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Coded Aperture Imaging for Fast Neutron Activation Analysis

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Coded Aperture Imaging for Fast Neutron Activation Analysis Coded Aperture Imaging for Fast Neutron Activation Analysis by Li Zhang B.S., Electrical Engineering, Peking University (1992) Submitted to the Department of Electrical Engineering and Computer Science, and the Department of Nuclear Engineering in partial fulfillment of the requirements for the degrees of Master of Science in Electrical Engineering and Computer Science and Master of Science in Nuclear Engineering MASSACHUSETTS INSTITUTE OF TECHNOLOGY at the - 129;- MASSACHUSETTS INSTITUTE OF TECHNOLOGY February 1996 LIBRARIES © Massachusetts Institute of Technology 1996. All rights reserved. Signature of A uthor ......... ........ .............................................. January 12, 1996 C ertified by ... , .................... .. ....... ................................ Richard C. Lanza Principal Research Scientist of Nuclear Engineering Thesis Supervisor C ertified by ........................ .................................................................. Berthold K. P. Horn Professor of Electrical Engineering and Computer Science Thesis Supervisor Read by ................ ....................................... ........... .. Shaoul Ezekiel Professor of Electrical Engineering and Computer Science, and Aeronautics and Astronautics R ead by .................. ...... ........ ..... .. ................................................... Lawrence M. Lidsky >fessor of Nuclear Engineering A ccepted b - ·-................................. Frederic P •i n- EECS Committee on Graduate Students Accepted by ............ ........................... Jeffrey P. feiF g,fhairman, NE Committee on Graduate Students Coded Aperture Imaging for Fast Neutron Activation Analysis by Li Zhang Submitted to the Department of Electrical Engineering and Computer Science, and the Department of Nuclear Engineering on January 12, 1996, in partial fulfillment of the requirements for the degrees of Master of Science in Electrical Engineering and Computer Science and Master of Science in Nuclear Engineering Abstract In this thesis, we show by Monte Carlo simulations that fast neutron activation anal- ysis techniques have unique advantages in identifying and localizing nuclear elements, which can be used to detect materials such as explosives or drugs. Fast neutron ac- tivation analysis methods in explosive detection have been simulated, and signature gamma-ray energies for special nuclear elements in explosive detection have been suggested. Coded aperture imaging methods, which combine high sensitivity with tomographic capability, have been employed in this project. Theoretical calculations, Monte Carlo simulations, and system performance analysis have been implemented for an imaging system which has been partly built for future experimental tests. The results have shown that a combination of fast neutron activation analysis techniques and coded aperture imaging methods is a solution for nuclear elemental identification, especially in explosive and drug detection at an airport. Thesis Supervisor: Richard C. Lanza Title: Principal Research Scientist of Nuclear Engineering Thesis Supervisor: Berthold K. P. Horn Title: Professor of Electrical Engineering and Computer Science Thesis Reader: Shaoul Ezekiel Title: Professor of Electrical Engineering and Computer Science, and Aeronautics and Astronautics Thesis Reader: Lawrence M. Lidsky Title: Professor of Nuclear Engineering Acknowledgments First of all, I would like to thank my thesis advisors Dr. Richard C. Lanza and Prof. Berthold K. P. Horn for their valuable suggestions and guidance on my thesis project. They are among the brightest and nicest people I have ever met. I would like to thank Dick for his generous continued financial support and very helpful tech- nical training. His optimism, humor, intelligence, and encouragement have made it a pleasure to work with him. I have benefited from him in both research and personal life. I also appreciate Berthold for his valuable discussion with me about my thesis. I thank my thesis readers Prof. Shaoul Ezekiel and Prof. Lawrence M. Lidsky for their input. I also thank my colleague Erik B. Iverson for his help when I first used the MCNP software. I have benefited from a talk by Dr. Tsahi Gozani from SAIC on explosive detection using time-of-flight methods. I owe my father, Prof. Qicheng Zhang, and my mother, Prof. Liuying Huang, for their long-term support and understanding while they could not have their only son with them. Their continued encouragement and care about my personal and academic life give me power in my striving. I especially thank my sister, Dr. Yanching Zhang, for her uninterrupted care, suggestions, and encouragement in the past few years. I