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Title:Photon scattering cross sections below the photoneutron threshold for Pb206, Pb208, and Bi209
Author(s):Stein, Nelson
Doctoral Committee Chair(s):Axel, P.
Department / Program:Physics
Subject(s):photon scattering
photoneutron threshold
Bremsstrahlung monochromator
monoenergetic photons
Abstract:The elastic scattering of photons has been measured as a function of energy below the (r,n) threshold. A bremsstrahlung monochromator was used to provide nearly monoenergetic photons of energy resolution between 1 and 2%. Cross sections were obtained for Pb 206 between 5.9 and 8.2 MeV, for Pb 208 between 6.6 and 7.5 MeV, and for B~0 209 between 5.1 and 7.7 MeV. In all three cases, structure and sharp fluctuations with energy were observed. The Pb208 cross section contains three strong peaks centered at 6.72, 7.03~ and 7.29 MeV. Each peak is consistent with the experimental resolution and is most likely dominated by a simple strong level whose width is much less than the resolution. The quantity r 2/r (r and r o 0 are the total and ground state radiative widths) for each peak is 14 ~ 7 eV at 6.72 MeV, 30 t 13 eVat 7.03 MeV, and 41 ~ 10 eV at 7.29 MeV. Additional resonant absorption measurements indicate that r o/ r is very close to 1. The widths associated with the structure in Pb206 and Bi209~ere generally greater than the resolution. In Pb 206 a well defined resonance about 400 kev wide is centered at 7.85 MeV with a peak cross section of 20 mb. Another maximum occurs at 7.45 MeV, and further structure superimposed on a generally decreasing cross section is suggested as the energy decreases. In ii2 09 the scattering strength is concentrated into two large peaks with maximum cross sections of 21 and 27 mb at 5.5 and 7.3 MeV, respectively. Separating the peaks is a broad valley in which the average cross section is about 1/7 of the observed maxima. o 208 . 206 In contrast w~th Pb ,the structure observed in the Pb and ii2 09 cross sections represents the average contribution from many unresolved narrow levels included within the resolution. Resonant absorption measurements indicate that at 7.3 MeV in ii209 the average spacing, D, between levels having the ground state spin is about 1.5 kev, and the average radiative width, r-o, to the ground state is about 0.2 eVe Estimates at 7 MeV for Pb206 indicate approximately similar values. Since ~(1,1) averaged over the resolution is proportional to (r 2/r )/D, it can be shown that similar structure is very likely present o in the ground state photon transition strength function» ro/D. The widths of only several hundred kev that are involved in this structure may be important for the general question of the distribution of nuclear transition strength over available energy levels. Accepted values of the imaginary part, W, of the optical potential would imply strength function widths much larger than those observed in this experiment. Fluctuations in rolD, rather than the monotonic energy dependence usually expected, may require a more cautious interpretation of measurements of gamma ray strength than has appeared necessary up to now. Where structure is present, results obtained either in isolated narrow regions or over broad regions with poor resolution may not be reliable for determining the general behavior of r-o/ D. The results provide lower limits for the photon absorption cross sections below the (r,n) threshold; for Pb 208 ,upper limits are also given. Similarities among the three nuclei, both in scattering and in the photoneutron cross sections just above the threshold, suggest the likelihood that the observed photon strength involves primarily the Pb208 core. The magnitude of the total strength observed throughout the threshold region is consistent with the two estimates of electric dipole strength that are closely related to the giant dipole resonance.
Issue Date:1964
Genre:Dissertation / Thesis
Rights Information:1964 Nelson Stein
Date Available in IDEALS:2011-05-19
Identifier in Online Catalog:2858669

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