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Polarized Photofission Fragment Angular Distributions of 232Th And Polarized Photofission Fragment Angular Distributions of 232Th and 238U Jeromy Ryan Tompkins 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 2012 Approved by: H. J. Karwowski M. W. Ahmed T. B. Clegg J. Engel F. Heitsch Oc 2012 Jeromy Ryan Tompkins ALL RIGHTS RESERVED ii ABSTRACT JEROMY RYAN TOMPKINS: Polarized Photofission Fragment Angular Distributions of 232Th and 238U. (Under the direction of H. J. Karwowski.) A study of photofission fragment angular distributions on 232Th and 238U has been completed for the first time using linearly-polarized, quasi-monoenergetic photon beams of near-barrier energies. Large polarization asymmetries are observed in both nuclei. For 232Th, fragments are emitted in the direction of beam polarization with more than fifteen times greater preference than perpendicular to it when the beam energy is 6.2 MeV. These asymmetries are roughly a factor of two times greater than are character- istic of 238U. The large observed fragment asymmetries are responsible for asymmetries in the accompanying prompt-fission neutron angular distributions that have recently been measured [1]. A smooth decrease in the polarization asymmetry has also been observed as beam energies increase from 5.9 MeV to 7.6 MeV. The smooth variations reveal no resonant fission processes in the same energy range. iii Contents List of Tables..................................viii List of Figures................................. ix List of Abbreviations............................. xii 1 Introduction................................. 1 1.1 The Importance of Fission.........................1 1.2 The Discovery of Fission..........................2 1.3 Overview of Contents............................3 2 Fission Theory............................... 5 2.1 General Fission Characteristics......................5 2.2 Photofission.................................7 2.2.1 Key Characteristics of Photoabsorption..............7 2.3 Theoretical Overview............................9 2.3.1 Macroscopic-Microscopic Calculations............... 10 2.3.2 Statistical Fission Calculations.................. 13 2.3.3 Fission Dynamics.......................... 15 iv 3 Fission Fragment Angular Distributions................ 17 3.1 The Bohr Channel Formalism....................... 18 3.2 The Kadmensky Fragment Angular Distribution............. 26 3.3 Comparison of Approaches......................... 31 3.4 Effect of a Multiple-Humped Fission Barrier............... 32 4 Previous Measurements of Photofission Fragment Angular Distribu- tions...................................... 33 4.1 Implications of Photon Beam Characteristics on Results........ 34 4.2 Dipole Photofission............................. 37 4.3 Correlation of Angular Anisotropy and Mass-Asymmetry........ 41 4.4 Quadrupole Photofission.......................... 41 4.5 Barrier Systematics............................. 43 4.6 Prompt-fission neutrons.......................... 48 5 The Experiment............................... 50 5.1 Description of the Experiment....................... 50 5.2 Experimental setup............................. 51 5.3 Targets.................................... 54 5.4 Silicon Strip Detectors........................... 56 5.5 Signal Processing and Readout...................... 57 5.6 Photon Beam Measurements........................ 62 5.7 Geant4.................................... 63 v 6 Analysis I.................................. 68 6.1 Overview of the Analysis.......................... 68 6.2 The Data.................................. 69 6.3 Data Reduction............................... 70 6.4 Live Time Fraction Correction....................... 75 6.5 Results.................................... 76 7 Analysis II.................................. 80 7.1 Overview................................... 80 7.2 Fitting procedure.............................. 81 7.3 Monte-Carlo Calculations......................... 83 7.3.1 θ and φ Determinations...................... 83 7.4 Simulated Geometry Optimization..................... 84 7.4.1 Relative Target-to-Detector Position............... 86 7.4.2 Λ probability distributions..................... 88 7.4.3 Charged Particle Transport Validation.............. 90 7.4.4 Robustness of Results....................... 91 8 Fit Results and Discussion........................ 94 8.1 Fit Results for 232Th............................ 94 8.2 Dipole Photofission of 232Th........................ 95 8.3 Quadrupole Photofission of 232Th..................... 99 8.4 Energy Dependence of Angular Distribution Parameters........ 100 vi 8.5 Comparison of 232Th and 238U....................... 101 8.6 Polarity................................... 