The Development of a
Novel Silicon Microdosimeter for
High LET Radiation Therapy

A thesis submitted in fulfilment of the requirements
for the award of the degree
Doctor of Philosophy
from

University of Wollongong
by
Peter D. Bradley,
B.E. (Elec., 1st class honours, UNSW)
M.E. (Biomed. Eng., UNSW)
Grad. Dip. (Biomed. Eng., UNSW)

Department of Engineering Physics
2000
 

Abstract

This work is the first comprehensive investigation of the issues confronting silicon microdosimetry and its application to radiotherapy. Four main problems requiring investigation are identified and addressed including requirement specification with particular emphasis on device shape, tissue equivalence, noise minimization, and sensitive volume definition.

Analysis of device shape showed that a rectangular parallelepiped with a tissue equivalent converter on top of device (i.e. a silicon microdosimeter) provides a lineal energy spectrum that is closely equivalent to a sphere using the criteria of equal dose mean lineal energy. The tissue equivalent study demonstrated that under appropriate geometrical scaling (dimensions multiplied by 1/0.63) silicon detectors with well known geometry will record energy deposition spectra representative of tissue cells of equivalent shape. A novel prototype device using silicon-on-insulator (SOI) is presented. Silicon-on-insulator technology assists in defining the sensitive volume depth although the current device still suffers from lateral diffusion effects. I-V and C-V testing are performed and a noise optimization design model is presented. Methods for characterizing the collection efficiency and radiation hardness of silicon microdosimeters are presented and compared including alpha and proton microbeam spectroscopy, broadbeam alpha spectroscopy and 2D and 3D device simulation.

Results from testing the low noise prototype SOI device at several high LET clinical facilities including BNCT, proton therapy and fast neutron therapy facilities are presented. In the BNCT experiments, a simultaneous thermal neutron flux and microdosimetric measurement at a high spatial resolution is demonstrated. The use of SOI technology in experimental microdosimetry offers simplicity (no gas system or HV supply), high spatial resolution, low cost, high count rate capability and the possibility of integrating the system onto a single device with other detector types. The device also offers applicability in radiation protection and electronic single event upset (SEU) studies.
 
 
 

Download Thesis Options:
You may (a)download and view the entire thesis PDF or
               (b) view PDF files of each chapter from the Table of Contents below:

Complete Thesis (5.4MB)

