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Fast Neutron Sputtering from Polycrystalline and Monocrystalline Gold Crystals

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Fast Neutron Sputtering from Polycrystalline and Monocrystalline Gold Crystals 70-14,012 FAIRAND, Barry Philip, 1934- FAST NEUTRON SPUTTERING FROM POLYCRYSTALLINE AND MONOCRYSTALLINE GOLD CRYSTALS. The Ohio State University, Ph.D., 1969 Physics, radiation University Microfilms, Inc., Ann Arbor, Michigan THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED FAST NEUTRON SPUTTERING FROM POLYCRYSTALLINE AND MONOCRYSTALLINE GOLD CRYSTALS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Barry Philip Fairand, B.Sc., M.Sc, Vc -k * * * * The Ohio State University 1969 Approved by (~rn> Adviser Department of Physics ACKNOWLEDGMENT The author wishes to express his appreciation to Dr. M. L. Pool of The Ohio State University for his encouragement and guidance in the pursuit of this research. In addition the author wishes to thank Dr. E. M. Baroody of Battelle Memorial Institute for his helpful suggestions in the preparation of this manuscript. The author is also indebted to Battelle Memorial Institute for the Battelle Fellowship given to support his research. ii VITA May 20, 1934 . Born - Watertown, New York 1955 B.Sc., LeMoyne College, Syracuse, New York 1957 M.Sc., The University of Detroit, Detroit Michigan 1957-1969 Physicist, Battelle Memorial Institute, Columbus, Ohio PUBLICATIONS "Least Square Matrix Method for Analyzing Neutron Spectra", American Nuclear Society Transactions, Volume 7, p. 371 (1964). "Sputtering of Polycrystalline Gold by Fast Neutrons", Journal of Applied Physics, Volume 37, pp. 621-623 (1966). FIELDS OF STUDY Major Field: Physics Studies in Mathematics. Professor Zuber Studies in Classical Mechanics. Professors Wave H. Shaffer and Jan Korringa Studies in Quantum Mechanics. Professor Andrew Sessler Studies in Electromagnetism. Professor Albert L. Prebus Studies in Thermodynamics. Professor Clifford V. Heer Studies in Modern Physics. Professor Marion L. Pool iii TABLE OF CONTENTS Page ACKNOWLEDGMENT ...................................................... ii VITA ................................................................... iii LIST OF TABLES ........................................... vi LIST OF ILLUSTRATIONS ...................................... vii INTRODUCTION ........................................................ 1 I. THEORY OF HIGH ENERGY NEUTRON INTERACTIONS IN GOLD AND SPUTTERING MODELS ........................................ 6 Primary Knock-On Atom (PKA) Energy Spectrum .... 6 Displacement Cascade Model in Gold ................... 10 Sputtering Models .................................... 17 II. FAST NEUTRON SPUTTERING FROM A POLYCRYSTALLINE GOLD TARGET AND A MONOCRYSTALLINE GOLD TARGET WITH THE <100> DIRECTION NORMAL TO THE TARGET SURFACE ........... 22 Introduction ........................................... 22 Experimental Procedure ............................... 22 Experimental Results .................................. 35 Discussion of Results ............................... 41 Discussion of Experimental Error ..................... 52 III. FAST NEUTRON SPUTTERING FROM A MONOCRYSTALLINE GOLD TARGET WITH THE <111> DIRECTION NORMAL TO THE TARGET SURFACE ............................................. 56 Introduction ........................................... 56 Experimental Procedure ............................... 57 iv Page Experimental Results .................. 64 Discussion of Results ................................ 68 Discussion of Experimental Error .................... 72 IV. C O N C L U S I O N S ...................................................... 74 APPENDIX A DISCUSSION OF SPUTTERING MODELS DEVELOPED BY THOMPSON . 77 B LEAST SQUARE MATRIX METHOD FOR ANALYZING NEUTRON SPECTRA .........................................................85 BIBLIOGRAPHY ........................................................ 89 v LIST OF TABLES Table Page 1 Results of Neutron Energy Spectrum Measurements Above 0.4 M e V ...................................................... 37 2 Results of Thermal Neutron Flux and Gold Activity Measurements .............................................. 39 3 Parameters Used in the Calculation of the Sputtering R a t i o s ...................................................... 40 4 Estimated Errors in Factors Determining the Poly­ crystalline Sputtering Ratio ............................. 53 5 Estimated Errors in Factors Determining the Mono­ crystalline Sputtering Ratio ............................. 54 6 Parameters Used in the Calculation of the Sputtering Ratios .......................................... 67 7 Specified Impurity Contents in Spectrosil ................ 71 8 Estimated Errors in Factors Determining the Sputtering Ratio .......................................... 