defects in, and volume
of the semiconductor material are the primary dependencies.
A major noise parameter in an SSPM is the dark current. Even though no light is
present some current will still flow between the terminals of the device. The magnitude of
this current differs, dependent on device physical design (layout, isolation trenches, pixel
size, pitch, etc.). Dark current arises from random electron-hole pair generated thermally
or by tunneling. A dark current generated electron will have the same effects as a photo
generated one, thereby multiplication. Tunneling is mostly a matter of design and choice
of materials. It is therefore not a process the user can alter by some means of external
stress, unless compromising vital properties like bias. However, the rate of thermally
generated carriers can be influenced by material temperature. The rate of thermally
generated carriers in an intrinsic material is given by the generation rate[Bar-Lev, 1993,
Sze, 2006]
G(T ) = κn2i (T ) [carrier pairs 1/m3 s]
(2.30)
where κ is a proportionality constant. Note that G(T ) ≈ n2i r(T ) where r(T) is the carrier
recombination rate.
At low temperatures (77 K) carrier mobilities are greater than at room temperature (300 K); as temperature is reduced Vbreakdown decreases.[Sze, 2006, Bar-Lev, 1993]
The function G(T) has a temperature dependency, decreasing with less temperature and
increasing with greater temperature. Decreasing the temperature decreases the dark current, thus cooling the semiconductor material increases low light photon detection ability.
The dark current in an SSPM is approximated by
I = Ie + Ih = q A
n2i (T )
C(T )
−qVbias
−qVbias
exp
− 1 = Idark exp
−1
kT
kT
(2.31)
where Ie and Ih are the electron and hole currents, A is the SSPM junction area, C(T) is a
24
constant including diffusion lengths, depletion region widths, and material doping levels.
ni (T ) is given by
ni (T ) = 2(
2πkT 3/2
) (me mh )3/4 eEg /2kT
h2
(2.32)
where Eg is the material energy gap and me , mh are the electron/hole mobilities respectively.
Thus, the thermal probability of carrier production can be approximated as
Thermal Probability ≈ T 3/2 eEg /2kT
(2.33)
Note I = dQ/dt = nq/∆t where n is the number of electrons, q is the electronic charge,
and ∆t is a time interval (typically ns).[Quimby, 2006] The number of electrons passing
a cross-section of material in 1 ns is
n=
I∆t
= 6.26 · 10−9 I
q
(2.34)
From equation 2.34 one can see that less than one electron per ns is needed to realize
a dark current in the sub-nanoampere range.
Temperature Dependence
SSPMs are similar to standard diodes with increased photo sensitivity, where the
output current contains a term which is dependent on the incident light intensity on its
surface in the operating wavelength range. SSPM output current is given by
− qVop
Isspm = Iphoto + Idark e
kT
−1
(2.35)
Here Isspm is the SSPM output current, Iphoto is the photo current, Idark is the
saturation dark current, and Vop is the operating voltage.
25
At a constant bias voltage and wavelength, the SSPM responsivity increases by
decreasing the device temperature. Cooling an SSPM will increase its detected photocurrent. SSPM dark current, given by the second term of equation 2.35, is also dependent
on the device operating temperature. In general, decreasing the SSPM temperature will
decrease the dark current.
SSPM signal amplitude temperature stability is a function of three (3) factors as
shown in equation 2.36
Signal Amplitude(T ) = Nphotons × G(T ) × P DE(T )
(2.36)
Note that gain G and PDE are strong functions of temperature, additionally both
PDE and G are functions of Vbias .
SSPM breakdown voltage Vbreakdown increases linearly with temperature[Bar-Lev,
1993]. The breakdown voltage can be approximated for pure avalanche processes as
Vbreakdown = Vbreakdown0 (1 + β(T − T0 )
(2.37)
where Vbreakdown0 is the reverse breakdown voltage at room temperature (T0 ) and β is the
linear growth constant, typically greater than 10−3 K −1 .[Petasecca et al., 2008]
Signal amplitude increases with Vbias when SSPM temperature is lowered because G
and PDE are negatively correlated with temperature.[McNally and Golovin, 2009] Temperature variation dependencies of bias voltage, G, and PDE are given in Table 2.2 (table
from [Shushakov et al., 2008]). A temperature controlled isolation chamber is needed to
calculate these variations from measured data; none was available for use in this work.
Table 2.2 is included to illustrate these variations are important parameters that must be
included in SSPM system design.
Afterpulses
Afterpulsing occurs when carriers are trapped during one avalanche and undergo a
delayed release, after which a new avalanche is triggered. Afterpulses with short delay
26
TABLE 2.2: G, PDE, and Bias variations as a function of temperature.
Parameter
Definition
Unit
Typical value
Temperature coefficient of Vbias
dVbias
dT
mV /◦ C
20 − 30
Temperature coefficient of G
dG
dT ·G
%/◦ C
0.1 − 0.5
Temperature coefficient of PDE
d(P DE)
dT ·P DE
%/◦ C
0.1 − 0.6
contribute little because the cells are not fully recharged but have an effect on the SSPM
recovery time.[Renker and Lorenz, 2009, Vinogradov et al., 2009] Long delay afterpulses
contribute to DCR.
Optical Crosstalk
Interpixel crosstalk occurs during avalanche breakdown between adjacent pixels or
nearly adjacent ones. Once a pixel fires, its photons can trigger neighborhood pixels adding
to the DCR or an artificial increase in signal amplitude. Approximately three photons per
105 carriers are emitted when the incoming photon energy is greater than 1.14 eV.[Lacaita
et al., 1993] It is this optical crosstalk problem that prevented the practical development
of SSPMs until the last decade of the 20th century. A typical SSPM physical topology
uses trenches to isolate each pixel on an island. Filling the trenches in the pixel array with
opaque optical material helps eliminate crosstalk. Crosstalk rate is a function of Vov .
2.5.6
SSPM S/N Ratio and Noise Considerations
The power SNR at the output of an optical receiver is given by:
S/N =
photocurrent signal
SSP M noise power + amplif ier noise power
(2.38)
To obtain the greatest S/N the SSPM must have high η(λ) and any subsequent
amplification noise should be low. The S/N sensitivity of an SSPM is described in terms
27
of the minimum detectable optical power. This is the optical power necessary to produce a
photocurrent of the same magnitude as the total rms noise current (or a SNR of 1).[Palais,
2005, Motchenbacker and Connelly, 1993, Sze, 2006]
The photodetection process for a SSPM is shown in Figure 2.8. The SSPM gain
multiplies three important currents: the signal current, the background current, and the
dark current or dark current noise.[Sze, 2006] Noise currents have mean square rms valSSPM
Amplifier
Current
Input
Output
Signal
Optical signal
Photo−
electric
effect
Background
Dark
Background
signal
Input−
gain−
output
circuit
Avalanche
gain
Excess
noise
Signal
+
noise
Thermal
noise
FIGURE 2.8: Photodetection process in SSPM. (source: Sze)
ues donated by the < > symbol and statistical variance σ 2 .[Motchenbacker and Connelly,
1993] When a modulated optical power P(t) impinges on a SSPM, the primary photocurrent is i(t)photo =
ηq
hν P (t).
This current has two components, a dc average photocurrent
Ip and a signal power component ip (t). For a SSPM the mean square signal current is
< i2s >= σs2 =< i2p (t) > M 2 . M here is the multiplication gain as discussed in Section 2.5.1.
As stated above, the primary sources of noise in SSPMs are: quantum noise, dark
current noise generated in the bulk semiconductor material without incident light, and
multiplication noise.[Palais, 2005, Motchenbacker and Connelly, 1993, Sze, 2006, Kasap,
2001] Dark current is further divided into two components: bulk dark current < i2BDark >
and surface dark current < i2SDark >.
1. Quantum noise also known as shot noise stems from the statistics governing production and collection of photoelectrons when light is incident upon a photodetector.
28
Shot noise obeys the axioms of Poisson processes. Shot noise is given as:
2
< i2quantum >= σquantum
= 2qIp BW M 2 M x
(2.39)
where BW is the bandwidth and M x is F(M) as defined in equation 2.20.[Kasap,
2001, Sze, 2006, Moura and Darwazeh, 2005] Note that for PIN diodes, F(M) and
M are equal to 1.
2. Bulk dark current noise originates from electrons/holes thermally generated in pn
junctions. However in an SSPM any charge carriers are accelerated and multiplied
by avalanche gain. The mean-square value of the bulk dark current from thermally
generated electrons/holes is given as:
< i2BDark >= σBDark = 2qIBD BW M 2 M x
(2.40)
where IBD is the unmultiplied SSPM bulk dark current.[Bielecki, 1997, Becker and
Johnston, 2004, Sze, 2006]
3. Multiplication noise was discussed previously in Section 2.5.1.
4. Surface dark current noise also called surface leakage current more commonly simple
leakage current, is a function of material surface defects and surface area. It is also
a function of bias voltage. Guard ring structures are used in SSPMs to reduce this
current. The mean square value of this current is given as:
2
< i2SDark >= σSDark
= 2qISLeak BW
(2.41)
where ISLeak is the surface leakage current. Note the surface dark current is not
multiplied by avalanche gain whereas bulk dark current is.[Bielecki, 1997, Becker
and Johnston, 2004, Sze, 2006]
All the dark currents and signal currents are uncorrelated, so the mean square SSPM
29
noise current is:
2
< i2sspm−noise > = σsspm−noise
=< i2quantum > + < i2BDark > + < i2SDark >
2
2
2
2
+ σSDark
= σsspm−noise
+ σquantum
+ σBDark
(2.42)
= 2q(Ip + IBDark )M 2 M x BW + 2qISLeak BW
The load resistor RL contributes a mean square thermal noise current, also known
as Johnson noise current, which is given as:
2
= 4kT
< i2thermal >= σthermal
BW
RL
(2.43)
Here, T is absolute temperature in K◦ and k is Boltzmann’s constant.[Palais, 2005, Kasap,
2001, Sze, 2006, Motchenbacker and Connelly, 1993, Moura and Darwazeh, 2005] Johnson
noise can be reduced by using a load resistor which is large mindful of overall bandwidth
requirements. A trade-off is involved.
Substituting equations 2.42 and 2.43 into equation 2.38 gives:[Bielecki, 1997, Sze,
2006, Palais, 2005, Motchenbacker and Connelly, 1993]
S/N =
< i2p > M 2
2q(Ip + IBDark )M 2 M x BW + 2qISLeak + 4kT BW/RL
(2.44)
Here the thermal noise is of lesser importance and the SSPM noise typically dominates.
Note that the signal power is multiplied by M 2 and the quantum noise plus bulk dark
current is multiplied by M 2 M x (or F(M)). [Bielecki, 1997, Palais, 2005, Kasap, 2001, Sze,
2006, Motchenbacker and Connelly, 1993]
One SSPM tested in this work (Voxtel) is mounted on a 3-stage thermo-electric
cooler (TEC) allowing it to be operated at −20◦ C (or lower depending upon the cooling
circuit setpoint). This can reduce the dark rate by an order of magnitude (or more) below
its room temperature value.[Voxtel, 2008]
2.5.7
Comparison of SSPM vs. Vacuum Tube (PMT) Technology
Table 2.3 summarizes the salient differences between vacuum tube and solid-state
technology for optical photon detection circa 2009.
30
TABLE 2.3: Optical photon sensing: vacuum vs. solid-state technology.
Device Characteristic
PMT
PIN
APD
SSPM†
Gain (G)
106 − 107
1
≈ 200
105 − 106
Dark Count (DCR)
< 0.5 Mhz
< 0.5 Mhz
< 1 Mhz
< 1 Mhz
Sensitivity
1 p.e.
100s p.e.
10 p.e.
1 p.e.
Speed
100s of ps
10s of ns
< 10 ns
100s of ps
Vbias
500 V - 1.2 kV
10 V - 100 V
100 V - 500 V
15 V - 100 V
Battery Operation
difficult
Yes
difficult
Yes
PDE (Red)
≤ 10%
≈ 90 − 100%
≈ 75%
≈ 40%
PDE (Green)
≈ 40%
≈ 80 − 90%
≈ 60 − 70%
≈ 40 − 50%
PDE (Blue)
≈ 20%
≈ 50%
≈ 40 − 50%
≈ 20 − 30%
Temperature Sensitive
Yes
Yes
Yes
Yes
Magnetic Sensitive
Yes
No
No
No
Mechanical
fragile
small
small
small
bulky
rugged
rugged
rugged
Rate
†Approximate die size 1mm x 1mm.
2.6.
Important Characteristics of Optical Fibers - Glass, Plastic
Figure 2.9 shows an optical fiber waveguide (hereafter simply optical fibers or OFs).
There are different indices of refraction for the core and cladding n1 , ncore or n2 , nclad ,
respectively, and ncore > nclad always. Refractive index may decrease abruptly or gradually from core to cladding depending upon how a fiber is manufactured. A gradual index
change means having a diffuse material interface between core and cladding. This results
in rays that do not change direction abruptly upon reflection by the material interface.
31
FIGURE 2.9: Optical fiber waveguide.
An abrupt decrease in index corresponds to a stepped interface where a ray changes direction abruptly upon reflection by the core-cladding interface. Fibers with abrupt changes
in index are called step index fibers, while fibers with a gradual change in index are
called graded index fibers (GRIN). GRIN fibers give sharper output pulses with less pulse
distortion in response to an input pulse, when compared with stepped index fibers. Figure 2.10 shows index and light ray profiles for multimode step index fiber used in this
work. GRIN optical fibers exhibit a gradual refractive index shift from the core outward
to the cladding as opposed to step index optical fibers where the index changes in a discrete step.[Palais, 2005, Kasap, 2001] An optical fiber has different core diameters. Small
n2
n2
n2
n1
core
θ
n
n1
cladding
n1
no
n2
2a
FIGURE 2.10: Index and light ray profiles for step index optical fiber.
cores (e.g., 5 µm) diameter mean that only rays parallel to the fiber axis travel completely
through the fiber; off-axis rays need to be reflected many times during fiber travel and
escape. Large core (e.g., 500 µm) diameters mean that both off-axis and on-axis rays
travel through the fiber. Fibers with large cores are called multimode while fibers with
32
small cores are called single-mode. Multimode fibers have greater pulse distortion than
single-mode fibers. Light intensity traveling through single-mode fibers is less than for
multimode fibers. Single mode fibers are stepped index, while multimode fibers may be
either stepped index or graded index. Table 2.4 summarizes the characteristics of the
three types of optical fiber.
TABLE 2.4: The three types of optical fiber.
Fiber type
single mode
multimode
multimode
stepped index
graded index
stepped index
Light travel
on-axis (||) to fiber
both on- and off-axis
both on- and off-axis
Pulse distortion
lowest
higher
highest
Size
small < 5 µm
large > 500 µm
large > 500 µm
2.6.1
Light Propagation in OFs
When an incident ray traveling in a material of refractive index n1 encounters the
interface with a material of refractive index n2 , as shown in Figure 2.11, it is partly
reflected and partly transmitted. The reflected ray is symmetric with the incident ray
around the normal to the material interface, such that θ1 (angle between the incident
ray and the normal) equals the angle between the reflected ray and the normal, which
is the case for mirror reflection. The transmitted ray is not in the same direction as the
incident ray, if n1 is not equal to n2 . This is known as refraction. The angle of refraction
θ2 (angle between the refracted ray and the normal) is not equal to the angle of incidence
θ1 . Snell’s Law describes the relationship between the angle of incidence θ1 and the angle
of refraction θ2
n1 sin θ1 = n2 sin θ2
(2.45)
33
Refracted Ray
Material refractive
index n 2
Material n 1
refractive
index
θ
1
θ2
θ1
Reflected ray
Incident ray
FIGURE 2.11: Snell’s Law of reflection and refraction.
From Snell’s Law one can see that if n1 > n2 , then θ2 > θ1 . A larger refractive index
means lower speed in a material, if n1 > n2 , then v2 > v1 . The material with the larger
speed is associated with a larger angle between the internal ray and the normal as shown
in the figure.
When θ2 = 90◦ , the refracted ray travels along the material interface, this condition
is satisfied when θ2 in equation 2.45 is 90◦ , see Figure 2.12. This value of θ1 corresponding
ο
θ 2 = 90
Material refractive
index n 2
Material n 1
refractive
index
θ
c
Incident ray
Refracted Ray
θ
c
Reflected ray
FIGURE 2.12: Critical angle for fiber.
to θ2 = 90◦ is called the critical angle (θc ).
