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The Use of Cosmic-Rays in Detecting Illicit Nuclear Materials

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The Use of Cosmic-Rays in Detecting Illicit Nuclear Materials The use of Cosmic-Rays in Detecting Illicit Nuclear Materials Timothy Benjamin Blackwell Department of Physics and Astronomy University of Sheffield This dissertation is submitted for the degree of Doctor of Philosophy 05/05/2015 Declaration I hereby declare that except where specific reference is made to the work of others, the contents of this dissertation are original and have not been submitted in whole or in part for consideration for any other degree or qualification in this, or any other Univer- sity. This dissertation is the result of my own work and includes nothing which is the outcome of work done in collaboration, except where specifically indicated in the text. Timothy Benjamin Blackwell 05/05/2015 Acknowledgements This thesis could not have been completed without the tremendous support of many people. Firstly I would like to express special appreciation and thanks to my academic supervisor, Dr Vitaly A. Kudryavtsev for his expertise, understanding and encourage- ment. I have enjoyed our many discussions concerning my research topic throughout the PhD journey. I would also like to thank my viva examiners, Professor Lee Thomp- son and Dr Chris Steer, for the time you have taken out of your schedules, so that I may take this next step in my career. Thanks must also be given to Professor Francis Liven, Professor Neil Hyatt and the rest of the Nuclear FiRST DTC team, for the initial oppor- tunity to pursue postgraduate research. Appreciation is also given to the University of Sheffield, the HEP group and EPRSC for providing me with the facilities and funding, during this work. Many thanks to my friends and family for there continuous support throughout this process. In particular, recognition must be given to James Gibson for his willingness to listen to my questions and his ability to solve just about any of them. Other close friends I would like to mention are Chris Shaw, Chris Martyn, David Slack and James Gould. Thank you for listening, offering me advice, and supporting me through this entire process. I want to thank my parents, brother and nephews for always being there for me. Their love and support mean very much to me. Finally my greatest appreciation must be saved for my girlfriend, Rachel. Her continuous love, caring and patience throughout the many months it has taken to complete this work, is what has kept me going. Abstract Muon tomography represents a new type of imaging technique that can be used in de- tecting high-Z materials (such as shielded HEU). As muons pass through materials, they continuously lose energy via ionisation and can stop within a material. Upon stop- ping secondary processes can occur, which result in the production of excess neutrons and g-rays. Due to their high energies most muons can pass through large depths of different materials. These muons will also undergo multiple scattering. Previous radio- graphic methods that looks to differentiate high-Z materials from low-Z clutter tend to be based on multiple scattering of muons only. Presented is the development of two new types of analysis algorithm, which makes use of the different interactions cosmic-ray muons undergo when passing through a mate- rial. The first algorithm developed makes use of the multiple Coulomb scattering thata muon will undergo. This involves using a novel density based clustering approach that ascertains regions of high-Z material placed within an inspected volume. The capability of this new type of algorithm has been tested under realistic scenarios. These scenarios involve placing shielded HEU in cargo containers filled with a variety of different clut- ter. The algorithm has been shown to be efficient in detecting shielded HEU amongst low-Z clutter, but struggles upon the introduction of blocks of aluminium, or materials with densities higher than this. Whereas previous radiographic methods were based on multiple scattering of muons only, the second developed algorithm uses muon absorption on nuclei to enhance the detection capabilities of the scattering technique. In particular, the goal is to improve on the distinction between high-density materials and low-density clutter. Muons will more readily lose energy in higher density materials. Therefore multiple muon disap- pearances within a localised volume may signal the presence of high-density materials. Muons that disappear have their track evaluated using a 3D line extrapolation algorithm, which is in turn used to construct a 3D tomographic image of the inspected volume. The viii ability to differentiate between materials using the 3D line extrapolation algorithm is established. The technique of muon disappearance has been applied to identifying