also thank my sister, Dr. Dahua Zhang, for her various kinds of help. I should thank Huifeng Lin for her understanding and support. She was with me during my happiest and gloomiest times. I also thank Mingsheng Gao. Her optimism always cheers me up and her help has made my life easier. I appreciate the financial aid provided by the Department of Nuclear Engineering during my first year at MIT. I am grateful to the Department of Electrical Engineer- ing and Computer Science for the cooperation during my thesis work. I also thank the Harvard-MIT Division of Health Sciences and Technology for offering me opportuni- ties to study at the Harvard Medical School, and the Federal Aviation Administration for the financial support of the project (FAA Grant 93-G-053). This thesis is dedicated to my father, Prof. Q. Zhang, my mother, Prof. L. Huang, and my grandmother, M. Zhou. Contents 1 Introduction 1.1 Background and Problems . ....................... 1.2 Contributions of This Thesis Project . ................. 1.3 Thesis Outline ............................... 2 Methods for Explosive Detection 14 2.1 Detection Requirements ... ..... .. ............... 14 2.2 Characteristics of Explosives ... ..... ............... 15 2.3 Detection Methods .......... ... ............... 15 2.3.1 Physical Methods ..... .... ............... 16 2.3.2 Chemical Methods ...... ... ...... 16 2.3.3 Electrical Methods ....... .. ............... 17 2.3.4 Nuclear Methods ........ .. ........... 18 2.3.5 Other Methods ....... .... .. 21 2.3.6 Conclusions .......... ... ...... 22 2.4 Fast Neutron Activation Analysis Methods ............ 2 2 3 Monte Carlo Simulations of Fast Neutron Activation Analysis Tech- niques 3.1 Simulation Model Geometry . ...................... 3.2 Neutron Sources .............................. 3.2.1 Optimal Neutron Sources in Explosive Detection ........ 3.2.2 Our Neutron Source ....................... 3.3 Simulation Results . .. ... .. .. .. .. 31 3.3.1 Expected Signature Gamma-Ray Spectra ... ... .. 31 3.3.2 Suggested Signature Gamma-Ray Spectra of Explosives 32 3.3.3 Discussion . ... ..................... 33 3.4 Conclusions .... ........................ 34 4 Theory of Coded Aperture Methods 4.1 Introduction . .... .................. .. .. 35 4.2 Concept of Coded Aperture Methods .. .. .. ... .. 35 4.2.1 Pinhole Cameras . ..... ........... .. 36 4.2.2 Multihole Collimator Systems . .. .. .. .. 36 4.2.3 Coded Aperture Systems . .. .. .. .. .. .. 36 4.2.4 Conclusions . ..... .... ... .. 38 4.3 Fresnel Zone Plate Coded Aperture Methods .. .. .. .. 40 4.4 Uniformly Redundant Array Coded Aperture Methods . .. .. .. 40 4.4.1 URA Coded Aperture Concept .. .. ... .. 40 4.4.2 URA Coding Methods . ...... ....... .. 41 4.4.3 URA Decoding Methods . .. .. ...... .. .. 42 4.4.4 URA Coded Aperture System Response . .. .. .. 47 4.5 Current Implementations of Coded Aperture Methods . .. .. 47 4.6 Implementations of URA Coded Aperture Methods . .. .. 48 4.7 Digital Realization of URA Coded Aperture Methods . .. .. 50 5 Coded Aperture Imaging System Design and Performance Analysis 51 5.1 URA Pattern Design . ..................... 51 5.2 System Point-Spread Function and Response Analysis . .. 57 5.3 Monte Carlo Simulations of System Response . .. .. 58 6 Complete Structure of Our CAFNA Imaging System 7 Monte Carlo Simulation Tests of Our Complete Imaging System 7.1 Lead Coded Aperture Plane . ...................... 7.1.1 1.25 MeV Photon Simulations . .. .. .. .. .. 65 7.1.2 6 MeV Photon Simulations . 65 7.2 Uranium Coded Aperture Plane . .. .. .. .. 65 7.2.1 1.25 MeV Photon Simulations 7.2.2 6 MeV Photon Simulations . 66 7.3 Discussion . ............... .. .. 71 7.4 Results ......... ... 73 8 Results, Conclusions, and Summary A Neutron and Gamma-Ray Data B Coded Aperture System Data 82 C Simulation Results 87 D Source Codes Bibliography 107 List of Figures 2-1 Neutron interaction and the time scale. ................. 24 3-1 Fast neutron activation analysis Monte Carlo simulation model geometry. 28 4-1 Diagram of a coded aperture system for photon imaging. ....... 39 5-1 URA coded aperture design 1: a pseudo-noise pattern and its SPSF.. 53 5-2 URA coded aperture design 2: an optimal pattern and its SPSF. ... 55 5-3 URA coded aperture design 2: an optimal pattern and its SPSF based on a balanced decoding algorithm ................... 56 6-1 Our complete CAFNA imaging system structure. ........ .. 61 6-2 Our actual coded aperture imaging system. ............ .. 62 7-1 Reconstructed images for lead shielding: a point source ......... 67 7-2 Reconstructed images for lead shielding: various source geometries. 68 7-3 Reconstructed images for uranium shielding: a point source. ...... 69 7-4 Reconstructed images for uranium shielding: various source geometries. 70 A-1 Mass absorption coefficients of the elements for X rays and neutrons.
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