34
When θ1 > θc , there is no refracted ray at all and the incident ray is completely
reflected; this is call total internal reflection (TIR), see Figure 2.13. TIR allows a solid
optical fiber to guide light within it such that light rays remain internal even when the
fiber is bent. Maximum bending radii are part of the specifications for optical fibers.
The reason is that optical fiber has a core with refractive index n1 and a cladding with
Material refractive
index n 2
Material n 1
refractive
index
θ
1
θ1
Incident ray
Reflected ray
FIGURE 2.13: TIR for fiber when incident angle exceeds the critical angle.
refractive index n2 , where n1 > n2 ; TIR occurs when θ1 > θc . The critical angle is given
by equation 2.46 for TIR.[Palais, 2005]
θc = sin−1 (
n2
)
n1
(2.46)
Provided the refractive index is lower on the other side of the core/cladding interface, no
light can pass through, thus most of the light is reflected back into the core. For light to
remain constrained within the fiber core an incident ray must have an angle of incidence
less than θc . Figure 2.14 shows the structural cross section of an optical fiber waveguide
illustrating the angle of acceptance, θaccept and the critical angle θc for an optical fiber.
The result is that incident rays greater than angle θna from the normal axis of the fiber
are not constrained by the core. The optical fiber acceptance angle is defined as
θaccept = 2 θc
Rays at angles within θaccept are completed constrained within the fiber core.
(2.47)
35
Acceptance
angle
n2
θ NA
Cladding
n1
θc
Core
n2
Cladding
Core Refractive Index (n 1) = 1.49
Cladding Refractive Index (n 2) = 1.39 − 1.42
Numerical Aperture (NA) = sin θ c= 0.3 − 0.6
Acceptance Angle = 2 θ c= 60 degrees
FIGURE 2.14: Important angle relationships for an OF.
Numerical aperture (NA) is a dimensionless number that describes the range of
angles over which an optical fiber can accept or emit light. NA is defined as
N A = n1 sin θna
(2.48)
From figure 2.14 θna = 90◦ − θc , thus
N A = n1 sin θna = n1 sin(90 − θc ) = n1 cos θc
(2.49)
From equation 2.46 and sin2 θ + cos2 θ = 1, substituting θc for θ gives NA in terms of the
core/cladding indices of refraction
cos2 θ = 1 − sin2 θ → cos θc = 1 − n22 /n21
r
√ 2 2
p
n1 −n2
n21 −n22
2
2
cos θc = 1 − n2 /n1 =
=
n1
n21
p
thus N A = n1 cos θc =
n21 − n22
(2.50)
Meridional rays are guided down the fiber only if their external angle of incidence
on the fiber end-face is smaller than sin−1 (N A), the acceptance angle from equation 2.47.
Equation 2.51 gives θaccept in terms of core and cladding indices respectively.
q
θaccept = sin−1 (N A) = sin−1 ( n2core − n2cladding )
(2.51)
Light rays impinging on a fiber at angles greater than the maximum acceptance angle
are refracted into the fiber, but are not guided for a long distance. This is because they
36
are not totally reflected at the core/cladding interface, but instead are partially refracted
back into the cladding. The maximum acceptance angle, 2θaccept , is a function of both
fiber NA and refractive index of the external material. Note that these equations apply
only for meridional rays; skew rays have greater acceptance angles.[Kasap, 2001, Palais,
2005]
Light is absorbed as it travels through any material (gas, liquid, solid); the intensity
I at a distance x is related to the intensity Io at x = 0 by
I = Io exp−αx
(2.52)
where α is the absorption coefficient, which varies from one material to another.
Define
attenuation loss(dB) = −10log(I/Io )
(2.53)
For example, when I/Io = 0.1 the attenuation loss is 10 dB, when I/Io = 0.01 the
attenuation loss is 20 dB, when I/Io = 0.001 the attenuation loss is 30 dB, and so on.
Rewriting equation 2.52 after taking logarithms and converting to base 10 gives
attenuation loss(dB) =
10αx
2.3
(2.54)
Note that in equation 2.54 the attenuation loss is directly proportional to x, the distance
light travels in the fiber.
The attenuation loss per unit length of an optical fiber varies with wavelength
because the absorptivity and light scattering out of a material are functions of wavelength.
Absorption losses occur when the frequency of the light is resonant with oscillations in the
electronic or molecular structure of the fiber material. Different ions and functional groups
have characteristic absorption peaks at well-defined frequencies. Scattering that occurs at
inhomogeneities in the fiber is linear and there is no change in frequency. Scattering due
to phonon-phonon interactions or Raman scattering is nonlinear and there is a frequency
change. Typical losses for glass fibers are approximately 1 - 10 dB/km@650 nm. While
37
polymers are not as robust as glass for use as a fiber material due to their high attenuation
loss, for example 190 dB/km@650 nm, they are considerably more rugged.
Another source of attenuation loss called coupling loss, typically 10-15 db, occurs
due to poor coupling between the light source and an optical fiber or between optical fiber
and detector. The loss is the result of light from the source that has rays greater than
θaccept for the optical fiber, illustrated by Figure 2.15. For example, when a light emitting
diode (LED) or scintillator with exit rays within 80◦ to 100◦ is directly attached to the
optical fiber, coupling loss still occurs. Using a laser less coupling loss occurs, because
laser light diverges less as it travels than diffuse light.
Lost power area
11111111111111111111111111111111111111
00000000000000000000000000000000000000
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11111111111111111111111111111111111111
Optical power source 11111111111111111111111111111111111111
00000000000000000000000000000000000000
00000000000000000000000000000000000000
(LED, fiber, scintillator) 11111111111111111111111111111111111111
000000000000000000
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11111111111111111111111111111111111111
000000000000000000
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11111111111111111111111111111111111111
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00
11
00000000000000000000000000000000000000
11111111111111111111111111111111111111
000000000000000000
111111111111111111
00
Emitting11
000000000000000000
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00
11
Optical power coupled in
00000000000000000000000000000000000000
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000000000000000000
111111111111111111
00
11
area
000000000000000000
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00
11
00000000000000000000000000000000000000
11111111111111111111111111111111111111
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00
11
000000000000000000
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000000000000000000
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000000000000000000
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00000000000000000000000000000000000000
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00000000000000000000000000000000000000
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00000000000000000000000000000000000000
11111111111111111111111111111111111111
00000000000000000000000000000000000000
11111111111111111111111111111111111111
Junction
Cladding
Fiber
acceptance
angle
Core
Cladding
Source radiation envelope
FIGURE 2.15: Illustration of fiber coupling issues: light source coupled to fiber.
2.6.2
Metalized Air-Core Capillary Tube Cerenkov Light Removal
Various Cerenkov light reduction schemes proposed by different groups typically use
one of several methodologies: a background fiber in parallel with the signal fiber, optical
filters to attenuate the Cerenkov light which is centered at a different frequency than the
signal of interest, temporal methods, spectral discrimination, or more recently the use
of metalized air-core capillary tubing.[Matsuura and Miyagi, 2004, Beddar et al., 2004,
38
Justus et al., 2006, Frelin et al., 2006, Lee et al., 2007a]
The generation of Cerenkov light within OFs when using electron beams degrades
the S/N ratio significantly, introducing additional noise into the system.[Beddar et al.,
1992a, Elsey et al., 2007] This light varies with the speed of the particle in a material and
the index of refraction of the material, and is a function of impinging electron beam angle
on the OF. [Law et al., 2007] However, it is an important noise source only for electron
beams above 200 keV and can be neglected for diagnostic radiation detection purposes and
low energy radiation detection. In this work metalized air-core capillary tube was chosen
to reduce the Cerenkov light generated within OFs, as shown in Figure 2.16.[Lambert
et al., 2008] No Cerenkov light is generated within the air-core. Cerenkov light generated
within the glass material of the capillary tube is reflected by the metal film. Within the
air-core itself, scintillation light is guided by grazing angle reflections from metal film into
the attached optical fiber.
Other Cerenkov Removal Methods
The time signature of light signals originating within optical fibers when irradiated
by high energy electron beams is different, thus providing a way to separate the Cerenkov
noise from the radiation dependent signal: Cerenkov - ps, native fluorescence - ns, phosphorescence - 100s of µs. [Justus et al., 2004, 2006] In theory, careful time discrimination
allows one to exclude the Cerenkov noise signals when using a high energy pulsed radiation
source (as a LINAC).
[Beddar, 2007] describes a PMMA fiber coupled to plastic scintillator dosimeter that
does not require optical stimulation for reading. This material has one principle advantage
in that it is near tissue equivalent.[Beddar et al., 1992a,b] Cerenkov noise signals are
removed using two adjacent identical fibers, one with a scintillator and one without, and
mathematically subtracting the different signals.[Beddar et al., 2001]
39
(a) Cross section of metalized air-core capillary tube. (Source:
Polymicro Inc.)
Plastic Optical Fiber
111111
000000
000000
111111
000000
111111
Glass
Cladding
000000
111111
000000
111111
000000
111111
111111111111
000000000000
000000
111111
1111111111111111111
0000000000000000000
00000000
11111111
000
111
11111111111111111111111111111111
00000000000000000000000000000000
1010
00000000
11111111
000
111
Air Core
Fiber Core
00000000
11111111
000
111
1010
0000000000000000000
1111111111111111111
00000000
11111111
000
111
000000000000
11111111111111111111111111111111 111111111111
00000000000000000000000000000000
0000000000000000000
1111111111111111111
000000000000
111111111111
Scintillator
Air−Core Silver Coated Capillary Tubing
Metal Film Silver Coating
(b) Interfaces between metalized air-core glass capillary tube and scintillator.
FIGURE 2.16: Illustration of metalized air-core glass capillary tube.
2.6.3
Other Optical Fiber Radiation Effects
An extensive overview of radiation effects on optical fibers in various environments
(Space, Nuclear Industry) was completed by NATO.[Berghmans et al., 2007] Radiation
Induced Attenuation (RIA) in optical fibers is the result of trapping radiation generated
electrons and holes at defect sites creating color centers. Typically, optical fiber color
centers absorb photons at specific wavelengths but can be annealed out, decreasing RIA,
by thermal and optical means. Many factors influence optical fiber response to ionizing
radiation, among them:[Berghmans et al., 2007]
• previous irradiation episodes and thermal cycling,
40
• chemical composition,
• ambient temperature,
• type of radiation exposure, dose-rate, and total dose,
• test parameters (optical wavelength and power),
• time lag between radiation exposure and optical measurement.
In the past, simple power-law and complex nth-order kinetics have been used to
model RIA in optical fibers without much success. At this time there is no commonly
accepted dose effect model.
To date there have been studies investigating standard SiO2 fiber material for use
as Thermo-Luminescent-Dosimeters (TLDs) with mixed results.[Espinoza et al., 2006]
2.7.
SiO2 : Cu2+ Material
Justus and Huston from the U.S. Naval Research Laboratory describe scintillation
and OSL from Cu1+ doped fused quartz fibers for use as a dosimeter.[Benevides et al.,
2007, Huston et al., 2001, 2002, Justus et al., 2004, 2006] This material has an OSL peak at
500 nm and a stimulation peak at 390 nm. Due to issues with the OSL mode of operation
they abandoned this method is favor of a more passive system using the material drawn
into a fiber used as a scintillator. The resulting signal is similar to OSL but no optical
stimulation is needed when using the fibers as radiation sensors or dosimeters.(This is
what other OSL researchers call the Radio Luminescence or (RL) signal.)
For this work the SiO2 : Cu2+ fibers themselves will be used as radiation sensors in
RL mode; no OSL mode of operation is utilized.
41
3.
3.1.
EXPERIMENTAL METHODS
SSPMs and Device Characterization
The primary SSPMs tested in this work include Hamamatsu MPPC S10362-11050C, Photonique 0810G1, and Voxtel SQBF-EKAA/SQBF-EIOA. All are Si devices
utilizing an MRS structure for pixel quenching where a typical broad spectrum device
extending from the UV/blue range through red-yellow range is a p+pn+ structure. Sizes
tested in this work had 1 x 1 mm2 or 1.1 x1.1 mm2 active areas divided into different array
sizes. Individual pixels are separated by grooves filled with an optically opaque material
for suppression of optical crosstalk which reduces SSPM noise. Each photosensitive area
is protected with 1.58 index of refraction transparent epoxy.
Custom electronic charge and transimpedance (MAR-8ASM+ RF) amplifiers were
constructed for the three SSPMs tested: MPPC S10362-11-050C, Photonique 0810G1, and
Voxtel SQBF-EKAA/SQBF-EIOA.[Hamamatsu, 2007, Voxtel, 2008, Photonique, 2006]
Two versions of Voxtel SSPMs were evaluated: one a room temperature device (SQBFEIOA) in a T0-8 package and another using the same semiconductor die mounted to
a 3-stage thermoelectric cooler in a modified T0-8 package (SQBF-EKAA) to minimize
DCR.[Dhulla et al., 2007] Dedicated circuitry (Maxim MAX1978) was used for the temperature controller of the SQBF-EKAA for laboratory tests. In addition to the Photonique 0810G1, four other Photonique SSPM devices were evaluated: 0701BG, 0611B1,
050701GR, and a Custom Low Noise Sample (CLNS). The amplified SSPM pulse or
charge signal was coupled to a custom designed discriminator circuit feeding an ORTEC
996 counter/timer or host computer running analysis software, as shown in Figure 3.1.
The photocurrent for SSPM tests was generated using blue and red light from high
intensity LEDs using a custom designed pulser circuit with extensive modifications based
upon [Kapustinsky et al., 1985]. This pulser outputs 1 ns pulses which gave a few photons
42
Scintillation crystals
Reference PMT
H5783P
Custom fiber
and
scintillator
connector heads
LED Pulser
1) GH4001 POF with BCF12 scintillator
2) Glass OF with SiO2:Cu2+ scintillator
USB connected PC
Running analysis software
SSPM Detectors:
MPPC
Photonique,
Voxtel
Amplifier
Threshold Discriminator
Frequency
Counter
Ortec 996
Counter/Timer
Oscilloscope
FIGURE 3.1: Block diagram of laboratory and clinical measurement system.
43
from the LEDs after attenuation. A reference photomultiplier tube (Hamamatsu PMT
H5783P) was used as a benchmark photon detector coupled to the same photon counting
electronics used with the SSPMs. A Hamamatsu S1223 Si PIN photodiode was used
as a reference photodetector for calibration purposes due to its known responsivity. A
Tiffen P Series ND.6 Grad SE Neutral Density Filter mounted to an optical xyz stage was
used to attenuate the photon flux from the LEDs and LEDs coupled to test fibers from
the pulser. A Newport Digital Power Meter Model 815 was used to measure the optical
power. Special light-tight mating jigs were used to hold in place the different scintillation
crystals or POFs to each SSPM device and PMT for laboratory testing purposes. Signal
c
acquisition was also done using LabVIEWsoftware
controlling Tektronix oscilloscopes.
3.1.1
Photon Counting Electronics
A brief overview of the photon counter operation is provided with reference to
Figure 3.2. The window comparator circuits (U4, U5 in Figure 3.6) allow a lower threshold
level to be set that screens out SSPM dark noise (0.5 p.e. black line in Figure 3.2). The
amplitude of this threshold is arbitrary, depending upon the application. This same circuit
allows an upper threshold level to be set. Both upper and lower threshold levels are used
to create a window than can be used for energy discrimination purposes. This feature was
not used for this work. Figure 3.3 shows scope photos of MPPC S10362-11-050C dark
noise pulses (note the individual p.e. levels), and Figure 3.4 shows scope output pulses
from the photon counter.
Because each SSPM tested had a different reverse breakdown voltage and for portability reasons (battery operation), the circuit shown in Figure 3.5 was designed and built
for laboratory and clinical testing purposes. This is a switching power supply circuit that
provides stable, extremely low ripple SSPM power from 18 V to 80 V for biasing purposes.
Figure 3.6 shows the single channel analyzer (SCA - photon counter) used in this
work. This circuit gives an output pulse when an input voltage from the MAR8+ amplifier
44
FIGURE 3.2: Photon counter operation. (source: Hamamatsu)
FIGURE 3.3: Dark counts showing p.e. levels.
FIGURE 3.4: Typical output from photon counter.
45
FIGURE 3.5: Circuit diagram for battery operated SSPM power supply.
