Modeling global cosmogenic nuclide production through 1st principles David C. Argento A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2013 Reading Committee: Dr. John O. Stone, Chair Dr. Erika Harnett Dr. Eric Steig Program Authorized to Offer Degree: Earth and Space Sciences ©Copyright 2013 David C. Argento University of Washington Abstract Modeling global cosmogenic nuclide production through 1st principles David C. Argento Chair of the Supervisory Committee: Dr. John O. Stone, Associate Professor Earth and Space Science The work contained in this thesis is focused on utilizing radiation transport code software as the basis for developing a well validated, first-principles model of global terrestrial cosmogenic nuclide production rates. The state-of-the-art radiation transport code, MCNPX, is utilized to model the terrestrial radiation field. Folding the radiation field neutron and proton results with cosmogenic nuclide production cross-sections yields production rates. This comprehensive, first- principles model is used to investigate characteristics of cosmogenic nuclide production. The goal of the work is to constrain uncertainties in cosmogenic nuclides by better understanding production systematics. Greater understanding of cosmogenic nuclide production rate systematics will assist in constraining uncertainties in cosmogenic nuclide production rate scaling, thereby reducing uncertainties in calculations based on sample nuclide concentrations iii exposure ages, erosion rates, and burial dating. Furthermore, novel uses of cosmogenic nuclides, currently unachievable due to uncertainties, will be enabled by further constraining these. The model is benchmarked against Dr. Paul Goldhagen’s ER-2 aircraft neutron monitor measurements, the Knyahinya meteorite in-situ cosmogenic nuclides, the Beacon Heights sandstone core measurements, and estimated sea level production rates. In this work, I examine: the production rates of each commonly used cosmogenic nuclide as a function of altitude and latitude; the angular distribution of nuclide-producing cosmic-ray particles as a function of altitude and latitude; subsurface production rate systematics; and the production of 36Cl in both the atmosphere and the oceans. iv Acknowledgments I would like to thank my advisor, Dr, John Stone, for his time, energy and guidance in developing my skills as a thorough and effective researcher. John played a critical role in my growth and provided essential perspective throughout this process. I would like to thank the members of my committee: Drs. Erika Harnett, Eric Steig, Bruce Nelson, and Greg Hakim for their guidance and insight. I would also like to thank Dr. Robert Winglee, the Chair of the Earth and Space Sciences Department for his support. I owe a debt of gratitude to Drs. Paul Goldhagen, Gregg McKinney and Laurie Waters whose MCNPX modeling expertise was essential to the success of this project. This work benefited greatly from collaborations with Dr. Robert C. Reedy, Keran O’Brien and Dr. L. Keith Fifield. I would also like to thank the members of CRONUS-Earth who were supportive of my efforts and leant advice and knowledge regarding cosmogenic nuclide formation and applications. My gratitude goes to my colleagues in the Earth and Space Sciences Department, my friends and my family who have supported me through this journey. Above all, I am grateful to my wife, Valerie, for her support, mentoring and love. Finally, I would like to dedicate this dissertation to my parents, Joseph Argento and Susan Millington, particularly my father who passed away before seeing me finish this journey. For their unwavering faith in me, their generosity, their never-ending support, and especially their unconditional love, I am deeply grateful. This work was supported by the CRONUS-Earth Graduate Fellowship (NSF EAR-0345949). v Table of Contents Acknowledgments ................................................................................................... v Table of Contents .................................................................................................. vi Chapter 1- A model of global cosmogenic nuclide production .......................... 1 THESIS STATEMENT ............................................................................................................ 2 BACKGROUND ...................................................................................................................... 2 GOALS & OBJECTIVES ........................................................................................................ 5 CONTRIBUTIONS .................................................................................................................. 6 OUTLINE OF THESIS............................................................................................................. 9 REFERENCES ....................................................................................................................... 11 Chapter 2 - Modeling the Earth’s cosmic radiation ..........................................14 ABSTRACT ............................................................................................................................ 15 INTRODUCTION .................................................................................................................. 16 METHODS ............................................................................................................................. 18 RESULTS ............................................................................................................................... 20 Deep atmosphere ............................................................................................................... 20 Simple planet .................................................................................................................... 24 DISCUSSION ......................................................................................................................... 26 CONCLUSION ....................................................................................................................... 28 REFERENCES ....................................................................................................................... 30 Chapter 3 - A physics-based system for determining cosmic-ray produced nuclide production rates .............................................................................33 ABSTRACT ............................................................................................................................ 34 INTRODUCTION .................................................................................................................. 35 METHODS ............................................................................................................................. 37 GCR models ...................................................................................................................... 37 Radiation transport ............................................................................................................ 40 Application of angular rigidity cut-offs ............................................................................ 42 Normalizations .................................................................................................................. 44 Model geometry ................................................................................................................ 45 Importance values ............................................................................................................. 47 MCNPX Physics options .................................................................................................. 48 MCNPX particle flux output tallies .................................................................................. 48 Atmospheric composition ................................................................................................. 49 Production rates: cross section folding ............................................................................. 49 RESULTS ............................................................................................................................... 50 vi Validations ........................................................................................................................ 50 Terrestrial cosmogenic production results ........................................................................ 57 DISCUSSION ......................................................................................................................... 60 Validations ........................................................................................................................ 60 CONCLUSION ....................................................................................................................... 65 APPENDIX A – MCNPX settings ......................................................................................... 67 REFERENCES ....................................................................................................................... 71 Chapter 4 - Key aspects of in-situ cosmogenic nuclide production: Insights from a physics based model ........................................................................74 ABSTRACT ...........................................................................................................................