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Title:Photoneutron spectra from monoenergetic photons on Pb208 in the giant resonance
Author(s):Calarco, John Richard
Doctoral Committee Chair(s):Hanson, A.O.
Department / Program:Physics
Subject(s):photoneutron spectra
monoenergetic photons
giant resonance
time-of-flight of photoneutrons
Abstract:The time-of-flight of photoneutrons from Pb208 was observed at 1150 for incident gamma ray energies of 9.90 MeV to 16.87 MeV. The University of Illinois 25 MeV betatron and bremsstrahlung monochromator facility were used to produce the gammas and determine their energies within 1.7% (full width). Neutrons above 200 keV were detected with a 5 inch diameter by 2 inch deep glass cell filled with NE213 liquid sCintillator, viewed by three RCA 8575 photomultipliers. Coincidences between the photomultipliers were used to reject random noise pulses', and neutrons were identified against the high gamma background by pulse-shape discrimination. The neutron energy resolution at 1.46 m was +3.5% at 6 MeV and +2.5% for en~rgies less than about 2.5 MeV. The total cross section was extracted as well as partial cross sections fur those final states which could be resolved. The gamma absorption cross section reached a maximum of 650 mb at 13.25 MeV, with a width of about 4 MeV. The shape was generally Lorentzian, but structure was observed at 10 MeV, 11 MeV, and 12 MeV. The peak at 10 MeV was found to be due to neutrons leaving Pb207 in the (P3/2)- state. The spectra were analyzed by assuming that the gamma ray is absorbed by exciting a single, well defined dipole state, which is a linear combination of elementary particle-hole excitations. The decay of this state was assumed to be either by direct neutron emission from a particle-hole state or by thermalization and subsequent neutron evaporation. At low gamma energies, individual final states were resolved, but the partial cross sections were not accounted for in detail by the model. At higher energies, between 12 MeV and 17 MeV, the majority of emitted neutrons were distributed as would be expected from neutron evaporation characterized by a nuclear temperature of 1.0 MeV. An excess of fast neutrons was observed in all the speotra which increased from about 10% at 12 MeV to about 15% at 14.5 MeV where it leveled off. This excess was found to be consistent with that expected from direct emission using available particle-hole amplitudes, it was dominated by emission to the (f5 / 2 )- and (f7/ 2 )- states in Pb207 • The fast excess did not increase as rapidly as expected. This indicated that the total width of the dipole state increased by about a factor of 3 between 12 MeV and 17 MeV. A crude estimate of the absolute magnitude of this total width, based on the observed 15% fast exoess, was smaller than the observed width of the giant resonance by about a factor of 2. This discrepancy and the observed Variation in the direct excess indicated either a breakdown of the assumption of a single dipole state or structure in the statistical width.
Issue Date:1969
Genre:Dissertation / Thesis
Rights Information:1969 John Richard Calarco
Date Available in IDEALS:2011-07-13
Identifier in Online Catalog:6074316

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