46
falls between upper and lower threshold voltage levels (set by the user using potentiometers
or host computer interface). It is a high speed window comparator design with feedback
hysteresis to prevent oscillation when switching from high to low voltage levels and low
to high levels.
Figure 3.7 shows the reference power supplies for the window comparator and logic
circuits. The DC reference level is provided by U12, a variable shunt regulator driven from
a current source U11, R11. The maximum DC reference level is 10 V for compatibility
reasons with NIM-BIN systems. With this circuit the reference voltage levels for the
window comparator circuit may be adjusted to screen out SSPM noise ( photo-electron
equivalent, p.e.), thereby setting a lower limit of detection.
Measured absolute photon counts (cpm), detector efficiency, and scintillator coupling issues are extremely important for low level environmental radiation sensing. Measured activities based on photon counts are not reported for two reasons: coupling issues
(area mismatch between scintillators and SSPM die, OF coupling inefficiencies) and the
lack of suitable calibration radiation sources. (The exception was when using the reference
PMT together with the BGO scintillator directly attached as the BGO diameter closely
matched the PMT diameter.)
3.1.2
I-V and CV Measurements
Current-Voltage (I-V) curves for all SSPMs were measured with an Agilent 4156
semiconductor parameter analyzer, pixel capacitance was measured with an HP 4263B
LCR meter - both measurements with the Device Under Test (DUT) SSPM in total
darkness. These measurements were used to assist with characterizing the DCR and gain.
Operating temperature for all SSPM tests was ambient, 22.5 ◦ C. For the SQBF-EKAA,
operating DUT temperature was set at −20 ◦ C. Thirty (30) engineering samples of the
room temperature Voxtel SQBF-EIOA SSPM were I-V tested to optimally select a device
for low dark current.
47
FIGURE 3.6: Circuit diagram for photon counter.
48
FIGURE 3.7: Power supply circuit diagram for photon counter.
49
The multiplication gain of each SSPM was calculated using the measured die capacitance and the photocurrent/dark current measurement results using equation 2.26.
The C-V measurement is used to determine the parasitic capacitance of the SSPM
at a given reverse bias voltage. It is also used to determine the punch-through voltage
of the SSPM, when its value is ambiguous from the I-V results. The SSPM capacitance
is extremely sensitive to the reverse bias due to the strong dependence on the depletion
layer thickness. For an SSPM device whose punch-through is difficult to determine by I-V
measurements, C-V measurement is an effective diagnostic tool. Another parameter one
can extract from C-V data is the doping level in the absorption region of the SSPM. How
fast the capacitance decreases with the bias depends on the doping level of the absorption
region. Capacitance, hence doping level ND is calculated from equation 3.1 where ǫ is the
permittivity of the absorbing material.
s
Csspm = A
ǫǫ0 qND
[Farads]
2(Vbias − Vovervoltage )
(3.1)
Above the breakdown voltage, capacitance cannot easily be measured. It is assumed
that the total capacitance after breakdown is the same as was measured before breakdown.
From this information the capacitance per pixel is calculated as Cpixel = Ctotal /n, where n
is the number of pixels in the SSPM array. The actual SSPM photosensitive area is found
by dividing the total area by (1 - fill factor), where “fill factor” is the total area of active
pixel elements. For the Photonique, Hamamatsu, and Voxtel devices this is approximately
1 mm2 , thus A = Aπ/(1 − f illf actor). Now the thickness of the avalanche region can be
determined:
tavalanche =
Aǫ0 ǫSi
[m]
Ctotal
(3.2)
50
3.1.3
SSPM Gain Measurement
The definition of SSPM gain typically uses a charge amplifier coupled to an Analog-
to-Digital Converter (ADC) that produces a frequency distribution of charge vs. channel
number; Figure 3.8 shows an example. This is essentially the same functionality used in
energy spectroscopy Multi-Channel Analyzers (MCAs). The LeCroy 4300B 16-channel
charge ADC is an example with a sensitivity of 0.25 pC/count. SSPM gain is given by:
Gain =
ADC conversion rate × number of ADC channels between two peaks
q
Note that this equation is the same as equation 2.26. For example, if the distance
between two adjacent peaks is 100 channels (Figure 3.8), then the gain is 100 × 0.25 ·
10−12 C/1.6 · 10−19 C = 156250.
A charge amplifier ADC and MCA were not available for this work. Instead the
method described here was used. In this work the SSPM charge signal was measured using
an oscilloscope to determine area. Together with equation 2.26 and a constant 50Ω load,
Qsspm was determined by integrating the area on the oscilloscope.
Z
V (t) dt =
Z
R · I(t) dt = 50Ω · Qsspm(t)
where gain is proportional to the overvoltage Vov .
Note that gain (G) can also be calculated using the leakage current Ileak and the
DCR as shown in equation 3.3.[Petasecca et al., 2008]
G=
3.1.4
Ileak
q DCR
(3.3)
SSPM DCR Measurement
DCR was measured using the setup shown in Figure 3.9. Referring to Figure 3.3,
threshold was set to 0.5 p.e. for this measurement. Frequency was recorded using a
51
FIGURE 3.8: ADC output charge frequency distribution. (source SensL)
52
Tektronix DC 502 frequency counter. Counting the number of pulses exceeding the 0.5
p.e. threshold gives the number of times one or more photons has been detected.
Optical Fiber
LED
Pulser
Optical Fiber
Neutral density filter
in xyz optical stage
SSPM MAR8+ Amplifier
Oscilloscope
Pulser
trigger signal
Frequency Counter
FIGURE 3.9: SSPM DCR measurement setup.
3.1.5
SSPM PDE Measurement
PDE(λ) was measured using the setup shown in Figure 3.10. In this measurement
the S1223 photodiode was used first to measure the photon flux, then replaced by the DUT
SSPM. PDE is determined by equation 3.4. Because the number of photons detected
are calculated from a photocurrent, afterpulsing and optical crosstalk effects are also
included.[Gomi et al., 2007, Hamamatsu, 2007]
P DE =
Optical Fiber
LED
Pulser
P IN Active Area SSP M detected photons
SSP M Active Area P IN incident photons
Optical Fiber
Neutral density filter
in xyz optical stage
(3.4)
Dark Box
SSPM
or
photodiode
Ammeter
MAR8+ Amplifier
Oscilloscope
FIGURE 3.10: SSPM PDE measurement setup.
Light power, P, is the number of photons impinging on a photodetector (PIN, SSPM,
PMT) each second, Nphotons , multiplied by the power of each photon. The power of each
photon is h ν. Thus P = Nphotons h ν and Nphotons = P/h ν = P λ/h c. The number
53
of optical photons incident on a photodetector (PIN, SSPM, PMT) can be approximated
using an optical power meter; this is given by equation 3.5
Nphotons ≈
P (W ) λ(m)
Rate(Hz) hc(Jm)
(3.5)
where P(W) is the power measured using a Newport Digital Power Meter Model 815, λ
is the peak wavelength of the LED, and Rate is the pulse repetition rate for the LED.
Alignment was done on an optical rail.
3.1.6
GRIN Lens Design for the Voxtel SQBF-EKAA
The photosensitive die of the SQBF-EKAA is recessed 2.73 mm behind the clear
glass window in the Peltier-cooled T0-8 package, see Figure 3.11. To optimally couple
FIGURE 3.11: Voxtel SQBF-EKAA mounted on 3-stage Peltier-cooler showing recessed
die. Can diameter is approximately 0.5 inch.
54
light from a scintillator crystal or OF to the die, some type of lens system is needed. For
this work a GRIN lens system was chosen. A bi-convex lens system was evaluated in the
optical lab but found too difficult for practical use with the cooled TO-8 package and 1
mm or 400 µm diameter OFs. Figure 3.12 shows the key GRIN lens system parameters
used for the SQBF-EKAA. The design information for SELFOC GRIN lenses from NSG
America was used to calculate parameters for the SQBF-EKAA SSPM.[NSG, 2008]
GRIN optic area
Diam(fiber)
Diam(sspm) active area
θ1
θ2
Grin Lens
n1
n2
Fiber core
SSPM
Z
L1
L2
FIGURE 3.12: GRIN lens system showing important dimensions.
The on axis refractive index with λ in µm is given in equation 3.6.
N0 (λ) = 1.5868 +
8.14 · 10−3
λ2
The index gradient constant for the SLW-1.0 with λ in µm and
√
A(λ) = 0.5945 +
A(λ) = 0.3238 +
√
A in mm−1 :
3.936 · 10−3 5.539 · 10−4
+
λ2
λ2
The index gradient constant for the SLW-1.8 with λ in µm and
√
(3.6)
5.364 · 10−3 2.626 · 10−4
+
λ2
λ2
For pitch P and lens length Z the relationship is:
(3.7)
√
A in mm−1 :
(3.8)
55
√
2πP = Z A
(3.9)
Table 3.1 shows calculated values for SLW GRIN lenses.
c
SLW GRIN lenses at λ = 440 nm and 580
TABLE 3.1: Calculated values for SELFOC
nm.
SLW-1.0 (440 nm, 580 nm)
SLW-1.8 (440 nm, 580 nm)
N0 - on axis refractive index
√
A in mm−1
1.6288, 1.6101
1.6288, 1.6101
0.6296, 0.6111
0.3585, 0.3420
Pitch length P - 0.23 (mm)
2.2953, 2.2364
4.0301, 4.2247
Pitch length P - 0.25 (mm)
2.4948, 2.5705
4.3814, 4.5920
For the image distance itself L2 :
√
√
√
−(n1 n2 / A sin(Z A) − n2 N0 L1 cos(Z A))
√
√
√
L2 =
n1 N0 cos(Z A) − N02 L1 A sin(Z A)
(3.10)
For the transverse magnification MT :
MT =
n1
√
√
√
n1 cos(Z A) − N0 L1 A sin(Z A)
(3.11)
For this system the height of the object on the SSPM die Hobj is:
Hobj =
Diamimage
|MT |
(3.12)
To simplify the optical system construction L2 was set equal to 2.73 mm; the rear
face of the GRIN lens is butted directly against the SQBF-EKAA cover glass centered
over the photosensitive die area. The image size is also a constant equal to 1.0 mm for
SQBF-EKAA (the diameter of the photosensitive area).
The object distance L1 can be found by solving equation 3.10 for L1 :
56
√
√
√
−(n1 n2 / A sin(Z A) − n1 N0 L2 cos(Z A))
√
√
√
L1 =
n2 N0 cos(Z A) − N02 L2 A sin(Z A)
(3.13)
Because SELFOC GRIN lenses have an active area on the frontal plane equal to
60% of the lens diameter, the maximum fiber diameter that can be used is
Diamf iber = 0.6Diamlens − 2L1 tan(θ1/2 )
(3.14)
The NA for the GH4001 POF is 0.50; the half-angle theta1/2 used in equation 3.14 is
arcsin(0.5) = 30◦ .
Figure 3.13 shows the results for object height Hobj , object distance L1 , and fiber
core diameter Diamf iber for both 400 µm and 1.0 mm cores.
Looking at the graphs one can make some design decisions:
• The height of the projected image object on the SQBF-EKAA photosensitive die
when using the SLW-1.8 GRIN is greater than for the SLW-1.0 GRIN.
• Where the object distance L1 and object height Hobj lines cross in Figures 3.13(a),
and 3.13(b), there are solutions which give lens lengths Z = 2.65 mm and 1.85 mm,
respectively.
• Overall, there is a poor GRINS lens solution for 1 mm core diameter optical fibers
when using off the shelf SELFOC lenses.
The end result of the GRIN lens optical analysis is that there is no optimum solution
when using off the shelf lenses for 1.0 mm core diameter optical fibers coupled to the 1.0
mm recessed die SQBF-EKAA. To maximize the light collection efficiency, a custom GRIN
lens must be designed and manufactured to optimize three parameters: optical fiber core
diameter, projected object height on the SQBF-EKAA die, and object distance. Given
these constraints the system was laboratory tested using the off the shelf SLW18-S0250130-NCO and SLH10-S0250-130-NCO GRIN lenses coupled directly to the SQBF-EKAA
57
Dependencies for SLW18 Grin Lens
2.5
L1, Hsspm, Dfiber (mm)
2.0
1.5
1.0
0.5
L1 Object Distance
0.0
Hsspm
Dfiber
-0.5
2
3
4
Z (mm)
(a) SLW-1.8
Dependencies for SLW10 Grin Lens
2.0
1.5
L1, Hsspm, Dfiber (mm)
L1 Object Distance
Hsspm
Dfiber
1.0
0.5
0.0
-0.5
1.5
2.0
2.5
3.0
Z (mm)
(b) SLW-1.0
FIGURE 3.13: GRIN lens design parameters for the Voxtel SQBF-EKAA.
58
glass cover using special light tight mating jigs. It was prohibitively expensive to do a
custom GRIN lens manufacturing run for this project. However, Voxtel has available a
custom GRIN lens solution for 65/125 GOFs for use in DNA testing.
3.2.
Optical Coupling using Adhesives
To ensure durable yet light transmissive joints between scintillators and optical
fibers (or capillary tubes) different types of UV cement were evaluated: Norland NOA68, TRI-Con F114, Dymax OP-52/OP-4-20639, and Master Bond EP21-7P-Clear. These
optical cements attempt to balance the need for rigid connections with refractive index
matching between two different materials. When curing these cements, a UV light source is
required. A 337 nm UV laser (Laser Science model VSL-337-NDS) was used for this work
with an initial curing time of 4 hours. Setting was completed overnight. After repeated
laboratory tests with pieces of GH4001 optical fiber, plastic scintillators, and glass optical
fiber; Norland NOA-68 cement was chosen for this work. Note that polyacrylate glue
(“Crazy Glue”) degrades OF material if not applied with extreme care.
3.2.1
Characteristics of Optical Fibers used in this work
Table 3.2 shows key parameters for the PMMA ESKA GH4001 optical fiber cable
and the glass SiO2 optical fiber used in this work.
3.3.
Scintillators
Two different inorganic scintillator materials and two organic materials were used
during SSPM laboratory testing: 1 mm x 1 mm x 5 mm square cylindrical Prelude 420
(Lu1.8 Y.2 SiO5 : Ce), 8 mm x 10 mm long cylindrical BGO (Bi4 Ge3 O12 ) crystals, 1 mm x
3 mm long cylindrical BC430 plastic scintillator, and 1 mm x 3 mm long lengths of cylin-
59
TABLE 3.2: Characteristics of GH4001 cable and SiO2 optical fiber.
Specification†
GH4001
SiO2
Fiber Diameter
1 mm
400 µm
Refractive indices
ncore = 1.492
ncore = 1.458
nclad = 1.402
nclad = 1.441
Angle of acceptance
(θcritical )
60◦
25.4◦
Numerical aperture
0.50
0.22
400 - 1000 nm
200 - 1600 nm
190db/km@650 nm
10db/km@650 nm
25 mm
275 mm
-55◦ C - 70◦ C
-40◦ C - 85◦ C
Black polyethylene
Black Nylon
Spectral range
Attenuation
Minimum bend radius
Temperature range
Jacket
†- Data are from Mitsubishi and US Naval Laboratory.
60
drical BCF-12 plastic scintillating fiber (Saint-Gobain Crystals, Hiram OH.). Inorganic
scintillators were first painted with BaSO4 reflective paint, wrapped with Teflon tape, and
covered with black epoxy before being coupled to each SSPM active area using optical
grease (Saint-Gobain Crystals, Hiram OH.).
BC430 plastic scintillator was chosen to test the greater SSPM PDE in the visible
light ranges; it was tested in addition to BCF-12 scintillating fiber during laboratory tests.
BCF-12 and BC430 scintillators were polished (both ends), painted with BaSO4 reflective
paint, wrapped with Teflon tape, and covered with black epoxy before coupling to each
SSPM active area using optical grease.[Webber and Christ, 2003] The count rate from
each SSPM-scintillator combination was measured when exited by low activity radiation
sources: (1.842 µCi
60 Co,
0.661 µGy/s and 1 µCi
137 Cs,
0.201 µGy/s) were used to
stimulate the BGO and BCF-12 scintillators in the laboratory. The Prelude 420 crystals
are self-poisoned with radioactive
176 Lu,
providing ≈ 1.3 Hz scintillations that can be
detected without using the low activity external radiation sources (see Appendix D3).
