AN ABSTRACT OF THE THESIS OF RICHARD HENRY BOSSI for the degree of DOCTOR OF PHILOSOPHY in Nuclear Engineering presented on December 3, 1976 (Major Department) (Date of Examination) Title: HIGH SPEED MOTION NEUTRON RADIOGRAPHY Abstract approved: Redacted for Privacy C-'L 'Alln H. Robinson A system to perform neutron radiographic analysis of dynamic events taking place over the order of several milliseconds has been developed at Oregon State University. The design consists of a TRIGA reactor capable of pulsing to 3000 MW peak power, a neutron beam collimator with an L/D of about 30:1, a lithium fluoride-zinc sulphide (LiF-ZnS) neutron scintillator screen, an image intensifier and a 16 mm high speed movie camera. The TRIGAreactor pulse and beam port collimator combine to yield a peak thermal neutron flux of about 11 2 4.2 x 10 n/cm-sec at the object with a pulse width (FWHM) of 9 milli- seconds. During this pulse duration sequential neutron radiographs are taken by the imaging system which detail any movement occurring in the object. The imaging system operates by converting the neutron signal to light, amplifying the light signal and recording the information on 16 mm Rapid Arts Recording film. LiF-ZnS neutron scintillator screens of 2/1 weight ratio of ZnS to LiF and 0.13 mm thickness constructed at OSU with sodium silicate binder are found to be very successful. The construction of the scintillators plus both an analytical model and experimental study of their performance is described. These screens have good light yield, good resolution and sufficiently rapid light decay characteristics to allow individual framing at over 5000 frames/second. The image intensifier is a two stage, 40 mm diameter tube. An 5-20 photocathode and a P-11 output phosphor are employed in the intensifier for their rapid decay characteristics and spectral response. The tube has a fiber optics input faceplate onto which the scintillator screen is pressure held for radiography. Actual imaging through the intensifier is superior to the conventional scintillator screen in contact with film method, apparently due to the non acceptance of wide angle light by the fine fiber optics. The high speed movie camera focuses on the intensifier output screen. Modulation transfer function analysis of the system indicates that the scintillator screen and the collimator L/D ratio are the two weakest areas of image quality in the system. The intensifier and high speed camera exhibit much better quality. Image quality can also be subject to statistical limitations due to the very short exposure times (about 80 psec at 5000 frames/second) per frame in the imaging system. Exper- imental studies indicate that the image quality becomes statistically 7 2 limited for general detail below about 10 n/cmfluence incident on the object. Subject to the object thickness and level of detail desired, the OSU system may be operated at up to 10,000 frames/second without experiencing this problem. For fine detail though it is necessary to enhance the object contrast by appropriate doping with a high contrast material such as gadolinium. This high speed motion neutron radiography technique may be applied to the interrogation of rapid dynamic events that have imaging charac- teristics suitable for neutron radiography inspection. High speed motion neutron radiographs have been taken of the ballistic cycle of both live and blank 7.62 mm munition rounds and of two phase flow as represented by air bubbles passing through water. O Richard Henry Bossi, 1976 High Speed Motion Neutron Radiography by Richard Henry Bossi A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Completed December 3, 1976 Commencement June 1977 APPROVED: Redacted for Privacy ProfessorOfncle:ar kngineering Redacted for Privacy Head of Department of Nuclear Engineering Redacted for Privacy Dean of Graduate School Date thesis is presented December 3, 1976 ACKNOWLEDGEMENT With the completion of this thesis and my degree I wish to ac- knowlege a few of the many who helped to make it possible. My thanks to Frankford Arsenal for their support in part of the work on "High Speed Motion Neutron Radiography". Also my thanks and appreciation to Dr. C.H. Wang, Director of the Oregon State University Radiation Center and Chairman of the Department of Nuclear Engineering, for his support of the work and to his staff, in particular Terry Anderson and Bill Carpenter of Reactor Operations, for their willing assistance. To the members of my graduate committee I would like to express special thanks. Foremost, to Dr. Alan Robinson, as my major professor, I am most appreciative, of his direction of my thesis investigation and guidance of my degree programs. To Dr. Bernard Spinrad, from whom I