73 9 Methods for Finding Fast Neutron Spectra From Activation Experiments...................................... 86 vi LIST OF ILLUSTRATIONS Figure Page 1 Diameter of Displacement Cascade Based on Von Jan Model and Vector Range of Gold Atoms in G o l d ............ 15 2 Vacuum Facility ........................................... 23 3 Sputtering Chamber for One of the Gold Targets ......... 27 4 Quartz Plate Holder for Thermal Neutron Activation E x p e r i m e n t .................................................. 32 5 Fast Neutron Energy Spectrum .............................. 36 6 Primary Knock-On Atom Spectrum in Gold Crystals .... 43 7 In-Pile Vacuum System ..... ................... ... 58 8 Sputtering Chamber for Gold Target ....................... 61 9 Integral Fast Neutron Energy Spectrum .............. 65 vii INTRODUCTION This dissertation describes experimental studies of atom ejec­ tion (sputtering) from the surface of polycrystalline and monocrystal­ line gold targets when these targets are bombarded by high energy neutrons that are produced in a nuclear reactor. These studies repre­ sent an extension of earlier work that was confined to fast neutron sputtering of polycrystalline gold.^ In these more recent experiments sputtering ratios (atoms of gold sputtered per fast-neutron-crossing the surface of the target) were measured for gold targets with the < 100> and < 111> crystallographic direction normal to the target sur­ face. Another measurement of the polycrystalline sputtering ratio was obtained by irradiating a polycrystalline gold target with the < 100> monocrystalline gold target. The present experiments were undertaken in order to study possible differences in the sputtering due to the crystalline state of the target. It was hoped in this manner to obtain additional informa­ tion on the mechanisms responsible for the number of sputtered atoms observed in the initial experiment. The sputtering ratio of -4 (1.0 + 0.3) x 10 observed in that experiment was much larger than would be expected on the basis of theory of sputtering by high energy ^D. W. Norcross, B. P. Fairand, and J. N. Anno, J. Appl. Phys. 37, 621 (1966). 1 ions (> 50 keV) developed by Goldman and Simon and modified for fast 3 neutron bombardment by Taimuty. Since no adequate theory to explain the magnitude of the observed sputtering ratio existed at the time these experiments were performed, an experimental study designed to provide additional information on the nature of the sputtering process was considered to be important. Ejection of atoms from a metal surface by high energy neutrons may provide a useful tool for studying the secondary collision pro­ cesses which occur in a collision cascade in the bulk of the material. This may be the case since the primary knock-on atoms (PKA) resulting from neutron interactions in the solid are formed as a volumetric source rather than as a distribution near the target surface. There­ fore, events occurring near the target surface which lead to atom sputtering may be similar in nature to those events occurring in the bulk."* ^D. T. Goldman and A. Simon, Phys. Rev. 14, 383 (1958). ^S. I. Taimuty, Nucl. Sci. Eng., JJ3, 403 (1961). 4 A collision cascade develops in the material when a neutron interaction in the solid creates a recoil atom (usually referred to as the PKA or primary knock-on atom) which in turn interacts with other atoms in the solid creating additional knock-on atoms. "*The sputtering ratio by itself can only provide rather limited information as to the details of the collision cascade. However, once a satisfactory theory of sputtering has been developed, data on sputtering ratios together with data on the spatial distri­ bution and energy spectra of ejected atoms can lead to a comprehensive understanding of secondary collisions. Sputtering of atoms from a material under fast neutron bombard­ ment is also of interest in thermonuclear research and may be applied to problems encountered in atom ejection from surfaces in the vicinity of high power fast reactor systems. In addition to the work reported in this dissertation and an earlier publication on fast neutron sputtering in polycrystalline gold the author is aware of only three other investigations of neutron 7 8 9 sputtering in gold. ’ * Two experiments involved the use of a 14 MeV beam of neutrons and a Pu-Be neutron source was employed in the other experiment. Within the context of the present understanding of fast neutron sputtering the difference in the sputtering ratios for gold when bom­ barded by a 14 MeV neutron beam and the neutrons from a reactor source is not unreasonable. However no'satisfactory explanation could be found to describe the difference in the sputtering ratio in gold for fast neutrons from the Pu-Be source and the other neutron sources.^ Possible theories
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