102 8.7 Implications to Prompt-Fission Neutron Polarization Asymmetries... 104 8.8 Considerations for Future Experimental Investigation.......... 106 9 Conclusions.................................108 A Electromagnetic Coupling to Matter..................110 B Relevant Equations.............................115 B.1 Wigner d functions............................. 115 C Tabulation of Strip Angles........................116 D Geant4 Code.................................118 D.0.1 Stored Straggling Data....................... 118 D.0.2 The Si Strip Detector Model.................... 119 Bibliography...................................151 vii List of Tables 3.1 Fission channel angular distributions................... 24 4.1 Table of previous measurements...................... 35 5.1 Characteristics of photon beams...................... 52 5.2 Target characteristics............................ 56 6.1 Four-point calibration alpha decays.................... 74 7.1 Geometrical adjustments in robustness investigation........... 93 8.1 Tabulated results.............................. 95 C.1 Angular positions - 232Th.......................... 116 viii List of Figures 2.1 Mass yield curve for 238U..........................6 2.2 Potential energy surface for 232Th..................... 12 2.3 Triple humped barrier........................... 13 2.4 Schematic multihumped barrier...................... 14 3.1 Euler angles................................. 20 3.2 Definition of J, K, and M ......................... 21 3.3 Schematic collective excitation level scheme............... 23 4.1 b=a of 238U.................................. 38 4.2 b=a of 232Th - bremsstrahlung sources................... 39 4.3 Comparison of b=a between 238U and 232Th................ 40 4.4 c=b of 238U - bremsstrahlung sources.................... 43 4.5 c=b of 232Th - bremsstrahlung sources................... 44 4.6 Diagram of multihumped barrier with different channels......... 46 4.7 Prompt fission neutron asymmetries.................... 49 5.1 Experiment layout............................. 53 5.2 Scattering chamber layout......................... 54 5.3 Silicon strip detector............................ 57 ix 5.4 Electronics block diagram......................... 59 5.5 Block diagram of the full circuit...................... 60 5.6 Beam energy measurement......................... 66 5.7 Geant4 detector geometry........................ 67 6.1 Analysis diagram.............................. 69 6.2 ADC spectrum............................... 72 6.3 232Th alpha partice spectrum....................... 73 6.4 232Th fragment yield ratios......................... 78 6.5 232Th fragment yield ratios......................... 79 7.1 Optimization adjustments in Geant4 model............... 85 7.2 Optimized relative geometry comparison................. 86 7.3 Marginal η function............................. 88 7.4 h24 probability function........................... 90 7.5 Comparison of alpha particle range.................... 92 7.6 Comparison of alpha particle angular straggling............. 93 8.1 Polarization asymmetry of 232Th...................... 96 8.2 θ asymmetry of 232Th............................ 97 8.3 Comparison of 232Th b=a to previous data................ 98 8.4 Theta-dependent asymmetry comparison - 232Th............. 99 232 8.5 c40 trend of Th.............................. 100 x 8.6 Comparison of polarization asymmetry between 238U and 232Th.... 103 8.7 Comparison with prompt-fission neutron asymmetries.......... 105 xi List of Abbreviations ADC Analog-to-digital converter BPM Beam pickoff monitor CEBAF Continuous Electron Beam Accelerator Facility CODA CEBAF Online Data Acquisition system CFD Constant-fraction discriminator DAQ Data acquisition system FAD Fragment angular distribution FEL Free electron laser HPGe High-purity germanium HIγS High Intensity γ-ray Source GDR Giant dipole resonance GS Ground state JLAB Thomas Jefferson National Accelerator Facility LD Liquid drop LDM Liquid drop model LED Leading-edge discriminator LTF Live time fraction NAD Neutron angular distribution PFAD Photofission fragment angular distribution xii SSD Silicon strip detector TDC Time-to-digital converter TKE Total kinetic energy TOF Time of flight TUNL Triangle Universities Nuclear Laboratory UTR Upstream target room xiii Chapter 1 Introduction 1.1 The Importance of Fission Few people in the world have never heard the word “fission", and for good reason. Fission has shaped the twentieth and twenty-first centuries in ways that no other nu- clear reaction has. It has influenced the world both politically and economically. It has ended wars and nearly begun others. In light of this, one might believe that fission is well understood. To say
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