received the major part of my nuclear engineering instruction, I am very grateful for the opportunity to have studied under him. Similarly to Dr. William McMullen, I am appreciative of his instruction of the majority of my metallurgical engineering studies. Special thanks go to Dr. John Barton for his friendship, assistance and support of my career in neutron radiography and nondestructive testing. Among the many others who have helped to shape my career I wish to especially thank my parents and relatives for their support, moral and financial, of my numerous years of education. And finally, to Peg; thanks so very much for waiting, sweatheart. TABLE OF CONTENTS Chapter Page I. INTRODUCTION 1 1.1 Historical Background 1 1.2 General Neutron Radiography 1 1.3 Review of Neutron Radiography Applications and Special Techniques 6 1.4 Scope of this Study 9 II. SYSTEM DESIGN 11 2.1 Radiography Requirement 11 2.2 Neutron Source 13 2.3 Collimation 14 2.4 Objects 20 2.5 Neutron Scintillator Screen 21 2.6 Image Intensifier 22 2.7 High Speed Camera and Film 25 2.8 Synchronization of Events 27 III. TRIGA PULSING CAPABILITY 32 3.1 OSTR Characteristics 32 3.2 Nordheim-Fuchs Model 35 3.3 Pulsing Operation 36 3.4 Increasing Peak Power 41 3.5 Synchronization with High Speed System 44 IV. SCINTILLATORS 47 4.1 Introduction 47 4.2 Lithium Fluoride-Zinc Sulphide Scintillators 49 4.3 Boron-10 Zinc Sulphide Scintillators 49 4.4 Rare Earth Scintillators 50 4.5 Glass Scintillators 51 4.6 Scintillator Selection 52 4.7 Scintillator Construction 55 4.8 Scintillator Computer Model 59 4.9 Computer Optimized LiF-ZnS Scintillator 68 4.10 Image Quality in LiF-ZnS Scintillators 73 4.11 Light Transmission through LiF-ZnS Scintillators 76 4.12 Statistical Effects 81 4.13 Conclusions 83 Chapter Page V. SYSTEM ANALYSIS 85 5.1 Introduction 85 5.2 Modulation Transfer Function 85 5.3 Measuring the MTF 90 5.4 Numerical Technique for MTF Measurement 93 5.5 Experimental Results of MTF Measurement 98 5.6 Conclusions 106 VI. NEUTRON STATISTICS 109 6.1 Introduction 109 6.2 Image Detail Visibility 109 6.3 Experimental Determination of the Statistical Limitation 112 6.4 Results 116 6.5 Conclusions 119 VII. RESULTS AND CONCLUSIONS 122 7.1 High Speed Motion Neutron Radiographs of Ordnance Devices 122 7.2 High Speed Motion Neutron Radiographs of Two Phase Flow 131 7.3 Conclusions 134 VIII. BIBLIOGRAPHY 137 APPENDIX A 146 APPENDIX B 151 APPENDIX C 153 APPENDIX D 156 LIST OF TABLES Table Page 2-1 ASTM Beam Purity Indicator 18 2-2 Characteristics of Rapid Arts Recording Films 26 3-1 TRIGA Pulse Data 40 3-2 TRIGA Fuel Characteristics 43 3-3 Test of Time to Peak Power 46 4-1 Neutron Radiography Scintillators 48 4-2 Comparison of Scintillator Screens 53 4-3 OSU Constructed LiF-ZnS Scintillators 60 4-4 Particle Ranges 67 5-1 Test Cases for the Modulation Transfer Function Program 97 6-1 Radiographic Exposure at Different Frame Rates for a 115 3000 MW Pulse 6-2 VISQI Sensitivity Readings for Neutron Statistics 118 Comparison LIST OF FIGURES Figure Page 1-1 Neutron Radiography 2 1-2 Mass Absorption Coefficients of the Elements 4 1-3 A. Direct Foil Imaging 5 B. Transfer Foil Imaging C. Scintillator Imaging 2-1 High Speed Motion Neutron Radiography System 12 2-2 Beam Port #3 Collimator 16 2-3 Radiographic Density Contours of Beam Port #3 19 2-4 Photocathode Spectral Response 24 2-5 Synchronization 29 2-6 Start Up Curves for High Speed Camera 30 3-1 Power Traces for OSTR Pulses 33 3-2 OSTR Core Loading 38 4-1 Construction of LiF-ZnS Scintillators 57 4-2 LiF-ZnS Scintillator 61 4-3 Slab Geometry Scintillator Model 63 4-4 Spherical Geometry Scintillator Model 64 4-5 Typical Bragg Curve for Slowing Down of Heavy Particles 66 4-6 Relative Light Yield Versus Weight Ratio for LiF-ZnS 69 Scintillators 4-7 Relative Light Yield Versus Weight Ratio as a Function 72 of Grain Size 4-8 Scintillator Neutron Absorption Efficiency Versus 74 Weight Ratio 4-9 Scintillator Model with 40% Binder and 50% Packing Density 75 Figure Page 4-10Blue Light Transmission in LiF and ZnS 78 4-11 Light Distribution at Scintillator Screen Surface 80 4-12 Histogram of the Number of Histories Versus Energy in 82 ZnS as a Function of Weight Ratio 5-1 Point Spread Function; Line Spread Function 88 5-2 Edge Spread Function 92 5-3 Edge Spread Function for a High Speed Motion Neutron 99 Radiograph 5-4 Modulation Transfer Functions for the High Speed Motion 101 Neutron Radiography Components 5-5 Modulation Transfer Functions for Neutron Radiography 104 Foils 5-6 Modulation Transfer Functions for Scintillator Screens 105 5-7 Modulation Transfer Functions for Geometric Unsharpness 107 6-1 Statistical Limitation Study 114 6-2 Statistical Limitation in VISQI Radiographs 117 6-3 Statistical