Figure 3.14 shows the BCF-12 scintillating fiber emission spectrum and the BC430
emission spectrum measured with an Ocean Optics PC1000 spectrometer. The peaks (Hg)
in the emission spectrum are from the room lights on during the measurement.
Table 3.3 shows the industry published emission characteristics of scintillators used
in this work for comparison purposes.
The photon count rate (cpm) with 0.5 p.e. threshold was measured (5 samples each
averaged over 1 minute) for each SSPM with Prelude 420, BGO, BC430, and BCF-12
scintillators directly attached to the transparent window surface of the package. These
same scintillators were then attached to 2.5 m lengths of Mitsubishi Eska GH4001 POF,
coupled to each SSPM, and the photon counting measurements repeated.
Clinical measurements with the GOF coupled SiO2 : Cu2+ and POF BCF-12 scintillators were done using the MPPC S10362-11-050C SSPM together with a USB-based
photon counter and PC software supplied by Hamamatsu. A Varian Clinac 2100CD Lin-
61
BCF-12, BC430 Scintillator Absorption Spectrum
2500
BCF12
BC430
Intensity (counts)
2000
1500
1000
500
0
400
500
600
Wavelength (nm)
FIGURE 3.14: BCF-12, BC430 scintillator emission spectrum
62
TABLE 3.3: Summary of published scintillator emission characteristics.
Scintillator
Material
Emission
Photons
Decay time
Density
λ (nm)
(per keV)
(ns)
(g/cm3 )
SiO2 : Cu2+
500
†
†
2.634
BCF-12
435
≈8
3.2
1.05
BC-430
580
≈9
16.8
1.032
Lu1.8 Y.2 SiO5 : Ce
420
32
41
7.1
480
8-10
300
7.13
(Prelude 420)
Bi4 Ge3 O12
(BGO)
†- Not available.
Sources: http://www.detectors.saint-gobain.com/MaterialsGasTubes.aspx
http://scintillator.lbl.gov
63
ear Accelerator was used for percent depth dose, dose linearity, and angular dependence
measurements. All clinical measurements were verified with a benchmark calibration using a Wellhofer CC13 ion chamber with 0.13 cm3 detector volume (hereafter called the
expected value). Unless otherwise noted 10 cm x 10 cm radiation fields (photon, electron
beam) were used during measurements.
3.4.
3.4.1
Optical Fiber System Efficiency Considerations
Estimated Detected Light from Scintillators
Light output from scintillators when using
60 Co
and
137 Cs
low activity sources was
used to estimate the number of photons detected by each SSPM when directly coupled.
The number of detected photons is:
Nphotons = Energy × scintillator light yield × P DE(λ) × area f actor
(3.15)
where the area factor is the ratio of the SSPM die area to the scintillator area, PDE is
the manufacturer’s given photon detection efficiency, and scintillator light yields are from
Table 3.3. For the S10362-11-050C SSPM directly coupled to the Prelude 420 scintillator,
the number of detected photons is: Nphotons = 662 keV × 32 photons/keV × 0.45 × 1 =
9532.3 p.e.. Assuming a conservative Gain M = 2 × 105 for each SSPM, the number of
electrons at the SSPM output is; Nphotons = 9532.3 p.e. × 2 × 105 = 1.9 × 109 , multiplying
by q gives the charge Q = 1.9 × 109 × 1.6 × 10−19 = 3.05 × 10−10 C. Now using an
appropriate time integration window, which is a function of the scintillator decay time τ ,
one can estimate the SSPM output current I:
I(t) =
dq
−q −t/τ
=
e
= 3.05 × 10−10 /50 × 10−9 = 6.1 mA
dt
τ
(3.16)
for this example. These calculations predict that oscilloscope signals in the 10s of mV
range should be observed without external amplification when using a 50 Ω load. During
laboratory tests the reference PMT scintillator combination exhibited oscilloscope signals
64
in the range that could be viewed without external amplification. This illustrates that
light coupling losses between the SSPM die and various scintillators is significant; measured
PDE for the SSPMs was sometimes less than indicated by the manufacturer.
Table 3.4 summarizes the expected number of detected photons and output current
I when using
137 Cs
as stimulator for photoelectrons.
TABLE 3.4: Estimated Scintillator Outputs: number of photons and SSPM current I.
Scintillator Material
BCF-12
PH0701BG
PH0810G1
S10362-11-050C
SBQF-EIOA
79440
52960
238320
66200
254.2 mA
84.7 mA
762.6 mA
42.37 mA
119160
119160
104265
52132
152.5 mA
152.5 mA
133.46 mA
66.73 mA
Lu1.8 Y.2 SiO5 : Ce
3177.6
2118.4
9532.8
2648
(Prelude 420)
2.0 mA
1.35 mA
6.1 mA
1.69 mA
Bi4 Ge3 O12
215.15
182.05
324.38
165.5
(BGO)
19.7 uA
16.65 uA
29.65 uA
15.13 uA
BC-430
3.4.2
Efficiency: Overall Light Coupling
Details of the overall light collection efficiency calculations for an OF radiation
sensing system are in Appendix B.[Beddar, 2007, Lacroix et al., 2008] Figure 3.15 shows
the coupling efficiencies for the components in the optical signal chain. The coupling
65
efficiencies at each material junction in the signal path, when combined together, provide
a gross estimate of (fiber-only and metalized air-core capillary tube-fiber) overall optical
system losses.
Fiber Coupling Efficiencies Contributing to S/N Ratio
scintillator
optical fiber
εtransmit−OF
εaccept
εscint−OF
scintillator
εaccept
SSPM
εPDE−sspm
εOF−sspm
capillary tube
optical fiber
εtransmit−CAP
εtransmit−OF
εscint−cap
εcap−OF
SSPM
εPDE−sspm
εOF−sspm
FIGURE 3.15: Light collection efficiencies for components in the optical signal chain.
For the non-capillary tube system:
ǫlight−collection = ǫaccept · ǫscint−OF · ǫtransmit−OF · ǫOF −sspm
and for the metalized air-core capillary tube OF system:
ǫlight−collection−captube = ǫaccept · ǫscint−cap · ǫtransmit−cap · ǫcap−OF · ·ǫtransmit−OF ·
ǫOF −sspm where ǫaccept is the fraction of light photons produced by the scintillator traveling
towards the optical fiber that fall within the optical fiber core acceptance cone. It is
estimated by the solid angle of isotropic light originating from the scintillator center which
falls within the acceptance cone of the OF. Here there is no assumption that light produced
in the scintillator itself is 100% reflected back by reflector materials (BaSO4 optical paint,
reflective tape). Worst case calculation gives ǫaccept = 0.01.
ǫscint−OF is the fraction of light coupled from the scintillator into the optical fiber
at the interface between the two. Here a value of 0.5 was assumed.[Attix, 1986]
ǫtransmit−OF is the transmission efficiency of the optical fiber itself. For 2.5 m of
66
Eska GH4001 fiber ǫtransmit = 0.896 (see Appendix B).
ǫtransmit−cap is the transmission efficiency of 30 cm of capillary tube. This value was
measured using the reference photodiode (green LED), = 0.6.
ǫscint−cap is the fraction of light coupled from the scintillator into the capillary tube
distal end. Measurements using the reference photodiode and Prelude 420 crystal as
scintillator were inconclusive. A course estimate was obtained using (green LED) = 0.27.
ǫcap−OF is the fraction of light coupled from the proximal capillary tube end into
the optical fiber. This value was measured using the reference photodiode (green LED),
= 0.4.
ǫOF −sspm is the fraction of light coupled from the OF to the SSPM photosensitive
die. Attempts to measure this value with consistency in the laboratory were not conclusive.
A value of 0.5 was assumed. This may be further broken down into a component that
includes the OF-FC fiber connector interface. This was not done in this study. However,
during laboratory testing the FC fiber connector needed to be positioned nearly flush
against the SSPM photosensitive die to achieve a measurable signal with low level light
sources.
Combining these numbers gives:
ǫlight−collection = 0.01 · 0.5 · 0.896 · 0.5 = 0.0022
and for the capillary tube/optical fiber system,
ǫlight−collection−captube = 0.01 · 0.27 · 0.6 · 0.4 · 0.896 · 0.5 = 0.00029.
It is clear from this analysis that ǫaccept is the limiting term. Robust fiber coupling
techniques are needed to optimize this value (polishing, optical concentrators, etc.). Further, the inclusion of the capillary tube reduces that light coupled into the fiber by a factor
of 0.6.
67
3.4.3
Optical Signal-To-Noise-Ratio (S/N) Considerations
Whether Cerenkov light is produced is a function of material refractive index and
incident electron energy given by equation 2.12. Figure 3.16 shows the Cerenkov threshold
energy vs. material refractive index for different OFs and scintillating materials used in
this work. As the material refractive index increases, the threshold energy decreases.
Cerenkov Threshold Energy vs. Material Refractive Index
Cerenkov Threshold Energy (MeV)
3.0
2.5
2.0
1.5
Cerenkov Energy
1.0
0.5
0.0
1.0
1.2
1.4
1.6
1.8
2.0
Index of Refraction
FIGURE 3.16: Cerenkov threshold energy vs. material refractive index.
Using equation 2.13, Figure 3.17 plots the angle where Cerenkov light begins to be
an important noise source vs. incident energy for different OFs and scintillating materials
used in this work. Note that at diagnostic imaging energies and for low level radiation
detection, Cerenkov photons can be ignored.
68
Cerenkov Angle vs. Incident Electron Energy
Cerenkov Angle (degrees)
50
45
GH4001 POF, ncore = 1.4902
SiO2:Cu2+ OF, ncore = 1.458
BC-430, ncore = 1.58
BCF-12. ncore = 1.6
40
0
2
4
6
8
10
12
14
16
18
20
Energy (MeV)
FIGURE 3.17: Predicted Cerenkov angle vs. incident energy.
69
For light in the visible range (λ2 − λ1 , 650 nm - 400 nm) one can calculate the
expected number of light photons emitted by Cerenkov radiation in optical fiber. For the
BCF-12 and SiO2 : Cu2+ scintillators with center signal wavelengths between 420 nm - 440
nm equation 3.17 gives,[Jelly, 1958]
Nphotons = 2παl(
1
1
1
−
)(1 − 2 2 )
λ2 λ1
β ncore
(3.17)
where
α = e2 /hc ≈ 1/137 is the fine-structure constant.
l is electron depth in the scintillator material.
λ2 is the upper wavelength of interest.
λ1 is the lower wavelength of interest.
ncore is the refractive index of the fiber core.
β is the ratio of velocity in the fiber to the speed of light in a vacuum. For a
2
given electron energy its velocity is calculated using KE = √ mo c2
1−v /c2
− mo c2 . For a 2
MeV electron this gives v = 0.979c. β = vparticle−in−f iber /c, thus 0.979c/c = 0.979 and
β 2 = 0.9584. Table 3.5 shows v for a few energies.
TABLE 3.5: v for a few energies.
Energy (MeV)
Particle speed v
0.1
0.5482c
0.25
0.7410c
2
0.9790c
6
0.9969c
9
0.9985c
For 6 MeV electrons moving through a depth l of plastic or glass optical fiber (1
70
mm or 400 µm, repsectively) with core refractive indices of 1.492 or 1.458 in the visible
light range, the results are:
Nplastic ≈ 24.2 light photons
Nglass ≈ 9.3 light photons
which shows that Cerenkov light is a more significant noise contributor in plastic
fibers than in glass fibers. At higher energies more Cerenkov photons are produced,
reaching a maximum as β 2 approaches c, which degrades the S/N ratio. Figure 3.18
shows the number of Cerenkov photons produced vs. energy for different material indices
of refraction (1 mm x 1 cm long material lengths).
Using the refractive index information from Table 3.2, one can calculate the critical
angle for the plastic fiber and the maximum Cerenkov angle from equation 2.46 and
equation 2.14 respectively. The results are 70◦ and 47.91◦ , respectively.
Figure 3.19 shows the situation when the electron beam is perpendicular to the
optical fiber, and the cone of Cerenkov radiation is less than the angle θcritical for the
optical fiber.
Figure 3.20 shows the cone of Cerenkov radiation when it approaches the θcritical
angle for the optical fiber as the electron beam gantry angle from the LINAC is changed.
A response maximum is observed when the cone of Cerenkov radiation rotates with the
electron beam, and the Cerenkov cone angle approaches θcritical for the optical fiber itself.
The inclusion of metalized air-core capillary tubing to mitigate Cerenkov produced
light photons attenuates further the optical signal as described in the previous section.
3.5.
SiO2 : Cu2+ Fiber Optic Probe Design
A 400 um diameter x 5 mm long round cylindrical length of SiO2 : Cu2+ scintillator,
supplied by the Naval Research Laboratory, Washington DC., was butt coupled using
Norland UV cement and cyanoacrylate glue to a 2.5 m length of GOF. An FC fiber
71
Cerenkov Photons Emitted vs. Material Refractive Index
Cerenkov Photons between 400nm and 650nm
28
26
24
22
20
GH4001 POF, ncore = 1.4902
18
GH4001 POF, nclad = 1.402
16
SiO2:Cu2+ OF, ncore = 1.458
SiO2:Cu2+ OF, nclad = 1.441
14
BCF-12, ncore = 1.6
12
BCF-12, nclad = 1.49
BC-430, ncore = 1.58
10
8
6
4
2
0
0
2
4
6
8
10
12
14
16
18
20
Energy (MeV)
(a) 1 mm material depth
Cerenkov Photons Emitted vs. Material Refractive Index
Cerenkov Photons between 400nm and 650nm
280
260
240
220
200
GH4001 POF, ncore = 1.4902
GH4001 POF, nclad = 1.402
180
SiO2:Cu2+ OF, ncore = 1.458
SiO2:Cu2+ OF, nclad = 1.441
BCF-12, ncore = 1.6
160
BCF-12, nclad = 1.49
BC-430, ncore = 1.58
140
0
5
10
15
20
Energy (MeV)
(b) 1 cm material depth
FIGURE 3.18: Number of Cerenkov photons produced in visible light spectrum.
72
Electron beam
cladding
n2 = 1.492
Optical fiber core
θ critical
θ Cerenkov−Max
= 47.91
= 70
cladding
n1 = 1.402
FIGURE 3.19: Cone of Cerenkov Radiation: electron beam ⊥ to fiber.
Electron beam
cladding
n2 = 1.492
Optical fiber core
θ critical
Cerenkov
cone angle
changes
with beam
angle
= 70
cladding
n1 = 1.402
FIGURE 3.20: Cerenkov cone approaching θcritical for the optical fiber.
73
connector was used to mate this fiber with the photon counting electronics. The scintillator
head and fiber junction were coated with black epoxy and covered with Dupont TefzelTM to
shield light. Due to the stimulation energy needed to generate an RL signal using this
material, it was not laboratory tested with the low activity gamma radiation sources but
was tested clinically using the linear accelerator as noted above. Figure 3.21 shows the
construction of the SiO2 : Cu2+ dosimeter cable. Clinical testing was done using the linear
accelerator as noted above.
FC Connector
BCF−12 Scintillator
BaS04 Reflective Paint
Dupont Tefzel
SH4001 Plastic Optical Fiber Jacket
Cladding
0.98mm Core GH4001 Plastic Optical Fiber
1mm
3mm
2.5m
Jacket
Opaque Glue
Dupont Tefzel
FC Connector
SiO2Cu2+ Scintillator
Cladding
400um
400um Core SiO2 Optical Fiber
5mm
2.5m
FIGURE 3.21: Schematic of BCF-12 and SiO2 : Cu2+ dosimeter cables.
74
3.6.
BCF-12 Fiber Optic Probe Design
A 1 mm diameter x 3 mm long round cylindrical BCF-12 scintillator was butt coupled using Norland UV cement and cyanoacrylate glue to a 2.5 m length of Mitsubishi
Eska Premier GH4001 jacketed POF using an FC fiber connector to mate with the photon counting electronics. BCF-98 clear light guide was also evaluated but rejected due
to permanent radiation damage and RL considerations within the fiber itself.[Wick and
T. Zoufal, 2001, Nowotny, 2007] Each scintillator was covered with BaSO4 reflective paint,
wrapped in Teflon tape, and further covered with Dupont TefzelTM to shield light.[Webber
and Christ, 2003] POF and BCF-12 scintillator ends were prepared following the general
procedures outlined in [Lee et al., 2004, Ayotte et al., 2006]. Figure 3.21 shows the construction of the BCF-12 fiber optic probe and Figure 3.22 shows the completed BCF-12
fiber optic probe with FC connector and scintillator end.
