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Measurement of Krypton Fission Product Yields from 14 Mev Neutrons on 238U

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Measurement of Krypton Fission Product Yields from 14 Mev Neutrons on 238U Measurement of Krypton Fission Product Yields from 14 MeV Neutrons on 238U by Ellen Edwards A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Engineering { Nuclear Engineering in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Edward Morse, Chair Professor Lee Bernstein Professor Yasunori Nomura Dr. Charles Yeamans Summer 2018 Measurement of Krypton Fission Product Yields from 14 MeV Neutrons on 238U Copyright 2018 by Ellen Edwards 1 Abstract Measurement of Krypton Fission Product Yields from 14 MeV Neutrons on 238U by Ellen Edwards Doctor of Philosophy in Engineering { Nuclear Engineering University of California, Berkeley Professor Edward Morse, Chair Precisely-known fission yield distributions are used to determine a fissioning isotope and the incident neutron energies in nuclear security applications. 14 MeV neutrons from DT fusion at the National Ignition Facility (NIF) induced fission in depleted uranium (DU) contained in the target assembly hohlraum. The fission yields of Kr isotopes (85m, 87, 88, 89, and 90) were measured relative to the cumulative yield of 88Kr. The fission gas was pumped from the target chamber, collected, and analyzed in the Radiochemical Analysis of Gaseous Samples (RAGS) diagnostic. Isotopes with half-lives ranging 8 s-9 hr can be measured. Kr fission yields have been measured both from the fission of DU in the hohlraum and DU doped into the capsule ablator. Since the mass of U was not known, the relative amounts of Kr isotopes were calculated and compared to existing fission product distribution tables. It was found that measurements can be performed with high precision for isotopes with half lives longer than 4 minutes. A more precise quantification of gas transport needs to be achieved to quantify isotopes with shorter half lives to a precision of the published tables. i To my family: Debbie, Ken, and Rachel Edwards. ii Contents Contents ii List of Figures iv List of Tables vi 1 Fission 1 1.1 Motivation . 1 1.2 Background . 2 1.3 Fission Theory . 5 2 Nuclear Forensics 10 2.1 Fission Product Yields . 10 2.2 Comprehensive Test Ban Treaty . 11 2.3 Debris Analysis . 13 2.4 Chronometry . 13 3 National Ignition Facility 17 3.1 NIF Basics . 17 3.2 Capsules . 18 3.3 Diagnostics . 20 3.4 Neutron Spectrum . 21 3.5 NIF for Fission Experiments . 23 4 Measurements of Gaseous Fission Products 24 4.1 Radiochemistry Experiments . 24 4.2 On-Line Isotope Separation . 24 4.3 This Experiment: NIF Radiochemical Analysis of Gaseous Samples . 25 5 Data Analysis 31 5.1 Data . 31 5.2 Energy and Efficiency Calibration . 37 5.3 Peak Fitting . 39 iii 5.4 Determination of Isotope Activity as Function of Time . 41 5.5 Fit Exponential Decay and Extrapolate to Shot Time . 44 5.6 Ratios . 45 5.7 Correction for Pumping Speed and Time . 47 6 Model Analysis and Fission Yield Determination 51 6.1 Distribution of Independent Yields . 51 6.2 Pumping Time Constant . 52 6.3 Delayed Neutrons . 53 6.4 85mSe........................................ 54 7 Results 56 7.1 Overall Results . 56 7.2 85mKr ....................................... 57 7.3 87Kr ........................................ 58 7.4 89Kr ........................................ 59 7.5 90Kr ........................................ 61 7.6 Uranium-Doped Capsules . 62 7.7 Comparison to SRCs . 63 7.8 Conclusions and Future Work . 64 Appendix A Gamma Ray Data 65 A.1 85mKr ....................................... 66 A.2 87Kr ........................................ 67 A.3 88Kr ........................................ 68 A.4 89Kr ........................................ 71 A.5 90Kr ........................................ 79 A.6 138Cs........................................ 82 A.7 138Xe........................................ 85 Bibliography 88 iv List of Figures 1.1 Independent and cumulative yields. 3 1.2 Fission Process. From [8]. 4 1.3 Potential energy barrier of fission as a function of deformation. 6 1.4 Diagram showing the splitting of nuclear energy levels with deformation. 7 1.5 Mass of the two fission products for different fissioning masses. 8 2.1 Fission product distributions change based on neutron energy and fissioning mass. 11 2.2 Kalinowski diagram showing ratios of Xe isotopes for different sources of Xe. 12 2.3 4n+2 nuclides . 14 2.4 Time since 238Pu separation using different isotopes in its decay series. From [12]. 15 2.5 Spontaneous fission products from 240Pu. From [12]. 16 3.1 NIF target. Figure from [24]. 18 3.2 Cross section view of the outer edge of a lined depleted uranium hohlraum. From [25]. 