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Atomic Number and Gamma-Ray Measurements from Neutron

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Atomic Number and Gamma-Ray Measurements from Neutron Atomic Number and Gamma-ray Measurements from Neutron-induced Fission at the ILL and n ToF A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Science and Engineering CERN-THESIS-2020-108 17/06/2020 2020 Nikolay V. Sosnin Department of Physics and Astronomy in the School of Natural Sciences The University of Manchester Blank page Contents List of Figures 5 List of Tables 11 Abbreviations 13 Abstract 15 Declaration of Authorship 16 Copyright Statement 17 Acknowledgements 19 1 Introduction 21 2 Nuclear Fission 27 2.1 LiquidDropModel............................... 27 2.2 TheShellCorrectionMethod . 29 2.3 FissionCross-section . 31 2.4 FissionProducts ................................ 33 2.4.1 Fissionfragments. .. .. .. .. .. .. .. .. 34 2.4.2 Neutrons ................................ 36 2.4.3 Gammarays .............................. 37 3 STEFF 41 3.1 STEFF ..................................... 41 3.1.1 2-Energy 2-velocity Measurements . 43 3.1.2 START ................................. 43 3.1.3 Scintillators............................... 45 3.1.4 Gas-filled Detector Principles . 48 3.1.4.1 FillGases........................... 51 3.1.5 Multiwire Proportional Counters . 52 3.1.5.1 STEFFMWPCSTOPs . 54 3.1.5.2 STEFF Bragg Chambers . 56 4 FiFI Characterization at Lohengrin 59 4.1 Fission Fragment Stopping in Gas . 61 3 4 Contents 4.2 Lohengrin .................................... 64 4.3 FiFIatLohengrin ............................... 65 4.4 DataProcessing ................................ 68 4.4.1 Differentiator .............................. 70 4.4.2 Integrator................................ 73 4.4.3 ChebyshevFilter ............................ 74 4.4.4 Ballistic Deficit Correction . 76 4.4.5 Moving Window Deconvolution . 78 4.4.6 Frisch Grid Inefficiency Correction . 79 4.5 DataAnalysis.................................. 80 4.5.1 DefiningParameters . 81 4.5.2 FittingtheData ............................ 83 4.5.3 ComparisonwithSRIM-2013 . 87 4.5.3.1 Filter Correction . 91 4.6 Conclusion ................................... 94 5 235U Campaign and Data Processing at n ToF 95 5.1 Neutron Time-of-Flight Facility . ..... 95 5.1.1 n ToFDataProcessing . .101 5.2 2016CampaignDataProcessing . 104 5.2.1 PSARoutineResults. .109 5.2.2 WaveformRoutine . .111 5.2.3 FissionEventBuilder . .113 5.2.4 Database ................................117 5.3 ConclusionandFutureImprovements . 119 6 235U Gamma-ray Measurement Results with STEFF 121 6.1 Prior STEFF Gamma-ray Measurements . 121 6.2 Gamma-ray Data Time Cuts and Calibration . 122 6.3 Scintillator Gain Effects . 129 6.4 Quantifying the NaI Gain Variations . 133 6.5 NaIGainCorrectionResults . 144 6.6 PSA Fit Quality Investigation . 148 6.7 LaBr3 Gain Investigation . 160 6.7.1 Countratecorrection . .160 6.7.2 Background peak correction . 162 6.8 Conclusion ...................................170 7 Conclusion 173 7.1 Futurework...................................174 7.1.1 STEFF .................................174 7.1.2 FiFI ...................................175 Bibliography 177 List of Figures 5 Word count: 41,662 Blank page List of Figures 2.1 Fissionbarrier ................................. 30 2.2 Potentialenergysurface . 31 2.3 235U(n,f)cross-sectionfromJEFF3.3 . 33 2.4 Fission fragment mass distributions . ...... 34 2.5 Brosamodes .................................. 35 2.6 Promptfissionneutronenergyspectrum . 36 2.7 Prompt fission neutron multiplicity correlated with FF mass........ 37 2.8 Fissionproduct................................. 38 2.9 Promptfissiongamma-rayspectrum . 39 3.1 STEFFdiagram ................................ 42 3.2 STARTdiagram ................................ 45 3.3 HPGe, LaBr3 andNaIresponsecomparison . 47 137 3.4 LaBr3 and NaI response to Cs....................... 48 3.5 Regions of gas detector operation . 50 3.6 Electrion-ion pairs produced in fill gases . ....... 52 3.7 Electric potential in MWPCs . 53 3.8 Avalanchearoundananodewire . 54 3.9 MWPCSTOPdiagram ............................ 55 3.10 MWPCwiregridstackdiagram. 56 3.11 FiFIBraggchamber .............................. 56 3.12 Bragg arm gas-filled detector . 57 3.13 Hipps arm gas-filled detector . 58 4.1 Charge yields of 236U in inverse kinematics . 60 4.2 Energy loss by accelerated and unaccelerated 90Krnucleus. 61 4.3 Regions of stopping power effects as a function of ion energy ....... 63 4.4 Lohengrinlayout ................................ 65 4.5 A typical spectrum of FF energies at the Lohengrin focal plane . 65 4.6 AschematicdiagramofFiFI . 66 4.7 FiFImountedatLohengrin . 67 4.8 Foil effect on energy resolution in FiFI . 68 4.9 Anodesignalparameters. 69 4.10 Sequence of filters used for anode signal processing . .......... 