FIGURE 3.22: Eska GH4001 POF and BCF-12 scintillator dosimeter cable.
75
Laboratory testing of absolute count rate was measured for each of the MPPC,
Photonique, and Voxtel SSPMs with the BCF-12 scintillator stimulated using low activity
radiation sources as noted above. Clinical testing was done using the linear accelerator as noted above with appropriate buildup material to approximate Charge Particle
Equilibrium (CPE).[Attix, 1986]
3.6.1
Angular Measurements
To minimize the effect of Cerenkov Radiation generated in POFs when using electron
beams clinically, a 980 nm internal diamater x 30 cm length of cyclindrical silver coated (inside surface coating thickness 100 nm to 200 nm) air-core capillary tubing (TS1000 series,
PolyMicro Technologies LLC, Scottsdale AZ.) was inserted between the BCF-12 scintillator and Mitsubishi Eska GH4001 POF (Norland UV cement, Polyacrylate glue).[Lambert
et al., 2008] Figure 3.23 shows the completed cable.
BaS04 Reflective Paint
BCF−12 Scintillator
Silver Capillary Tubing
GH4001 Plastic Optical Fiber Jacket
FC Connector
Dupont Tefzel
Cladding
Air Core
1mm
3mm
0.98mm Core GH4001 Plastic Optical Fiber
0.5mm
10−20cm
2.5m
FIGURE 3.23: BCF-12 Air-Core Capillary Tube Dosimeter.
Clinical measurement of signal magnitude was recorded as a function of electron
beam angle for BCF-12 and BCF-12 capillary tube probes. The measurement setup
shown in Figure 3.24 was used. Two 25 cm x 25 cm styrofoam blocks spaced 30 cm apart
positioned the dosimeter at the isocenter of the electron beam. The dosimeter was fixed
in position between the blocks using Micropore tape and small cylindrical paper tubes to
76
keep scattered radiation to a minimum. A constant 4 cm x 20 cm radiation field size was
used throughout the angular dependence measurements. Five readings each at dose rate
400 MU min−1 with 160 MU total dose delivered were integrated and averaged for each
type of fiber optic probe, each with an electron beam (9 MeV) using angles from 0◦ to
100◦ in 10◦ steps.
FIGURE 3.24: Photo of electron beam angular measurement setup.
77
3.7.
Dose Linearity Measurements
Dose linearity measures the ability of the OF probes to accurately determine dose
between scaled monitor units (MUs) or activity from ionizing radiation sources. The signal
ratio to MU from the linear accelerator should be a constant value for each probe tested,
but will not be the same value between probes using different scintillators. For example,
ideally each raw signal value should be 2x the previous value ±3% (20 MU = 2 x 10 MU)
but also (4 x 5 MU), etc. The variation between the signal ratio within the same data
set was analyzed using the slope of the linear best fit line. The slope of the best fit line
is the average of the signal/MU ratio. The y-intercept is not zero because the accelerator
is not linear when delivering dose at the low end of the range. Accelerators typically
under-deliver in the low MU (< 4 MU) ranges which is why a lower limit of 5 MUs per
segment is used for IMRT radiation fields. A typical clinical specification requires that
the linearity results are between ±3%.
Dose linearity measurements were done on top of an acrylic 20 cm x 30 cm x 5 cm
phantom placed on top of the Clinac 2100CD patient table as shown in Figure 3.25. These
were repeated for both types of BCF-12 probe, and the SiO2 : Cu2+ probe. The Prelude
420 crystal was tested at 6 MV. Different electron (6 MeV, 9 MeV, 12 MeV, 16 MeV, 20
MeV) and photon (6 MV, 18 MV) energies were used. Five readings each were integrated
and averaged, then compared against the expected value over a dose range of 5 MU to
160 MU (400 MUmin−1 dose rate).
78
FIGURE 3.25: Photo of dose linearity measurement setup.
79
3.8.
Percent Depth Dose Measurements
Percent depth dose, equation 3.18, measures the dose deposited at a particular depth
in water, normalized to the depth of maximum dose.
PDD =
Dd
x100
Ddmax
(3.18)
The depth dose distribution (or profile) is a function of two parameters: type of
radiation used and its energy in a radiotherapy treatment setting. Each of the depth
dose figures shows the results for both types of BCF-12 probe and the SiO2 : Cu2+ probe
measured in an acrylic water tank placed on top of the Clinac 2100CD patient table as
shown in Figure 3.26. Different electron (9 MeV) and photon (6 MV, 18 MV) energies were
used at 400 MUmin−1 dose rate, then compared against the expected value. Each percent
depth dose data set was normalized to its own dmax depth: 1.5 cm for 6 MV photons,
3.0 cm for 18 MV photons, and 2.0 cm for 9 MeV electrons. Positional accuracy error
for percent depth dose was ±2mm from the effective point of measurement in the water
bath. A water phantom/tank from Med-Tec, Inc., model MT-DDA with a hand-cranked
gear driven depth adjustment (in increments of 0.1 mm) was used.
80
FIGURE 3.26: Photo of electron beam percent depth dose measurement setup.
81
4.
RESULTS AND DISCUSSION
The overall SSPM signal shape is approximately a decaying exponential characterized by a time constant, τRt Ct , which is technology dependent (Rquenching , Rs , Cj , etc.) and
also depends on other factors discussed in section 2.5.3. Signal rise time is fast, typically
less than 2 ns for tested devices. Signal rise/fall times vary by manufacturer, number of
pixel elements in the array, overall die capacitance, and external circuit elements. SSPM
devices with die sizes ranging from 1 mm2 , 3 mm2 , 4 mm2 , 6 mm2 , and 9 mm2 exhibit increasing capacitance from 10 pF to hundreds of pF, with pixel element numbers
ranging from 100 to nearly 10000.[Hamamatsu, 2007, Photonique, 2006, Voxtel, 2008, Zecotek, 2008, SensL, 2006] Figure 4.1 shows SPICE simulation results of signal shape vs.
changing SSPM die capacitance in parallel with fixed load resistances simulating typical
preamplifier or oscilloscope impedance’s.
SSPM Signal Shape vs. Die Capacitance
0
2e-09
4e-09
6e-09
8e-09
2e-05
1e-08
2e-05
Ct = 0 pf
Ct = 10 pf
Ct = 20 pf
Ct = 35 pf
1.5e-05
1.5e-05
Ct = 45 pf
Ct = 100 pf
Current (A)
Ct = 300pf
1e-05
1e-05
5e-06
5e-06
0
0
2e-09
4e-09
6e-09
Time(s)
8e-09
0
1e-08
FIGURE 4.1: SSPM signal shape capacitance SPICE simulation.
82
An exponential current signal with fast rise and fall times is used to stimulate the
passive network. What a load impedance actually sees is the SSPM die capacitance in
parallel with two resistances: one the oscilloscope or preamplifier input impedance (50
Ω used in the simulation) and the second the SSPM external circuit load resistance, RL ,
which has a unique recommended value for each SSPM by manufacturer (50 Ω used in
the simulation, see Figure 3.5). Results show that SSPM die capacitance significantly
influences rise time and pulse height.
Table 4.1 shows the manufacturer data for the SSPMs tested in this work; FF is
the pixel fill factor in %, C is the device capacitance prior to breakdown, and gain (G) is
quoted for the recommended operating voltage Vop .
TABLE 4.1: Summary of published SSPM parameters.
SSPM
Area
#cells Pixel size FF
Idark
C(pf) Vop (V)
G
PDE
PH050701GR
1mm2
516
44µ2
60
2µA
35
40
0.8·106
30@600nm
PH0701BG
1mm2
556
44µ2
60
10µA
40
20
0.4·106
40@560nm
PH0611B1
1mm2
516
44µ2
60
10µA
40
30
0.18·106
20@440nm
PH-CLNS
1mm2
516
44µ2
60
5µA
40
30
0.18·106
20@440nm
PH0810G1
1.1mm2
556
44µ2
70
630nA 40
34.5
0.5·106
40@520nm
MPPC
1mm2
400
50µ2
61.5
50nA
70
0.75·106
50@400nm
SQBF-EIOA
1mm2
1024 32µ2
†
100nA 13
43.5
1.3·106
29@500nm
SQBF-EKAA 1mm2
1024 32µ2
†
25nA
43.5
1.3·106
29@500nm
†- not available.
35
10
83
4.1.
Photonique SSPM Comparisons
Figure 4.2 shows the measured pixel capacitance up to the breakdown voltage for
each Photonique SSPM.
Photonique SSPM Die Capacitance
75
70
050701GR
0701BG
Die Capacitance (pF)
65
0611B1
CLNS
60
0810G1
55
50
45
40
35
0
5
10
15
20
25
30
35
Voltage (V)
FIGURE 4.2: Die capacitance for five Photonique SSPMs.
All Photonique DCR measurements were performed at 22.5◦ C. The following figures
show DCRs for the Photonique: PH050701GR, PH0701BG, PH0611B1, PH-CLNS, and
PH0810G1 Figure 4.3(a) shows measured DCRs while Figure 4.3(b) shows calculated
DCRs based upon measured I/V curves and pixel capacitance.
All Photonique gain measurements were done at 22.5◦ C. The following figures show
the measured gain for the: PH050701GR (Figure 4.4(a)), PH0701BG (Figure 4.5(a)),
PH0611B1 and CLNS (Figure 4.4(b)), and PH0810G1 (Figure 4.5(b)).
Table 4.2 shows the PDE summary for the Photonique SSPMs. PDE increases with
84
SSPM Dark Count Rate (Log10 Hz)
Measured Dark Count Rate: Photonique SSPMs
1E7
1000000
100000
PH050701GR
PH0811B1
10000
PH0701BG
PH0611B1
PHCLNS
1000
100
0
2
4
6
8
Overvoltage (V)
(a) Measured DCR for Photonique SSPMs.
Dark Count Rate Per Pixel (Log10 Hz)
Photonique DCR vs. Overvoltage
100000
10000
PH0701BG
1000
PH0611B1
PH050701GR
PH0810G1
100
0
1
2
3
4
5
6
Overvoltage (V)
(b) Calculated DCR for Photonique SSPMs.
FIGURE 4.3: Dark count rates for Photonique SSPMs.
85
Photonique 050701GR Gain vs. Bias
1.0
Equation
y = a + b*x
Weight
No Weighting
0.01249
Residual Sum of
Squares
0.98165
Adj. R-Square
0.8
Value
Intercept
Gain (10^6)
B
Slope
Standard Error
-7.38365
0.35478
0.1911
0.0087
0.6
0.4
0.2
PH050701GR
Linear Fit of PH050701GR
0.0
39
40
41
42
43
Bias (V)
(a) Gain dependence for 050701GR.
Photonique CLNS, 0611B1: Gain vs. Bias
2.0
PH0611B1
1.8
PHCLNS
Linear Fit of PH0611B1
1.6
Linear Fit of PHCLNS
Gain (10^5)
1.4
1.2
1.0
0.8
0.6
Equation
y = a + b*x
Weight
No Weighting
0.02685
Residual Sum of
0.04058
Squares
0.98987
Adj. R-Square
0.4
0.98439
Value
Intercept
0611B1
Slope
0.2
Intercept
CLNS
Slope
Standard Error
-48.0508
1.5675
1.62891
0.05207
-47.32818
1.92725
1.60909
0.06402
0.0
29.6
29.8
30.0
30.2
30.4
30.6
Bias (V)
(b) Gain dependence for 0611B1 and CLNS.
FIGURE 4.4: Gain dependence for two Photonique SSPMs.
86
Photonique 0701BG Gain vs. Bias
4
3
PH0701BG
Gain (10^5)
Linear Fit of PH0701BG
2
1
Equation
y = a + b*x
Weight
No Weighting
0.10306
Residual Sum of
Squares
0.99083
Adj. R-Square
Value
Intercept
B
0
18
Slope
19
Standard Error
-15.8775
0.61225
0.92167
0.03133
20
21
Bias (V)
(a) Gain dependence for 0701BG.
PH0810G1 Gain vs. Bias
0.7
0.6
PH0810G1
Linear Fit of PH0810G1
Gain (10^6)
0.5
0.4
0.3
0.2
Equation
y = a + b*x
Weight
No Weighting
0.00194
Residual Sum of
0.1
Squares
0.99481
Adj. R-Square
Value
Intercept
0.0
B
31
32
Slope
33
34
Standard Error
-3.79365
0.09387
0.1226
0.0028
35
36
Bias (V)
(b) Gain dependence for 0810G1.
FIGURE 4.5: Gain dependence for two Photonique SSPMs.
87
bias voltage; here the bias voltage was fixed at manufacturer suggested Vop .
TABLE 4.2: PDE measurements for Photonique SSPMs.
Photon Detector
475nm (Blue LED)
640nm (Red LED)
PH050701GR
13.1%
39.7%
PH0701BG
25.4%
33.8%
PH0611B1
20.2%
11.4%
PH-CLNS
21.3%
12.1%
PH0810G1
19.7%
39.0%
∗ - All biased at manufacturer Vop .
Breakdown voltages (Vbreakdown ) and dark current (Idark ) ranges for Photonique
SSPMs are shown in table 4.3,
TABLE 4.3: Summary of measured breakdown voltages and device currents for Photonique SSPMs.
Photon Detector
Vbreakdown (V)
Idark @Vbreakdown (A)
Vbias (V)
Idark @Vbias (A)
PH050701GR
38.2
2.0 · 10−8
44.0
8.1 · 10−2
PH0701BG
16.5
2.2 · 10−9
23.0
3.2 · 10−5
PH0611B1
28.2
2.6 · 10−8
35.0
5.18 · 10−5
PH-CLNS
28.2
2.3 · 10−8
35.0
5.0 · 10−5
PH0810G1
27.7
2.2 · 10−9
40.0
4.32 · 10−5
Table 4.4 summarizes the measured Photonique SSPM characteristics. Gain was
first measured then calculated using equation 2.26. τ ·DCR shows the expected number of
dark counts added to a detected signal when using a τ = 1 µs sampling time. DCR was
88
first measured then calculated using equation 4.1.[Pavlov et al., 2005] The DCR for the
PH050701GR is greater than measured by Pavlov05 but consistent in magnitude.
DCR(Vov ) = I(Vov )dark /Vov Cpixel Npixel
(4.1)
To decrease the DCR contribution to any signal the sampling time τ must be shorter
or device DCR must be smaller. When using fast scintillators such as BCF12 or Prelude
420 (table 3.3, 3.2 ns and 41 ns, respectively), τ can be shortened. This can be done
primarily by lowering I(Vov )dark holding the other parameters constant. For the SSPMs
tested in this study, I(Vov )dark was indeed decreased with each new generation of device. Vop (hence Vov ), Cpixel , and Npixel often change with each device improvement and
topology.[McNally and Golovin, 2009]
Based upon these test results the Photonique PH0810G1 was chosen for further
testing in part due to its wider Vov range and smaller dark count rate. The PH0701BG
had greater PDE in the blue/green light range, but a higher DCR and restricted Vov range.
Note that the calculated gain based upon the measured pixel capacitance was consistently
larger than measured when using the pulser.
Table 4.5 shows the measured counts for the reference PMT and each Photonique
SSPM type with BGO, Prelude 420, and BCF-12 scintillators directly attached and attached using 2.5 m Mitsubishi Eska GH4001 POF.
89
TABLE 4.4: Summary of measured Photonique SSPM characteristics.
SSPM
PH050701GR
I(Vov )dark (A) Pixel Cap. (fF) Gain (105 )
1.15 · 10−5
77.9
0.98
DCR
τ ·DCR
15.9 kHz/pix
8.2
8.2 MHz†
PH0701BG
6.35 · 10−7
78.2
0.89
9.74 kHz/pix
5.41
5.41 MHz†
PH0611B1
9.0 · 10−6
75.6
0.95
14.24 kHz/pix
7.35
7.35 MHz†
PH-CLNS
9.0 · 10−6
81.4
1.02
13.27 kHz/pix
6.83
6.83 MHz†
PH0810G1
7.1 · 10−7
80.0
0.91
4.2 kHz/pixel
2.33
2.335 MHz†
†- At Vov : 1.8V, 1.5V, 1.8V, 1.8V, 3.8V, respectively, vertically down DCR column.
