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A HIGH POWER LASER ABLATION ION SOURCE FOR PENNING TRAP STUDIES OF RARE ISOTOPES By Deborah Ann Davies A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 2006 ABSTRACT A HIGH POWER LASER ABLATION ION SOURCE FOR PENNING TRAP STUDIES OF RARE ISOTOPES By Deborah Ann Davies A high power laser ablation ion source has been developed characterized and implemented as part of the Low Energy Beam and Ion Trap LEBIT experiment at the National Superconducting Cyclotron Laboratory LEBIT was developed to make precision measurements of rare isotopes including mass measurements with a Penning trap The system relies on thermalizing nuclear reaction products in a helium filled cell and then extracting them from the gas through ion manipulation and differential pumping Atomic ions and clusters are needed to calibrate various aspects of the entire system such as transport efficiency and the main magnetic field in addition to understanding properties of the helium filled gas cell High power laser ablation has proven to be a successful method for producing a wide range of ions under various conditions including atmospheric pressure Laser ablation studies of C Al Ag Au Cu Fe and Zn were carried out in two separate chambers one under vacuum conditions and one under buffer gas conditions with the second harmonic 532 nm from a Q switched Nd YAG laser A series of calculations were done to model the ablation process and various ablation parameters of the system and the ion trajectories of the ablated ions were modeled with the computer program SIMION Many studies were carried out under vacuum using an ion drift system and mass analysis in a residual gas analyzer RGA The ablation target and laser optics were moved to the gas cell and several ablation studies were performed including measurements of the extraction time and the ion mobility for ions traveling through the gas cell for Andrew iii ACKNOWLEDGMENTS There are multiple people I would like to acknowledge for their contributions making this dissertation possible First thank you to my advisor Dave Morrissey for your guidance support and allowing me to work independently while always being available to help I am also grateful for your patience The NSCL has been a fantastic place to work particularly with such an excellent staff Thank you especially to Jack Ottarson and Renan Fontus for your design work help getting everything assembled and continued interest and assistance The faculty and staff of the NSCL are inspiring constantly pursuing excellence Thank you also to my second reader Paul Mantica particularly for your encouragement and listening ear during my start in grad school Also thanks to my committee members Gary Blanchard and Lynmarie Posey for sharing laser wisdom so I wouldn t have to reinvent the wheel I would also like to thank my undergraduate thesis advisor Niva Tro for the research experience the encouragement to attend graduate school and the advice along the way Thank you to my research group and fellow students Elaine and Chandana I ve appreciated your friendship in addition to your willingness to listen to multiple practice talks and for your help along the way Thanks to former group member Pat Lofy for all your help with getting started and answering my endless questions Georg Bollen and the LEBIT group deserve a lot of thanks as well Thanks especially to Stefan Ryan and Pete for patiently answering my questions and helping with calculations I would especially like to acknowledge my fellow graduate students for their friendship Thanks Michelle for the ballet and Orchesis memories and Heather for our conversations and fun outings Thanks also to Jeremy Sean Bryan and Jon Thanks to my friends both here in Michigan and far away and my family for iv your encouragement and support Thanks especially to my parents for your love and always being there through everything To my husband Andrew I would not have made it through without your love and endless patience and encouragement Finally thanks to God for giving me the strength to run this race v Contents 1 Introduction 1 1 Precision Mass Measurements 1 2 The NSCL 1 2 1 The Gas Stopping Station and LEBIT Overview 1 3 The Gas Cell Laser Ablation System Overview 1 3 1 Motivation 1 3 2 Laser Ablation Definition and Brief History 1 3 3 Laser Ablation Systems at Other Facilities 1 3 4 Introduction to the Laser Ablation System 1 4 Organization of Dissertation 2 Laser Ablation Theory and Calculations 2 1 Processes Involved in Laser Ablation 2 2 Evaporation Mechanism 2 2 1 Thermal Conduction Calculation Results 2 2 2 Threshold Fluence for Ablation 2 2 3 Surface Temperature Approximation 2 3 Photon Absorption Mechanism 2 3 1 Plasma Screening 2 4 Plasma Expansion 2 4 1 Plasma Properties 2 4 2 Velocity 2 4 3 Plume Length 2 4 4 Plume Splitting 2 5 Laser Ablation Theory and Calculations Conclusion 1 2 2 4 5 5 7 8 9 10 12 13 15 17 22 25 26 27 29 29 29 30 35 36 3 Ion Transport Calculations 3 1 SIMION Calculations 3 1 1 SIMION Calculations of Laser Ablation in the High Vacuum System 3 1 2 SIMION Calculations of Laser Ablation in the Gas Cell 3 2 Ion Mobility Overview 3 3 Ion Transport Simulations Conclusion vi 37 37 38 42 46 48 4 High Vacuum Laser Ablation System 4 1 Experimental Details 4 1 1 High Vacuum Ablation Chamber 4 1 2 Ring Electrodes 4 1 3 Laser Operation 4 1 4 Laser and Optical Configuration 4 1 5 Laser Ablation Target Assembly 4 2 RGA Operation and Modifications 4 3 Detection and Parametric Studies of the Ablation Targets 4 3 1 Mass Scan Studies 4 3 2 Isotopic Abundances 4 3 3 Data Acquisition and Analysis 4 3 4 Metal Targets in Vacuum 4 3 5 Ablation Stability and Reproducibility 4 3 6 SigradurTM Target in Vacuum 4 3 7 Target Comparison and Target Properties 4 4 Mass Loss Calculations and Results 4 5 Initial Voltage Test for Gas Studies 4 6 Data Summary 49 49 50 52 53 56 64 69 71 71 72 75 78 94 95 98 103 105 105 5 The Gas Cell Laser Ablation System 5 1 Experimental Details 5 1 1 The Gas Cell 5 1 2 Transport and LEBIT Instrumentation 5 2 Laser Ablation in the Gas Cell 5 2 1 Ablation Targets and Holders 5 3 The Laser Ablation of Metal Targets in the Gas Cell 5 3 1 The Laser Ablation of Iron in the Gas Cell 5 3 2 The Laser Ablation of Copper in the Gas Cell 5 4 The Laser Ablation of SigradurTM in the Gas Cell 5 4 1 Parametric Studies 5 4 2 Drift time Studies 5 4 3 Discussion and Significance


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