Yield, Kinetic Energy, Pairing Effect, and Shell Effect of Light Fission Products for Thermal-Neutron Fission of Uranium-233
Lee, Shengdar
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https://hdl.handle.net/2142/70887
Description
Title
Yield, Kinetic Energy, Pairing Effect, and Shell Effect of Light Fission Products for Thermal-Neutron Fission of Uranium-233
Author(s)
Lee, Shengdar
Issue Date
1983
Department of Study
Nuclear Engineering
Discipline
Nuclear Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Nuclear
Abstract
We used the mass spectrometer HIAWATHA in conjunction with a thin-windowed gridded ionization chamber to determine the yields of light fission products for thermal-neutron fission of U-233. HIAWATHA has an energy resolution of 0.3%, a mass resolution of 0.5 amu, and an atomic-number resolving power of 38. The yields were measured as a function of mass number, atomic number, and kinetic energy. Detailed comparison shows that the present results are in good agreement with the evaluated yield data (ENDF/B-V). Moreover, when incorporating the present results, the accuracies of the ENDF yields will improve significantly. The accurate results also allow a detailed study in fission physics.
The nuclide yield distributions are modulated strongly by proton pairing, but weakly by neutron pairing. The magnitudes of proton and neutron pairing enhancements were determined to be 23.8 (+OR-) 0.3% and 3.7 (+OR-) 0.3%, respectively. The proton odd-even effect increases as kinetic energy increases. The neutron odd-even effect, however, varies more complicatedly with energy. It is least prominent around 102 MeV, and increases as kinetic energy either increases or decreases. The complication is probably caused by prompt neutron emission, which increases as fragment kinetic energy decreases. If so, we conclude that the neutron emission enhances the neutron odd-even effect.
An analysis of proton odd-even effect as a function of proton number was performed by using the method of Tracy. This analysis shows that the proton odd-even effect is suppressed around atomic numbers 38 and 42. These locations correspond to 38 and 50 proton shells at scission. A similar analysis for neutron odd-even effect also shows that it is suppressed by neutron shell effects at scission.
The neutron displacement, which describes the deviation of mean neutron number for isotopic fission products from the unchanged charge distribution, increases as atomic number increases. Besides, the displacements for even-Z fragments are slightly smaller than their odd-Z neighbors. As fragment kinetic energy increases, the magnitude of neutron displacement decreases, but the prominence of odd-even fine structure increases.
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