90
TABLE 4.5: Photonique laboratory photon count rates†(cpm)
60 Co:
direct scintillator-
SSPM attachment and using POF.
Photon Detector†
BGO
P450
BCF12
BGO-POF
P450-POF
BCF12-POF
H5783PMT-ref
5713
3672
2378
732
1466
639
PH050701GR
683
1948
746
87
273
∗
PH0701BG
1015
3572
1489
114
407
310
PH0611B1
830
3069
965
110
351
271
PH-CLNS
919
3021
1006
103
380
284
PH0810G1
927
3109
1127
92
385
302
∗ - Undetectable
†- At Vop : 41.5V, 19.5V, 31V, 31V, 31.5V, respectively, vertically down Photon Detector
column.
91
4.2.
Photonique, MPPC, and Voxtel SSPM Comparisons
Based upon the measured results from the previous section, the Photonique PH0810G1
was compared against the MPPC and Voxtel SSPMs.
Figure 4.6 shows the measured I-V characteristics for each SSPM and Figure 4.7
shows the measured pixel capacitance up to the breakdown voltage for each SSPM.
I-V Comparison: Four Different SSPMs
1E-4
S10362-11-050C
1E-5
PH0810G1
SBQF-EKOA
SBQF-EIOA -20 deg. C
Current (A)
1E-6
1E-7
1E-8
1E-9
1E-10
1E-11
10
20
30
40
50
60
70
Bias (V)
FIGURE 4.6: IV-characteristics for MPPC, Photonique, and Voxtel SSPMS.
All DCR measurements were done at 22.5◦ C. The following figures show DCRs
for the Photonique PH0810G1, Voxtel SQBF-EIOA/EKAA, and MPPC S10362-11-050C.
Here Figure 4.8(a) shows measured DCRs while Figure 4.8(b) shows calculated DCRs
based upon measured I/V curves and pixel capacitance.
All gain measurements were done at 22.5◦ C. The following figures show the measured
gain for the: Voxtel SQBF-EIOA/EKAA (Figure 4.9(a)), and MPPC (Figure 4.9(b)).
92
SSPM Die Capacitance vs. Voltage
70
60
Capacitance (pF)
50
40
30
SBQF-EIOA
SBQF-EKOA -20 deg. C
PH0810G1
20
S10362-11-050C
10
0
10
20
30
40
50
60
70
Voltage (V)
FIGURE 4.7: Die capacitance for MPPC, Photonique, and Voxtel SSPMs.
93
Measured Dark Count rate: 4 SSPMs
SSPM Dark Count Rate (Log10 Hz)
1E7
1000000
100000
10000
1000
PH0810G1
SBQF-EIOA
SBQF-EKOA
100
10362-11-050C
10
0
2
4
6
8
10
12
Overvoltage (V)
(a) Measured DCR for (4) compared SSPMs.
Dark Count Rate vs. Overvoltage
Dark Count Rate Per Pixel (Log10 Hz)
1E7
PH0810G1
S01362-11-050C
1000000
SBQF-EIOA
SBQF-EKOA, -20 C
100000
10000
1000
100
10
0
2
4
6
8
10
Overvoltage (V)
(b) Calculated DCRs for (4) compared SSPMs.
FIGURE 4.8: Dark count rates for Photonique, Voxtel and MPPC SSPMs.
94
Voxtel SSPMs: Gain vs. Bias
0.8
SBQF-EIOA
SBQF-EKOA
0.7
Linear Fit of EIOA
Linear Fit of EKOA
0.6
Gain (10^6)
0.5
Equation
y = a + b*x
W eight
No W eighting
0.00406
Residual Sum of
Squares
0.98922
Adj. R-Square
Value
Intercept
0.4
C
Standard Error
-2.15541
0.07907
0.05993
0.00189
Slope
0.3
0.2
Equation
y = a + b*x
W eight
No W eighting
0.00132
Residual Sum of
Squares
0.1
0.99398
Adj. R-Square
Value
Intercept
B
Slope
Standard Error
-2.30566
0.0662
0.05969
0.00155
0.0
36
38
40
42
44
46
48
Bias (V)
(a) Gain dependence for Voxtel SSPMs.
MPPC S10362-11-050U: Gain vs. Bias
1.2
MPPC
1.0
Linear Fit of MPPC
Gain (10^6)
0.8
0.6
0.4
Equation
y = a + b*x
Weight
No Weighting
Residual Sum
0.02531
of Squares
Adj. R-Squar
0.2
0.97412
Value
Intercept
B
Slope
Standard Err
-22.9830
1.35082
0.33733
0.01941
0.0
68.0
68.4
68.8
69.2
69.6
70.0
70.4
70.8
71.2
Bias (V)
(b) Gain dependence for MPPC SSPM.
FIGURE 4.9: Gain dependence for Voxtel and MPPC SSPMs.
95
Table 4.6 shows the PDE summary for the four SSPMs. PDE increases with bias
voltage, here the bias voltage was fixed at manufacturer suggested Vop .
TABLE 4.6: PDE measurements for PH0810G1, Voxtel, and MPPC SSPMs.
Photon Detector∗
475nm (Blue LED)
640nm (Red LED)
PH0810G1
19.7%
39.0%
S10362-11-050C
38.0%
18.4%
SQBF-EIOA
18.1%
14.3%
SQBF-EKAA
19.0%
15.6%
∗ All biased at manufacturers suggested Vop .
Breakdown voltages (Vbreakdown ) and dark current (Idark ) ranges for the PH0810G1,
S10362-11-050, SQBF-EIOA, and SQBF-EKAA are shown in table 4.7,
TABLE 4.7: Summary of measured breakdown voltages and device currents.
Photon Detector
Vbreakdown (V)
Idark @Vbreakdown (A)
Vbias (V)
Idark @Vbias (A)
PH0810G1
27.7
2.2 · 10−9
40.0
4.32 · 10−5
S10362-11-050C
68.0
6.0 · 10−10
72.5
6.38 · 10−3
SQBF-EIOA
36.5
1.0 · 10−9
49.0
1.29 · 10−5
SQBF-EKAA
33.0
4.2 · 10−11
49.0
1.36 · 10−6
Table 4.8 summarizes the measured characteristics. Gain and DCR were calculated
as described in section 4.1..
Table 4.9 shows the measured count rate (cpm) using the reference PMT (H5783)
and each SSPM type with Prelude BGO, Prelude 420, and BCF-12 scintillators directly
attached and attached using 2.5 m Mitsubishi Eska GH4001 POF.
96
TABLE 4.8: Summary of measured characteristics for Photonique, Hamamatsu, and Voxtel SSPMs.
SSPM
Npixel I(Vov )dark (A) Pixel Cap.(fF) Gain (105 )
PH0810G1
556
2.5 · 10−7
80.0
DCR
10.0
τ ·DCR
2585.4 Hz/pixel
1.44
1.44 MHz†
S10362-11-050C 400
1.0 · 10−7
92.75
8.69
1796.9 Hz/pixel
0.71
718.7 kHz†
SQBF-EIOA
1024
1.5 · 10−7
12.3
2.30
3969.8 Hz/pixel
4.06
7.2 MHz†
SQBF-EKAA
1024
2.5 · 10−8
8.88
3.88
392.8 Hz/pixel
≈ 0.4
402.2 kHz†
†- At Vov : 2V, 1.5V, 3V, 7V, respectively, verticlly down SSPM column.
TABLE 4.9: Laboratory photon count rates†(cpm) 60 Co: direct scintillator-SSPM attachment and using POF.
Photon Detector
BGO
P450
BCF12
BGO-POF
P450-POF
BCF12-POF
H5783PMT-ref
5713
3672
2378
732
1466
1039
PH0810G1
927
3109
1127
92
385
302
S10362-11-050C
1082
3281
1682
182
526
413
SQBF-EIOA
358
2613
173
∗
∗
∗
SQBF-EKAA
∗
138
∗
∗
∗
∗
∗ - Undetectable
†- At Vop : 30.5V, 70V, 39.5V, 39.5V, respectively, vertically down photon detector
column.
97
Laboratory measurements confirm that all three room temperature SSPM types
may be used with inorganic scintillators coupled directly to their photosensitive die areas. These results suggest that uses for photon counting, PET, CT, diagnostic radiology,
gamma spectroscopy functions, and remote environmental radiation monitoring are possible.
The poor count rate measurement performance when using OFs, as with the Photonique SSPMs described earlier, has its roots in a number of areas. Chief among them is
the size mismatch between the OF and the active optical area of the SSPM (approximately
1 mm2 for all). The H5783PMT reference PMT has a photocathode effective area that
closely matched the BGO crystal, thus optical coupling was good. The Prelude 450 and
BCF-12 scintillators, being 1 mm in diameter, did not couple well directly to the large
photocathode area reference PMT when using the custom machined mating jigs. When
using custom machined FC fiber connectors with the reference PMT the problem persisted
as the GH4001 fiber cable has a core diameter of 1 mm.
Prelude 450 and BCF-12 scintillators, each being approximately 1 mm in diameter,
exhibited significant losses when coupled to the GH4001 fiber cable. To improve the S/N
ratio industry standard fiber preparation and scintillator cleaving/polishing techniques
must be used. In this work GH4001 fiber ends, BCF-12, and BC430 scintillators were cut
with a razor blade beneath a microscope and hand polished. Figure 4.10 illustrates the
types of junction defects for optical fibers and scintillators that result in signal loss due
non-standard preparation techniques. BGO and Prelude 420 scintillators were supplied
professionally polished by Saint-Gobain. Increased count rates due to the professional
polishing were immediately apparent when coupling Prelude 420 and BGO scintillators
directly to SSPMs without using GH4001 cable. BGO crystals being of larger diameter
suffered the most performance loss when coupled to GH4001 cable, as no optical concentrator other than Teflon tape and plastic mating jigs were used (see Figure 5.1). Being
larger, the BGO crystals actually absorb the most radiation from the low activity sources.
98
However due to GH4001 fiber coupling losses the count rates are smaller than Prelude 450
and BCF-12 fiber coupled probes.
When testing SSPM/scintillator combinations in the laboratory using low activity
gamma radiation sources, FC fiber connectors were observed to cause excessive signal loss.
It was necessary to place the fiber core in direct contact with the SSPM photosensitive
area, eliminating any air gap to maximize the acquired signal. The Voxtel (SQBF-EIOA)
and Photonique (0810G1) use clear epoxy windows as a protective material covering their
photosensitive areas. This material is hard yet scratchable when butting sharp crystal
edges or OF cores against its surface, however shallow scratches may be polished out.
The MPPC (S10362-11-050C) uses a clear gelatin-like polymer as a protective material
covering its photosensitive area which easily scratches whereby sharp crystal edges or OF
cores can penetrate to the photosensitive area, potentially damaging the die or breaking
the bonding wires. When using the S10362-11-050C or its many newer variants, it is
necessary to design a coupling solution that prevents sharp edges from crystals or OF
cores from actually touching the material - adding to system cost.
SSPM-scintillator coupling was an issue for the cooled TO-8 packaged Voxtel (SQBFEKAA) with its transparent glass window where the photo sensitive die is set back approximately 2.75 mm from the coupling surface. A custom lens design (GRIN or convex
system) is needed to take full advantage of this cooled SSPM when used in large OF core
diameter systems. Indeed even when using the SELFOC GRIN lens for 1 mm POF-SSPM
coupling, it was not possible to transmit usable amounts of light to the photosensitive die
for POF testing purposes during laboratory tests. Voxtel supplied a 62.5/125 GRIN fiber
coupling jig for testing purposes. Laboratory tests with the LED pulser confirmed that
the cooled SQBF-EKAA can be used successfully with a custom designed optical system,
as the 62.5/125 GRIN improves the coupling substantially. However the 62.5/125 fiber
coupler is suboptimal for used with 1 mm diameter fibers and large scintillators.
A simple package redesign, which places the photosensitive die nearer (< 0.05 mm)
99
the OF core or scintillator crystal surface while eliminating the need for custom lenses
(inert gas is a requirement to prevent condensation), will take advantage of the superior
dark noise performance of the SQBF-EKAA making it an ideal candidate for low-noise
(G,P)OF-SSPM coupled systems or direct scintillator-coupled (diagnostic, CT, PET, environmental) monitoring systems.
When using the uncooled TO-8 packaged Voxtel (SQBF-EIOA) with 1 mm core
POF centered directly over the photosensitive die (butted against the protective epoxy),
the lower PDE of this device when compared with the MPPC (S10362-11-050C) and
Photonique (0810G1) during laboratory tests (as shown by lower photon count rates),
rendered it unusable for a (G,P)OF-SSPM clinical detection system. However, it can be
used successfully with die sized bright scintillators coupled directly to the photosensitive
area.
Semiconductor device theory predicts[Sze, 2006] that thermally generated free carriers contributing to SSPM dark count noise are reduced by approximately a factor of 2 for
every 8 ◦ C drop in temperature.[Renker and Lorenz, 2009] A similar dependency was noted
for the SQBF-EKAA (see I/V curve Figure 4.6). This metric alone and others that depend
upon temperature (I/V, Gain) are not sufficient by themselves to choose one SSPM over
another for a given fiber-based application as noted above. The MPPC (S10362-11-050C)
is a lower noise, higher gain device at small overvoltage (Vov ) [with better PDE at the
scintillator emission frequencies tested here,] than the Photonique (0810G1) and the Voxtel SQBF-EIOA (Figure 4.6, table 4.8) at room temperature. However, the Photonique
(0810G1) has greater dynamic latitude for overvoltage (Vov ) than the MPPC (S1036211-050C) which can be seen from the slope of their respective I/V curves in Figure 4.6.
This characteristic is important, for example, when trading off noise against gain (bright
scintillator light source, long fiber length, remote monitoring of isotopes) or signal acquisition without a preamplifier where high gain is a requirement. Test results indicated
these two devices are closely matched. The room temperature Voxtel SQBF-EIOA, de-
100
spite a large dynamic overvoltage (Vov ) range, exibited lower gain with higher noise that
the MPPC and Photonique counterparts, table 4.8). Optical signal coupling with 1 mm
POF/scintillator combinations was poor for this device. Based upon these results (measured I/V, DCR, and Gain at Vov ), PDE at scintillator wavelength, and system analysis
considerations (fiber coupling issues, and availability of portable electronics), the MPPC
(S10362-11-050C) was chosen for the initial clinical portion of this work.
101
(a) Optical fiber junctions resulting in signal loss.
(b) BCF-12 scintillator tip under 10x magnification: tapered end is 485.5
µm diameter.
FIGURE 4.10: Optical fiber and plastic scintillator junction ends.
102
4.3.
4.3.1
Clinical Results
Mitsubishi Eska GH4001 POF Cable Clinical Background Measurements
Optical fibers exhibit luminescence while in photon orelectron beams.[Clift et al.,
2002, Archambault et al., 2006] This is a noise source which must be subtracted from the
measured count rate to obtain the true count rate.
To gain information about the background counts generated in GH4001 POF cable,
it was tested without an attached scintillator using the Clinac 2100, for both photon and
electron beams (dose rate 400 MU/min−1 ). Figure 4.11(a) shows the background photon
response measured for the GH4001 POF cable and Figure 4.11(b) shows the background
electron response measured for the GH4001 POF cable at 0◦ gantry angle. Measurements
confirm the literature reports that POFs generate optical noise when in photon beams as
cited above. Electron beam noise is due in part to the generated Cerenkov radiation.
4.3.2
Angular Dependence
Figure 4.12 shows the results for the electron beam (9 MeV) angular dependence
tests. Using equation 2.14 the Cerenkov generated maximum peak at approximately
47.91◦ is clearly visible for the standard BCF-12 dosimeter, as is a 40% reduction when
using the BCF-12 capillary tube POF dosimeter. The theoretical Cerenkov intensity curve
is also plotted for comparison using equation 2.15. Earlier published work reports that
the spectrum of light generated is independent of the angle between the beam and the
fiber axis.[Lambert et al., 2009] Measured background counts were subtracted from the
subsequent clinical measurements when using the GH4001 POF cable.
