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SUB-BARRIER PHOTOFISSION MEASUREMENTS in 238U and 232Th

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SUB-BARRIER PHOTOFISSION MEASUREMENTS in 238U and 232Th SUB-BARRIER PHOTOFISSION MEASUREMENTS IN 238U AND 232Th Jack Andrew Silano A dissertation submitted to the faculty of the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics and Astronomy. Chapel Hill 2016 Approved by: Hugon J. Karwowski Nalin R. Parikh Joaquin E. Drut Reyco Henning Mohammad W. Ahmed Thomas B. Clegg ○c 2016 Jack Andrew Silano ALL RIGHTS RESERVED ii ABSTRACT JACK ANDREW SILANO: SUB-BARRIER PHOTOFISSION MEASUREMENTS IN 238U AND 232Th. (Under the direction of Hugon J. Karwowski.) A study of photofission of 232Th and 238U was performed using quasi-monoenergetic, linearly-polarized γ-ray beams from the High Intensity γ-ray source at TUNL. The prompt photofission neutron polarization asymmetries, neutron multiplicities and the photofission cross sections were measured in the sub-barrier energy range of 4.3-6.3 MeV. This data set constitutes the lowest energy measurements of those observables to date. Large polarization asymmetries are observed in both nuclei, consistent with the only other measurement of this kind made at a higher energy range. Over the newly probed energy range the asymmetries, multiplicities and cross sections all vary smoothly, revealing no new resonant fission pro- cesses. The present cross sections are consistent with a triple-humped 232Th fission barrier and a double-humped 238U fission barrier. Previous experimental evidence of a third min- imum in the 238U fission barrier has been identified as an accelerator-induced background. Applications of the present data will also be discussed. iii ACKNOWLEDGMENTS Though this thesis is my own work, I could not have completed it without the help and support of my friends, family and colleagues. My experiences with them have shaped every part of my dissertation, and it wouldn't have been the same without them. First and foremost, I would like to thank Hugon Karwowski for his guidance and support throughout my graduate career. I owe nearly everything I have learned through my research experience to his mentorship, and for that I will always be grateful. In my time at the University of North Carolina at Chapel Hill, I have had the opportunity to take classes and engage with many faculty members of the Physics & Astronomy Depart- ment. I would especially like to thank Tom Clegg and Reyco Henning for their thoughtful and thorough comments on my dissertation. A special thanks to Nalin Parikh and Adriana Banu for assisting during the experiment. This work would not have been possible without their help. Thank you to Jonathan Mueller, Kevin Wierman, Mohammad Ahmed, Brent Fallin, Krishichayan, and Erin Hansen for their helpful discussions. Thanks to Grayson Rich for laying the groundwork for this project and getting the HIγS beam time approval. I would like to thank Stepan Mikhailov and the HIγS staff for providing the γ-ray beams that made this work possible, and for their willingness to put up with my endless questions about the facility. Thank you to Bret Carlin and the TUNL staff for sharing their expertise, and for their patience with my many questions and requests over the years. Finally I would like to thank my family and friends for all of the love and friendship they have shown me throughout my life. This work was supported in part by the United States Department of Energy (DOE) under Grant DE-FG02-03ER41-041. iv TABLE OF CONTENTS LIST OF FIGURES ................................... xiii LIST OF TABLES .................................... xiv LIST OF ABBREVIATIONS AND SYMBOLS ................. xv 1 INTRODUCTION .................................. 1 1.1 Nuclear Fission . 1 1.2 Overview of Contents . 3 2 FISSION THEORY ................................. 5 2.1 General Aspects of Fission . 5 2.2 Liquid Drop Model . 6 2.3 Strutinsky Hybrid Model . 8 2.4 Modern Fission Models . 11 3 PHOTOFISSION THEORY ............................ 15 3.1 Photoabsorption . 15 3.2 Fission Barrier Interpretation . 16 3.3 Fission Fragment Angular Distributions . 21 3.4 Fission Neutrons . 25 4 PREVIOUS PHOTOFISSION MEASUREMENTS ............. 31 4.1 Introduction . 31 4.2 Photofission Cross Sections . 