Investigation of the photoneutron cross section in 206Pb and 208 Pb from 9 to 16 MeV
Morford, Larry Joe
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https://hdl.handle.net/2142/25281
Description
Title
Investigation of the photoneutron cross section in 206Pb and 208 Pb from 9 to 16 MeV
Author(s)
Morford, Larry Joe
Issue Date
1985
Doctoral Committee Chair(s)
Debevec, Paul T.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
photoneutron
incident photon beam
neutron energies
Language
en
Abstract
The time of flight of photoneutrons from 206Pb and 208Pb was observed at scattering angles of 55 0 90 0 , and 125 o. Photons with energies between 9 and 16
MeV were used to excite the target nuclei. The incident photon beam was provided by the University of Illinois Tagged Photon Facility and had an effective energy resolution of around 400 keY. Neutron energies were determined by time of flight through a 1.5 meter path. The effective energy resolution of the neutrons was between 7.6% at 1 MeV and 12.5% at 5 MeV. Neutron detector efficiency was determined by measuring the photo-disintegration of deuterium. The resulting efficiency was found to be constant across the neutron energy range of interest. The fraction of neutrons with energy insufficient to pass the detector threshold was estimated to be 10.7%.
The total cross section (or both 206Pb and 208Pb are well described by
Lorentzian curves with widths of 4.12 ± 0.24 MeV for 206Pb and 4.09 ± 0.38 MeV
for 208Pb. The peak positions of the curves were determined to be 13.70 ± 0.06
MeV for 206Pb and 13.53 ± 0.05 MeV in 208Pb. No angular asymmetry about 90 in the differential cross sections was detected in either element above 11 MeV.
Below 11 MeV the asymmetry varied with energy between alternate positive and negative values for each element.
The time-of-flight spectra could be parameterized for low energy neutrons by a statistical decay with a nuclear temperature of O,g MeV. There was an excess of high energy neutrons above this distribution. A more detailed analysis of the spectra using the Hauser-Feshbach formalism suggests that this excess is also from the statistical decay of the nucleus.
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