4.3.3
Dose Linearity
An initial dose linearity test was done to check the clinical measurement setup using
a Prelude 420 scintillation crystal attached to 2.5 m GH4001 POF. The result is shown
103
Eska GH4001 POF: Photon Response
18000
16000
Equation
y = a + b*x
Weight
Equation
No Weighting
y = a + b*x
26269.84826
No Weighting
Residual Sum of
Weight
Squares
14000
Residual Sum of
Adj. R-Square
Squares
Response (Counts)
Intercept
12000
Slope
Intercept
6xbg
18xbg
0.99971
Value
0.98072
Adj. R-Square
6xbg
47889.61487
26269.84826
0.98072
Intercept
Slope
Slope
Standard Error
147.53731
46.08477
Value
Standard Error
9.76453
147.53731
0.61108
46.08477
-39.57164
9.76453
62.22277
0.61108
107.65089
0.82507
10000
8000
6MV
18MV
6000
Linear Fit 6MV
Linear Fit 18MV
4000
2000
0
0
20
40
60
80
100
120
140
160
Dose (MU)
(a) GH4001 POF photon background response.
Eska GH4001 POF: Electron Response
10000
6 MeV, Linear Fit of 6 MeV
9 MeV, Linear Fit of 9 MeV
12 MeV, Linear Fit of 12 MeV
8000
18 MeV, Linear Fit of 18 MeV
Response (Counts)
20 MeV, Linear Fit of 20 MeV
6000
4000
2000
0
0
20
40
60
80
100
120
140
160
Dose (MU)
(b) GH4001 POF electron background response.
FIGURE 4.11: GH4001 POF cable background response.
104
BCF-12 Dosimeter Response vs. Angle
1.0
Theoretical
BCF12
BCF12 Cap Tube
Normalized Response
0.8
0.6
0.4
0.2
0.0
0
20
40
60
80
100
Angle between beam axis and POF axis (degrees)
FIGURE 4.12: Angular dependence of standard and capillary tube POF dosimeters: BCF12 scintillator: measured and theoretical results
105
in figure 4.13.
2100CD Clinac 6MV Photon Dose Linearity: Prelude 420 Crystal
25000
6MV
Linear Fit of 6MV
Response (Counts)
20000
15000
10000
Equation
y = a + b*x
Weight
No Weighting
375069.7299
Residual Sum
2
of Squares
5000
0.99893
Adj. R-Square
Value
6x
0
0
20
40
60
80
Standard Erro
Intercept
14.73134
174.13444
Slope
157.4575
2.309
100
120
140
160
180
Dose (MU)
FIGURE 4.13: 6 MV photon dose linearity: Prelude 420 scintillator.
The 6 MV ratio and linear fit analysis to data points at doses of (5, 10, 20, 40, 80,
& 160 MUs) gave percent errors of (-34.0, -16.2, 2.10057, 5.2, 2.1, -0.9)%, respectively.
Note the large percent error at the 5 MU and 10 MU dose ranges, indicating a large
under-response (hence nonlinearity) in this range.
The linear dose responses of the OF dosimeters in the RL scintillating mode are
shown in the following figures.
Figure 4.14(a)shows the measured photon dose linearity for the standard POF
dosimeter and GOF dosimeter: BCF-12, and SiO2 : Cu2+ scintillators at 6 MV and 18
MV. Figure 4.14(b) shows the measured photon dose linearity for the capillary tube POF
dosimeter: BCF-12 scintillator at 6 MV and 18 MV.
Figure 4.15 shows the measured electron dose linearity (9 MeV - 20 MeV) for the
standard POF dosimeter, GOF dosimeter: BCF-12, and SiO2 : Cu2+ scintillators.
106
2100CD Clinac Photon Dose Linearity
400000
6MV,
350000
Linear Fit of 18MV
6MV SiO2:Cu2+,
300000
Response (Counts)
Linear Fit of 6MV
18MV,
Linear fit of 6MV
250000
200000
150000
100000
50000
0
0
20
40
60
80
100
120
140
160
Dose (MU)
(a) BCF-12 and SiO2 : Cu2+ Dosimeters
2100CD Clinac Photon Dose Linearity
120000
6MV Cap Tube,
18MV Cap Tube,
Linear Fit of 6MV
Linear Fit of 18MV
Response (Counts)
100000
80000
60000
40000
20000
0
0
20
40
60
80
100
120
140
160
Dose (MU)
(b) BCF-12 Capillary Tube Dosimeter
FIGURE 4.14: Photon dose linearity: standard and capillary tube dosimeters
107
2100CD Clinac Electron Dose Linearity
300000
250000
6MeV,
Linear Fit of 6MeV
9MeV,
Linear Fit of 9MeV
12MeV,
Linear Fit of 12MeV
16MeV,
Linear Fit of 16MeV
20MeV,
Linear Fit of 20MeV
Response (Counts)
9MeV SiO2:Cu2+,
Linear Fit of 9MeV
200000
150000
100000
50000
0
0
20
40
60
80
100
120
140
160
Dose (MU)
FIGURE 4.15: Electron dose linearity: BCF-12 and SiO2 : Cu2+ Dosimeters.
Figure 4.16(a) shows the measured electron dose linearity for the capillary tube
POF dosimeter (6 MeV, 9 MeV): BCF-12 scintillator. Figure 4.16(b) shows the measured
electron dose linearity for the capillary tube POF dosimeter (12 MeV, 16 MeV, 20 MeV):
BCF-12 scintillator.
The figures are non-linear in dose ranges of interest for radiation oncology treatments. Table 4.10 compares the measured photon and electron dose linearity for the
GOF dosimeter, standard POF dosimeter, and capillary tube dosimeters: BCF-12, and
SiO2 : Cu2+ scintillators.
4.3.4
Depth Dose Measurements: Photon and Electron Beam
Section 3.8. describes depth dose as measured in this work. Each of the depth dose
figures shows the results for the OF dosimeters measured in a water tank, using different
electron and photon energies, and comparing against the reference ion chamber.
Figure 4.17 shows the result of the 6 MV photon depth dose measurements for the
108
2100CD Clinac Electron Dose Linearity: Capillary Tube
180000
160000
Response (Counts)
140000
6MeV,
Linear Fit of 6MeV
9MeV,
Linear Fit of 9MeV
120000
100000
80000
60000
40000
20000
0
0
100
200
300
400
500
Dose (MU)
(a) BCF-12 Capillary Tube Dosimeter: 6 MeV, 9 MeV.
2100CD Clinac Electron Dose Linearity: Capillary Tube
16000
14000
Response (Counts)
12000
12MeV,
Linear Fit of 12MeV
16MeV,
Linear Fit of 16MeV
20MeV,
Linear Fit of 20MeV
10000
8000
6000
4000
2000
0
0
20
40
60
80
100
120
140
160
Dose (MU)
(b) BCF-12 Capillary Tube Dosimeter: 12 MeV, 16 MeV, 20 MeV.
FIGURE 4.16: Electron dose linearity: capillary tube dosimeters.
109
TABLE 4.10: Photon and Electron Dose Linearity Differences (range extrema) from Reference Ion Chamber (in %).
Dosimeter
6MV
SiO2 : Cu2+
-5.5
18MV
†
6MeV
†
+0.5
BCF-12
BCF-12 Cap. Tube
9MeV
-4.4
12MeV
16MeV
20MeV
†
†
†
+0.2
+9.0
-3.0
-0.8
-16.0
-6.5
-5.1
-6.2
+0.6
+6.4
+7.1
+8.2
+0.8
+9.4
+5.0
-21.1
-19.2
-17.8
-15.7
-20.3
-15.4
-18.2
-0.8
+22.4
+1.6
+7.1
+14.0
+8.0
+6.6
†- Not tested.
standard POF dosimeter, using both BCF-12 and SiO2 : Cu2+ scintillators (always used
with GOF) as well as the capillary tube dosimeter using a BCF-12 scintillator.
Figure 4.18 shows the result of the 18 MV photon depth dose measurements for the
standard and capillary tube dosimeters: BCF-12 and SiO2 : Cu2+ scintillators.
Figure 4.19 shows the result of the 9 MeV electron depth dose measurements for
the standard BCF-12 POF dosimeter, the GOF SiO2 : Cu2+ dosimeter and the BCF-12
capillary tube dosimeter.
Overall the figures show the agreement is good for the SiO2 : Cu2+ and standard
BCF-12 dosimeters; however, in a clinical setting no more than 3%-5% error is permitted. The capillary tube dosimeter measured a large error, 21%. The errors are too large
for actual clinical measurements using the current laboratory manufactured OF dosimeters and further characterization is needed. Table 4.11 compares the measured photon
and electron depth doses for the GOF dosimeter, standard POF dosimeter, and capillary
tube dosimeters: BCF-12, and SiO2 : Cu2+ scintillators. However, the accuracy is certainly closeenough to use these small, all solid state dosimeters for real-time detection of
110
Relative Response
2100CD Clinac - Reference % Depth Dose: 6 MV Photons
100
IonChamber
90
SiO2:Cu2+
80
BCF12
70
BCF12 Cap Tube
60
50
40
30
0
5
10
15
20
% Difference from Reference Ion Chamber: 6 MV Photons
0
SiO2:Cu2+
Residuals (%)
-2
BCF12
-4
BCF12 Cap Tube
-6
-8
-10
-12
0
5
10
15
20
Depth (cm)
FIGURE 4.17: 10 cm x 10 cm 6 MV photon depth dose profile in water tank
equipment malfunctioning. The equipment could be shutdown before injuring the patient.
4.4.
Summary of Clinical Results
Angular measurements with the capillary-tube-coupled BCF-12 scintillator in an
electron beam show a Cerenkov signal reduction of 40% at the measured peak angle of 50◦ ,
while not completely eliminating the Cerenkov noise. Due to a scarcity of material to meet
the clinical testing schedule, the length of air-core capillary tube used for this measurement
was not of sufficient length to completely avoid Cerenkov effects (as described in [Lambert
et al., 2008]) from the beam. This greater variation is likely due to three factors: the
shorter length of air-core capillary tubing used during the angular measurements, the
direction of the beam incident to the POF (refer to Figure 3.24) positive or negative
111
2100CD Clinac - Reference % Depth Dose: 18 MV Photons
Relative Response
100
90
80
70
IonChamber
60
SiO2:Cu2+
50
BCF12
40
BCF12 Cap Tube
30
20
0
5
10
15
20
% Difference from Reference Ion Chamber: 18 MV Photons
15
Residuals (%)
SiO2:Cu2+
10
BCF12
BCF12 Cap Tube
5
0
-5
-10
-15
0
5
10
15
20
Depth (cm)
FIGURE 4.18: 10 cm x 10 cm 18 MV photon depth dose profile in water tank
gantry angle), and effects from the 2100CD Clinac generated noise. As the 2100CD
Clinac rotates to extreme angles more of the POF is in the beam, coupling issues between
the POF-air-core capillary tubing-scintillator junctions, and manufacturing differences
between the dosimeter cables are significant. There remains a measurable Cerenkov effect
when irradiating the BCF-12 capillary tube POF dosimeter.
The dose ranges discussed below and shown in the tables are range extremas (extremes of the measured results). When using SSPMs-OFs as a photon detector, variation
in dose linearity for the SiO2 : Cu2+ (6 MV only) and standard BCF-12 dosimeters (6
MV, 18 MV) confirms previous studies on the accuracy of OF dosimetry.[Beddar, 1994,
Beddar et al., 2001, Huston et al., 2001] Here the variation in dose linearity ranged from
−5.5, +0.5% for SiO2 : Cu2+ dosimeters and ranged from +9.0, +0.6%, -3.0, +6.4% for
BCF-12 dosimeters respectively, when compared against the expected values from the ion
112
Relative Response
2100CD Clinac - Reference % Depth Dose: 9 MeV Electrons
100
80
60
IonChamber
40
SiO2:Cu2+
BCF12
20
BCF12 Cap Tube
0
0
1
2
3
4
5
% Difference from Reference Ion Chamber: 9 MeV Electrons
10
Residuals (%)
5
0
-5
-10
SiO2:Cu2+
-15
BCF12
-20
BCF12 Cap Tube
-25
0
1
2
3
4
5
Depth (cm)
FIGURE 4.19: 10 cm x 10 cm 9 MeV electron depth dose profile in water tank
chamber. For the BCF-12 air-core capillary tube dosimeter (6 MV, 18 MV), the variation
in dose linearity ranged from -21.2, -0.8% and -19.2, +22.4% when compared against the
expected values from the ion chamber.
When using SSPM-OFs as electron detectors variations in dose linearity for the
SiO2 : Cu2+ dosimeter (9 MeV only) ranged from -4.4, -0.2% and for the standard BCF12 dosimeter (6 MeV, 9 MeV, 12 MeV, 16 MeV, 20 MeV) ranged from -0.8, +7.1%,
-16.0, +8.2%, -6.5, +0.8%, -5.1, +9.4%, and -6.2, +5.0%, respectively, when compared
against the expected values from the ion chamber. For the BCF-12 air-core capillary tube
dosimeter, the variation in dose linearity ranged from -17.8, +1.6%, -15.7, +7.1%, -20.3,
+14.0%, -15.4, +8.0%, and -18.2, +6.6% when compared against the expected values from
the ion chamber.
Depth dose range discussed below and shown in the tables are range extrema. When
113
TABLE 4.11: Photon and Electron Percent Depth Dose Differences (range extrema) from
Reference Ion Chamber (in %).
Dosimeter
6MV
18MV
9MeV
SiO2 : Cu2+
-7.4
-12.9
-7.3
-1.2
+1.8
+4.9
-9.1
-1.8
-11.3
-1.3
+7.4
+6.4
-11.0
-2.7
-21.2
-1.4
+15.3
+3.9
BCF-12
BCF-12 Capillary Tube
using SSPMs-OFs as a photon detector for depth dose (6 MV, 18 MV), variation in accuracy for SiO2 : Cu2+ , BCF-12, and BCF-12 capillary tube dosimeters ranged from: -7.4,
-1.2%, -9.1, -1.3%, -11.0, -1.4% and -12.9, +1.8%, -1.8, +7.4%, -2.7, +15.3% respectively,
when compared against the expected values from the ion chamber.
When using SSPMs-OFs as an electron detector for depth dose (9 MeV), variation
in accuracy for SiO2 : Cu2+ , BCF-12, and BCF-12 capillary tube dosimeters ranged from:
-7.3, +4.9%, -11.3, +6.4%, and -21.2, +3.9% respectively, when compared against the
expected values from the ion chamber.
The SiO2 : Cu2+ dosimeter had the most accurate response in this study when compared against the expected values from the ion chamber. Both BCF-12 dosimeter depth
dose measurements were consistently below the reference ion chamber measurements,
indicating a S/N ratio, coupling, or scintillator efficiency issue when compared to the
SiO2 : Cu2+ dosimeter.
114
4.5.
Uncertainty Analysis
SiO2 : Cu2+ and BCF-12 material dose linearities have been well characterized [Huston et al., 2001, Justus et al., 2004, 2006, Beierholm et al., 2008] while a capillary tube
dosimeter using a PMT optical photon sensor showed much better linearity than measured here in a previous study using a 60 cm capillary tube.[Lambert et al., 2008] In an
SSPM-POF coupled radiation sensing system sources of signal degradation are numerous:
three optical junctions in a standard cable (scintillator-POF, POF-FC, FC-SSPM), four
optical junctions in a capillary tube cable (scintillator-capillary tube, capillary tube-POF,
POF-FC, FC-SSPM). Each of these junctions has associated with it a signal coupling
attenuation factor, which when multiplied together greatly reduce the number of optical
photons reaching the SSPM.[Beddar, 2007]
In this work scintillators of varying sizes (1 mm square, 1 mm diameter circular, and
4 mm diameter circular) were used to gain insight into coupling onto 1 mm2 die area SSPMs
and 1 mm diameter POFs for remote monitoring applications. Photon counting results
indicate that significant light is lost when using a larger diameter scintillator without an
optical concentrator system. Using a smaller scintillator (smaller than die area) introduces
excess noise into the system due to DCR considerations.
ESKA GH4001 plastic fiber cable attenuates light at 0.19 db/m (650 nm) and 0.22
db/m (400 nm). For example, at 15 m this is 2.85 db with a signal transmission factor
of 0.518 (2.85db = −10logT , see Appendix B). For remote radiation monitoring applications even this short POF length degrades the signal substantially (fewer optical photons
reaching SSPM), underscoring the need for a low DCR SSPM with high gain and high
PDE at scintillator emission wavelengths. Similarly, in clinical applications it is desirable
to have electronics physically separated from noise sources (removed from Linac vaults,
x-ray generator rooms, etc.). At long POF cable lengths needed to achieve this separation, signal attenuation is significant; multiple junctions further degrade the S/N ratio.