32 4.2.1 Photofission of 232Th........................... 32 v 4.2.2 Photofission of 238U............................ 35 4.3 Photofission Neutron Polarization Asymmetries . 40 4.4 Photofission Neutron Multiplicities . 43 5 THE EXPERIMENT ................................ 45 5.1 Introduction . 45 5.2 Experiment . 45 5.3 HIγS γ-ray Beams . 46 5.4 γ-ray Beam Diagnostics . 48 5.5 Neutron Detector . 53 5.6 Data Acquisition . 54 5.7 Targets . 56 6 DATA REDUCTION & ANALYSIS I ..................... 59 6.1 Introduction . 59 6.2 HPGe Detector . 59 6.3 Digital Waveforms from the INVS . 60 6.3.1 Pulse Shape Discrimination . 62 6.4 GEANT4 Model of the INVS . 65 6.4.1 Introduction . 65 6.4.2 GEANT4 . 65 6.4.3 Detector Efficiency Calibration . 67 6.4.4 Neutron Asymmetry Calibration . 76 6.4.5 Model Validation . 77 6.5 γ-ray Beam Flux Monitor Calibration . 80 7 MODELING PHOTOFISSION NEUTRONS ................. 84 7.1 Introduction . 84 7.2 FREYA . 84 vi 7.2.1 Pre-fission Neutrons . 85 7.2.2 Fragment Masses and Charges . 85 7.2.3 Fragment Kinetic Energies and Excitations . 87 7.2.4 Neutron Emission . 90 7.3 Photofission Neutron Spectra . 91 7.4 Neutron Angular Correlations . 93 8 DATA REDUCTION & ANALYSIS II ..................... 98 8.1 Introduction . 98 8.2 Moderated Neutron Time Distribution . 98 8.3 Background Analysis . 101 8.3.1 Cosmic-ray Induced Neutrons . 102 8.3.2 Neutrons Produced by the D(γ,n) Reaction . 102 8.3.3 Bremsstrahlung Contamination of the HIγS Beam . 104 8.3.4 Measuring the Compton and Bremsstrahlung Backgrounds . 107 8.3.5 238U(γ,n) Reaction Background Neutrons . 113 8.4 Prompt Photofission Neutron Asymmetry Analysis . 115 8.4.1 Neutron Asymmetries in the INVS Detector . 115 8.4.2 Simulated Asymmetries in the INVS Detector . 116 8.5 Photofission Neutron Multiplicity Analysis . 118 8.5.1 Rossi-alpha Method . 120 8.5.2 Monte Carlo Fitting of Photofission Neutron Multiplicities . 123 8.6 Photofission Cross Section Analysis . 127 9 RESULTS & DISCUSSION ............................ 130 9.1 Introduction . 130 9.2 Prompt Photofission Neutron Polarization Asymmetries . 130 9.3 Prompt Photofission Neutron Multiplicities . 135 vii 9.4 Photofission Cross Sections . 137 9.4.1 Photofission of 232Th . 137 9.4.2 Photofission of 238U............................ 141 9.5 Concluding Remarks . 146 Appendix A DEUTERIUM PHOTODISINTEGRATION KINEMATICS . 149 Appendix B TABULATED RESULTS ....................... 151 B.1 Prompt Photofission Neutron Polarization Asymmetry Data . 151 B.2 Prompt Photofission Neutron Multiplicity Data . 153 B.3 Photofission and Photoneutron Cross Section Data . 154 BIBLIOGRAPHY .................................... 164 viii LIST OF FIGURES 2.1 238U(n,f) reaction fragment mass distribution . 6 2.2 Liquid drop model fission barrier . 8 2.3 Strutinsky hybrid model . 11 2.4 Macroscopic-microscopic calculation of the 232Th fission barrier . 12 2.5 Self-consistent calculation of the 232Th fission barrier . 13 2.6 Effects of neutron number on the fission barrier . 14 3.1 Energy level diagram for an even-even nucleus . 17 3.2 Isomeric and transition excitations within the fission barrier . 18 3.3 Barrier penetration probability . 19 3.4 Photofission resonance schematic . 20 3.5 Bohr formalism . 21 3.6 238U(γ,f) reaction neutron Watt spectrum . 27 3.7 Fission neutron boosted by fragment . 28 3.8 Fission neutron multiplicity distribution . 29 3.9 Fission neutron multiplicity vs. fragment mass . 30 4.1 232Th(γ,f) reaction cross section over the GDR . 32 4.2 232Th(γ,f) reaction cross section and fission barrier fit . 34 4.3 Inferred fission barrier structure for 232Th . 35 4.4 238U(γ,f) reaction cross section over the GDR . 36 4.5 238U(γ,f) reaction cross section and fission barrier fits . 38 4.6 Inferred fission barrier structure for 238U...................... 39 4.7 Neutron detector arrangement for previous work . 40 4.8 232Th and 238U photofission neutron polarization asymmetries . 41 4.9 Photofission neutron multiplicities for 232Th and 238U . 43 4.10 Photofission neutron spreads for 232Th and 238U.................. 44 ix 5.1 Experimental geometry . 46 5.2 Schematic of the HIγS facility. 46 5.3 Paddle flux monitor . 48 5.4 Image of HPGe detector . 49 5.5 HPGe spectrum of the γ-ray beam . 50 5.6 HIγS beam imager . 51 5.7 Collimator alignment . 52 5.8 γ-ray beam alignment . 52 5.9 INVS detector . ..
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