FY04&05 LDRD Final Report Fission Fragment Sputtering

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Fission fragments born within the first 7 {micro}m of the surface of U metal can eject a thousand or more atoms per fission event. Existing data in the literature show that the sputtering yield ranges from 10 to 10,000 atoms per fission event near the surface, but nothing definitive is known about the energy of the sputtered clusters. Experimental packages were constructed allowing the neutron irradiation of natural uranium foils to investigate the amount of material removed per fission event and the kinetic energy distribution of the sputtered atoms. Samples were irradiated but were never analyzed after irradiation. Similar experiments ... continued below

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PDF-file: 29 pages; size: 7.6 Mbytes

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Ebbinghaus, B; Trelenberg, T; Meier, T; Felter, T; Sturgeon, J; Kuboda, A et al. February 22, 2006.

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Fission fragments born within the first 7 {micro}m of the surface of U metal can eject a thousand or more atoms per fission event. Existing data in the literature show that the sputtering yield ranges from 10 to 10,000 atoms per fission event near the surface, but nothing definitive is known about the energy of the sputtered clusters. Experimental packages were constructed allowing the neutron irradiation of natural uranium foils to investigate the amount of material removed per fission event and the kinetic energy distribution of the sputtered atoms. Samples were irradiated but were never analyzed after irradiation. Similar experiments were attempted in a non-radioactive environment using accelerator driven ions in place of fission induced fragments. These experiments showed that tracks produced parallel to the surface (and not perpendicular to the surface) are the primary source of the resulting particulate ejecta. Modeling studies were conducted in parallel with the experimental work. Because the reactor irradiation experiments were not analyzed, data on the energy of the resulting particulate ejecta was not obtained. However, some data was found in the literature on self sputtering of {sup 252}Cf that was used to estimate the velocity and hence the energy of the ejected particulates. Modeling of the data in the literature showed that the energy of the ejecta was much lower than had been anticipated. A mechanism to understand the nature of the ejecta was pursued. Initially it was proposed that the fission fragment imparts its momenta on the electrons which then impart their momenta on the nuclei. Once the nuclei are in motion, the particulate ejecta would result. This initial model was wrong. The error was in the assumption that the secondary electrons impart their momenta directly on the nuclei. Modeling and theoretical considerations showed that the secondary electrons scatter many times before imparting all their momenta. As a result, their energy transfer is more isotropic than directional. It was therefore concluded that the nuclei are set in motion not by direct collisions with the secondary electrons, but by repulsive forces cased by the temporary net positive local charge of the nuclei. This is cased by ejected electrons and by the reduced bonding nature of the nuclei cased by many of the local electrons being in excited and in nonbonding states.

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PDF-file: 29 pages; size: 7.6 Mbytes

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  • Report No.: UCRL-TR-219743
  • Grant Number: W-7405-ENG-48
  • DOI: 10.2172/928835 | External Link
  • Office of Scientific & Technical Information Report Number: 928835
  • Archival Resource Key: ark:/67531/metadc899197

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  • February 22, 2006

Added to The UNT Digital Library

  • Sept. 27, 2016, 1:39 a.m.

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  • Dec. 5, 2016, 4:34 p.m.

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Ebbinghaus, B; Trelenberg, T; Meier, T; Felter, T; Sturgeon, J; Kuboda, A et al. FY04&05 LDRD Final Report Fission Fragment Sputtering, report, February 22, 2006; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc899197/: accessed September 21, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.