115
Multiple junction POF systems necessitate that precision manufacturing techniques be
used on each component to avoid optical photon attenuation and loss. Current state of
the art SSPMs have greater DCRs than PMTs (200 nA vs. 2 nA).[Hamamatsu, 2008,
2007, Zecotek, 2008, SensL, 2006] Replacing PMTs in radiation monitoring applications
using POFs as signal conduits requires that the system S/N ratio budget be carefully
considered.
For direct coupled, low-level radiation detection systems, the size of the SSPM die
and scintillator diameter must be closely matched or an optical concentrator system must
be used. Scintillator end faces must be carefully polished to optical industry standards,
and index matching gel should be used at the interfaceto the SSPM. Finally, the PDE
of the SSPM must be high at the emission wavelength of the scintillator. Once POF is
introduced into the system the importance of these constraints is magnified as noted above.
During laboratory tests, length extensions of POF beyond 2.5 m with low PDE SSPMs
resulted in undetectable signals when using low activity radiation sources. Glass optical
fiber which is less lossy but fragile, (400 µm or 500 µm diameter) was not invistigated in
this work.
116
5.
5.1.
CONCLUSIONS AND RECOMMENDATION FOR FUTURE
WORK
Optical and Electrical Characterization
Use of SSPMs as photon detectors for laboratory, environmental, diagnostic radiation sensing and dosimetry applications have many advantages over PMTs: SSPMs are
rugged, small size, have high gain, operate at low bias voltages, are non-magnetically sensitive, have high quantum efficiency, and (potentially) much lower cost. This work has
shown that, based upon their performance as radiation sensors and optical fiber dosimeters, SSPMs can be used to replace PMTs for laboratory, clinical, and environmental
applications.
The MPPC S10362-11-050C and Photonique 0810G1 had the best laboratory and
clinical performance of the SSPMs tested. Their gain measurements and photon counts
with scintillators directly attached indicate that these two devices are somewhat closely
matched in the low Vov range, with the Photonique 0810G1 exhibiting an advantage with
a much wider Vov range (Figure 4.6). The MPPC S10362-11-050C has a measurable edge
in dark current (6.0 · 10−10 A at Vbreakdown vs. the Photonique 0810G1 (2.2 · 10−9 A),
and SQBF-EIOA (1.0 · 10−9 A) respectively (Table 4.7). The two Voxtel devices ( SQBFEIOA/SQBF-EKAA) and the Photonique 0810G1 exhibited much wider usable Vov ranges
than the MPPC S10362-11-050C, allowing the user greater flexibility to trade off gain vs.
noise at a given Vov . However, the MPPC S10362-11-050C exhibited a higher PDE at
the scintillator wavelengths used in this work (Table 4.6). Since the same techniques were
used to attach scintillators to optical fibers for use with the SSPMs, the performance
degradation relative to each device was observed to be similar.
Much time was spent testing the Voxtel SQBF-EIOA (room temperature) and
SQBF-EKAA (cooled) devices in the laboratory in an effort to approximately match the
117
performance of the MPPC and Photonique SSPMs. Thirty engineering samples of the
SQBF-EIOA were tested for minimal dark current, with the four best chosen for further
laboratory work. The SQBF-EKAA (cooled, −20◦ C) SSPM had the lowest dark current
of any device tested, 4.2 · 10−11 A at Vbreakdown (Table 4.7). This result is expected; in
semiconductors much of the dark current is thermally generated. On the test bench the
photon counts for the SQBF-EIOA were lower than MPPC and Photonique SSPMs (Table 4.9). Further discussion with Voxtel disclosed that PDE was not directly measured for
the sample devices - PDE was measured by an outside vendor at the initial design and fabrication stage. Although PDE was also not measured through the 380 nm - 650 nm range
in the laboratory for MPPC and Photonique SSPMs, photon counting results indicate
that the Voxtel device suffered from a significantly lower PDE at the scintillation wavelengths of interest (420 nm - 450 nm) when compared with its Hamamatsu and Photonique
counterparts. When used with the 580 nm BC-430 plastic scintillator the SQBF-EIOA
demonstrated significantly larger photon counts, indicating that the decreased PDE for
this device was a significant factor in laboratory tests.
The SQBF-EKAA (cooled) SSPM shows much promise as a device with lowered dark
pulses when compared with the MPPC and Photonique SSPMs as shown in Figure 4.6.
Due to its current package design, robust optical coupling was not possible with off-theshelf GRIN lenses. Indeed Voxtel had an optical manufacturer fabricate a custom GRIN
lens for use with 62.5/125 diameter optical fibers (for DNA testing) coupled to their
Peltier-cooled T0-8 package. In this work budgetary and time constraints prohibited the
design of a custom GRIN lens for 1mm diameter optical fibers.
The introduction of metalized air-core capillary tubing in the optical signal path
essentially functions as an attenuator and Cerenkov noise reducer during radiotherapy
applications. Under clinical conditions the SSPM-air-core capillary tube-OF dosimeter
provided a noisy, fluctuating signal for analysis. Angular measurements confirm the value
of the air-core capillary tube for reducing Cerenkov radiation generated within the OFs.
118
Additional lengths of this material (up to 1 m) can completely eliminate measurable
Cerenkov noise generated in the OFs, whilst providing a simpler noise reduction methodology when compared with other noise reduction schemes discussed previously.[Lee et al.,
2007b, Frelin et al., 2006]
Depth dose profiles can be measured by the SiO2 : Cu2+ dosimeter, standard BCF-12
dosimeter, and BCF-12 metalized air-core capillary tube dosimeter for electon and photon
beams. The standard SSPM-OF dosimeters exhibit linear response characteristics to dose
and energy independence when using both plastic and SiO2 : Cu2+ materials, though not
within the commonly accepted +-3% error tolerance needed for clinical measurements.
The SSPM-air-core capillary tube-OF dosimeter while suffering from noise and accuracy
problems, can be improved with better manufacturing processes to approach the performance of the standard SSPM-OF dosimeters. The S/N ratio of the non-capillary tube
SSPM-OF system is adequate for both clinical dosimetry and short length environmental
radiation detection.
5.2.
Recommendations for Future Work
The two apparent limitations of the Voxtel SQBF-EKAA (cooled) device, PDE and
optical coupling, can be addressed by device testing/redesign, and an improved compact
package (modified T0-8 with inert gas atmosphere to prevent H2 O condensation) that
decreases the separation of the photosensitive die area for coupled light sources to distances
< 0.1 mm, eliminating the need for a custom GRIN lens. A thin optical coating similar
to that used on the MPPC and Photonique SSPMs is adequate for this purpose.
Laboratory photon-counting electronics used in this work can be improved by creating a portable USB based system that accommodates SSPMs with different working
voltages and gain requirements. The power supply and amplification stages for a portable
device were presented in this work. The USB interface and software blocks are currently
119
missing from a realization.
Use of a light guide when attaching large scintillators, large diameter optical fibers,
or fiber bundles to small SSPMs facilitates light concentration onto the small die area.
Larger (2 mm x 2 mm and 3 mm x 3 mm) SSPMs are available. However, at present,
these have significantly higher DCRs.[Hamamatsu, 2007, Photonique, 2006, Zecotek, 2008,
SensL, 2006] Figure 5.1 shows a proposed light guide schematic drawing and ray trace diagram. Importantly, calibrated low activity radiation source testing is needed to determine
accuracy for environmental radiation measurements when using SSPMs and POFs.
Testing of the fiber optic dosimeters and direct scintillator coupled SSPMs with
low energy diagnostic radiation sources (x-ray generators, mammography, CT scanners),
where the number of daily procedures is much greater than clinical radiotherapy, is needed
for system validation. Together with the low energy clinical testing regime, an x-ray
generator and mCi activity radiation sources are required for laboratory testing purposes.
Dose rate tests, both in the laboratory and in the clinic are needed to assess system
reproducibility. CT scans in particular are experiencing explosive growth worldwide. The
ability to easily record diagnostic radiation dose in patient charts is likely to be become
standard practice in the future. The low energies used in clinical diagnostics render the
need for Cerenkov noise cancellation moot; for these applications no capillary tube noise
reduction is necessary.
In addition, monte-carlo simulation studies using various simulators (Slitrani,
GEANT4, EGS4, MCNP) can be used to validate the scintillation light generated by
different radiation fields and in external light/environment shielding materials. In particular, studies to determine the effect of light shielding materials, light photon propagation,
capillary tube signal loss, and fiber junction signal loss characteristics are needed.
Though no long-term environmental or clinical study of the SSPM-OF system has
yet been performed, the first results presented here are encouraging. For future prototypes
the air-core capillary tube-dosimeter-OF coupling must be optimized by cutting/polishing
120
the OFs, scintillators, and capillary tubes with tighter manufacturing tolerances to improve performance and reduce signal variability. Moreover, computer simulation of the
optical photon signal path will assist in identifying coupling problems and in improving
the S/N ratio. Further laboratory and clinical studies with new OF-SSPM prototypes, using the Photonique (0810G1) and Voxtel (SBQF-EKAA) with modified package (outside
the LINAC vault, with and without a preamplifier), will include low energy (< 300 keV)
dose and dose rate characterization, long-term measurements, ion chamber OF dosimeter
calibration measurements, and improved software to operate the system.
121
(Large diameter scintillator
or fiber bundle)
Plastic
lens
material
SSPM die
cover material
o
0.9mm
SSPM die
approx. 4mm
10
111111
000000
000000
111111
SSPM case
(a) Light guide schematic drawing.
(b) Light guide ray trace diagram.
FIGURE 5.1: Proposed light guide for small area SSPMs.
122
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132
APPENDICES
133
A
APPENDIX A TEC Cooler Schematic for VOXTEL SQBF-EKAA
134
FIGURE 0.2: Circuit diagram for TEC cooler controller.
135
B
APPENDIX B Fiber Efficiency Calculations
• ǫaccept: For butt-joined OFs or OF-scintillator junctions, NA is modified by a term
n1 /no where n1 is the index of the OF core (1.492 for GH4001 fiber cable) and no is
the plastic scintillator refractive index (1.58). This gives a new NA value of 0.482.
Thus the new acceptance cone angle is 2 · sin−1 (newN A) = 57.62◦ .
Now ǫaccept is approximately the elementary solid angle that falls within the 57.62◦
R
acceptance cone angle. It is (1/4π) sinθdθdφ = 0.01.
• ǫtransmit−OF : GH4001 fiber cable has an attenuation of 190 db/km or 0.19 db/m @
650 nm. This is 0.475 db for the 2.5 m length used in this work. The attenuation
factor is: 0.475db = −10logT . Solving for T gives a transmission value of 0.896 or
89.6%.
Table 0.1 shows attenuation characteristics for GH4001 plastic fiber cable out to
100 meters.
136
TABLE 0.1: GH4001 plastic fiber cable attenuation characteristics.
GH4001 length (m)
Attenuation (db)
Attenuation factor
0
0
0
2.5
0.475
0.896
5
0.95
0.8035
7.5
1.425
0.7202
10
1.9
0.6456
12.5
2.375
0.5787
15
2.85
0.5188
20
3.8
0.4169
25
4.75
0.335
30
5.7
0.2691
35
6.65
0.2163
40
7.6
0.1738
45
8.55
0.14
50
9.5
0.1122
100
19
0.0126
137
C
APPENDIX C Laboratory Dose Calculations
Dose rate in Gy/s from a radioactive point source is given by:
Ḋ =
S
µen
·
· Eγ · 1.602 × 10−10 [Gy/s]
2
4πr
ρ
(C.1)
where the 1.602 × 10−10 term (conversion factor) is for unit conversion to Gy from MeV
and g etc., r is the distance to the point source (in cm), S is the isotope energy flux in
[d/s or dps] (from appendix D), µen /ρ, is the mass attenuation coefficient for the isotope
in (cm2 /g), and Eγ is the energy of the isotope (in MeV).
The
60 Co
source had 1.1 mCi activity on 06/09/1960 (age 48.6 years).
29284.6d/s
(0.0326
4π(0.2 cm)2
• For
137 Cs:
• For
60 Co: 68154.16d/s (0.0243
4π(0.2 cm)2
cm2 /g)(0.662 M eV )(1.602 × 10−10 ) = 0.201 µGy/s
cm2 /g)(1.25 M eV )(1.602 × 10−10 ) = 0.661 µGy/s
These calculations confirm the laboratory observations that the
stimulated the scintillators better than
137 Cs
60 Co
point source
using the same physical geometry.
138
D
APPENDIX D Decay of Laboratory Sources, Number of Photons
Emitted, and Prelude 420 Crystal Activity
D1
Decay of Laboratory Sources
Radioactive decay is governed by equation D.1:
A = Ao e−λt
ln 2
T1/2
where λ =
=
0.693
T1/2 ,
(D.1)
t is time, T1/2 is half-life, and activity A is in [Ci] or [Bq], often
expressed as disintegrations per second (d/s or dps) or transformations per second (t/s or
tps).
The
µCi
137 Cs
60 Co
source had 1.1 mCi activity on 06/09/1960 (age 48.6 years). For the 1
and 1.1 mCi
60 Co
sources that were 3 years and 48.6 years old, respectively,
when delivered from campus Radiation Safety. Using equation D.1:
137 Cs
60 Co
D2
= (1x10−6 Ci)(3.7x1010 dps/Ci)e
−0.693x3yrs
30.07yrs
= (1.1x10−3 Ci)(3.7x1010 dps/Ci)e
= 34528 dps
−0.693x48.6yrs
5.27yrs
= 68154.16 dps
Number of Photons Emitted by Decayed Sources
The total number of photons emitted by the decayed sources is given by equa-
tion D.2:
N = t × A × TB
(D.2)
where t is the time, A is the activity (equation D.1), and TB is the total branching ratio
for the isotope (the probability of specific transitions in the decay chain). TB itself is the
product of the branching ratio for that photon energy (BR) and the branching fraction
for the mode of decay (BF): T B = BR × BF .
N137 Cs = 1s × 34528 dps × 0.85 = 29248.6 662 keV photons in 1 s
139
N60 Co = 1s × 68154.16 dps × 1 = 68154.159 1.773, 1.332 MeV photons in 1 s 1 .
D3
Prelude 420 (Lu1.8 Y.2 SiO5 : Ce) Crystal Activity
Prelude 420 crystals contain
176 Lu,
a natural beta emitter, which emits a 307 keV
gamma ray as part of its decay cascade. Here the Specific Activity (SA) of the 1 mm x
1 mm x 5 mm crystals used in the laboratory is estimated using equation D.3. [Cember,
1998]
SA =
Activity
Nλ
λNa
=
=
m
m
Ao
(D.3)
where Na is Avogadro’s number (6.025E23 atoms/mol), λ is the same as equation D.1,
and Ao is the compound mass in g/mol.
• Mass of Lu1.8 Y.2 SiO5 : Ce = 7.1 g/cm3 .2
• Volume of 1 mm x 1 mm x 5 mm Prelude 420 crystal is 0.1 x 0.1 x 0.5 = 0.005 cm3 .
• Mass of 1 mm x 1 mm x 5 mm Prelude 420 crystal is 7.1 x 0.005 = 0.0355 g.
The mass percentage M of Lu in Lu1.8 Y.2 SiO5 : Ce is
M ass% =
=
1.8 M assLu
1.8 M assLu + 0.2M assY + M assSi + 5 M assO
1.8 × 174.967
= 71.44%
1.8 × 174.967 + 0.2 × 88.905 + 28.085 + 5 × 15.999
The mass of Lu in Lu1.8 Y.2 SiO5 : Ce is 0.714 x 0.0355 = 0.02536 g. There is 2.59% of
176 Lu
in natural Lu; the radioactive mass in each Prelude 420 crystal is 0.0259 x 0.02536
g = 6.57E-4 g.
1
Data for branching ratios are from Martin [2006], Chart of the Nuclides, and http://www.detectors.
saint-gobain.com
2
http://www.detectors.saint-gobain.com/PreLude420.aspx
140
Using equation D.3 the specific activity of each 1 mm x 1 mm x 5 mm Prelude 420
crystal is
SA =
ln2 × 6.025E23 atoms/mol
= 2002.3 Bq/g
1.192E18 s × 174.967 g/mol
thus the specific activity is 2002.3 Bq/g x 6.57E-4 g = 1.315 Bq.