Table of Contents

1. Introduction

1.1 Background
1.2 Thesis Outline 2. Literature Review and Background 2.1 Historical Foundations
2.2 Basic Principles and Microdosimetric Quantities
2.3 Experimental Methods in Regional Microdosimetry 2.3.1 Proportional Counter Microdosimetry 2.3.1.1 Basic Principles of Proportional Counters
2.3.1.2 Performance of Proportional Counters: Uncertainties and Limitations 2.3.2 Semiconductor (Silicon) Microdosimetry 2.3.2.1 Basic Principles of Semiconductor Detectors
2.3.2.2 Performance of Silicon Microdosimeter
2.3.2.3 Review of Semiconductors in Microdosimetry 2.3.3 Other methods 2.3.3.1 Microstrip Gas Counters (MSGC)
2.3.3.2 Cloud Chambers
2.3.3.3 Optical Ionization Chamber
2.3.3.4 Autoradiography
2.3.3.5 Three Dimensional Optical Random Access Memories (3D ORAM) 2.3.4 Summary of Experimental Microdosimetry Methods 2.4 Applications of Experimental Microdosimetry 2.4.1 Radiobiology
2.4.2 Radiotherapy Applications
2.4.3 Radiation Protection Applications
2.4.4 Space applications and Radiation Effects on Microelectronics 2.5 Summary: Key Problems in Silicon Microdosimetry 3. Silicon Microdosimeter Requirements and Shape Analysis 3.1 Intended Applications
3.2 Sources of Fluctuations in Regional Microdosimetry
3.3 Detector Shape Considerations 3.3.1 Comparison of Chord Length Distributions
3.3.2 Comparison of Segment Length Distributions and finite range effects 3.3.2.1 Segment length distributions for RPP, cube and sphere under m -randomness
3.3.2.2 Segment length distributions for infinite slab with converter overlayer
3.3.2.3 Segment length distributions for RPP with converter and overlayer 3.3.3 Equivalence of Shapes using Mean Energy per Event (ed) 3.3.3.1 Equivalence under m -randomness
3.3.3.2 Equivalence under nonisotropic conditions and c-randomness 3.3.4 Equivalence of Shapes using Frequency Mean Energy per Event (ef)
3.3.5 Equivalent Lineal Energy Spectra and Straggling
3.3.6 Summary: Criteria for the Equivalence of Various Shapes in Microdosimetry 3.4 Combining Criteria for Shape Equivalence and Tissue Equivalence
3.5 Summary of Requirements 3.5.1 Geometrical Requirements and Considerations
3.5.2 Operational Requirements and Considerations 4. Silicon Microdosimeter Design and Noise Analysis 4.1 Detector Description
4.2 Basic Electrical Characterization and Testing 4.2.1 Current-Voltage Measurements
4.2.2 Capacitance-Voltage Measurements 4.3 Probe Assembly #1
4.4 Probe Assembly #2
4.5 Design Equations, Noise Modeling and Optimization 4.5.1 Basic Design Equations and Noise Model
4.5.2 Optimization of Shaping Time
4.5.3 Optimization of Detector Voltage
4.5.4 Selection of Key Components and Parameters
4.5.5 Comparison with Device Performance
4.5.6 Ultimate Noise Performance 5. Tissue equivalence correction for silicon microdosimeters 5.1 Definition of Tissue and Detector Materials
5.2 Microdosimetry TE Correction Factor
5.3 Summary of Conditions for Tissue Equivalence
5.4 Boron Neutron Capture Therapy Tissue Equivalence 5.4.1 Comparison of Silicon and Tissue BNCT Range-Energy Relationships
5.4.2 Comparison of Silicon and Tissue BNCT Energy Deposition Spectra 5.4.2.1 Method: Monte Carlo Program
5.4.2.2 Results: Geometry Case 1 - Ion generation in RPP volumes with SV=GV.
5.4.2.3 Results: Geometry Case 2-Ion generation above RPP volume 5.5 Fast Neutron Therapy and Proton Therapy Tissue Equivalence 5.5.1 Geometrical TE scaling factor for FNT and PT
5.5.2 Effect of converter atomic composition in FNT
5.5.3 Silicon versus tissue interactions in FNT and PT and range of secondary products. 5.6 Summary of Tissue Equivalence Study 6. Charge Collection Modeling: Sensitive Volume and Radiation Hardness Characterization 6.1 Charge Collection Physics 6.1.1 Motion of Carriers 6.1.1.1 Carrier Drift
6.1.1.2 Carrier Diffusion
6.1.1.3 Recombination
6.1.1.4 Combined Model (Continuity and Poisson Equation) 6.1.2 Ion Interaction: Generation of Carriers
6.1.3 Ion Interaction: Motion of Carriers
6.1.4 Induced Currents by Carrier Motion (Ramos theorem) 6.2 Sensitive Volume Characterization 6.2.1 Spectroscopy Modeling Software
6.2.2 Numerical device simulation 6.2.2.1 Simulation Setup and Parameters
6.2.2.2 Collection Efficiency Simulations
6.2.2.3 Comparison of alpha and proton funneling in the 100 ´ 100 m m2 bulk diode.
6.2.2.4 Simulation of angled BNCT alpha generated strike 6.2.3 Microbeam Spectroscopy 6.2.3.1 Collection efficiency derived from complete spectroscopy model
6.2.3.2 Collection efficiency derived from microbeam image 6.2.4 Broadbeam Spectroscopy 6.2.4.1 Results for 10´ 10 m m2 , 2 m m SOI, undamaged and radiation damaged
