| Abstract: | An experiment is described which utilises the excellent energy resolution (1:10⁶) of neutron-capture gamma rays in order to determine nuclear energy-level widths between 6 and 10 MeV. Neutrons from the Texas A&M research reactor produce 7.277 MeV gamma rays in iron: these gamma rays strike a lead target and are scattered by resonance fluorescence. The energy of the emitted gamma rays varies continuously with the scattering angle as the result of recoil energy imparted to the target nuclei. This "variable energy" beam may now be used as an exciting beam to search for nuclear energy levels. As an initial experiment, the level in Pb²⁰⁸ responsible for the scattering was studied by scattering the emerging gamma rays a second time. It was found, as expected, that the resonance scattering was reduced as the first scattering angle was increased from near 0°. The linewidth can then be determined from these data. Another parameter that may be found is the separation energy between the incident line in the iron spectrum and the resonance level in lead. Denoting these quantities by Γ and δ, respectively, the following values have been observed in the present investigation: [line break] Γ = 0.77 ± 0.05 eV [line break] δ = -9.55 ± 0.40 eV [line break] These values are in agreement with the measurements made by other methods. |
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