Distribution of radiative strength in GD-156, 157 and 158 nuclei

The distribution of E1 and M1 strength Gd-156, 157 and 158 compound nuclei has been studied. Several formulations of strength functions are compared to average resonance capture and/or photo-absorption data. Further these prescriptions are used in calculations of the total average radiation width, radiative capture cross sections and gamma-ray spectra, and compared to available experimental information. As the calculated quantities critically depend on the magnitude and energy dependence of the gamma-ray strength functions, the results can be used to test the validity of their models. By analyzing the results for 155,156,157Gd(n,γ) reactions strong evidence was found for an E1 strength function which is based on a generalized Lorentzian, enhanced compared to spherical nuclei, with a energy dependent spreading width and a nonzero limit as the energy tends to zero. For M1 radiation the giant resonance spin-flip mode is favoured. The possibility of inclusion of a M1 scissors excitation at around 3 Mev in Gd-156 is discussed. (author). 48 refs.; 11 figs.; 4 tabs