The giant dipole resonance in highly excited nuclei: Does the width saturate

I have undertaken new measurements of 18O+ 100Mo reactions from E(18O) = 122 to 214 MeV bombarding energy in order to understand better the width evolution of the hot GDR versus excitation energy in near-Sn compound nuclei. My approach combines results from separate measurements of light-charged particles, gamma-rays and evaporation residues. First, I have measured light charged particle emission and deduced the effect of preequilibrium energy and mass loss prior to compound nucleus decay. Large preequilibrium losses of approximately 20% of the full fusion excitation energy and several mass units are observed for bombarding energies as low as 11 MeV/nucleon. Second, using a new array of three large NaI spectrometers along with a gamma-ray multiplicity array, I have measured the gamma-ray emission cross sections and angular distributions for five bombarding energies. These data permit a direct separation of the statistical GDR component from the underlying bremsstrahlung. Last, the measured evaporation residue excitation function is used to extract the GDR strengths with good accuracy.;The analysis of GDR spectra includes a simultaneous fit of statistical emission plus bremsstrahlung to both the measured gamma-ray strength function and the a1(Egamma) coefficient determined from the angular distributions. A careful account of the important dynamical effects of preequilibrium energy loss and bremsstrahlung emission is necessary for a reliable determination of the GDR parameters, while further insight into the excitation energy dependence of the GDR is gained by examining the dependence of the fitted parameters on the average excitation energy following giant-dipole emission rather than on the initial compound nucleus excitation energy. Results for the energy dependences of the GDR strength, width and centroid energy are presented and the measured widths are compared with the results of thermal fluctuation calculations of the width of the GDR. An additional contribution to the width of the GDR equal to twice the compound nucleus evaporation width may also become important at the higher energies. The result of this study is a new and qualitatively different understanding of the temperature evolution of the GDR in hot nuclei; namely, the width of GDR continues to increase up to at least T ∼ 2.4 MeV, which represents the highest temperatures for data measured here. Previously measured widths, which have, been reanalyzed here, give additional support for an increasing width up to T ∼ 3.2 MeV.