70 4.11 An example of a differentiator trace . 71 4.12 Effects of frequency on differentiated trace . ...... 72 4.13 Differentiated amplitude as a function of frequency . ......... 72 4.14 Differentiated amplitude as a function of frequency for various fragments . 73 7 8 List of Figures 4.15 Integratedanodepulse. 74 4.16 Chebyshevfilterfrequencyresponse . ..... 75 4.17 AnodepulseafterChebyshevfilter . 76 4.18 Integrated pulse with and without Chebyshev filter . ......... 76 4.19 Ballistic deficit-corrected pulse . ....... 77 4.20 Moving Window Deconvolution pulse . 78 4.21 The effect of a grid inefficiency correction . ..... 80 4.22 Averaged traces from energy gates . 81 4.23 Gaindriftduringthecampaign . 82 4.24 ǫ′ and τ plotsagainstvelocity . 85 4.25 ǫ′ and τ plots from SRIM ........................... 88 4.26 Compraison of FF ranges between this work, prior work and SRIM . 90 4.27 Filter effects on pulse risetime . 92 4.28 Corrected and re-fitted τ plotcomparedwithSRIM . 93 4.29 Corrected comparison of current measured ranges with prior measurements 93 5.1 Spallationstages ................................ 96 5.2 Target-beam orientation . 97 5.3 n ToFlayout .................................. 98 5.4 EAR1andEAR2neutronflux. 99 5.5 EAR2 gamma-ray background energy time distribution . ....... 99 5.6 γ-flashinNaI..................................100 5.7 Post γ-flashsignalinNaI ...........................100 5.8 ThermalsignaltraceinNaI . 101 5.9 Raw data conversion to ROOT ........................103 5.10 Uraniumtargetalignment . 104 5.11Uraniumtarget .................................105 5.12 Inputsforwaveformroutines . 107 5.13 Milestones of STEFF data processing for the 2016 campaign. .109 5.14 Relative positions of waveform timing variables showing on a pictogram ofananodepulse................................111 5.15 The outline of Data Proc functionality. .114 5.16 Time offset between START and Master signals . 115 5.17 NumberofPSAeventsperfission. 118 5.18 Processing times and data volume stages . 120 6.1 Gamma-ray fold distributions of the two previous STEFF 235U campaigns 122 6.2 FF time-of-flight in the main arms of STEFF . 123 6.3 FFvelocityinthemainarmsofSTEFF . 124 6.4 Time offset of the scintillators relative to the time of fission . 126 6.5 NaI1energycalibration . 127 6.6 Time difference between consecutive Master triggers . ........128 6.7 NaI gold resonance saturation . 130 6.8 Countrateeffectongain............................131 6.9 PMT linearity for increasing light input . 132 6.10PMThysteresis.................................133 6.11 PSAfitqualityversussignaltime. 134 List of Figures 9 6.12 Scintillator count rates . 135 6.13 Matrix of γ-ray spectra for various neutron ToF bins . 137 6.14 Gamma-ray spectra for both scintillator types at different neutron energies138 6.15 Gamma-ray energy spectrum fit parameters . 139 6.16 Gamma-ray HWHM and RMS values for each crystal and bunch type . 141 6.17 Gamma-ray HWHM and RMS values for each crystal and parameter type142 6.18 Fits to the evolution of the HWHM and RMS parameters . 145 6.19 Comparison of the effects on fission gamma-ray spectra of the HWHM and RMS corrections for parasitic pulses . 146 6.20 Comparison of the effects on fission gamma-ray spectra of the HWHM and RMS corrections for dedicated pulses . 147 6.21 Comparison of the NaI γ-ray spectra measured at n ToF and ILL . 148 6.22 Comparison of the PSA fits for NaI and LaBr3 scintillator data . 149 6.23 Comparison of the PSA amplitude fits for NaI data . 151 6.24 Comparison of the PSA fit amplitude differences as a function of neutron ToF .......................................152 6.25 Comparison of the PSA fit amplitude differences as a function of ampli- tudecut-off ...................................153 6.26 Comparison of the PSA fit amplitude differences for different pileup settings154 6.27 Comparison of the PSA fit amplitude differences for different NaI crystals 154 6.28 Parasitic NaI spectra corrected for PSA fit offset . .......156 6.29 Dedicated NaI spectra corrected for PSA fit offset . 157 6.30 A comparison of the ILL measured spectrum to the n ToF spectra cor- rectedforfittingerrors. .158 6.31 A comparison of the NaI PFG fold distributions for this work and the previousSTEFFexperiments . .159 6.32 Polynomial fits to LaBr3 countrates .....................160 6.33 Effect of the LaBr3 count rate correction . 162 6.34 Parasitic background spectra in LaBr3 andNaI1 ..............163 6.35 Enhanced view of the peak in the parasitic LaBr3 spectrum . .163 6.36 A comparison in peak shape and position for different proton pulses and neutronToF...................................164 6.37 An example of peak fitting in LaBr3 .....................165 6.38 A distribution of background peak positions in LaBr3 signals for different neutronToFcutsforbothproton pulsetypes . 165 6.39 A distribution
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