6.2.4.2 Results for 10´ 10 m m2 , 10 m m SOI, undamaged device 6.3 Radiation Hardness Characterization 6.3.1 Basic Radiation Damage Physics
6.3.2 Minority Carrier Lifetime Estimation
6.3.3 Damage Constant 6.4 Summary of Charge Collection Modeling 6.4.1 Collection Efficiency and Radiation Damage Summary
6.4.2 Implications for Mean Chord Length Estimation 7. Application to Boron Neutron Capture Therapy 7.1 Background to BNCT
7.2 Thermal Neutron Sensitivity of Microdosimeter Construction
7.3 Experiments at Brookhaven National Laboratory (Boron and U-235 Coating Investigation) 7.3.1 Materials and Methods 7.3.1.1 Description of Brookhaven Medical Research Reactor(BMRR)
7.3.1.2 Description of probe assembly and device coating (B-10 and U-235)
7.3.1.3 Description of phantom
7.3.1.4 Microdosimetric measurements
7.3.1.5 Simulation of microdosimetric measurements and MCL estimation 7.3.2 Experimental Results and Discussion 7.3.2.1 Evaluation of boron coating methods
7.3.2.2 Evaluation of device orientation effects
7.3.2.3 Measurement using U-235 coated diode array
7.3.2.4 Comparison with Simulation
7.3.2.5 Comparison with Proportional Counter Microdosimetric Measurements 7.4 Experiments at Kyoto University Research Reactor (Simultaneous microdosimetry and thermal neutron flux measurements) 7.4.1 Materials and Methods 7.4.1.1 Description of Kyoto University Research Reactor (KUR)
7.4.1.2 Description of probe assembly and phantom
7.4.1.3 Microdosimetric measurements
7.4.1.4 Simulation of microdosimetric measurements and MCL estimation 7.4.2 Experimental Results and Discussion 7.4.2.1 Comparison BNL and KUR microdosimetric spectrum
7.4.2.2 Simulation (ion implanted boron and nitrogen in air)
7.4.2.3 Thermal neutron flux depth profile
7.4.2.4 Microdosimetric comparison of irradiation modes
7.4.2.5 Comparison sensitive volume depth variation (2 and 5 m m SOI) 7.5 Experiments at Massachusetts Institute of Technology (Epithermal Neutron Beam Investigation) 7.5.1 Materials and Methods 7.5.1.1 Description of facility
7.5.1.2 Microdosimetric setup and measurements 7.5.2 Experimental Results and Discussion 7.6 Summary of BNCT Applications 8. Application to Fast Neutron Therapy 8.1 Background to Fast Neutron Therapy
8.2 Materials and Methods 8.2.1 Description of fast neutron beam at Harper Hospital, Detroit.
8.2.2 Description of probe assembly and phantom
8.2.3 Microdosimetric measurements 8.3 Experimental Results and Discussion 8.3.1 Comparison of 2 m m and 10 m m SOI devices with the proportional gas counter.
8.3.2 Microdosimetric characterization of neutron beam using silicon microdosimeter
8.3.3 Tissue equivalence issues using a silicon microdosimeter in fast neutron beams 8.4 Summary of FNT Applications 9. Application to Proton Therapy 9.1 Background to Proton Therapy
9.2 Experiment at PMRC-Tsukuba 9.2.1 Materials and Methods 9.2.1.1 Proton Therapy Facility at PMRC-Tsukuba
9.2.1.2 Experimental Setup at PMRC-Tsukuba 9.2.2 Results: Microdosimetric Spectra at PMRC-Tsukuba 9.3 Experiment at NPTC-Boston 9.3.1 Materials and Methods 9.3.1.1 Proton Therapy Facility at NPTC-Boston
9.3.1.2 Experimental Setup at NPTC-Boston
9.3.1.3 Simulation using SRIM and PRISM 9.3.2 Results 9.3.2.1 Microdosimetric Spectra at NPTC-Boston
9.3.2.2 Comparison with Simulation 9.4 Summary of Proton Therapy Applications 10. Conclusions and Future Recommendations 10.1 Key Findings
10.2 Future Recommendations and Proposed Design Outline 11. References

Appendices

Appendix A: Prototype Fujitsu Silicon Microdosimeter, Packaging and Processing A.1. Device Packaging
A.2. Device Processing Appendix B: Mathematica Notebook - Analytical and Monte-Carlo Calculations of Chord and Segment Length Distributions in Various Shapes
Appendix C: Low Noise Microdosimeter Probe Design (Prototype #2)
         Reviewer Concluding Comments: "..To summarize, I am sure that Peter's thesis would be readily accepted as an outstanding doctoral thesis at Harvard University, which is the razor I am using for my own evaluation, and therefore I would strongly recommend that he receive his degree with the highest honors."
Professor Robert Zamenhof, Beth Israel Deaconess Medical Center/Harvard University.
"..I must commend Mr. Bradley not only for the original work, but also for displaying an unusually comprehensive knowledge of microdosimetry and its various applications.Both Mr. Bradley and his mentor, Prof Rosenfeld, must be congratulated"
Professor Marko Zaider, Memorial Sloan Kettering Cancer Center/Colombia University