18 3.3 A typical CH capsule. From [25]. 19 3.4 X-ray radiographs of capsules with and without DU dopant. Photos courtesy of General Atomics. 20 3.5 NIF target chamber. From [32]. 21 3.6 Simulated neutron spectrum from NIF. 22 4.1 Schematic of RAGS. 25 4.2 RAGS diagnostic. 26 4.3 Coldhead with vacuum housing removed. The copper heat strap was originally aluminum. From [62]. 28 4.4 Comparison of activity in collector with and without a translating detector. 30 5.1 List mode data using 5 minute time bins. 32 5.2 A histogram created by taking a time slice from Figure 5.1. 33 5.3 Plotting the intensity of the photopeak at 402 keV vs time. This peak can also be observed in Figure 5.1. Error bars omitted for clarity. 34 5.4 List mode data using 1 second time bins. 35 v 5.5 A section of the Chart of the Nuclides [63]. Small squares indicate metastable states. Kr isotopes decay to Rb isotopes of the same mass. 36 5.6 A typical measured efficiency curve for HPGe detectors. 37 5.7 Fitting a photopeak from 87Kr. 39 5.8 Similar to Figure 5.7 but for 89Kr. The photopeak at 586 keV is not fully resolved so instead of a single Gaussian fit, a double Gaussian is fit to the background- subtracted data. 40 5.9 Relative activity of 87Kr vs time. This is the area calculated from the Gaus- sian peaks (Figure 5.7) at each time step, corrected for dead time and relative efficiency. It decays with the half life of 87Kr, 1.2 hr. 42 5.10 Converting activity from Figure 5.9 to atoms and extrapolated to shot time. The detector moves from 11.5" to 5" from the sample after 5 hrs. 45 5.11 Ratio of 87Kr / 88Kr atoms extrapolated to shot time. 46 5.12 Decay chain for mass 87 . 48 5.13 Illustration of the effect pumping speed and duration has on 85mKr collected in RAGS. Because the half life of 85Br is non-negligible compared to pumping time, not all 85mKr is transferred to RAGS from the target chamber. 50 6.2 Amount of 89Kr collected for 3 different pump speeds. The difference is most pronounced during pumping but the final amount differs by only 2.61% at the end of the collection period. 53 7.1 Cumulative yield ratios from RAGS experiments and evaluations. 57 7.2 Cumulative yield ratios (relative to 88Kr) measured by RAGS for individual shots. The weighted average is shown in black and JEFF value in red. The 1% error bar for the RAGS value is too small to see on this scale. 58 7.3 Cumulative yield ratios (relative to 88Kr) measured by RAGS for individual shots. The weighted average is shown in black and JEFF value in red. 59 7.4 Cumulative yield ratios (relative to 88Kr) measured by RAGS for individual shots. The weighted average is shown in black and JEFF value in red. 60 7.5 Example of a shot with a corroded Ge detector. Low-energy channels do not have data for approximately an hour, preventing the measurement of 89Kr and 90Kr. 61 7.6 Cumulative yield ratios (relative to 88Kr) measured by RAGS for individual shots. The weighted average is shown in black and JEFF value in red. 62 7.7 Measurement of surface roughness in a U-doped capsule. Courtesy General Atomics. 63 vi List of Tables 5.1 Energies used for energy and efficiency calibrations. 38 5.2 Coefficients at two different times for a Gaussian fit to the 577 keV and 586 keV photopeaks of 89Kr. 41 5.3 Nuclear data for the isotopes measured in this work. Additional nuclear data is listed in the appendix. Data from [66], [64], [65], [67], [69]. 41 5.4 Contributions to uncertainty for an individual data point in Figure 5.11. 46 5.5 Factors to account for pumping speed and duration on Kr collected in RAGS. 50 6.1 Errors of independent yields of Br isotopes [3] . 51 6.2 Change in amount of Kr collected by varying the pump time constant from 160- 200 s. A * indicates an average is taken from 780-900 s for 89Kr and 120-200 s for 90Kr......................................... 53 6.3 Probability of decaying to another element of the same mass. If the value is 100%, the isotope decays only by β− decay. If the value is less than 100% it partially decays by β− and neutron emission. Data from [7]. 54 6.4 Difference from including delayed neutrons as part of independent yields vs through- out pumping interval. The correction is less than 1% for all Kr isotopes studied. 54 6.5 Independent and cumulative yields from England and Rider [3] and JEFF 3.1 [1]. 55 7.1 NIF shots with DU doped into the capsule ablator. 63 vii Acknowledgments Thanks to: RAGS Engineers: Don Jedlovec, Allen Riddle, Jaben Root, and Tony Golod. For keeping RAGS running during data collection even if it meant going to work at 2 am. Bill Cassata and Carol Velsko: For all your help with familiarizing me with RAGS.
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