shielded HEU in realistic scenarios. Despite being capable of identifying shielded HEU in otherwise empty cargo containers, multiple additional regions (due to the lorry and container it- self) were misclassified as possible threat materials. This makes accurate detection systems that solely use this technique for shielded HEU unlikely. However, since the build-up of nuclear materials was apparent, we have demonstrated how this technique can be used as a supplementary technique to help enhance muon scattering tomography capabilities. This is done through first identifying ‘areas of interest’ using muon scat- tering tomography before confirming whether they are threats or other materials with the muon disappearance algorithm. It is concluded that while muon disappearance can slightly enhance the capabilities of muon scattering tomography, muon scattering still remains the most efficient method for detection of nuclear materials. Contents Contents ix List of Figures xiii List of Tables xxiii Nomenclature xxix 1 Introduction1 1.1 Introduction . .1 1.2 Thesis Outline . .3 2 Background5 2.1 Passive Interrogation . .5 2.2 Active Interrogation . 12 2.2.1 Interrogation Sources . 13 2.2.2 Background Interferences . 15 2.2.3 Techniques that Detect Prompt Signatures . 17 2.2.4 Techniques that Detect Delayed Signatures . 21 2.2.5 Summary of Active Interrogation Methods . 22 2.3 Radioactive Signatures for Passive and Active Interrogation Methods . 24 2.3.1 Radioactive Decay . 24 2.3.2 Induced Fission . 25 2.4 Cosmic Rays . 29 2.4.1 Cosmic Rays in the Atmosphere . 29 2.4.2 Cosmic-Ray Muon Spectrum at the Earth’s Surface . 31 2.4.3 Cosmic-Ray Neutron Spectrum at the Earth’s Surface . 32 2.5 Muon Interactions with Matter . 34 x Contents 2.5.1 Energy Loss . 34 2.5.2 Multiple Coulomb Scattering (MCS) . 36 2.5.3 Muon Capture . 39 2.6 Neutron Interactions with Matter . 43 2.7 Cosmic-Ray Muon Radiography . 46 2.7.1 History . 46 2.8 Cosmic-Ray Muon Tomography . 47 2.8.1 Previous Work . 47 3 Material Segregation using Muon Scattering 51 3.1 Concept . 51 3.2 Initial Parameters . 53 3.2.1 GEANT4 . 53 3.2.2 Muon Spectrum . 54 3.2.3 PoCA Algorithm . 55 3.2.4 Simulation Parameters . 58 3.3 Material Segregation using Muon Scattering . 61 3.3.1 Material Segregation using Realistic Muon Momentum Spread . 61 3.3.2 Comparison of Three Block Scenarios . 64 3.4 Summary . 72 4 Material Segregation using Stopping Muons 73 4.1 Concept . 73 4.2 Testing the Validity of GEANT4 . 74 4.2.1 Neutron Multiplicity from Muon Capture . 74 4.2.2 Muon Lifetime Measurements . 76 4.3 GEANT4 Simulations on Distinguishing Between Different Density Materials . 79 4.3.1 Simulation Parameters . 79 4.3.2 Gamma-ray and Neutron Spectra . 79 4.4 Material Segregation using Realistic Momentum Spread . 94 4.4.1 Simulation Parameters . 94 4.4.2 Tomographic Reconstruction for Muon Disappearance Tomog- raphy . 95 4.4.3 Material Segregation using Muon Capture . 99 4.4.4 Comparison of Three Block Scenarios . 101 Contents xi 4.4.5 Three Block Scenarios with Reduced Detector Coverage . 115 4.5 Summary . 121 5 Material Segregation using Cosmic-Ray Muons for Empty Cargo Container123 5.1 Simulation Parameters . 124 5.1.1 Shielding Calculation . 124 5.1.2 Simulation Package and Muon Generator . 125 5.1.3 Target Geometry . 126 5.2 Techniques for Muon Scattering . 128 5.2.1 Muon Tracking . 128 5.2.2 Analysis Algorithm . 128 5.2.3 Description of how the Optimum Parameters were Calculated . 134 5.2.4 Results for Cargo Container Housing 5 Shielded Threats . 135 5.2.5 Effect of Detector Spacing . 136 5.2.6 Number of Detector Planes . 139 5.2.7 Results for Cargo Containing 5 Shielded Nuclear Materials with no Clutter . 141 5.2.8 Comparison Between Four 5 cm × 5 cm × 5 cm Voxels and a Single 20 cm × 20 cm × 20 cm Voxel to Detect 5 Shielded Nu- clear Materials in a Cargo Container Housing 5 Shielded Threats 145 5.3 Techniques for Stopping Muons . 147 5.3.1 Muon Tracking . 147 5.3.2 Results for Cargo Container Housing 5 Shielded Threats using Muon Disappearance Tomography . 147 5.4 Results for Cargo Container Housing 5 Shielded Threats using Muon Disappearance and Muon Scattering Tomography . 150 5.5 Summary . 153 6 Material Segregation using Cosmic-Ray Muons for Cargo Containers with Clutter 155 6.1 Simulation Parameters . 156 6.1.1 Description of the Scenario Tested . 157 6.2 Shielded HEU Placed within Low-Z Clutter . 160 6.2.1 Results using Muon Scattering Tomography . 160 6.2.2 Discussion of the Muon Disappearance Algorithm . 170 xii Contents 6.2.3 Results using Muon Scattering and Muon Disappearance To- mography . 171 6.3 Shielded HEU Placed within Medium-Z Clutter . 178 6.3.1 Results using Muon Scattering Tomography . 178 6.3.2 Results using Muon Scattering and Muon Disappearance To- mography . 185 6.4 Summary . 188 7 Material Segregation using Cosmic-Ray Neutrons 189 7.1 Concept .
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